The Msquared Files

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msquared

Member
Hello everyone. In a effort to be sure each and everyone learns from my knowledge, fortunately I was able to save several of my posts to many questions. I am going to post them here.

Over the next several weeks I will be posting comments to many different questions that I have answered.

I will make no comment on these posts and do not expect any debate on them. You may debate on them in a new thread as you wish.

Maybe you guys could suggest to Bob to preserve these somewhere for all to learn from.

I would very strongly suggest that you copy/print or save these posts.

Here goes the first installment:

Installment 1

Speaking to setting toe-out.

It seems that no one can explain this one. OR they know it and just do not bother to tell anyone.

Here is your answer.

Since the right front is heavier than the left front, the right front will have a higher slip angle. Slip angle is the difference between the direction the wheel rotates and the direction the tires contact patch points. Remember, this is a rubber tire and it flexes in all directions. Real race tires are more flexible than DOT tires and will have higher slip angles due to their construction. The reason the slip angle is important to our toe setting is that we should align the slip angles rather than the wheels.

At the point in a turn where the right front is turned right in a drift, the wheel is dragging the tire, inducing a slip angle. With the wheels toed out, the tires contact patch is running truer to the direction of travel than the wheel. With this in mind, we need to set the toe out using the right front, which compensates for the degree of slip angle difference.

There is a method to the madness.
 

msquared

Member
Installemnt 2


Straight vs Z-bar Chassis

Well I promised more and here it is.

Straight bar vs s/z-bar has to do with dynamic loading of the chassis, where and how the weight moves around.

The s/z-bar chassis tend to be more stable and tend to be more positive steer than the straight bar chassis. On the other hand straight bar chassis tend to be stiffer and rely more on lateral weight transfer for cornering. The end result is s/z-bar karts tend to like more cross, less left side and more aggressive caster settings. Where straight bar karts tend to be a bit more stagger sensitive.

More important is where the tubes ties into the left frail and engine rail. As you move the left connection point forward or rearward you dramatically change how the kart works the LR. Same thing goes for the RR. I remember Mike Ward talking about how as much as 1/8” difference had very big effects on kart handling.

To be very general, karts that have the bar that connects close to the LR hanger (e.g. the older straight bar chassis) load the LR more and sooner on exit, which is why those karts like lower cross in them. The opposite goes for moving the connection forward.

You just cannot stop there. You also have to consider the waist of the kart and how the rails angle in then angle out. The distance from the front loop to the main crossbar (where the front of the seat mounts) also has to be considered. Todays karts tend to have a narrow waist and have become pretty short in the front loop/crossbar area. Then there is the relative stiffness of the various parts of the kart. There's the main rail tubing diameter and thickness, all the cross bars' diameters and thicknesses, the various bar lengths and bend types and locations, the termination points where the rails weld together, etc. If you look at the points on the rightside where the engine rail, the crossbar all tie in, there is a lot of stress at those points. You see a lot of chassis crack either the tubing or weld there.

Most chassis today have gotten stiffer. As a result the norm for tire duro has also gotten softer. Anyone notice that? Also a stiffer chassis is more consistent and consistency equals faster lap times. Also a stiffer chassis is going to “hold” the front geometry better than a softer chassis.

There is going to be an optimal front to rear roll stiffness for a kart. That's effectively the chassis torsional stiffness and it determines how effectively you can transfer loads between the front and rear axle in the corners. When I mention axles here think of the front wheels and rear wheels With a stiff chassis, most of the "twist" of the axles with respect to one another happens in the deflection of the chassis at the waist and other areas. That's good because that deflection is well understood and is controlled by the tires which act as dampers/springs. With a flexible chassis it's just the opposite. The chassis flexes and limits how well the tires can function. So in the end you can do a better job of controlling the tires if the chassis is stiff. A better job of controlling the tires means faster lap times... if you can retune the chassis to take advantage of it. Of course that assumes your original chassis setup was optimal.
 

msquared

Member
Installment 3

I remember this from a discussion about dynamic weight transfer.

Ignoring the effects of the human body, and chassis and tire reactions, here's how I look at it- on an Oval track:

There are 4 points of the kart where weight is transferred from the kart to the ground. Those 4 points are the connecting points of the hubs/rims.
If you draw this out on paper.(Looking down from above the kart). Connect the dots and you'll have a somewhat warped rectangle.

Scale the kart and write down the 4 corner weights.

By knowing your scaled weights in a horizontal plane you can find the approximate weight centers in both the x and y positions. If you don't know how to do this, ask.

The points for our discussion are;
"X" is Front to Rear,
"Y" is Left to Right (Driver side to Engine side),
"Z" is the vertical aspect.

Next,
Locate the weight center between the LF and RF (this point is XFront).
Locate the weight center between the LR and RR (this point is XRear).
Locate the weight center between the LF and LR (this point is YLeft).
Locate the weight center between the RF and RR (this point is YRight).

Now we are going to start our kart observation from a position already racing in a straight line, shortly after corner exit.

When the kart is accelerating forward, weight is transferred backwards, the X value will move towards the rear, the Y value will not change, and the Z value will rise upwards slightly (due to weight transfer and aerodynamics).

Next, the kart will decelerate slightly due to friction incurred from the front end tire grip while turning in at corner entry- the x value will move forward, the Y value will not change, and the z value will drop slightly. So, at corner entry the X value will decrease (usually to the apex) then increase with acceleration towards corner exit. The Y value will move to the Right until it reaches the point where maximum G's occur (apex), then it will begin transitioning back towards the Left towards corner exit. The Z value will roll slightly downward to the Right at corner entry, then back upwards from the apex to corner exit.

We've just looked at all of the 2D points in motion from a 3D aspect.

Now...Backing up a square, look at your original x,y,z values.

X and Y form 2D points on one plane, while the point for Z is above the X and Y plane.

Look just at the motions in X and Y for one lap.

Next, draw a line (A) from the weight centers of XFront to XRear in their static positions. Note how the location of the midpoint of line A moves (due to the motions along the X and Y plane) for that one lap. Next, draw a line (B) from the weight centers of YLeft to YRight in their static positions. Note how the location of the midpoint of line B moves (due to the motions along the X and Y plane) for that one lap. The intersection of A and B is our moving "Anchor point", ZZ (ZZero). Calculate the VCG and you'll have a "ZS" value (ZStatic). As the Z position rises and falls, we will have new values also (that moving 3D point is the Virtual "M"). Now look at the vertical rise and falling motion from point ZS for one lap. Next, draw a line (C) from our Anchor point (ZZ) vertically to the ZS position. Next, draw form any imaginary spherical radius with the center at ZZ and the outside radius at ZS.


In closing;
While racing around our oval track, the positions of ZZ and ZS will move, and can be calculated. For any track at any given speed, corner radius, and banking, G-Force values can be determined. If I can determine the G's and know the grip available at the tires**, I can calculate a maximum speed that I can put a kart through a corner before it will get loose and spin out (**Yeah, I know...there's a whole lot to the tire science, so let's not even go there!). If I can know the numbers, I can lock in on chassis analysis a little tighter. When I find "good numbers" for my chassis, I have a baseline to work from at any track, on any surface. Tracking this kind of stuff is how I learned to dial a chassis in AFTER it already "feels good".

