Basic tire pressure for autocross question

I am trying to figure out the basic rule of thumb for tire pressure. I am specifically interested in the tire pressure for rear wheel drive car (i.e., BMW 330Ci) for autocross with same size tires (i.e., 245/40/17 Kumho MX all around). The standard tire pressure posted on the door jamb for staggered tires is 30psi front and 35 psi rear. My logic says that this is recommended to reduce oversteer since its rear-wheel drive. Higher pressure means lower traction. Am I right? If this is the case, if I use 245/40/17 all around and put 40psi all around, will I be inducing oversteer or will I be making the handling more neutral since the tires are of the same size all around:?: Should I still have higher pressure in the rear even though I use same size tires:?: Am I missing something here:?:

I know the ideal pressure can only be achieved through lots of trial and error but I just want to know the rule of thumb to find the base line from which I'll adjust up or down. Thanks all.

40 psi is a good starting point. See how the car handles and then adjust accordingly. One thing that is often overlooked is that a tire has a spring rate. The more pressure, the higher the rate. 1psi equals about 40lbs spring rate. That's why the NASCAR guys are constantly changing air pressure in the tires.

I use 40 in my Z3. My recommended pressures are 32 and 35 front and rear respectively. You also need to consider the weight distribution of the car. Front wheel drive cars tend to have a greater amount of their weight on the front wheels. They require a bit more pressure in the front just to keep the tires from rolling over on themselves.

Your 330 is pretty neutral so your base of 40 all around sounds good to me as well.

I run at about 39F/36R in an E36 with 225/45-17 Azenis

ea_sport wrote: I am trying to figure out the basic rule of thumb for tire pressure. I am specifically interested in the tire pressure for rear wheel drive car (i.e., BMW 330Ci) for autocross with same size tires (i.e., 245/40/17 Kumho MX all around). The standard tire pressure posted on the door jamb for staggered tires is 30psi front and 35 psi rear. My logic says that this is recommended to reduce oversteer since its rear-wheel drive. Higher pressure means lower traction. Am I right? If this is the case, if I use 245/40/17 all around and put 40psi all around, will I be inducing oversteer or will I be making the handling more neutral since the tires are of the same size all around:?: Should I still have higher pressure in the rear even though I use same size tires:?: Am I missing something here:?: I know the ideal pressure can only be achieved through lots of trial and error but I just want to know the rule of thumb to find the base line from which I'll adjust up or down. Thanks all.

No higher pressure will prevent roll over of the sidewall AND make the tire more flat for more traction. Of course going to high on tire pressures and you will be only riding on the middle of the tire.

Always adjust tire pressures by making the front GRIP more, and then adjust the rear pressures to make the car comfortable for you to drive at the limit.

Most cars coming out of the show room have a safety margin of understeer since that is easier for Average Joe Schmoo to recover from on the street. Oversteer for the average consumer is bad.

So cheapest way to see if you have enough pressures in the front is to chalk the front tires on the shoulders. Go out and run your 1st run and see where the chalk rubbed off. If it rubbed off on the sidewalls you need to add more air. If it did not then take out air.

The next way is using a pyrometer to measure temperature across the tire. You would want even temperatures across the tread. Hot middle means too much air, hot shoulders means not enough.

I seem to recall those MX's having pretty soft sidewalls (compared to the other choice ST tires). You might end up well north of 40 psi.

CivicSiRacer; You wouldn't happen to auto-x a black Civic hatch with those sponsors listed in your signature on the sides of it, would you? Perhaps those on the passenger side are ¿sdrawkcab

Just wondering.

bludroptop wrote: I run at about 39F/36R in an E36 with 225/45-17 Azenis

Thanks for all the info guys. I'll probably start with 40 all around and adjust the front by 1psi. bludroptop, I am interested in your combination of 39F/36R with 225/45/17 all around. Was it done to induce more oversteer?

confuZion3 wrote: CivicSiRacer; You wouldn't happen to auto-x a black Civic hatch with those sponsors listed in your signature on the sides of it, would you? Perhaps those on the passenger side are ¿sdrawkcab Just wondering.

Why yes that would be me :) What's up?

Sorry for the delay in responding, busy weekend and then the board was down...

I've found that about 3 psi lower in the rear helps to make the care more balanced - so I guess less understeer is how I'd describe it. I also run a stock front bar and a slightly larger than stock rear bar. The car will still push, slightly, at the limit but a slight lift of throttle will tighten the line.

I can go lower - say 36F/33R - and slightly increase grip but at considerable expense to turn-in responsiveness. The Azenis also have very stiff sidewalls, so I've gone as low as 31 on the back without rolling over the tire.

I'm still learning myself and I don't tune the tire pressures much between runs. I decide on a number and stay consistent with that for the day, just bleeding off to get back to my baseline as the tires warm up.

Hope that helps a bit.

Standard rules don't always work.

I was struggling a lot with tire pressure on my Yugo, and couldn't even get close. It's weird turning 360's in every corner in a FWD car.

I found some advice from one of the major performance tire manufacturers (I forget which one) that helped a lot.

Turns out my issue was the car's weight. It only weighs 1200 lbs.

The rule of thumb they gave was 1 lb of pressure for every 100 lbs of vehicle, plus 2 lbs on the drive wheels.

That meant I needed to be running at 12 PSI in the rear, and 14 PSI in the front.

That sounded so ridiculously low that I couldn't bear it. I added my weight , plus a little, and used 15 and 17 PSI as my starting point (still less than half what any conventional wisdom was suggesting).

The car acted like it was on rails. I have run it like that for a year and a half now, on several manufacturers, with very little change.

On my 2000 Maxima I run 44 psi F, 38 psi R when auto-xing. It took me about a half-dozen auto-x events before I found (I think) a pressure that works well on my '99 Miata. 31 psi F, 29 psi R seems to be working OK. I was so used to having to pump up the tires on the Max I could not accept that the much lighter (and very different) Miata needed much lower pressures. Now that I think I have a idea on pressures I can start adjusing shock settings, KYB AGXs.