Larry Herman

What tire pressures should I run?

Probably the most oft asked question in the pits is: what tire pressures should I run? The most accurate answer to that question is: it depends. And that is what makes it so difficult to determine. It depends on a variety of factors including wheel size, tire profile, tire composition, tire brand, car weight, suspension setup, spring stiffness, track temperature and how hard you drive! No wonder so many are confused. I would like to explain some of the parameters, both objective and subjective, to help you make a more informed decision.

Let’s briefly examine what happens to a tire under cornering loads, as that is usually what you are optimizing your tires for. Under braking, the weight shifts to the front of the car and the front tires compress the sidewalls, spreading out the contact patch length-wise. As you begin to turn, the front wheels turn, twisting the sidewalls, and coercing the tire tread to turn as well. As the cornering load builds, the sidewalls flex more under the lateral forces, and the tread patch becomes distorted. The wheel will actually move over the tread, and the outer edge of the outside tire will roll under, with the inside edge rolling up and possibly lifting off of the road. The more this happens, the less grip that you have. How much the tire patch distorts will depend on a variety of factors, not the least of which is tire pressure.

It is the internal pressure which stiffens the sidewalls and supports the tire. Too much pressure and the tire will be firm, but will have less contact patch. Too little pressure and the tire will distort too much, also reducing the contact patch. The right amount will allow for as much contact patch as possible, with minimal distortion. The way to understand where to start is to consider how the variables listed above affect the optimal pressure:

Less < pressure < More
Wheel width – Wide < pressure < Narrow
Tire sidewall profile – Tall < pressure < Short
Tire type – Race < pressure < Street
Car Weight – Light < pressure < Heavy
Suspension stiffness – Soft < pressure < Stiff
Track Temperature – Hot < pressure < Cool
Driver Aggressiveness – Ragged < pressure < Smooth

All of these factors combine to determine what your optimal hot pressure should be. And I do mean HOT pressure; that is where your tires are operating. Cold pressure is merely the starting point at which to set the tire pressures when you start in the morning. The best way to figure out what you want is after you get your pressure where you want them, check them the next morning after they have cooled back down, and that will be your starting point. And this leads us back to the original question, what tire pressures should I run?

The ideal way is to take tire temperature readings. This will tell you how effectively the tire is working across the width of its tread. The outside and middle of the tread should be approximately the same temperature, with the inside usually being cooler. If the center of the tread is cooler than the outside, you need more pressure, and vice versa. Tire temps can also show the effectiveness of your camber and toe settings.

Many of us will just use feel to get the tires to close, and for DEs that is fine. I find that if my pressures are too low, the tire will brake well, but feel mushy and unresponsive in the corners. If the pressures are too high, then the tires feel crisp, but will feel greasy and slide too easily under both braking and cornering. Your best pressure and maximum grip lie right in the middle. Hope that helps; see you at the track!

Veloce Raptor

Very good! My only suggestion: delve more deeply into the tire temp issue, and what sort of spread in temps is optimal (and why) as well as what widely varying temps really tell you. Also, the last paragraph is very good. You may want to also explain the smartest way to adjust tires by feel, and which end to start at, etc.

Sean F

You might want to add something about ambient temp/track temp as well. Seems like there are always lots of questions/debates in the Northeast about what a starting temp should be if the ambient temp is 38 degrees vs. 86 degrees and how much of an increase in pressure to expect hot based on ambient temp.

Jim Child

You might also want to point out the value of keeping records on tire pressure. Having records to go back to for different tracks, ambient temperatures, etc. can help achieve the targeted hot pressure on the first try, thus minimizing the number of sessions it takes to get the car where you want it.

TD in DC

Nice Larry,

Although it might expand the scope beyond what you want, I think adding a little more information would be fascinating to newbies.

Specifically, it might be interesting to talk a little about the proper way to get your tires up to temperature, and about the fact that the way you drive can affect your tire temps. In other words, you can abuse your tires and fool yourself into thinking that you need to keep dropping the baseline pressures. At some point, you will have dropped the pressure too low and the tires will heat up even more. So, you really do not want to end up at either extreme of the pressure scale, and if you do, it might be time to reexamine what is really happening rather than just continuing to fool around with the pressures.

P.S. Only this year do I think I really learned the best way to heat up your tires. I wish I had learned earlier.

