“What tyre pressures should I run?” Always a hot topic of discussion among racing drivers and enthusiasts.
In circuit racing, the tyres build up from a cold pressure at ambient temperature, and then stabilize at the operating pressure (hot pressure). Here are your target hot pressures:
I want to show you some very simple rules for setting tyre pressures, and then go over some really useful insights we can get from real world tyre test data.
The operating hot pressure is your target. Immediately you come in from your first run of the day and your tyres are up to temperature, you will bleed the tyre pressure down to your target. As the day progresses you may be building up more pressure again due to ambient temperature increase, and heat from your brakes generated in longer runs. So you will adjust pressure accordingly.
The bottom line is that maintaining your target hot pressure for the major part of your race, whether it is a ten lap circuit race, or a short run in autocross - this is the number one priority. This applies under any track conditions, wet or dry.
In racing, the grip of the tyre depends on the vertical load, or weight on that tyre.
With weight transfer in cornering, the vertical load varies a lot. You have increasing load going to the outside tyre, up to 1.5 to 2 times the static load, and the equivalent decreasing load on the inside tyre. Operating pressure has an influence on the shape of the tyre contact patch. The vertical load and tyre pressure influence the shape of the contact patch. In matching the pressure to the weight of the car, we are trying to maintain the most suitable contact patch shape for the tyre.
Now before we go into the detail of this, let’s start with what we know about road tyres.
Tyres on the Road
Consider the “tyre placard” (or the relevant section in your Workshop Manual). Here you see the recommended tyre pressures from the manufacturer.
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 for 1-3:
31 psi front, 31 psi rear
4-5 passengers and luggage for 4-5 (load is biased towards the rear)
35 psi front, 41 psi rear
Note: For fully loaded car, 4 psi extra in the front and 10 psi extra at the rear, due to the extra rear load.
The thing about manufacturers recommended tyre pressures is that you set them cold in the morning, and then do not bleed them off when the tyres 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 tyre characteristics are such that any pressure build-up is accommodated.
Clearly, the manufacturer’s recommendation is to set the recommended pressure with the tyres at ambient temperature, the so-called “cold pressure”.
So, what does this say about setting the tyre pressures at the service station, when you’re on your way home after a trip? If you were to just re-set the tyres to recommended pressures, the hot tyres will be over pressured. You need to bleed the tyre pressures down the next morning, when the tyres have cooled to the ambient temperature.
One “advanced” option here. Say you are doing sustained high speed running on the highway. Say you determine your best pressure for tyre performance is 33 psi all round. In this case, with more knowledge about the tyre characteristics, you might bleed off the excess pressure. You could get your tyres up to running temperature first, and then bleed back to your target hot pressure of 33psi, just as you do on your race car.
You can read everything you need to know about tyres on the road on the Tire Rack website, including an interesting bit from Michelin about tyres aquaplaning in the wet with just 5 psi less than the manufacturer’s recommended pressure.
The Tyre Placard for a Road Car like the Ford Focus
Two very important lessons we can take away from this road car tyre placard…
The weight of the vehicle on the tyre determines what tyre pressure we need on the road. The same applies for racing.
Hot Tyre Pressures for Racing
For racers, our best source of tyre info is the Hoosier “Tire Care and Safety Guidelines”. You can Google and download it. Hoosier is the racers friend when it comes to tyre 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 tyres for the road.
Getting straight to it, we’ll be starting from a cold pressure, build up tyre temperature and pressure during the track session, then check tyre pressures immediately we come in. You will see then exactly how many psi you need to take out of each tyre to get to your target hot pressures in the next track session.
Hoosier tyre pressure recommendations for Dot radials (semi-slick racing tyres):
Light Vehicles 1800 – 2200lbs: Hot Pressure 32 – 34 psi
2200 – 2600lbs: 34 – 36 psi
2600 – 3000lbs: 36 – 40 psi
OVER 3000lbs 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 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 tyre pressure.
So, with your hot pressures set correctly, you could go out and win races and set FTDs, right? Pretty much.
But there is more. The diagrams below 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” (inside and outside tyres) in a corner for Formula SAE race car. The more heavily loaded outside tyre is shown with a vertical load of 350 lbs. The lightly loaded inside tyre has a vertical load of 150 lbs. The static weight on each tyre is 250lbs and, as shown in the diagram, the weight transfer from inside tyre to outside tyre is 100 lbs.
I don’t think I am speaking out of school with this, because the data is over 10 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 tyre pressure vs coefficient of friction. We have a coefficient of friction of over 2, for what we imagine is a tyre up to racing temperature. The coefficient of friction is our measure of cornering grip in this case. On our most important outside tyre, at 14 psi, we have 2.3 cof x 350 lbs = 805 lbf lateral force (cornering grip). (
The tyre pressure of 14 psi may seem low, but remember, an FSAE race car is very light. We would expect a hot tyre pressure in this range, just like your much heavier production car requires a hot tyre pressure about 35psi.) There is a direct relationship between weight of the car and tyre pressure – roughly twice the weight requires twice the pressure.
The most important observation is that for the outside tyre, tyre pressure doesn’t affect maximum lateral grip that much, in a normal range of tyre pressure – 12psi to 16psi is a huge percentage 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 tyre is unloading. I’ve seen some data that indicates there is a wide difference in the lateral force generated, loading vs unloading, especially for lower pressures. From what I can see, with higher tyre pressure, loading vs unloading lateral force is more consistent. Could indicate the higher pressure in the range is better.
