Sunday, January 1, 2012

Car Prep on a Budget (Part II): Tire Pressure

Continuing on our journey of upcoming season car prep on a shoestring, today we'll talk about an often overlooked setup component: Tire Pressure.

Planning on how you will approach your tire choices and pressure-to-be-used is one of your biggie considerations for next season. If it isn’t, it certainly should be.

You are all probably playing with your tire pressure throughout your track day or between autocross runs. At least I really hope you are. It doesn’t matter if you’re running Hoosier slicks or low-wear streets tires, these 3 rules will always hold true:

1. You will gain significant tire pressure throughout the day.
2. Pressure will greatly affect your contact patch and tire adhesion levels.
3. Pressure will greatly affect your under/overseer characteristics.

The art of air pressure is to achieve the largest contact patch, while maintaining lateral sidewall stiffness. You want to keep the sidewall from rolling over (dangerous and expensive), while allowing as much of the sticky stuff gripping the road surface. Both increase cornering agility and acceleration.

Lighter cars do not need as much air; heavier cars need more. I instruct mostly BMW drivers, and as a general rule, I recommend around 37psi to start an autocross event. Check pressure after each session/autocross run, and bleed off as pressure increases due to friction and ambient temperature changes. On a particularly warm day, I’ve gained over 10psi in a single autocross run! Pressure changes beyond 2psi will impact handling characteristics significantly. So keep an eye on it. Eventually, your pressures will even out, as you reach max heat in your tires. When that occurs, you don't have to bleed off any more air, but you may have to add some back.

I have seen a few cars attempt to use nitrogen for track and autocross events. Nitrogen expands and contracts much less than air, and therefore does not gain anywhere near the amount of pressure that air does. I firmly believe that nitrogen is just not practical for the amount of tweaking that we all should be doing, unless you at the level where you have a support crew.

Use chalk on the sidewall to find how much you are rolling the sidewall, which will lead your optimal pressure. You do not want your tire rolling over past the indicator, as displayed. Re-chalk after each session/run, check pressure immediately after coming in, and set pressure immediately before going out for your next session/run.

If you have a car that oversteers or understeers, and it’s not due to you ham-fisting the car controls, you can dial out much of this with your tire pressure. BMWs and Porsches usually have narrower tires up front, and that adds to the understeer built into the cars at the factory. By having a few extra PSI up front, you can easily make the car more neutral, allowing quicker and more controlled cornering. The opposite holds true for cars that oversteer... try a couple of psi more in the rear.

Hopefully, you asked Santa to bring you a QUALITY pressure gauge. If you don’t already have one, go buy one. Most speed shops sell them. I have one similar to the one in this picture.

We could certainly do an entire article on different gauges, but not today. Just make sure you get a decent one, and take good care of it. At a minimum, make sure it has an easy bleed mechanism, and test it against other quality gauges at events for calibration. It’s a very bad idea to save your $10s on this item.

At the end of the day or between sessions/runs, make sure you add to your log, as described in Part I of this series. These log entries are invaluable, and will definitely make you faster for almost no expense, other than your time. If you are using street tires, make sure you replenish the air you bled at the end of the event. If you don't, you'll be driving home with pressure in the mid-to-low 20's psi range.

Have fun preparing for next season!

Next up: Camber/Alignment

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