G forces

9

95 shaw

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G force affects lateral weight transfer. I look at g force being near zero until turn in. At that point, g forces ramp up quickly finally maxing slightly before apex, or whenever driver begins to unwind steering input. It may remain steady or fall off, returning to near zero at straightaway.
I've heard of dirt lto karts generating up to 2 plus g' s on high bite tracks. Coke syrup racing can generate even more side force.
I like to think of g force acting on the chassis on a line perpendicular to a line drawn through the vertical center of gravity to the center of the earth. This is important when considering how these forces affect chassis when banking is also in the mix. Some of the g force in converted to downforce acting on the inside tires.
 
modern high cross lto operation

95 shaw 95 shaw is online now
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Probably been discussed before, but, here is how I see modern lto karts operating.

Statically, right rear tire is least loaded. basically the same going down straightaway. As the least loaded tire on a solid axle, the right rear slips on straightaway. As entering corner, right rear loads, making stagger proper for turn in. At the apex, right rear is more loaded than left rear, allowing left to slip slightly, (maybe). After apex, left rear reloads from cross. At some point stagger is correct to let both tires pull equally out of corner. Left continues to reload, right rear unloads, allowing
RR to slip going down straight. Actually, the left rear is in control of the rear of the kart most of the time.

The right rear slipping is helped along by high cross, tire selection, thin cut rubber, tire prep, etc. The whole premise is built on making maximum roll speed with limited horsepower.

Hope that helps.



I wrote this some time ago in a discussion about stagger. I think it looks similar to the post above.
 
stagger in a high cross setup

If you followed the chassis math thread, I talked about tire contact patch shape in both the static and dynamic modes. Most emphasis is generally placed on the dynamic. I think a discussion of the static state is also warranted. As you know, we use different tire width combinations on our LTO chassis. I've often questioned why. A look at the contact patches can give some clues. I'll just talk about the rear for now.
If you drew out the static patches on sheets of paper, you may have noticed the right rear is wider than it is long. This shape is thought to be better for applying lateral traction and not as good for forward traction, but induces less drag from rolling resistance. The left rear is generally more square or even longer than it is wide, generally considered better for forward traction and less desirable for lateral traction. However, this shape gives more rolling resistance.
As we know using a staggered rear axle to go straight, one tire has to slip. Looking at the contact patches, it should be easy to see which it is.
The benefit of having the right rear slip is that its surface speed is faster than the left rear, so it is trying to help go faster instead of just dragging along slowing us down. Stagger plays multiple roles in both lateral and forward traction. It lets the rear axle help the front generate enough turning force to navigate the track. It also contributes to the shape of the contact patch in the dynamic mode through camber.
I personally think the camber help in the dynamic mode is more of a factor than the turning help in a high cross setup. I know I haven't talked about camber yet. I'll get to that later.
 
Balance of traction

Although maximum traction should be the goal, compromises have to be made to achieve a balance of traction to promote the best lap times. Drag on the straights is a compromise made to achieve better corner speeds which hopefully translate to better lap times. Stagger adjustments are such a compromise.
Lateral traction should also be balanced. Maximum lateral traaction is achieved when both end of chassis are equal. Adjustments to camber, tire size, corner weights, center of gravity and stagger are made to try to balance that traction. Compromises have to be made here also to achieve the best overall performance. There is no single correct answer, just as there is no single best stagger.
We are always struggling to find the best strategy to gain speed. High cross and low cross setups are examples of different means to that end.
 
Tire lateral performance

We know adding weight to any tire increases the traction the tire is able to generate. However, the weight gain to tractive gain is not linear. Simply adding weight does not always add the same amount of tractive force. Tire performance curves have been generated to give an idea of what is happening.
An example of a curve is shown here. This is the actual curve from Chassis Dynamics by Herb Adams.
https://www.google.com/imgres?imgur...O3QAhXEzVQKHQ_iDBUQMwglKAowCg&iact=mrc&uact=8
This curve is for a 3000 lb car. If you were inclined to use this information to evaluate the effect of weight to tire lateral performance, the following info can be used.
For simplicity, use 1 g. Race cars of this weight generate g forces in the range of .65 to 1.3 g.
Typical center of gravity heights are in the 15 to 20 inch range.
I like to use percentages that are typical for our chassis, just to get a feel for how it works.
First get dynamic corner weights. Then using the chart, get the lateral force traction weights. Comparisons paint an interesting picture.
 
Locked down

I missed this when talking about stagger and balancing traction on rear axle.
Locked down occurs when whichever rear tire that should be slipping has too much traction and cannot release. This binds the chassis, robbing speed . There are lots of thoughts on methods to prevent this. It will happen to you at some time if you race.
 