Bull

Looks good Larry. I was Technical Chair (Co-Chair) of our Region until '06, and agree that you have hit on a subject that never goes away. We found that there is so much information to provide on this subject, as others are indicating here, that we made it the subject of one of our annual Tech Sessions (we do 5-6 Tech sessions each year from November to April).

The Tire Tech Session is held at one of the tire/suspension shops that supports the Region, with Manufacturers' Reps in attendence when we can get them. It allows for a lot of show-and-tell, including used tires that weren't managed properly. After the first Tire Tech, we learned that it was impossible to do without being ready to dive into suspension/alignment discussions as well.

Good luck with your endeavors with this!

Larry Herman

Good Luck? Ha Ha, you just shot yourself in the foot, because you're helping me now! I'll PM you with my numbers so that I can get some mentoring from you.

993inNC

I'm not sure Larry, if what I have to say is worth mentioning in you write up.....but. I have always found that a correct tire pressure from start to finish, yeilds a consistent pressure increase (6lbs) as well as temps usually within about 10-15 degrees deviation across the tire ( even with agressive camber). I think from a laypersons point of view, it would be worth mentioning that even if they are not a mechanical wiz, they should be at least capable of keeping track of their tire pressures in that regard as well as paying attention to how the car reacts as they make changes, and that if they get more than roughly 6 lbs increase, something may be still wrong.

just my .0002 cents

'course then the question comes up, is it the driver or the car thats "off"

PogueMoHone

It's a good article, but the text suggests it is for novices; as the more experienced, though having the same ultimate quest, will be further along the learning curve.


The answer "it depends" while absolutely accurate will only frustrate the novice, when what in fact they need is a starting point to begin experimentation.

My suggestion to your article is to "encourage" the novice to begin the track day by dropping 2lbs all around from the recommended street pressures (assuming stock) and voila you have a practical starting point.

That's what I do, and as a novice it does the "trick" for me.

Further you might quantify the direct link between temperature rise and tire pressure ( e.g every x degrees increases y lbs approx). Lastly a comment or two on how the manufacturer sets the pressures cold, anticipating warm when in operation. Again, 36 lbs at 68 F translates into a temperature of X when at freeway speeds (no, I'm not suggesting you get out on the freeway to measure), the implication of this calculation is that less of an adjustment than one might think is required at the track.

It mystifies me to see people constantly playing with their pressures but then maybe they know a lot that I don't (or maybe it's a side hobby!)

Larry Herman

I knew it was a good idea to post this. You guys make a lot of good points, and I will massage it a little and then post the revisions. It is aimed more for the novice, and I like Colm's idea about suggested starting points for different types of tires. Maybe as I acquire more information, I'll split it into a 2 parter, with the second article delving into more detail for the more advanced driver. Keep the suggestions coming.

TD in DC

I would not get any more specific than suggesting something like "try dropping X from the manufacturer's suggested cold pressure to start" due to potential liability concerns . . . . The problem still is that a set amount of pressure is a different percentage of the overall pressure of different types of tires. You don't want to make too specific of a suggestion, have a particular tire fail, and then have to defend yourself against the suggestion that the tire failed because they followed your suggestion. Sorry to be the buzzkill here.

Larry Herman

Understood. I was thinking more on the lines of a range of pressures, like street tires between 36-42 lbs, R tires between 34-39 and slicks between 32-36 as a guideline. What say ye, counselor?

TD in DC

I wouldn't, but that's just me. Seriously, you just can't know what tires someone will bring, and there will be some freak tire for which your recommendation will not be accurate. Even if it is accurate, the fact that you made a specific recommendation at all could be risky, and it is just not necessary for your goals for the article.

I think you are safer suggesting that they look to the manufacturer's suggested pressure, and then start 3-5 lbs below that for the first run of the day. Then, you can adjust as necessary based upon hot pressures (and the cold pressure from the next day).

George A

Larry, the biggest problem I get down here in Texas is from newer 911 with 18 inch wheels that have a recommended COLD pressure of 44psi. It's really hard to convince a novice that the factory numbers don't make sense for the track. In the 100 degree heat, I can see them going over 50psi. I wish someone would address this issue in our region.

George

DanS911

Looks good Larry.
I wrote a similar article for our region a few years back. Many similar conclusions and variations.
One thing I added at the end was tread roll over.
Either chalking the tires or checking the manufacturers indicator (the little triangle) to see how much the tire is rolling while cornering.