Being fast for 10 or 20 laps is pretty simple, but when it's 50 or 100 laps, no amount of tire prep will beat selecting the proper tire duros and getting the chassis "right".
(That was tough to express on paper...hope it all came out straight!)

Y only changes due to g-forces during cornering- when motion is NOT is a straight line. Like I said, sometimes its difficult to write what I think and see in my mind.
I envision Z as rising and falling because its anchor (rotation) point during forward acceleration is the rear axle, and the center of the mass, "Z" (basically the driver's body) is forward of the rear axle. Due to the forces incurred during acceleration and aerodynamic lift, the nose gets "lighter" and it raises. I may be wrong. I'm not a genius, but that's how I envision it. It could also be that the rotation downwards behind the axle presses down so hard that it leverages the front up when pivot
 

msquared

Member
Here is the way I am going to answer your question ??????. First of all, for those who are new here let me give a bit of a lesson. No doubt as chassis builders have learned and are still learning, kart tires are very sensitive to camber changes as I have mentioned before. They are now making the front of the chassis even more stiff so the front tires are utilized more and the front tires, especially the RF does not see as much dynamic camber gain due to chassis flex and steering input (as little as it may be). Camber also has ties to cross.

Depending on how the chassis is built is going to determine the amount of camber that you run along with cross. This means that your chassis will be sensitive to the amount of camber and cross that you run. So, as you increase cross you will need to increase camber and vise versa. To a certain degree.

Due to front stagger, the chassis has some degree of natural camber in it. Adjusting cross using the LF (preferably) will change the angle of the front of the chassis (known as the rake). So, adding cross will add a little more negative RF and a little more positive LF camber. You also have to consider your caster settings. More caster acts as a reduction in front stagger meaning that if you run say LF 6 and RF 10 degrees of caster you may need 1 ¼” of front stagger where if you run LF 8 and RF 12 degrees of caster you may need 1 ½” of front stagger. Then you have to consider the steered camber gain between the two settings. I generally run very low caster settings to control the steered camber gain and the bump steer effect.

I know the question is going to come up about how much of a cross change until you need to change camber. The answer is it depends on many factors, your chassis and the track to name a few. As a general rule a change in cross of 5% or more is going to warrant a change to camber.

Let me explain the monkey-see monkey-do scenario. An old pro is at the track with his new 2012 chassis running 70% cross with RF camber at -3.0 and 10 degrees of caster. The new guy comes along with his 2004 chassis running 60% cross with -2.0 RF camber and 12 degrees of caster. The new guy asks, “How much camber you running?” The old pro answers, so now the new guy bumps up his camber. He goes out and runs like crap. Now he is scratching his head. My point is that you have to know your own chassis and what it likes, not what everyone else’s chassis is like. Same for tires.

On most chassis these days they are designed for higher cross settings, typically 60% to 70% or more! Camber, cross and front stagger are going to go hand-in-hand with each other. Typically on lower cross older chassis you will see less cross and camber and more front stagger. The higher cross chassis will be the opposite.
Here is why, typically as the cross range increases in adds grip to both the LR and the RF. The added static and dynamic weight on the RF causes the sidewall to flex more during cornering. This means that you must add camber to the RF to fully utilize the tires contact patch and change how the vertical loads are applied to the contact patch. It also helps the chassis to “rollover” a little more onto the RF, keeping the RR from becoming overloaded and delaying LR loading on turn exit.

As the track grips up the steering gets heavier and the chassis becomes more sensitive to steering input. This is due to additional grip in the RF. So to reduce this, you add a little RF camber. By doing this you change the way the RF contact patch loads moving it more towards the inside of the contact patch rather than the center or outside. I call this the grip balance.
Here is what I mean. Camber affects the vertical load distribution of the tires contact patch and can also impact the footprint of the contact patch. As lateral loads increase, the tire tends to distort, or "roll under". Depending on the chassis, camber change characteristics (due to chassis flex and steering input at the front), the “rolling” (all-be-it ever so slight) of the chassis about its longitudinal axis may tend to aggravate this. This is why understanding all these imaginary lines are important to understanding how and why a chassis works.
Imagine that your kart has zero right front camber. The result is that the inside edge of the tire tends to become more lightly loaded as cornering loads increase. The inside edge of the tire may even pick up off the track, losing contact with the track surface entirely. This results in less cornering force and speed.
However, if the chassis is set up so the right front has negative camber when the chassis is static, this will compensate for the distortion of the tire and the rolling moment of the chassis as I mentioned in my previous paragraph. In the static state, the outside edge of the tire will be lightly loaded and the inside edge will be heavily loaded, but as the lateral loads go up and the chassis rolls, the outside edge will be loaded more and the inside edge will be loaded less. It is a fine line that you are looking for. It is also why karts are camber sensitive. It is also why you add RF camber as the grip level goes up, you want to put the contact patch load back in favor of the inside edge instead of the outside edge.
If you run too much RF camber the kart will want to push, especially on turn exit due to less contact patch and the forces of weight transfer now pointing more towards the LR/RR due to acceleration. When you have too little RF camber the tire is getting most of its grip from the outside edge of the tire. The tire grabs, releases, grabs, releases and so on. These drivers are sawing on the steering wheel as if they were cutting wood. More like a front end hop. It can also just be plain loose in some cases.

This same camber sensitivity also happens at the RR. You just don’t have as much control over it.

I did not even cover camber thrust and tire slip angles. These topics and many more subjects are covered in my chassis manual.

Paul, as you would say, “I was thunkin bout this.” Guess it is like which came first, the chicken or the egg? Lets look at it this way, I took a metal ruler and put it on my desk. No matter how much weight I applied it moved pretty easily. I then took a rubber eraser, put it on the desk. It gripped with no weight added. I added more weight and it gripped even more. I’d conclude that in order to have weight transfer you first have to have some amount of grip. The grip allows weight transfer to occur. So, weight transfer is following grip. Once the gripping object cannot handle the weight, it is grip now following the weight.

Take a metal ball connected to a string. The string provides the “grip” when it is swung around your head and keeps the ball going in a circle. Once the string breaks (the amount of grip is exceeded) the ball just goes straight. Kind of like a push in a kart. The weight wants to go toward the RF but it can’t because there is no grip.
 

msquared

Member
I am not going to speak for Joey but am merely going to give the readers ”my” take on camber. A zero toe setting, for instance will lessen tire scrub and friction on the straights, which will lessen the rolling resistance of the kart. Rolling resistance lowers the acceleration and top speed the kart can produce, resulting in wasted engine power (which you've probably paid your engine builder lots of money for). I am using “zero” here merely as an example.

Zero (or very close to zero) camber settings will help to keep the full width of the front tire's tread in contact with the track surface when cornering, particularly at mid corner and corner exit on a flat track. As you corner the right side tires deform quite a bit and camber compensates for this.