The second observation is that the inside tyre loses grip fast as pressure increases. The inside tyre would work better with a lower pressure. But of course, it is doing less work than the inside tyre, so does it matter? We have already calculated the lateral grip on the outside tyre at 14 psi as 805 lbf. At 14 psi, the inside tyre 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 for speedway, ashphalt and dirt it, dropping the inside tyre pressure is a big deal. It would appear that inside pressure should be 60% to 70% of outside pressure.
Tyre Maximum Cornering Grip vs “Tyre Responsiveness”
Two race cars 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 extent of seconds per lap. So it works out that we are not so interested in the maximum cornering grip, more so in what happens as cornering grip in the tyres build up in corner entry and let go in corner exit.
In this next diagram, we show tyre pressure vs “cornering stiffness”. Increasing cornering stiffness means that the tyre builds slip angle faster, and therefore builds tyre grip faster. If this is within a range the driver can feel, we expect that the tyre will more responsive. An example. Initially, say the car does not rotate well on corner entry, resulting in understeer. The additional response from increasing cornering stiffness might get the car turned better, and if so, understeer will be reduced.
Notice it’s the outside (loaded) tyre that builds the cornering stiffness with increased pressure. The inside tyre actually loses cornering stiffness with increasing pressure.
If we increase pressure from 14psi to 16psi, the cornering stiffness goes up by 10lbf/degree of slip angle. Thus grip will build faster on corner entry (for example), although ultimate mid corner grip will be unchanged with the pressure increase.
Tyre Pressures for Circuit Racing
Under good track conditions, and reasonable length runs (five or 6 minutes at race speed in the corners), pressure build up from cold to hot should be 7 – 8 psi.
For cars with 50-50 weight distribution, we want the same hot tyre pressure at all 4 wheels. For vehicles with weight biased to the front or rear, we expect around 3 or 4 psi more for the heavy end of the vehicle. Porsche 911 with higher rear weight percentage, maybe higher pressure split again.
When you check after your first run, there may be different pressure build-ups – the outside tyres for the predominant direction of the circuit may get hotter than the other side. But bleed them all back to the same pressure. After the last race, you could wait a few hours for the tyres to cool, and then record your cold pressures. If you have to go straight out to qualifying at your next race meeting, without a warm-up, you have the cold pressures recorded that will give you the hot pressure you need to do your best lap.
For running in the rain, the target hot pressures are the same. Just start with higher cold pressures, but do be aware of increasing pressure build-ups on a drying track.
Though autocross hot target pressures are the same as those for road racing, you will need to start at a higher cold inflation pressure to compensate for the lower pressure build-ups in autocross racing.
Say that the target hot pressure is 34 psi. You know in a 60 second run, there is no way you’ll get your tyres up to temperature for the first run of the day. You might set a cold pressure of 32 psi and not actually bleed the tyres down until you see 36 psi after a run. The idea is to achieve the recommended pressure for the greater part of your run. Of course, on stinking hot days with high track temperatures, pressure build-ups will be greater and you will start with lower cold pressure.
With more experience, you get a better idea of your required starting pressures your starting pressures.
Wet running – target hot pressures are the same as for the dry. Start with your target hot pressures and see if you get any pressure build up at all. In particular, dropping tyre pressures can only hurt grip. Grip will not improve with lower pressures for all the reasons discussed earlier.
Adjusting Balance for Understeer/Oversteer with Tyre Pressure
Adjusting balance with the tyre pressures is a last resort. Our first preference is to adjust balance with the anti-roll bars. (For more details see our free on-line suspension tuning workshops at https://www.suspensionsetup.info/)
When adjusting tyre pressures for oversteer/understeer – think increase tyre pressure for the end you want to “stick” and decrease tyre pressure at the other end.
For circuit racing it’s not generally going to be a good idea to do this. If an extra 2 to 4 pound pressure split front to rear is working, then maybe not too much harm is done. A bigger split than that, and we must be outside the best grip range of the tyres.
However, for short run events (eg autocross, hill climb), tyre pressures will more often be used as a stop gap measure. Balance is king after all.
Google "2016 MX-5 Solo Development – CS" and you’ll see the article there on the Mazdamotorsports website. In this article, Ron Bauer, an SCCA National Solo Champion, is working with Mazda to develop the 2016 ND Mazda for autocross. In his initial outing, the car is standard, with the Brembo brakes and Bilstein shock options.
Initially, the car has way too much oversteer. As a stop gap measure, they needed a 7 lb split to balance the car. It is remarkable for a standard car, that the balance could be this bad.
An adjustable front anti-roll bar was required. The bar chosen has a replaceable centre section, with removable arms. Different wall thickness bars provide different roll rates. Apparently, the recommended way to fit the new one is the cut the original bar to remove it, to save many hours of work removing parts to allow removal in one piece! This new part, available from Mazdamotorsports, means that all the ND Mazdas in Solo can be properly balanced and run an even tyre pressure all round.
So where’s this leave us? Best advice is to stick with the recommended hot 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 tyre 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 tyre grip will drop off more sharply. You can see this drop off in any tyre data you look at. My thinking? Let’s go for drivability, and be on the low end of the recommendation.
This is supported by a comment in the Hoosier tyre information - higher pressure (than recommended) may improve grip, but could also require greater driver sensitivity.
To read about the characteristics of tyres, rubber compounding and the mechanics of grip (rubber friction) there is no better book than "The Racing and High performance Tire" by Paul Haney.
I will write a second article about tyres to deal with tyre temperatures - pyrometer readings - and camber settings.
But the important thing to take away for this article is that maintaining your target hot pressure for the major part of your race, whether it is a ten lap circuit race, or a short run in autocross - this is the number one priority. This applies under any track conditions, wet or dry.