One question I'm asked more than just about anything else is, "What tire pressures should I be running on my car?" Of course, there's not a one-size-fits-all answer to that, and that's why this week's feature is part two of Dale Thompson's article on car setup that he began last week. -Ross
“What hot tire pressures should I run?” Many drivers/racers consider this question, but get confused by all of the advice flowing around the paddock.
So let’s start with something we all know about - the tire placard that recommends tire pressures for your road car.
My wife’s Ford Focus is sitting in the driveway, so I’ve just popped outside to check the recommended pressures…
1-3 passengers and luggage: 31 psi front, 31 psi rear
4-5 passengers and luggage: 35 psi front, 41 psi rear
The thing about manufacturers’ recommended tire pressures is that you set them cold in the morning, and then do not bleed them off when the tires build up pressure, in the heat of the day, or with running on the road. On the road, as opposed to racing, we are working from a cold pressure and the design of the tire characteristics are such that any pressures build-up is accommodated.
So, what does this say about setting the tire pressures at the service station, when you’re on your way home after a trip? You need to over-pressure the tires and then bleed them down the next morning. You can read everything you need to know about tires on the road on the Tire Rack website, including an interesting bit from Michelin about tires aquaplaning in the wet with just 5 psi less than recommended pressure.
There are three wheel sizes for my wife’s Focus – 16s, 17s, and 18s, with 50, 55, and 40 series tires. Yet the same pressures are recommended for all tire and wheel combinations. That’s something we can think about for racing tires. On the other hand, look at the 10 psi pressure difference between light load and heavy load. That’s most certainly something we can think about for racing!
For racers, our best source of tire info is the Hoosier “Tire Care and Safety Guidelines.” Hoosier is the racer's friend when it comes to tire data and info. The reason, I guess, is that they don’t have to risk confusing people with road cars, because they don’t sell tires for the road.
A quick point before we proceed. Two racecars may have similar max cornering grip (max lateral grip) – same grip mid-corner. Yet one may be significantly quicker in corner entry and/or exit, even to the tune of seconds per lap. We’re not so interested in the maximum cornering grip, more so in what happens as cornering grip in the tires builds up in corner entry and lets go in corner exit.
Getting straight to it, we’ll be starting from a cold pressure, build up tire temp and pressure during the track session, then check tire pressures immediately when we come in. You will see, then, exactly how many psi you need to take out of each tire to get to your target hot pressures in the next track session.
Hoosier tire pressure recommendations for DOT radials (semi-slick racing tyres):
1800 – 2200lbs: Hot Pressure 32 – 34 psi
2200 – 2600lbs: 34 – 36 psi
2600 - 3000lbs: 36 - 40 psi
OVER 3000 lbs: 40 - 42 psi
That's it, really. There is an eight to ten psi difference in the pressure you need between the lightest and heaviest production-based cars, and as a general rule, the same pressures apply to DOT radials from other manufacturers. Generally speaking again, you can set your cold pressures to 26 psi and then bleed them back to your required hot pressure immediately after your first run. Of course, if you haven't reached your hot pressure, then consider adding air for your next run. You do need to keep an eye on the pressures as you go during the day. With higher ambient temperature and more track temperature, you might generate more grip, and therefore, build higher tire pressure.
So, with your hot pressures set correctly, you could go out and win races and set FTDs (Fast Time of Day), right? Pretty much.
But there is more. These diagrams help us understand why there is a broad range of acceptable pressures, yet there still could be small areas where we might gain performance.
The diagram shows a “wheel pair” in a corner for Formula SAE race car. The more heavily-loaded outside tire is shown with a vertical load of 350 lbs. The lightly-loaded inside tire has a vertical load of 150 lbs. The static weight on each tire is 250 lbs and, as shown in the diagram, the weight transfer from inside tire to outside tire is 100 lbs.
I don’t think I am speaking out of school with this, because the data is over ten years old. However, we should remember that data like this is expensive to produce, and the only reason it is available to us is through the work of the Formula SAE Tire Consortium, a volunteer-run organisation with the support of Calspan Tire Testing facility, and the senior vehicle dynamics engineers and FSAE judges that have made it all possible.
We are looking at tire pressure versus coefficient of friction. We have a coefficient of friction of over 2, for what we imagine is a tire that is up to racing temperature. The coefficient of friction is our measure of cornering grip in this case. On our most important outside tire, at 14 psi, we have 2.3 cof x 350 lbs = 805 lbf lateral force (cornering grip). (The tire pressure of 14 psi may seem very low, but remember, an FSAE race car is very light. We would expect a hot tire pressure in this range, just like your much heavier production car has a hot tire pressure range about 35psi.)
The most important observation is that for the outside tire, tire pressure doesn’t affect maximum lateral grip that much, in a normal range of tire pressure – 12 to 14psi is a huge range where grip is shown as virtually unchanged.
We do need to add one complexity here. The lateral grip we are looking at in the diagram is an average value. The lateral force is greater when the load is coming on, forcing the rubber into the road, and somewhat less when the tire is unloading. I’ve seen some data that indicate there is a wide difference in the lateral force generated, loading versus unloading, especially for lower pressures. With higher pressure loading versus unloading, lateral force is more consistent.
The second observation is that the inside tire loses grip fast as pressure increases. The inside tire would work better with a lower pressure. But of course, it is doing less work than the outside tire (because it’s unweighted), so does it matter? We have already calculated the lateral grip on the outside tire at 14 psi as 805 lbf. At 14 psi, the inside tire lateral grip is 2.55 x 150 lbs = 382 lbf. If we dropped the inside pressure 1 psi, we would gain 10 lbf of lateral grip – maybe not enough to get excited about.
But this next diagram may change the picture a bit. Here we have tire pressure versus “cornering stiffness.” We touched on this last week. Increasing cornering stiffness means that the tire builds slip angle and tire grip faster, the tire is more responsive – maybe even enough to influence the balance of the car in corner entry and corner exit, as we discussed last week. Here’s hard data that shows that…
Notice it’s the outside tire that builds the cornering stiffness with increased pressure. The inside tire actually loses cornering stiffness with increasing pressure.
So where’s this leave us? Stick with the recommended pressures unless you have the test time to prove anything different. My own thinking is to be on the low end of the recommendation, taking advantage of any improved inside tire grip. Another reason? The Hoosier recommendation is that a higher pressure may give a little more maximum grip, but will require greater sensitivity to drive. The peak tire grip will drop off more sharply. You can see this drop off in any tire data you look at. My thinking? Let’s go for drivability, and be on the low end of the recommendation.