As it's very common to see full sized racing cars using obvious negative camber settings, some karters conclude that if this is OK for NASCAR, F1 and Indycar teams, it must be the right thing to do. As is the case with karting, it is mostly moneky-see, monkey-do. Unfortunately for those karters thinking this way, the tires used in most other forms of motorsports are radically different in their construction being radials, while kart tires use cross-ply (bias-belted) construction. Radial tires have much more flexible sidewalls than cross-plies, and because of this can work well at larger camber settings. The stiffer sidewall of a cross-ply tire means it has to be kept very close to vertical to work correctly, meaning that you must match the camber to the banking of the track. The tire chalk method works very well in finding the optimal camber to match the track. To avoid much debate I will not mention tire temps.

Inaccurate camber incorrectly loads the tire and lessens the size of the contact patch. As a result this smaller tire 'footprint' will have a tendency to overheat. Especially in hot conditions, this contributes strongly to premature tire wear and inconsistent handling (ie. the kart will handle differently as the race progresses, probably tending to increased understeer).

At most races you will see plenty of front tires with substantial wear on the inside edge, yet virtually unworn on the outside. You won't see many, if any tires looking like this at the front of the grid for the final.

It's really very simple, if the rubber's not in firm contact with the track, it's not providing grip, and the rubber which is contacting the track is being asked to do the work of the entire tread width, which it wasn't designed to do. Poor camber settings can have a similar (but not identical) effect as fitting undersized (narrow) tires on your kart. I'm sure nobody would deliberately put narrow tires on their kart!

In racing, it takes only a tiny deficit to lose large amounts of track distance. For instance, if you're only losing 1% to the kart in front due to poor alignment (or any other reason), then in ten laps on a 1000-foot track you will lose 100 feet!

Looking at this another way; on the same track, assuming a 'hot lap' time of 20 seconds, then a 1% deficit is equal to losing 2 tenths of a second per lap. An expensive engine blueprint might gain you 2 tenths. So why waste this costly and valuable advantage with poor front-end settings?

Toe and camber are among the most important settings on the kart, but they are also the two settings most likely to significantly be altered when the driver's weight is placed in the seat. This shouldn't be surprising, since driver weight can easily be over half the on-track kart weight.

If a kart is aligned to zero toe and camber without the driver seated, it is certain to have some unpredictable amount of negative camber with the driver in the kart. Most karts (but not all) will gain some unpredictable amount of toe-in, up to 1/8” is quite possible. Toe-in can easily contribute to poor turn-in as it makes the kart more resistant to change of direction and lessens turn-in weight transfer.

As the kart will always be raced with the driver in it, it is strongly recommended both the toe and camber be adjusted to the baseline settings with the driver seated in the kart.

This is by far the best starting point for setting camber, and the exact setting can be fine tuned using tire wear as a guide. If you're lucky enough to have a tire temperature gauge, adjust from tire temperatures across the tread. Or, if using tire chalk you will have 1 ½” of chalk left on the outside of the right front and ½” left on the inside of the left front.
 

msquared

Member
Basically, LF caster, along with camber acts as a mechanical weight jacker. It controls how much and how fast the LR is “lifted off” the tracks surface. Now, when the steering wheel is turned on an oval kart, caster causes weight to jack from the RF/LR axis onto the LF/RR axis. This happens whether the kart is moving, or whether it's setting still on the scales. Just like on a sprint kart, weight jacking causes the LR tire to get lighter. When the LR is lighter, it is more likely to be able to “slip” or better yet lose some of its dominance in the corner. If you try to run ½” rear stagger on a 1/8 mile track, you don't have enough stagger for the track. You'll have to rely on caster and your weight percentages to allow the LR tire to “slip” through the corners. This is much like what sprint karts have to do since they run no rear stagger they have to rely on larger amounts of caster and camber to “unload” the LR.

Scrub radius (also called 'kingpin offset') and caster angle work together to produce a diagonal mechanical weight transfer from the LR tire and the RF tire, to the RR tire and the LF tire. This weight transfer causes the LR tire to be physically lifted from the track surface at turn in. If this weight transfer is not great enough, the combined grip of the rear tires can simply push the front wheels straight ahead.

This mechanical weight transfer means the LF tire is much more heavily loaded than the RF tire at turn-in. As a result, the LF tire provides most of the front-end grip at turn-in. Once into the middle part of the corner most of the kart weight is transferred to the outside tires (due to cornering force). The mechanical weight transfer then becomes far less important (and can even be counter productive) and is largely superseded by weight transfer due to cornering forces (lateral 'g' forces, causing frame flex). It is possible for the LF to actually have weight to reverse transfer to it. That is a different story. I mention it in my chassis manual.

Increasing either the caster angle or scrub radius will increase the LR wheel lift at corner turn-in, which is really what you are after (if the kart is turning in badly). Increased caster may require using more positive camber to keep the tires contact patch flat on the track during cornering. Be aware that increased scrub radius and / or caster can contribute to front tire overheating in some conditions. I have had this happen on dirt before at both the front and rear tires (rightsides). The kart just snaps loose when it happens.

Having said all of this, give this some thought. The less amount of grip that is available the less that you can involve the LR with the track. This means slower lap times. The more grip that the track has, the more that you can involve the LR with the track meaning faster lap times. Remember, the LR is the dominant tire because it is the heaviest tire on the kart. This dominance must be controlled.

Front-end settings have a huge effect on the overall handling of the kart. If your kart turns-in to a corner the way it should, then the rest of the corner will be easier and faster to negotiate as you are not having to catch up with the effects of poor turn-in. In addition, the rest of the chassis is likely to be easier and less confusing to tune if the front end is functioning properly.

Finally, many handling problems that may at first seem similar can easily stem from different causes. Any adjustment you make to your kart is only correct if it lowers your lap times, assuming that the basic alignment is at least close to accurate.

Having said this, here is where I feel racers fail. The chassis manufacturer builds a chassis and tests it for quite some time to gather data. They look over the data and come up with their baseline numbers. Most will consider these numbers to be set into stone and will not step outside of them. So they leave speed in the kart. The ones who are the fastest step outside of the “numbers” and get all they can out of the kart by maximizing their overall setup. In other words they match the setup to the geographical area, the driver (style and build), the tires, tire prep to name a few. They merely use the baseline as a starting point and where they end up is what is the fastest for them. There are no magic numbers but only a baseline to start from.

As a very general rule of thumb keep this in mind:

I tend to ignore deceleration and acceleration because the stockers do not have much ability to accelerate or decelerate, so it's all about momentum and rolling resistance. All I really care about is:

1) the front-to-rear balance of grip (so that handling is balanced),
2) overall grip (to maximize turning ability),
3) and minimization of rolling resistance.

#1 is all about proper chassis setup, this is the easy part. It isn't hard to get a balanced well-handling kart.

#2 is a combination of having the right tire compound (and prep), and chassis setup - primarily left side weight and VCG to get the proper amount of side bite. This part is also pretty easy.

