Question about tire pressure and tire wear.

[CaptainSquirts]

I was wondering what's a good tire pressure for DD and Autocrossing to get some nice even wear. OEM tire pressure specs for my vehicle is 32 fronts and 29 backs cold. Thing is I'm not running OEM tires/rims. I'm running 245/40/18 tires(oem 235/45/17). I noticed after some autocross events I would get more wear on the inner middle part of the tire and not much wear on the outer. I was running around 36-38 all around hot. When I last had my alignment done they were set to 0 toe and -1 camber all around.

Now that I have some new fronts I would like to get some nice even wear on them. Is this something that is achievable or is this just one of those you gotta pay to play things.

[Loren]

Nobody can give you the magic number for correct tire pressure. You're probably in the ballpark at 36-38.

Too many variables. Weight of car, size and construction of tires, wheel size, alignment, suspension geometry and stiffness, driving style, driving conditions, amount of straight-line driving vs autocross and aggressive cornering, etc.

In general, if you autocross frequently, you'll do better with more than 1 degree of negative camber. Up to about 2 degrees is fine for a street car that sees autocross duty. It won't cause undue tire wear unless you couple it with excessive toe.

Talk to one of the gurus who always has a tire gauge in their hand at an event. They can give you some great info.

[CaptainSquirts]

Oh this should've been in car set up, my bad.

I'll have to poke a couple of people on tire and suspension setup next time then.

[twistedwankel]

You have two sets of rims. Have two sets of tires

OR. Rotate your tires more often. I found wider shorter tires need about 2psi less cold pressure than tall skinny ones that come on the cars. Sounds like you might be running too much pressure in front. My old Jap turbo cars and a line of Foxbodies never allowed for more than -1deg camber and struts so learned to live with it. Your AWD so check with Tim or Terry.

[Loren]

Oh this should've been in car set up, my bad.

Moved.

Now that I'm not typing on my phone...

More camber if you can get it. That will help keep the tires from rolling over onto their sidewalls in a turn and chewing themselves to bits.

More tire pressure can be a band-aid to acheiving the same goal. So, it really comes down to "am I able to optimize the suspension enough to focus on truly optimal tire pressures". For most street driven cars, you compromise your alignment settings in the interest of good tire wear on the street, and general "friendliness". (a really agressive alignment will eat tires on the street, AND have traits like "twitchiness", "tramlining", etc.)

If you could fully optimize your alignment, then your ideal tire pressures (and alignment parameters) would be such that, in skidpad testing, the tire TEMPERATURES are relatively even across the face of the tread. Not too hot on the outside edge, the inside edge, or the middle.

But, since we're dealing with a compromised daily driver suspension and alignment, we're pretty much always going to have "not enough" negative camber, which means out outside tire temps will be high, and we're struggling to keep the tire from rolling over too much onto the sidewall. So, proper tire pressure for YOU will be attained by marking the side of your tire (right where the edge of the tread meets the sidewall) with chalk or shoe polish or whatever... and then go take a couple autocross runs. How far is the tire rolling over? If it's rolling too far (and you can't or don't want to dial in more negative camber), you're going to want to raise the tire pressure. Will this give you optimal handling? Possibly/probably not! It's merely self-preservation for that tire. If you don't add pressure, you're rolling over too far... look at the photo of Heather's tire after the last event. That's what you get.

So, Doug is probably right. 38 psi might be more than optimal for your tire on your car... in a perfect world. (one where you can get exactly the alignment that you need to keep the contact patch flat in a turn) But, in practice, if your tires are telling you that you're rolling over too much with less pressure, then you need more pressure.

Were it not for that, then tire pressure is usually tuning the TEMPERATURE of the center of the tread. If the inside and outside were even temps and you were too hot in the middle, less pressure. Too cool in the middle, more pressure. But, again, I don't think you're going to be worried about tire temps with only 1 degree of negative camber. You're in "compromised DD" mode and simply trying to save your tires.

[Jamie]

Don't expect to find a single pressure that'll work for daily driving and autocross, either -- it doesn't exist. You don't shouldn't be cornering as hard in daily driving, so running pressures that high simply sacrifices street performance and wears out the center of the tire faster. Running DD pressures on course will usually have you riding on the sidewalls. If you're using one set of tires for both, just plan on pumping them up before each event.

If you find me at the next event, I have a crayon you can mark your tires with, to check rollover.

[Loren]

Jamie's right, but if you're lazy, you can find a "close enough" tire pressure. I've been doing it that way for years. You might sacrifice a tiny bit on course by not constantly chasing "ideal", and you'll definitely sacrifice ride quality on the street by running tire pressures that are anywhere near correct for autocross. But, you "can" do it.

My philosophy: Get the car to handle consistently in a satisfactory manner and not eat its tires. (that's suspension changes, alignment and tire pressure) Set it, forget it... focus on DRIVING. For casual local-level autocross, this will serve you well.

[twistedwankel]

My philosophy: Get the car to handle consistently in a satisfactory manner and not eat its tires. (that's suspension changes, alignment and tire pressure) Set it, forget it... focus on DRIVING. For casual local-level autocross, this will serve you well.

OR...just co-drive one of The Mayor's cars which are tits and hand him his ass

[CaptainSquirts]

I'll be at the next event. I like the idea of getting a tire inflator and just inflating the tire some at the event and then drop the pressure when I'm leaving. I'll try and seek you out Jamie. Also who's Mayor?

[Loren]

Doug Adams. The Mayor of Dougopolis.

[ImpostorDan]

Does Dougopolis have a super hero?

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[Tim_M]

I use 37 and 33 front and rear cold, respectively on my 04 STI. This is very similar to OEM.

I run that pressure all the time and don't touch them during the events.

I'm sure there is more to gain, but the tire wear is dead even with my -1.8/-1.5 camber f/r that my car is able without special suspension bits.

I would expect yours would at least be less in the rear (rather then same all around) as both chassis's still mimic a front driver (front heavy) than not.

[Jamie]

Does Dougopolis have a super hero?

Don't get him going...he'll show up at the next event in a cape....

[ImpostorDan]

Does Dougopolis have a super hero?

Don't get him going...he'll show up at the next event in a cape....

I'll double register just to see that S*** all day long...

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[Rpwolf]

Does Dougopolis have a super hero?

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I don't think I've met "the mayor" but id image the hero goes around with a coffee cup and a hat that says "hero."

I'm with Loren. I ball park a PSI figure for my particular tire. I usually check it once or twice during an event. Mainly, I look at shoulder wear and that would help me gauge if it needs more or less.

Generally speaking, I think max front camber (at least more in the front than rear) works well with most cars with the exception of MR layouts (they may benefit from a rear traction bias). But alignments are car and driving style specific.

Im the guy that will daily an "autocross alignment" and since im in the business of performance, I cant expect even tire wear. What works for me and my car/setup is going to factory recommended tire pressure (32ish) for the street and jacking them up to 40 ish for autocross (my sidewalls are made of paper it seems) to help with tire wear.