Tire pressures and AutoCross

[wildbilly32]

I have read most, if not all, on this subject in this part of the forum and I'm still confused. Run-what-u-brung parking lot AutoX. Stock(except RMM) with Falken 510 tires 245-35/20 on OEM rims.

I get the 30 vs 35 sidewall load rating deal suggesting lower tire pressures. I get the stiffer sidewalls on the 510's compared to the all season Conti's I first ran on. I get the chalk deal.

This season ran 33# front with chalk indication of no roll-over on sidewall. Still pushed badly in tight corners and even in a higher speed sweeper last weekend. Even though I put 34# in the rears to loosen things up.

Thoughts: I could have been over-driving and pushing too hard(trying to carry too much speed into corners). Still learning this game. So there is that.

Question: Why do people recommend higher air pressure ie: 5-10#'s over "stock"? I see a lot of folks here saying that and even TireRack say for front engine FWD car run 40-45#!! See the below chart from their web page on tire pressures and handling adjustments:

I'm an old dirt tracker racer and this all seems counterintuitive to me. I.E.: more air equals smaller contact patch equals less traction...??

Maybe I just need to drive smoother(I do as I catch myself tossing the car into corners like a sprint car) and learn how all this works. On a sprint car it was so much easier to adjust by moving the appropriate wheel(s) in or out depending on what you were trying to improve...ah the good old days.

Advice is appreciated, but please don't suggest buying sticky tires that only come in 18 or 19 inch sizes. I am aware of the options and intend to stick with the 20's at this point. I also realize there are stickier 20" tires out there and plan to move that direction once I burn these and the OEM Conti's up.

Thanks in advance for suggestions.

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[AlphaDigital]

Just because your chalk indicator is telling you that youre not rolling the sidewalls doesnt mean you wont understeer corners. No amount of adjustments are going to cure understeer when coming into a corner too hot, thats up to you to control with your pedals and steering wheel and something that youll get better at as you continue autocrossing.

IMO, there is no blanket statement where people should be suggesting tire pressures to run for all FWD cars, thats just silly.

Ideal tire pressures change based on multiple factors from the tire itself, to the weather youre running in. Thats why you should be religiously checking your pressures after your run to make sure you dial them in correctly and how you need them.

 

[Champale]

Following this thread . . .

 

[Banshee]

Is your traction control on, partially off, or fully off? Turned on, it is more prone to understeer, as it is safer. You can tap on the brakes, get a little more traction, and get out. Unlike the RWD, where you can give a bit more gas and rotate that ass..lol
If you need a little more rotation, get a stiffer rear sway, like a Karcepts, and you won't have as much understeer at all.

If you want to stay in street class, you can get a staggered setup, to get more grip on the front. I believe you are allowed +/- 1".

 

[Methane Herder]

Since TW200 tires are off the table, your delta P front to rear is probably going to have to go up quite a bit. (Biased to the rear).
Understeer in this case is probably a result of relatively hard compound tires, front drive, front LSD, and a forward weight bias.(Oh and that 300+ hp thing).
In this scenario, for AutoX, in that car, the tires are the ultimate limiter.
Pumping up the rears will reduce the contract patch and help induce trailing throttle oversteer. (Good for long sweepers)It can also induce quick transition snap oversteer.(good luck with that).
Ultimately, you will destroy those tires quicker than running TW200 tires if you AutoX very much.

My ultimate suggestion is to buy some lightish summer AutoX wheels and put some TW200 rubber on them.

Hope this helps


MH

 

[NapalmEnema]

For what it's worth, when I went to the track I under inflated to get the tires up to temp so they would be 'hot' and be 38lbs all four corners. Tire wear and grip were great, I didn't have any weird tire issues after. I think running 36-38 is optimal, as that's what the tire calls for when up to temp.

 

[wildbilly32]

Is your traction control on, partially off, or fully off? Turned on, it is more prone to understeer, as it is safer. You can tap on the brakes, get a little more traction, and get out. Unlike the RWD, where you can give a bit more gas and rotate that ass..lol
If you need a little more rotation, get a stiffer rear sway, like a Karcepts, and you won't have as much understeer at all.

If you want to stay in street class, you can get a staggered setup, to get more grip on the front. I believe you are allowed +/- 1".

Nope. Traction control is off with 10 second button push.(didn't do the pedal dance). I've considered a stiffer RSB so thanks for the confirming vote. This is run what you brung just for fun AutoX so there ain't no classes!

 

[ArturCTR]

Im running 8psi less in the rear for more rotation, plus rv6 swaybar at max stiffness. But then again I’m on 265 a052 on a 10 inch wide wheel and ohlins coil overs lol

 

[wildbilly32]

So this season was an excellent learning experience. The OEM Conti's made a huge difference over the 35 profile Falken and it's 300tw. 36-38# front(depending on course) and 33-35# rear and understeer GONE. It is so much fun rolling into a corner, being able to feed throttle and have the car still turn the direction desired. It is nice to be able to post respectable times for an old guy AutoX newbie.

 

[AlphaDigital]

Glad you had a good season. I think you might find, the more you go up in pressure in the front, the more youll understeer. Typically, the rule of thumb is, putting more pressure in the fronts will result in more understeer than if you had less pressure in the fronts compared to the rear. Definitely keep playing with them, and maybe even chalk your tires before your runs to see where its wearing down to on the course.

 

[wildbilly32]

Thanks, appreciate the suggestions. I've been using chalk every session. Haven't gotten much into the sidewall ever. Next year I will keep tinkering with pressures. Keep in mind this is school parking lot courses so nothing like SCCA sized courses. Usually tight (for the small English sports cars) with some sweepers but speed is not high (maybe 45-50 max). It was so nice to find out that the "tight" obstacles weren't so tight after all with stickier tires, proper driving techniques and trusting the car.

 

[Dave B]

Huh ????

 

[wildbilly32]

Huh ????

TireRack...

 

[Dave B]

Don't know too many people who increase their front pressures to add grip. Might be ok with supersoft sidewall tires. I race front wheel drive cars on ice and we typically use only 15-18 PSI at the front (with inner tubes) and 25-30lb out back to help the car rotate i.e. decrease rear grip compared to the front.

 

[wildbilly32]

Don't know too many people who increase their front pressures to add grip. Might be ok with supersoft sidewall tires. I race front wheel drive cars on ice and we typically use only 15-18 PSI at the front (with inner tubes) and 25-30lb out back to help the car rotate i.e. decrease rear grip compared to the front.

Yeah that was my point. How can increasing front tire pressures on a FWD car increase grip. That chart is confusing to me. True my racing experience was all dirt ovals(no FWD) but that is hard for me to get my head around from my previous life.

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