#3 is what going fast is all about. This is a combination of having the right tire with the right air pressure, balanced with the right amount of #2 (side bite). The #3 balance is constantly changing as the track conditions change.

#4 more caster split will make the kart want to turn into the corner on its own. If you have too much caster split, then in the center, the kart may want to push because the LF isn’t jacking weight enough with respect to the RF to take drive away from the LR.

#5 Less caster split will make the kart more responsive, due to steering input getting in to the corner and in the center but may either make the kart too twitchy or make the kart "bind" up because its not wanting to travel the corner radius as much on its own.

You generally see caster splits of 2 or 4 degrees. With high cross setups being run these days, the RF carries much more pre-loaded than the LF, that more caster split is not generally needed to get the front end to want to turn left on its own.

Then you have reverse caster which changes those darn imaginary lines which seem to have a huge impact on handling. That has to do with scrub radius and caster trail. Front stagger acts like a reduction in caster which is why reverse caster makes sense.

My brain is depleted right now but you should get the idea, do not be afraid to step out of the box and try something different.
 

msquared

Member
I think cross is much like politics, everyone has an opinion. I've had chassis that were supposed to be low cross chassis (48 - 55%) but ran as much as 60%. I've has chassis that were high cross chassis and ran lower cross on them. So to me cross is just a reference number and is to be used to fine tune the kart and not to force it to handle.

Nose and leftside will have more of an effect on overall handling.

Older chassis pre 2000 nose 42-44%, cross 48-54% leftside 56-58%, 2000 to 2008 or so nose 44-46%, cross 55-60%, leftside 54-56%, 2009 to current nose 47-49%, cross 65-70%, leftside 57-59%. I believe Todd Goodwin has numbers close to this give or take a few percentage points.

For me I set the chassis up with a generic setup that I think will work and then start matching things like caster, camber and stagger to the overall setup.

Let me give you an example. I have had several guys on 2011 and 2012 chassis. All of them complaining about the handling. They just cannot get them to handle very well. Depending on the weight class and their location I'll suggest something outside the box like:

Nose 46%
LS 54%
Cross 65-70%
Front stagger 1.25"
Rear stagger 1"
RF Camber -2.25 Caster 10
LF camber +0.50 Caster 6

They will email me saying they have never handled better and are winning races. Others need more nose or leftside or both. It depends on the situation.

I even have a kid running on 32-38% cross on asphalt and winning!

Having said all this here is what I think of cross. It is very general, because there are a lot of factors that apply.

Here are some examples: Cross 65% and leftside 58%. Lowering cross to 60% will tighten the kart because it is now closer to equaling the leftside weight. Raising cross will free up the kart because you are moving away from leftside weight. So, if you now had 50% cross and 55% leftside, increasing cross, moving it closer the equaling leftside will tighten the kart. Lowering cross will free the kart.

On a high grip track, low cross tends to give you a kart that does not get tight and handles well, but does not give you much speed off the turn. Why? Because you are controlling the left rear but not maximizing the right front, the handling is somewhat sluggish. You are mainly running off the right rear and keeping the left rear from pushing the kart.

Medium cross (the dead zone) has inconsistent handling and is slow. Why? Not enough load on the right front to relieve the load on the right rear. The right front does not load enough and the left rear reloads too soon resulting in a tight condition. This is due to what is referred to as a fight for dominance of the rear tires. Generally high cross is for tracks that have a lot of grip. Low cross is for tracks with less grip. For running on coke syrup indoor tracks you actually want to go negative on cross running 48% or lower. Some will use 55% or lower.

High cross gives you a quick reacting kart that plants the right front hard enough to relieve the right rear and control the left rear. The higher the grip that the race track has (the more G’s pulled) the more you have to pre-load the right front corner of the chassis (spring) with the high cross to get the kart to store and re-disperse the energy that it is receiving from the racetrack. Without the cross, the right rear generally wastes the stored energy (binds the kart) of the spring (chassis). That is what is meant by “freeing the right rear up.”

Another thing with cross is that on dirt you need the higher cross to put more weight on the LR because of the lack of grip that is available in the “groove” that the LR is traveling in.. On asphalt the available grip is about equal in the groove both rear tires are traveling so you need less cross but more left.

I repeat, with a car there is a balance that has to be considered among the four corners of the chassis because each corner is independently suspended which is to say, it does not care about what connects one corner to the next. With a kart, each corner does have a suspension (the tires) but each corner, very much so, cares how it is connected to the next corner (the chassis itself). Because of this, you can’t think in terms of “balance” as it pertains to your setup percentages related to a car.

Cross is just a reference number that is a variable for adjustment. Run the most leftside (as much as you can) and nose weight (as little as you can) that you can get away with. Use cross to make the kart faster. Too much or too little cross is rarely if ever the real main problem. Adjusting cross is sometimes a means to overcome another problem. Adding cross will increase the right front load and decrease right rear load. Adding cross will usually stop an engine from bogging in the turn because this is normally a result of over grip in the right rear. As mentioned before there is a fine line between cross and leftside. If you lower leftside you can lower cross. Also the higher the caster, VCG and stagger the more cross that you will need as well as leftside. Higher cross set-ups need lower leftside than do lower cross set-ups. If you choose to run higher cross you will need higher caster, more right front camber and a higher VCG. About the only other thing to cover is this. There is an optimum setting for cross, if you have too much or too little you will be loose. There is a certain amount of cross that will make you the tightest. If you go over that, adding cross will loosen the kart. If you go under that, reducing cross will make you loose. You need to find where you are at on cross; above or below the optimum amount.

That is just my humble opinion on it.

Mike McCarty
Chassis manual (Only $17.95)
www.kartcalc.com
 

msquared

Member
Does anybody know what the manufacturers are talking about when they say 'hardness' of the axle? Are they actually talking about the material property that tells you about the hardness of the surface as in wearability, like Rockwell or Birnell hardness?

As I understand it, all steels have about the same modulus of elasticity (which Dutton pointed out) which is the inherent property of a material that dictates its stiffness. Different alloys have different yield and tensile strengths (which is what I believe karters are more interested in) but this doesn't make a difference in stiffness, it just dictates how far you can bend the material before it stays bent or breaks.

Does through hardening a material affect its modulus of elasticity? I'm pretty sure I have seen steels with very different hardness values that still had the same modulus of elasticity.

There are three principal operational definitions of hardness: Scratch hardness; Indentation hardness; and Rebound, dynamic or absolute hardness. The surface hardness measured with Rockwell or Birnell is Indentation hardness. This is done for tires. The hardness related to an axle bending is the Rebound, dynamic or absolute hardness, which is related to Young's modulus of elasticity. The modulus of elasticity can change with the amount and type of alloy material and with the crystal structure of the alloy. There are many crystal structures or phases of steel that can usually be created by the heat treating processes.

Also, the frequency response of an axle (and a chassis) is a function of several factors including mass, elasticity and damping. These are the 3 major factors in any dynamic structure. I have noticed that in karting very little is discussed about damping. Everyone tries for lightweight components, adjusts flex, but has no idea about damping. Some concerns regarding damping is seen with tires but nowhere else. Dampening is one of the reasons I always stress tire spring rates.

