Tire pressure quandary

In the FWD handling discussion there were suggestions of opposite rear pressure. Some higher than the front and some lower. Since the tire can be considerd a spring of sorts, then increasing pressure would increase the rate. This is the theory I have used for a long time, although never going to extreme. After reading some of the autocross gurus suggestion as to as much as 10 psi higher in the front is the way to go. One member mention he used that and liked it So what are some of the "pro" autocrossers doing ? Of course, with the weight distribution difference on FWD will make a difference to a nicely balance car. Just out of curiosity, what are some of you using ?

iceracer wrote: what are some of you using ?

A RWD car

I suggested the lower rear pressure, and I am not a pro. So theres my disclaimer.

Every local autocrosser I had talked to with more experience than me with FWD (and very built cars), suggested the opposite, that I should be running a higher rear pressure. But I toyed with it and for my application, I did prefer the behavor of my mostly stock car with the pressure lower in the rear.

It was trial and error and merely by "feel", I wasnt marking the tires with chalk to determine how much they were rolling over. nor did I use luxuries such as those thermal tools to measure tire temperatures, or a sprayer to wet the tires and cool them between runs. that stuff is serious buisiness.

http://www.youtube.com/watch?v=23_0cYlqpzw

Another very non-pro here...

There's gotta be a good flow chart for this out there somewhere...

Seems like the place to start is making all four tires as happy as possible, pressure-wise, and then adjusting balance by going either too high or too low at the less-important end as needed for balance, right?

I've historically thrown away traction at the "wrong" end by going low to induce rollover and squirm, but your spring rate comment reminds me that raising the rear pressure might also add a little rear roll stiffness, which would also help with getting the back end around...

Sorry, I think I sort of bounced back and forth between general application and FWD-specific there...

Front = whatever works best. Example: found that 43psi worked best on a D-Stock MINI on Hoosiers.

Rear = Whatever works best. Example: found that 60psi worked best on aforementioned MINI.

Generally speaking, I like to run more in the rear to promote more neutral handling---without being wishy-washy like it is with too little pressure. At times, the rear tire (note singular) has to support a fair amount of weight as the other tire is up in the air. If it's too little, the car tends to get wiggly/woggly when it's up on 3.

YMMV.

Per (multiple nats trophies, dozens of tour and pro wins in FWD cars)

Trial and error with the particualr car for sure and I can add that on 96 civic hatch with koni sports, ST sway bars, and ebiach sportline springs, on 195 55 14 star specs:

40 psi front and 32 rear is what works day to day for me. It's understeers just a bit if pressed, but rotates when you lift in a corner or really turn in really hard. As the rear pressure goes up the rear wants to rotate more and more. At 40 psi rear It will even 4 wheel drift with power on in some situations. At 36 psi ish or above the rear will step out in the rain if you lift on an on ramp or down hill on a twisty road at unremarkable speeds. Which is no good as the wife drives the car, and I dont like it either.

I prefer to hyper inflate the rears rather than underinflate, but it depends on the car, the tires, the course and the driver. I find that when over inflated, even though the traction limit is sharply defined, I can find it and maintain it closer to the limit easier. Scariest moment I ever had as a driver while autocrossing was not any of the umpteen spins I've done, or dim witted newbie course workers running out too close in front of me, but rather a very nasty tank slapping moment at Grissom while experimenting with low pressures in the rear. I'm done with low rear pressures in that car.

I've run both higher rear and lower rear ( Integra & crx ) if I understand the theory correctly, the under inflated rear causes a loss of grip ( due to the center of the tire not "grabbing" as much of the pavement as it would with higher pressure .... exact opposite when you raise the rear pressure... now the contact patch is smaller because the edges aren't in contact with the pavement...

on the CRX running BS RE-11's I ran 40 front and 36 rear .... with some small degree of success .... ( 2 regional championships last yr.... and no where near good enough for national tour )

this yr with Toyo R1R's it's 26 front and 28 - 30 rear, same sort of success as last yr ( 1 first place and one 2nd... and slightly .... very slightly .... closer to the times of the really fast STS drivers ( they all run in a "Pro" class )

'course YMMV...

So it appears that what ever the driver prefers and works for him/her. The type of tire may make a differnce . When I ran Hoosiers on my ZX2SR I had the pressures pretty near equal and according to their suggested pressures. Seemed to work very well. Street tires that are not as stiff may react differently.

Yup, sounds about right. On my protege I liked higher pressure because it still would rotate but it wouldn't be sloppy, which it was when running lower pressure.