I can't help believing that if both rear tires are locked down in a turn, if the stagger is right, that that would be an ideal situation. If the stagger is right, and both tires are rolling, (a rolling tire has more traction than a sliding tire) side bite would be at maximum. Turning ability would be at maximum. Forward acceleration would be at maximum. Tell me I'm not believing correctly.
 
Last edited by a moderator:
alvin l nunley said:
I can't help believing that if both rear tires are locked down in a turn, if the stagger is right, that that would be an ideal situation. If the stagger is right, and both tires are rolling, (a rolling tire has more traction than a sliding tire) side bite would be at maximum. Turning ability would be at maximum. Foreword acceleration would be at maximum. Tell me I'm not believing correctly.
Click to expand...

Locked down hurts the most down the straightaway. Yes you are looking at it correctly for a turn. Even if we are quickest around the turn, we can actually loose speed in the straights by being locked down.
 
Maybe a look at strategy will help. As you know, asphalt sprint chassis must release one rear tire in order to turn. Staggered axle oval chassis strategy must release a rear tire to go straight. My belief which tire releases has to do with which cross strategy is involved.
 
95 shaw said:
Locked down hurts the most down the straightaway. Yes you are looking at it correctly for a turn. Even if we are quickest around the turn, we can actually loose speed in the straights by being locked down.
Click to expand...
I'm afraid I can't understand how you can get locked down on the straights. Maybe I don't understand what lockdown is. I know, with stagger, one tire is being dragged down the straightaway, robbing horsepower from the other tire.
 
Thinking hard about how to convey this. Simple answer is the tire does not release down the straights so all power is consumed with tires fighting each other.
Front tires provide enough traction to keep stagger from turning kart. Some will say that is bound up. My answer to that is this is with a normally neutral chassis. Usually, track bites up and/or too much/wrong prep. My one experience, I was able to up all air pressures by 2 psi and at least partially overcome this. Next week, track was its normal self.
 
I always believed that the inside rear tire had to lift for the kart to be able to turn, usually when weight transfers to the right front on corner entry the left rear lifts, allowing the kart to make the entry and get thru the corner, then on corner exit the weight transfers from the RF back to the LR and both rear tires would drive the kart down the strait using forward bite...am I wrong with this thinking? On some of the small bullrings its pretty common to see the leading karts pick the LF wheel off the ground coming out of the corner, sometimes packing it halfway down the strait on a track that has good bite, especially indoor concrete racing on syrup.

95 Shaw, some of your post really remind me of Paulkish...are you sure you 2 aren't related lol
 
W5r I also always believed that. A post last year by JWD about adding cross always tightening the chassis caused the epiphany I spoke of. Also led to my evaluation of where weight goes during weight transfer. Al also made me think that grip by both rear tires creates more lateral traction. Stagger makes a tire have to slip on the straights in order to go straight. My mathmatic evaluation shows this is possible.
Or at least in my mind.
 
Let's carry it to the extreme, maybe we can get a better look at it.

Let's picture the kart going down the straight but the LR is being held in place, i.e. not turning, although the axle is still turning and the RR is rolling. Can you see how that would slow the kart down, and can you see how that would make the kart want to turn left? Now, can you see if the RR is being held in place, and the LR is rolling, how that would make the kart want to turn right?

Depending on the stagger, that's pretty much what is happening. As I see it, stagger is pretty much a compromise depending on the radius of the turns and the length of the straights.
 
I agree with most of your premise. However, you must evaluate the whole system in order to see what is actually happening. The tire widths, compounds, tread thickness, air pressures and weight on tire all influence the traction a tire can make. Tuning requires manipulation of the whole system.
 
Picture a segway with a solid axle on a skid pad. We can calculate the perfect stagger. If I can stand perfectly still, we can circle the skidpad at maximum speed for the traction available. If I step to the left, the segway will veer off the track. What changed? the stagger is still perfect. I simply changed the traction capabilities of the tires.
 
95 shaw said:
Picture a segway with a solid axle on a skid pad. We can calculate the perfect stagger. If I can stand perfectly still, we can circle the skidpad at maximum speed for the traction available. If I step to the left, the segway will veer off the track. What changed? the stagger is still perfect. I simply changed the traction capabilities of the tires.
Click to expand...

should have read "If I can stand perfectly still, in the right position, we can circle the skidpad at maximum speed for the traction available."
 
One of the things which made me think the right rear was slipping was the premise that adding left side weight always frees the chassis. Just seems to make sense if thinking about it that way.
 
95 shaw said:
One of the things which made me think the right rear was slipping was the premise that adding left side weight always frees the chassis. Just seems to make sense if thinking about it that way.
Click to expand...
Believe it or not.
kish has tossed that out . One or the other rear tires is slipping.
Along with surface speed.
The weight on the right also makes sense on a segway, or ski's, even a shopping Cart.
 
I'm not offering anything new. Just sometimes we get caught up thinking about things a certain way. Even if we don't agree, it can make some things more clear.
 
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