Whether a chassis responds well is a function of how new it is, or if the designers got lucky by trial and error. I'm not saying the designers don't know what they're doing, they just can't do what race car designers do, which is to shoot for a stiff chassis and adjust elasticity with springs and damping with shocks. With karts, they have a very difficult job involving a lot of empirical observations of what seems to work.

I believe that much of why axles (and chassis for that matter) do what they do is related to what is happening in the frequency (dynamic) realm, and cannot be shown by a simple static test. It is my belief that the dynamic stiffness and damping is really what is at play.

While the rebound hardness can be affected by the modulus of elasticity, or Young’s modulus, most steels have less than 10% variation in stiffness, so that is not the effect one would see with a rebound test on steel.

What this really measures is elastic strain energy and specific damping. While these two effects can confound each other, it is not a good test to study the damping of the material.

Consider two samples of a low/medium carbon steels. One hardened, one annealed. The harder steel will exhibit a higher yield and ultimate strength than the annealed counterpart. When one compares a rebound test on each they will measure the elastic strain energy which causes the rebound. The plastic strain energy and the damping will act to lower the rebound because that energy was not recovered in kinetic energy of the rebound. If both steels have the same modulus, which they will within a close margin, the one with the higher yield strength, the hard one, will have more elastic strain energy (area under the stress strain curve - slope is the same, the modulus, but the triangle with the higher yield has more area, and thus more elastic strain energy) and cause the slug to rebound higher.

At any rate, it does little to tell us the damping of the material, which does change as a function of carbon content and hardness. Consider grey cast iron vs. carbon steel. Cast iron is a very high carbon content 'version' of steel. The micro structure is different but it is very well know that cast iron has much higher damping capacity than steels, and hence why it makes excellent engine block material (good for NVH). By this crude example we can infer more carbon and different micro structures have different damping properties. Cast iron has a different modulus but that is irrelevant because as I will stress, we know different steels modulus does not vary significantly.

Damping is the key, but structural damping has some interesting properties. It is often called 'complex stiffness' in that the damping force is proportional to amplitude but in-phase with the velocity. This is a popular model for understanding the effects of structural, or material damping, which is the prevalent mechanism in both the kart chassis and the axle.

Many materials science papers will outline that a steels damping increases with hardness and carbon content. So in effect a steel axle with the same geometry but higher hardness or carbon will exhibit higher structural damping. Because of the unique property of being proportional to amplitude (remember the axle is a rotating beam so under flex it is actually a sinusoidal vibration) it 'feels' more like stiffness.

At least that is the way my metal man explained it to me. I also had a very interesting conversation with one of my pals at Birel Karts. Once I have time to decipher those notes I’ll explain what he was saying about the 40mm and 50mm axles. It is very interesting and a little opposite of what I thought.
 

msquared

Member
OK, I am going to get a bit long winded here. I understand you want a short simple answer but I believe to fully understand the answer you have to look at the whole picture. This topic comes up often and I want everyone to understand it. I am sure some sort of can of worms will be opened over this but I want racers to learn and think about what they are doing and why they are doing it.

In a nut shell a softer axle will slow weight transfer and allow for less chassis flex keeping the RF planted more. This can in some instances free up the kart. A stiffer axle will increase weight transfer and plant the RR more.

I’m definitely not an expert on the practical application of different axle stiffness, but I suspect the ‘correct’ axle stiffness will very much depend on available grip and the basic rigidity of the rest of the chassis. I tend to think that ideally you want the axle to be as stiff as possible for the application and the chassis setup. However, if the chassis is too rigid (mechanically and/or geometrically) for the forces involved (basically the force created by available grip and driver weight) and the LR is tending to reload prematurely because of this excessive chassis rigidity (creating a high front roll stiffness) then short of changing to a different chassis it might be advantageous to use a softer axle. As such, I suspect needing to use a softer axle is to some degree ‘making up’ for a chassis that isn’t quite right for the grip conditions and driver weight.

I feel that this is an area of chassis tuning that is very overlooked by most and could be a very useful tuning tool not only for class but also for driver size.

My best take on this subject is that a stiffer spring will transfer more weight and a softer spring will transfer less weight. This is accepted roll couple theory. So, softening the load path (spring) from the CG to the RR by using a softer axle will reduce the DIFFERENCE in the front / rear load path stiffness (and thus the difference in roll stiffness). This should take some percentage of the transient weight transfer away from the RR and redistribute it to the RF. The CG/RF load path will now be relatively more rigid compared to the softened CG/RR load path, so somewhat more weight will transfer to the RF. However the CG/RF load path will be no more rigid in ABSOLUTE terms so the additional weight transfer may be enough to increase flex at the RF corner of the chassis or the waist and improve LR unloading. Improved LR rear unloading should lessen understeer and create a higher acceleration (G force) which will further assist in twisting the chassis and keeping the LR unloaded. To be perfectly clear, I’m talking here about LR unloading due to acceleration and weight transfer, not from the jacking effect. To be even more perfectly clear, I’m speculating on the mechanics of this to some degree.

The basic idea in chassis tuning as far as the rear end is concerned, is to get the chassis axle combo to put force on the RR at the correct rate and achieve the optimum amount of axle flex to hook up just right for a given situation. More flex is achieved through the use of shorter hubs and/or a thinner axle, and less flex achieved with a stiffer axle/more hub length. For a given frame stiffness, you will usually want a softer axle hub combo for high grip situations, and stiffer for low grip, however it is also possible to use a very stiff axle and tune the chassis for the correct amount of flex, as the overall flex is a combination of the two.

Axles have a sweet spot where they flex enough and give a free rear end but not so much that they create binding. The best way to know if your axle is too stiff/soft is to change your hub length. The same holds true for softer, that is, shorter hub makes it softer and if it feels better with the shorter hub, you are probably too stiff on the axle. I have put on a very long hub at the RR after a rain storm and simply killed the competition.

I think the answer to this question has to do with a few factors and so it can't be a generic answer. If you think about a softer or stiffer axle as it relates to spring rates and their effect on weight transfer it will help with finding a solution.

Softer springs tend to slow the weight transfer down and not transfer as much weight to the tire. This sometimes produces less grip, most of the times I've seen this in slower classes. On the other hand the slow weight transfer sometimes produces more grip when there's not much grip in the tire or the surface. It's one of those things that depends on a lot of other variables. You are changing roll stiffness and roll couple distribution.

With a stiff spring rate weight transfers faster and more of it transfers. This can also either produce more grip or lose you more grip depending on the variables involved. A tire produces more grip the more weight you put on it but at some point the amount of weight overcomes the amount of available grip. This is why sometimes if you stiffen the kart up a lot in high grip conditions it frees up. You have put enough weight into the tire that it breaks free. Highly unlikely on high grip dirt tracks.

