What tire pressures should I run?

[Larry Herman]

Here is an advanced copy of an article I am going to put in Der Gasser

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.

But many of you still want to know what pressures to run.  I hope that you can see that there is no one right setting.  It is something that you will have to experiment with.  That being said, there are ranges which most tires seem to operate in.  Slicks have the stiffest sidewalls, are run with the least amount of pressure, and can start cold at as low as 24 lbs, and increase to between 30~34 lbs hot pressure.  R compound tires need a little more, starting between 26~30 lbs cold and settling between 34~38 lbs hot and street tires need the most pressure, starting cold between 30~34 lbs and getting up to 38~44 lbs hot.  As I mentioned above, you will need to experiment to fine that exact pressures that suit you and your car.

Hope that helps.  I'll see you at the track!

[Phokaioglaukos]

Larry Herman wrote:

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.

Good stuff, Larry, thanks for posting.

When I ran the OEM Pirelli Rosso tires I changed from the owner's manual 32F/39R cold to 32F/32R cold and looked for the tires to be 38-40F/38-40R hot. I kept doing that when I changed to Toyo RA-1s about the middle of last year (I drive to/from the track on the same tires).

Now I have invested in a contact pyrometer that has a memory function. The operator presses the probe into the carcass of each tire at three locations across the face of the tire to evaluate the alignment. It also lets me see if my tires are at the right temperature. Manufacturers advise temperature ranges for their R tires, even the semi-R tires like the Toyo RA-1s (which I have) and the Nitto NT01s (which I will install after Shenandoah).

According to Toyo, the RA-1s want to be 160° F. to 220° F. Toyo also says that the target hot inflation pressure for these tires is high 30s to low 40s. I think over 40 is a bit much. Toyo also recommends negative camber of from 2.0° to 5.0° and advises that inside shoulder wear will result. Toyo says to maximize tire life , the tire should be dismounted from the wheel and flipped when the tire is about half way worn.

According to Nitto, the optimum operating temperature for the NT01s is also 160° F. to 220° F. Some on Rennlist have asserted that the NT01s have the same rubber compound as the RA-1s. I sure do not know.

Others can discuss Michelin Pilot Sport Cups, Hoosiers and slicks. I'm too inexperienced (and cheap) to move to those tires, and also lack the second set of wheels, trailer and tow vehicle they can require.

[smankow]

I started running the MPSC's last season.  I talked with people, did research and even talked with Michelin Motorsports Division for optimal tire pressures.

The overall consensus (confirmed by Michelin) was to run 26f/28r cold to achieve 32/36 hot.  Even on colder days, the starting pressures yielded the proper hot pressures.  I bleed down to 32/36 if they get too high.  The tires feel greasy to me if they get above this range.

Generally, after the first session, the pressures after a run stay consistent with the range that I'm looking for.

I don't have a pyrometer, so I don't know the actual tire temps.  Maybe I'll have to borrow Chris's.

 

Steve

[Darren]

I've found tire temps to be a bit confusing sometimes, Larry maybe you have some advice/guidance here.  In the case of using a memory pyrometer that takes 3 readings per tire -- assuming that you pit in and have a friend check these right after a hot lap, they may be as accurate as you can measure, but don't seem to tell the full story.  Each turn or straight section are going to generate different heat gradients across the tire.

Also, since braking is where we see the highest g-force, wouldn't that be the time that we generate more heat than in other case? And if that is true, wouldn't the inside heat up faster because of the negative camber?

[Larry Herman]

Darren, what you say is true, but I think that most of the temperature in a tire comes from the distortion and sliding that occurs in a turn.  There is where the highest sustained loads are, and checking tire temps in the pits gives us a muddled average of what is actually happening, weighted mostly from cornering.  This is a fact recognized by the pro race teams, and the latest strategy is to put 12 IR sensors (3 per tire) on the car to get real-time temperatures.  Then they can map the tire temps over the course of an entire lap, and make better determinations.  Still, like us, they have to make most of their adjustments to improve cornering because that is where most of the speed comes from.

[Phokaioglaukos]

I'm looking forward to seeing Geoffrey Ring's track car again (a 964?). He'll be at the Cups and Saucers at the Glen next month. He now has IR sensors so that he can record the temperature of the track, tires and brake rotors while underway.

For those of us with less tricked-out cars looking for tire temperatures, make sure to get a contact pyrometer. The IR ones measure surface temperatures which cool quickly. The termperatures a ¼" into the carcass of the tire cools much more slowly, and that is what you want to measure. (I'm great on the theory--let's see how I do on the practice, measuring temperatures after coming in on a hot lap.)

[Larry Herman]

Hey, I'll let you practice on mine!

Actually, we can probably help each other, as I assume that you will be running in saucers, and I will be with the cups.

[Phokaioglaukos]

That's a deal, Larry. I plan to start in saucers, but I also plan to do some time in cups so that I get an idea of what it will be like at Kojote where there will be at least two 962s on the track with us. Got to practice my mirror-use and track awareness!

But we can certainly practice with the pyrometer there. I'm hoping to get to Shenandoah this weekend, too, but a deal is kicking up now that I did not expect....

[smankow]

I'm set for cups and saucers, too.  I'm more than willing to help and learn.

 

Steve