Need a little help figuring something out

[Ron Sutton]

I see that now after watching the video again, things look pretty decent and the car is sticking into the corner then with the extra bit of steering input for the sharpest part of the corner it all goes to hell.
Yup! Sure does. Glad you can see it now too.

You can really tell when it happens by watching the tire roll under the rim.
Correct. The initial "tightness" you feel in the steering wheel before it breaks into a push indicates the tires have gripped at their limit & the front end of the car is at it's maximum roll angle ... which COMPOUNDS the problem. If your car is rolled over 2.5 degrees ... that is 2.5 degrees of camber lean the wrong direction.

When the KPI/SAI being greater than the caster ... more steering input makes outside tire & wheel LEAN OUT at the top, known as losing camber (technically called going positive). My rough calcs say it is probably at 2.7+ degrees (at 20 degrees of steering input) ... the wrong way. Not only does the tire "roll under" ... but the contact patch narrows immediately ... and a lot. Probably going from 8" of tire contact patch to 5-6" ... maybe less.

If you were at the limits of tire adhesion ... with 8" of contact patch ... and that shrinks to 5-6" (25% less tire) ... at speed, under maximum load, in the corner ... well ... the front tires immediately break traction and the car goes into a push/understeer condition.


It doesn't look like the ride height is changing much at this point, so am I correct in assuming the KPI has more to do with causing this than camber loss because of suspension compression?
You are correct again. More KPI/SAI than Caster is the cause, not more body roll.

Here's why it happens.



Before the car breaks into a push ... the tires were gripping at their absolute limit ... therefore the front end of the car is at it's maximum roll angle ... MEANING the outside front spring is loaded as HARD as it is going to be. The spring is at maximum compression for that corner.

Then, as you give it more steering input ... and more KPI/SAI than caster causes the tires contact patches to shrink ... causing the tires to break traction & push. The car does not roll over any further. It can't ... it just lost traction. No traction = no body roll.

The outside corner will actually start to come up a little & the car will flatten out a little ... meaning the chassis/body roll actually reduces ... but only a small amount, say fraction of a degree ... until you unwind the steering wheel to exit the corner.



And does the camber gain on the inside tire happen at the same point that the camber loss on the outside tire happens?
Yes. So you're losing contact patch on the inside tire too.

I like to think of it this way ... there is no way a car driven at the true limits of tire adhesion ... with 16" of contact patch (both front tires) ... suddenly shrinking to 10-12" (again, both tires) ... at speed, under maximum load, in the corner ... can hold. The front tires are going to break traction to some degree and the car goes into a push/understeer condition.

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And then ... (sounds like a camp fire story)

When the front tires do regain traction, the outside front corner dips again, unloading the inside rear tire ... causing it to lose a degree of grip ... and you get the loose condition on exit.

As you can tell, push/understeer conditions are the worst thing for competition cars. I hate them, so I've spent my racing career understanding causes & cures.

The good news is your car is already WAY better than it was stock. With the changes you already made, you have made big steps at improving the handling. The even better news will come when you get the suspension fully optimized, and cure the small ills it has now. Better lap times & more fun at the track will be our result.

**back to reading**

Me too. I have more of your questions & comments to read ... plus some spindle research to do.