I think a lot of people overlook what the axle flex does to the chassis. You have to remember, the axle is connected to the frame via the bearing hangers, and so by changing the axle, you are also changing the amount of flex that the frame will have at the rear end based on axle stiffness. This is why I believe some karts like soft axles to free the kart up because a stiffer kart needs a softer axle to allow enough frame flex which will free up the kart. Conversely, a softer kart would need a stiffer axle to allow the rear end to stay stiffer instead of folding up (too much flex) and therefore causing excessive side bite, making the kart tight. I believe this is also why on some karts changing the hubs can do the opposite, that is, putting on a shorter hub on a stiffer axle will actually free the kart up since it is changing the flex on the ends of the axle, BUT on that same kart, going to a softer axle will tighten the kart up because we're assuming its a softer kart and it would cause too much flex in the rear. Really, if the kart is either too stiff or too soft, it will cause different kinds of tight (maybe one is caused by having a lot of side bite and too much LR lift and the other could be so rigid that there is no lift at the LR at all) but clearly it is not ideal either way.

If properly used, different axle stiffness and different rear hub lengths can be another valuable tuning tool for the karter intent on getting the most out their set-up. But, as with so many things, just buying the pieces and trying them “on the fly” won’t get you very far. The best approach is to devote plenty of dedicated testing time to learn what each change does. You won’t get that kind of time on a race weekend. You’ll have to get out there by yourself when you can commit enough hours to learning what these tools can do for you. Once you have a grip on how each change affects your kart’s handling, then you can use them on race weekends to put your very best package on the grid.

I hope this helps and did not confuse anyone too much.

The bottom line is test it and see what happens.
 

msquared

Member
Caster Part I

There are two primary things which affect the weight of the steering, the first is the vertical loading of the tire working against the trail and the second is the lateral loading of the tire working against the trail. On the RF the vertical load causes the kart to want to turn left and the lateral load causes it to want to turn right. Because the tire can make quite a bit more lateral force than vertical how the castor affects steering weight will vary at different points on the track.

Additionally, there are two components of trail: the first is the geometric trail created by castor and the second is the pneumatic trail created by the slip of the tire as it makes lateral force. As a tire is worked harder and harder it will operate with more pneumatic trail which will make the kart harder to steer in the corner. At some point the tire will be at peak lateral force and after this the pneumatic trail (and steering effort) will do down somewhat.

With respect to the feel of the wheel, the best choice is to find the settings which feel best to the driver and leave them. The actual feel to the driver will still vary with how hard the tires are biting and which ones are doing most of the work but if you use castor to tune the feel away you are sacrificing valuable knowledge of what your tires are doing that you could be using to make more speed. I am a little different, I prefer to run very low caster. Depending on the chassis I like 4 degrees of caster minimum. Sometimes I run LF caster at 8 and RF at 4 or reverse it depending on the bite in the track. With the lower caster numbers I can better feel how the tires are biting the track. It is much more of a fine tuning thing.

So, on a race car, the only reason the tires roll in a somewhat straight line is because they are connected to each other with tie rods. There is always a strain on the tie rods because the two tires are always fighting each other. On a rear steer car, they are stretching the tie rods and on a front steer car, they are compressing the tie rods. But the car will go in a straight line as long as the caster setting is the same on both sides. Put more caster on one side and that side will be trying harder to turn and will overcome the lesser caster on the other side. Now the car will try to veer to one side.

So, I suspect a long time ago before power steering was common on race cars, someone realized he could make it easier to turn left on an oval track by adding more caster to the right side. He found he didn't tire as easily in a long race. I also suspect he discovered the car handled better too and came up with some better lap times. All of this has of course been translated to karts.

Scrub radius (also called 'kingpin offset') and caster angle work together to produce a diagonal mechanical weight transfer from the LR tire and the RF tire, to the RR tire and the LF tire. This weight transfer causes the LR tire to be physically lifted from the track surface at turn in. If this weight transfer is not great enough, the combined grip of the rear tires can simply push the front wheels straight ahead.

This mechanical weight transfer means the LF tire is much more heavily loaded than the RF tire at turn-in. As a result, the LF tire provides most of the front-end grip at turn-in. Once into the middle part of the corner most of the karts weight is transferred to the outside tires (due to cornering force). The mechanical weight transfer then becomes far less important (and can even be counter-productive) and is largely superseded by weight transfer due to cornering forces (lateral 'g' forces, causing frame flex). It is possible for the LF to actually have weight to reverse transfer to it. That is a different story. I mention it in my chassis manual.

Increasing either the caster angle or scrub radius will increase the LR wheel lift at corner turn-in, which is really what you are after (if the kart is turning in badly). Increased caster may require using more positive camber to keep the tires contact patch flat on the track during cornering. Be aware that increased scrub radius and / or caster can contribute to front tire overheating in some conditions. I have had this happen on dirt before at both the front and rear tires (rightsides). The kart just snaps loose when it happens. You also have to consider camber gain. More caster equals more camber gain. This is why when you roll caster into the kart and do not reset camber, the kart will push.

Having said all of this, give this some thought. The less amount of grip that is available the less that you can involve the LR with the track. This means slower lap times. The more grip that the track has, the more that you can involve the LR with the track meaning faster lap times. Remember, the LR is the dominant tire because it is the heaviest tire on the kart. This dominance must be controlled.

Front-end settings have a huge effect on the overall handling of the kart. If your kart turns-in to a corner the way it should, then the rest of the corner will be easier and faster to negotiate as you are not having to catch up with the effects of poor turn-in. In addition, the rest of the chassis is likely to be easier and less confusing to tune if the front end is functioning properly.

Finally, many handling problems that may at first seem similar can easily stem from different causes. Any adjustment you make to your kart is only correct if it lowers your lap times, assuming that the basic alignment is at least close to accurate.

Having said this, here is where I feel racers fail. The chassis manufacturer builds a chassis and tests it for quite some time to gather data. They look over the data and come up with their baseline numbers. Most will consider these numbers to be set into stone and will not step outside of them. So they leave speed in the kart. The ones who are the fastest step outside of the “numbers” and get all they can out of the kart by maximizing their overall setup. In other words they match the setup to the geographical area, the driver (style and build), the tires, tire prep to name a few. They merely use the baseline as a starting point and where they end up is what is the fastest for them. There are no magic numbers but only a baseline to start from.
 

msquared

Member
Caster Part II

As a very general rule of thumb keep this in mind:

I tend to ignore deceleration and acceleration because the stockers do not have much ability to accelerate or decelerate, so it's all about momentum and rolling resistance. All I really care about is:

1) the front-to-rear balance of grip (so that handling is balanced),
2) overall grip (to maximize turning ability),
3) and minimization of rolling resistance.

#1 is all about proper chassis setup, this is the easy part. It isn't hard to get a balanced well-handling kart.

#2 is a combination of having the right tire compound (and prep), and chassis setup - primarily left side weight and VCG to get the proper amount of side bite. This part is also pretty easy.

#3 is what going fast is all about. This is a combination of having the right tire with the right air pressure, balanced with the right amount of #2 (side bite). The #3 balance is constantly changing as the track conditions change.

