You made it to your first track day. It is something you have been waiting to do for a long time, and you finally pulled the trigger, pre-registered for an HPDE event and are at the track. Congratulations! Before the big day, you took the time to wash your car so it will look shiny and fast as you drive around the race course. You find a place in the paddock to park next to another HPDE driver who looks like she has been to the track a few times. Her car has a few track map stickers on the rear window. The two of you exchange pleasantries and start to talk about where the driver’s meeting will be held and then she asks, “What tire pressures are you running?”
You pause for a moment, because the fact of the matter is you don’t know what your pressures are. You know that you have tires and they have air in them, otherwise your pesky tire-pressure monitoring system would have lit up the dashboard like a Christmas tree. But, you don’t know exactly what the tire pressures are and that could be a problem for high performance driving.
The good news is the information about what your HPDE tire pressure should be (at a minimum) is stuck to your car inside the driver’s door. Open your door and check the decal to determine what the manufacturer recommends for each tire. In the case of a Ford Fiesta ST, the front and rear pressures are different: 39 p.s.i. front; 36 p.s.i. rear. A general recommendation for track days/HPDE is that tire pressures should be — at an absolute minimum — what the manufacturer recommends on the tire loading and information decal, and should be between two to five pounds above that minimum for each tire. The reason for this is your car is going to be driven aggressively around the track, and for safety reasons NASA does not want you to have a tire come off the bead of the rim because you don’t have enough air pressure inside the tire.
Owning a good tire-pressure gauge is a great tool to have with you at an HPDE event. You will find that tires are the most important thing to improving a vehicle’s performance. Knowing what the tire pressure is exactly is important to help you make decisions to raise or lower the tire pressure. I use a digital gauge that has a memory in it to keep track of tire pressures before and after a track session. But you don’t need a super fancy digital tire gauge to do the job. A much less expensive analog gauge works just as well.
It is best to plan ahead and arrive at the track with the correct pressures in your tires, but if you need a few pounds of air you can usually find a tire shop at the track or somebody with an air tank that will be kind enough to help you add a few pounds of air in your tires. After you make your first runs around the track it is crucial to take a look at your sidewalls and see what kind of rollover you are getting. If the tire wear is rolling over right to the edge of the tread blocks on the tire, that is good. If you are rolling over onto the sidewall, you will want to add tire pressure. Start with a 3 pound increase. If your tire isn’t rolling over at all toward the edge you can consider lowing your pressures 2 pounds. When you check your pressures after a track session, they will naturally be higher in pressure due to warm air expanding in the tire. Remember the tire pressures listed on your door jamb are for cold tire pressures. The increase is OK and normal.
Another tool for making adjustments to tire pressure, besides visually inspecting the sidewall rollover and using a tire gauge, is a pyrometer. A pyrometer is simply a thermometer for a tire. You jam a small probe into the tire to measure the heat within the rubber. This measurement is done at three different locations along the tire — outboard, middle, inboard — of the lateral tire tread. Looking at the difference in those temperatures along the tire can help indicate if you have too much or too little air pressure, or if you need to make an alignment change to the car.
When using a tire pyrometer, it helps to know what to do with the information. A basic guideline is that if the middle of the tire is hotter than the inboard and outboard of the tire then the tire is ballooning because it has too much pressure inside it. If you find that type of heat data I would lower the tire pressure in that tire. Conversely, if the outboard and inboard of the tire are warmer than the middle of the tire I would add tire pressure.
Once you have tire pressures adjusted for straight line speed based on the chart above, you can begin to drive the car aggressively through a number of corners and then use the pyrometer again to determine if your alignment can use some adjustment. If you have an extreme tire temperature difference — more than 30 degrees — between the inboard and middle of the tire versus the outboard portion of the tire that may mean you need to add more negative camber at an alignment shop. Your tire pressure may be just fine. The outboard heat may be a function of suspension geometry and not tire pressure.
If the outboard edges of your tires are running warmer than the rest of the tire, that is a natural thing for street car tires to do at an HPDE event. It is simply a function of the vehicle leaning through the curves, which means the outboard of the tire is doing more of the work through the corners. To try and get more surface area of the tire to help with those cornering forces, you can add negative camber to your alignment. It depends on the vehicle, but these adjustments usually aren’t terribly difficult to make.
By collecting tire data using a pyrometer and a tire gauge, you may decide you do want to make an alignment change. You may not have the tools or know-how to do it at the track, but you can take the data you gleaned from your time at HPDE and have the car given a performance alignment at a shop before you come back for another HPDE. Chances are you will be back. Track days are addictive.
When it comes to tire pressures for HPDE, start with safety first, ensure your car has at least what the manufacturer mandates and then I would suggest going up from there to start. Inspect your outer edges of your tires after every session. Use your tire gauge to make minor adjustments. Use a pyrometer to fine-tune your pressures for optimal performance. And most importantly have fun on track!