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Discussion Starter #1
I've been seeing people mark their tires with chalk to see how much the sidewall flexes, and then adjust their air pressure accordingly.
I've been running 40 in front and 35 in the rear, kind of adjusting based on how loose the rear end gets with the varying pressure, but I'm not sure how much roll in the tires is "good."
So when I check my sidewall for the amount of rollover, how much am I looking for in the front and rear? I run 40 aspect tires all around (I know, suboptimal, but I don't have a choice). 1" of rollover? More, less? Or am I doing it correctly now (adjusting for rear end looseness)?
Thanks.
 

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Re: Sidewall flex for auto-x? (iamdoingthat)

whoa... hang on a sec.
you should mark from like 1" down the sidewall up and over onto the tread about 1"
at the end of your run, the chalk should be gone to JUST the edge of the tread. you're looking for all of the chalk on the sidewall to still be there.
if you are rolling onto it, you need to slow down before getting into the turns more, and adjust your driving, or get a different tire (which probably won't help if you're going in too hot.)
 

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Re: Sidewall flex for auto-x? (John A)

Welp, I'm even more clueless than I thought. I am definitely not driving optimally (very much a newbie), but it always seemed like people were chalking their tires to see if there was enough pressure?
So if I'm looking to not rub off any chalk on the sidewall (meaning I'm driving properly) that means the chalk has nothing to do with tire pressure adjustments? It's just a way to see if I'm driving optimally?
Sorry for the newb questions. John A, I've seen your rabbit before, I appreciate your help.
 

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Re: Sidewall flex for auto-x? (iamdoingthat)

no, it's definitely to adjust pressures -- let me reword that...
yeah, basically they're checking to see if they're rolling the sidewall -- if you are, then the pressures need to go up. if you get to the tire's MAX PSI (probably in the 40's) and are still rolling it, then you are overdriving it.
with some tires (and in certain applications) people will chalk them and find that they're NOT using the full width of the tread -- in those cases most of the time the pressure can be lowered to get more tread on the ground...
 

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Re: Sidewall flex for auto-x? (John A)

Quote, originally posted by John A »
if you get to the tire's MAX PSI (probably in the 40's) and are still rolling it, then you are overdriving it.
Many auto-xers inflate their tires a bit past what it says on the sidewall since some (most) tires aren't up to the abuse an auto-x can dish out. Be safe and use your head. If the max is 40, 45 is no biggie, but 50 or more might be much.
 

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Re: Sidewall flex for auto-x? (vuu16v)

Quote, originally posted by vuu16v »
Many auto-xers inflate their tires a bit past what it says on the sidewall since some (most) tires aren't up to the abuse an auto-x can dish out. Be safe and use your head. If the max is 40, 45 is no biggie, but 50 or more might be much.

even then, i mean -- i ran my street tires to 48 one time (44 max) and it couldn't make up for the fact that i was just pushing them too hard, and coming into turns too hot. this was on my jetta a few years back.
 

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Re: Sidewall flex for auto-x? (John A)

When you look at how far you are rolling your tires, keep in mind that the tread actually continues over onto what you would call the "sidewall" about half an inch. That is still useable area.
Autocrossing FWD is mainly about managing the grip at the front end. Keep in mind that you have more grip at smaller slip angles (sliding just a bit) than you do at large slip angles (when the tire is sliding a lot) In short, going in a bit slower, keeping the tires hooked up, rather than charging in too hot, scrubbing off a lot of speed sliding, and winding up slower. Smooth is fast!
Also, grip drops of faster with too much pressure than it does with too little, and the breakaway tends to be sharper as well. On the rear, that means that lower pressures might actually help keep the back end stuck better. Generally on street tires the front needs a lot more pressure, like 45-50 psi. i have never had a problem exceeding the max inflation pressure by large amounts, but then i always run Pirellis: my P6000s are rated for 51 psi max
 

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Lack of negative camber can also influence how far the tire rolls over. Measuring tire temps is the most accurate way to tell what needs to be changed, but marking the "corner" of the tread is a great starting point.
 
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