More caster split will make the kart want to turn into the corner on its own. If you have too much caster split, then in the center, the kart may want to push because the LF isn’t jacking weight enough with respect to the RF to take drive away from the LR.

Less caster split will make the kart more responsive, due to steering input getting in to the corner and in the center but may either make the kart too twitchy or make the kart "bind" up because its not wanting to travel the corner radius as much on its own.

You generally see caster splits of 2 or 4 degrees. With high cross setups being run these days, the RF carries much more pre-loaded than the LF, that more caster split is not generally needed to get the front end to want to turn left on its own.

Then you have reverse caster which changes those darn imaginary lines which seem to have a huge impact on handling. That has to do with scrub radius and caster trail. Front stagger acts like a reduction in caster which is why reverse caster makes sense.

Most will think that increasing caster will make the kart jack more weight thus freeing it up if it has a push. Maybe so with a sprint kart but not with an LTO kart. If a kart is pushing you can fix it by increasing caster. The only way that will work is if the push is caused by rear stagger that is too low. Increasing caster works in that case by unloading the LR more.

If the kart has plenty of rear stagger for the turn, and the kart is pushing, then the front end is simply not making enough grip compared to the rear. In that case, increasing caster will only worsen the push because more load is put on the RR and less on the RF. You'll get less grip in the front and more in the rear. Been there, done that.

Like I said most think that increasing caster will increase front grip, which it won't. Increasing caster *might* fix a push, but even then it doesn't fix it by increasing front grip.

What does caster do?...

Because of the solid rear axle on a kart, the rear tires will always try to make the kart go in a particular direction. If there is no rear stagger on the kart, the rear tires always try to go in a straight line. This is the situation with a sprint kart that has to turn left and right and has no stagger. The way that a sprint kart manages to turn involves using excessive caster angles and a very wide front track width.

When the steering wheel is turned on a sprint kart, the outside front wheel and the inside rear wheel get lighter. This lets the inside rear wheel slip when the sprint kart starts to turn. The slipping of the inside rear tire eliminates the tendency of the solid rear axle to drive the kart in a straight line.

How does it apply to an oval kart?...

Now, when the steering wheel is turned on an oval kart, caster causes weight to jack from the RF/LR axis onto the LF/RR axis. This happens whether the kart is moving, or whether it's setting still on the scales. Just like on a sprint kart, weight jacking causes the LR tire to get lighter. When the LR is lighter, it is more likely to be able to slip in the corner. If you try to run 1/2 rear stagger on a 1/8 mile track, you don't have enough stagger for the track. You'll have to rely on caster and your weight percentages to allow the LR tire to slip through the corners.

So caster can fix a kart that is pushing, so long as the push is caused by low rear stagger!

But what does caster do if you have plenty of rear stagger?

If you have enough rear stagger - say, 1.25" on a 1/6 mile track, the LR tire does not have to slip for the kart to go around the corners. If the kart is pushing, the push is not caused by rear stagger!

To figure out what increasing caster will do in this situation, ask one question - what happens if left side weight percentage is reduced? The right side tires will grip more than they did before. In my experience this is always true with a kart. The more load you put on the right side tires, the more they will grip.

OK, so what does that have to do with caster? Remember that increasing caster unloads the LR and RF tire more. The load that was previously on the LR tire is transferred over to the RR tire, so the RR tire will grip more. The load that was previously on the RF tire is transferred over to the LF tire, so the RF tire grips less. So increasing caster caused the RF to grip less and the RR to grip more.

What happened as a result of the increased caster? The kart will push even more than before.

So the results of a caster change depend primarily on how much rear stagger you are running, and secondarily on whether the inside rear tire is slipping in the corner.

If you are running a 1/8 mile track with 1/2" rear stagger, and the kart is pushing (no surprise there!) increase caster and the push should lessen. You'll likely go faster by increasing rear stagger and leaving caster alone. If you are running 1.25" on a 1/4 mile track and the kart is pushing, increasing caster isn't going to fix it. You're going to load up on the RR even more and the push will get worse.
 

msquared

Member
Caster Part III

More caster split increases rotation on entry but will reduce rotation on exit. Increase RF caster.

Less caster split reduces rotation on exit but will increase rotation on exit. Increase LF caster. Provides good turn down on exit.

Increases caster on LF and RF simultaneously will add bite to the rear of the kart.

Decreasing caster in LF and RF simultaneously will add bite to front of the kart.

RF caster (along with camber) will dictate weight transfer to RF corner of chassis. More caster means more weight to RF. Helps free chassis up. Less weight gets to RR

LF caster will dictate mechanical weight transfer to RR. More LF caster means more weight to RR. Helps tighten kart up.

If I have a method it'd be to run less castor when I need more turning power and more when I need less.

Decreasing Castor adds bite to front of the kart.

Increasing Castor adds bite to rear of the kart.

Less castor will typically help the kart make more turning power and will help it steer more easily. The opposite does the opposite.

If it's pulling hard and you want to stop it then you can take some castor out of the RF. However, a kart that pulls hard to the left doesn't bother me if it's fast.


“Kart is pulling hard to the left” The trick with the pull is to understand where it's coming from. Most of a leftward pull comes from either castor split (the force itself being the result in differences in trail), the weight difference between the LF and RF (most often related to cross) or a combination of the two. Additionally, LF and RF camber and rear stagger can contribute as well. In the end, assuming the kart is properly assembled and setup, most of the things that can be done to reduce the pull will hurt the kart in the corner. My kart pulls pretty hard to the left (as I'd expect with 66% cross and a 3 or 4 degree castor split) and I don't worry about it. Additionally, several of my friends' karts pull plenty hard as well. If I were you I'd make sure my kart is assembled well and has a good baseline and I'd ride it.
 

