Part I
From my past experience I have solved a handling issue using caster split. I agree with Todd in those particular situations it was a complement to the overall setup. I say this because I was curious and went back to the old caster setting at another track and the kart became way too sensitive to small changes like cross and air pressure. However, the split does make a difference and so does the overall settings. I have gained an awful lot of speed in running caster as low as I possibly can and running reverse caster. I believe that running low caster allows you to better feel what the tires are doing in relation to the track.
If caster were not an issue then why even have it on a kart as an adjustment? Is it just there for selling purposes?
Junior drivers need more caster due to having to mechanically get weight transfer started. Lite to Heavy drivers need caster in the ranges Todd mentioned earlier. Where as super heavy drivers need more caster because of the amount of steering that gets soaked up due to chassis flex. Same with a thinner tubing wall chassis which will need more caster than a thicker wall chassis.
Two degrees of split is not going to be the same as 4 degrees and caster of RF at 10 and LF at 6 will not be the same as RF 6 and LF 10 and RF at 14 and LF of 10 will not be the same either. You have to factor in the rate of steered camber gain and the wheel height change. Along with KPI and caster trail there are many factors to consider.
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.
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 in a car is this still needed with power steering? 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.
Generally speaking:
1) More caster (both sides) will make the front end jack more weight and thus, make it grip harder.
2) Less caster (both sides) will make the front end jack (transfer) less weight and therefore, make it grip less (tend to push).
3) More caster split (more in RF than LF) will make the kart want to turn into the corner on its own. If there is too much caster split, in the center, the kart may want to push because the LF isnt de-wedging the kart enough with respect to the RF to free up the LR.
4) Less caster split will make the kart more responsive (to driver input) gettin in and in the center but may either A) make the kart too twitchy or B) make the kart "bind" up because its not wanting to travel the corner radius as much on its own.
A good rule of thumb for caster split is 2*. Some manufacturers run more but, with the high cross weights being run these days, the RF is so much more pre-loaded than the LF that more caster split isnt generally needed to get the front end to want to turn left on its own.
Too much caster split, imo, is worse than not enough as it will tend to make the kart want to be pushy lazy getting in and through the center of the corner.
my rule of thumb. 2* split - never run less than 8* on the RF (high biting racetrack) or more than 13* on the RF (low biting, slow racetrack).
But...
The way I read #1 and #2 of the above, 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 getting 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.
IMO #1 and #2 are a gross oversimplification of the effects of caster, and will cause a lot of confusion to a person trying to understand kart setups. Actually, it's worse than an oversimplification, because it says 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.
I'll try and explain what I know about caster more fully.
From the top...
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.
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 a 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.
Mike McCarty
Chassis Manual (Only $17.95)
www.kartcalc.com