msquared

Member
In all reality there really is no such thing as front stagger. Front stagger is used mainly to set only your cross. Or is it? Each front wheel is independent upon the other tire and rotates independently from each other. They are not linked together turning independently at a different rate than the rear tires which are tied together by a common solid axle. A circumference change in theory on one front tire will not affect the other. You will notice the size difference and a difference of your corner height on the side of the larger or smaller tire. For example going from 1” of front stagger to 1.75” of front stagger will be a difference of about ¼” of ride height LF to RF or rake. This is the same as weight jacking or even your cross settings. Sure a lot of people “think” front stagger is only used to help set cross. Maybe? More often than not, you will hear racers refer to the difference in RF to LF tire diameter as stagger. A smaller left side tire rollout is 1” to 1.5”s smaller than the RF tire and is often used on most kart chassis.
In a car, the tire size difference does, however, affect the breaking torque applied to both the front tires during entry into the turns. As brake torque is applied to the front tires, the left front tire has more brake torque applied to it due to the size of the tire and will apply more brake, pulling the car to the left into the corner. Hummmm. Maybe LTO karts need a LF brake, but braking on an under powered over tired kart will kill your corner speed. But, I remember using rear trail barking on asphalt to smoke the competition. Momentum is everything in karting. PERIOD! This brake torque (front or rear) has the same effect as caster split. As I recall caster and front stagger go hand in hand, especially in a kart. That is why when running reverse caster going from 1.5” of front stagger to about 2.25” of front stagger works. If front stagger acts as a reduction in caster then it makes sense to run more LF caster than RF caster when increasing front stagger.
Sure maybe a few thousandths here and there does not make a big difference in the chassis itself. But, if that were the case, then saying a few thou here or there in the engine makes no difference… Most pay hundreds of dollars to get a few thou from the engine. A deficit of .020 per lap over time and distance equates to .400 in the end. So is it OK to be .020 off the leader? Maybe a few thou in the “engine” and a few thou in the chassis makes no difference at all? Hummm… My point is it all adds up and it all makes a difference.
Here is an item that never seems to be explained too well. This discussion is like camber, the amount of camber and stagger that you use is directly related to the degree of caster in the kart. You can suggest to someone to use say -3.25 right front camber, Works good for old Joe. But what if Joe is using 10 degrees of right front castor and 1” of front stagger and the other person has 13 degrees of caster and 1.5” of front stagger? With steering input, as little as it is, that -3.25 camber makes a big difference with a bias ply tire. No manufacturers seem to have spindles built to the same kingpin angle or SAB. All of these different kingpin angles, means that there will be different steered camber gains. All of this translates into the entire front-end geometry.
I’ve seen Josh Philpott be very particular about air pressure changes. Some would laugh at those changes. But think about NASCAR, only very minute changes makes a very big difference in handling in those huge cars. In a kart it is magnified.
Sure, going from 1” of front stagger to 2” of front stagger translates to a radius diameter difference of .31831”, or more than a ¼” of rake. Does that ¼” amount to much? How does that relate to the front roll center and roll axis of the kart? What about body roll? Sure karts have it. Does ¼” of rake make that much difference? Well does a reed valve difference of .002 make a difference in a 2-stroke or a jet difference of .002 make a difference in a 4-stroke?
A good starting point (and a standard in karting) is to use 1.25"-1.5" front stagger and use a common caster split of 4 degrees. What this will allow you to do is to use less caster split with a larger size front tire, so you do not have to make a large caster split allowing for little steering effort to steer to the right. With the larger right front tire, it adds more wedge to the left rear.
When you decrease your tire size on the left rear by a circumference of 1" over the right rear, you gain turn ability, most chassis require at least ¾” – 1.25” of stagger in the rear to help the chassis to turn.
One other way of making your kart wanting to turn is to change the length of your RS wheelbase (moving the RF back or the RR back). Moving the RF back moves it more towards the CG which causes it to load more faster, moving the RR back delays the RR from loading but adds “rear steer”, not in the way a dirt car does. If you lengthen the right rear or shorten the left rear you are making an adjustment to the rear axle in the way it tracks to the chassis (if you re-square the chassis the handling will be different). The rear wheels will always be turning to the left by a small amount. The benefit is better turning in the corners. The down fall is your kart will drive sideways down the straight away which will scrub off speed. Remember momentum is everything in karting.
In a car (as most of these discussions seem to veer off onto, we want to know about karts not cars), one aspect you will want to look at is the type of rear suspension that you are using, depending upon using a 2 link, 3 link, 4 link or a cantilever (rear steer included).

The reality is it is not the front stagger that makes the difference, it is what it does to caster, caster trail, scrub radius and many other imaginary lines with the front-end geometry.
In the end, with a kart, every thou or tenth of a second makes a difference.
 

msquared

Member
I want to point out a two fine technical points in regards to caster. I was going through some old notes on the many different chassis that I fooled with over the years trying to find some answers. Like everyone else I am always learning. I’ve really been pondering this caster business. Then it hit me because I keep very technical details about the karts that I work on. I am going to share some info with you all and we will see where it leads.

First I believe some of the confusion with caster comes from many reading articles on the Internet relating to sprint karts. I myself did the same and thought the same. You can even find the same material on many LTO racers sites describing caster, weight jacking, etc. What many do not realize is sprint karts use caster and scrub radius to literally lift or jack one of the rear tires off the ground depending on the direction they want to go. Us oval racers do not want to do this at all. We want to have the left rear unload a little but not completely lift. We want to control tire presentation and dynamic weight distribution.

Our karts are setup to basically turn them selves through the turn. Us as drivers are merely the steering lock. We use minimal weight jacking due to such small steering inputs. It is still important to know what weight jacking is and does.

I mentioned controlling tire presentation and that has to do with the front-end geometry settings, air pressure, tire cutting profile and even tire prepping to name a few. Dynamic weight distribution has to do with our setup percentages, seat and weight placement, and the stiffness of each corner of the chassis to name a few.

Here are my fine technical points:
I looked at toe and on some karts, it changes some where on others it changes a lot.

I looked at my notes on those karts again, and found that the karts where toe changes a lot have steeper tie rod angles. The karts where toe didn't change as much have flatter tie rod angles. Steep angles aren't necessarily bad as long is it's done properly, and compensated for in the rest of the steering geometry. For example, tie rod angles can be used to create the Ackerman effect.

Tie rod angle is critical in car suspension design because of how the wheels travel vertically compared to the chassis. It's called bump-steer. The steered angle of each wheel will change as the suspension compresses or rebounds. In a car you have to minimize bump-steer as much as possible. Some cars that are improperly designed in this area are deadly in extreme handling situations because they are unpredictable.

Kart wheels don't travel much if at all, so bump-steer isn't really an issue. But, caster angle changes move the steering arm in the vertical axis, which affects the tie rod angle, causing the same kind of change in steering angle that you get with bump-steer.

The one thing I did notice about rolling caster in and out of the kart was if the track grips up and I roll caster out my lap times pick up. I just wonder if it because the steering is lighter and I do not get as tired. That was the first thing I noticed with running reverse caster. The steering was very different and I was not tired at all. Lap times were super consistent and about 1.5 seconds faster than before.

I think the majority of the lap time gain is probably due to the improved sensitivity in feel you get from the lower caster angles. Not only do you need less force to steer the kart, you also get a better feel through the steering wheel for what the tires are doing. Through the steering wheel you can feel changes in the track, camber, texture, or grip that you can't feel with higher caster angles. Additionally, hitting a bump in the corner will be less likely to jerk the steering wheel in your hands, so you can maintain more consistent steering input.

That last sentence reminds me of a comment I got one time after a race. I was running around 4 degrees caster, while I'm sure everyone else was running the standard 10 to 12 degrees. There was a hump on the edge of the track right at the apex of turns 1 and 2. It wasn't a bump - it was a smooth transition that the left side tires would run over, putting the kart a bit off-camber for just a moment. After a heat race, one guy told me that my kart was tracking across that hump like a Formula 1 car with my steering wheel perfectly stable, while everyone else was about to have the steering wheel jerked out of their hands every lap on that hump. My low caster was the reason. I waxed the field that day.

I believe it just may be the toe and angularity of the tie rods that is making the difference and causing part of the confusion with caster.

This is just what I have experienced and have noticed.
 
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