Tire Pressure November 22, 2005
The cheapest handling upgrade you can perform on a car is to find the “right” tire pressure. The recommended tire pressure for a car is a compromise based on factors that include ride comfort and noise. At an autocross event the only factor that matters is traction, on street tires increasing the tire pressure can shave a second or more off your time.
Unfortunately there is no magic number that’s right for every car and tire, you have to figure out what works best through trial-and-error. Start by splitting the difference between the car manufacturer’s recommended tire pressure and the maximum pressure listed on the tire itself. For my car the owners manual recommends 32PSI all-around and the Kumho MXs I use for autocross list 50PSI as the max, so 41PSI is where I would start from.
The goal is to increase the sidewall stiffness so that they don’t “roll” during hard cornering. Traction on the tread is good, on the sidewall not so much. Take some white chalk or shoe polish and make some large markings where the tread meets the sidewall at the 12, 3, 6, and 9 o’clock positions. After your first run check the markings. If the markings haven’t rubbed off the tread then the tires are over-inflated — let some air out. If the markings on the sidewall are gone, add a few more PSI. Re-mark the tires and repeat, keeping in mind that tire pressure will increase as the tires heat up.
If you aren’t hyper-competitive you can eventually settle on a cold tire pressure that provides good traction and won’t result in over-inflation as the tires warm up.
Once you’ve found the “right” tire pressure for maximum traction you can experiment further by using different pressures in the front or rear. Less pressure will generally mean less traction at the limits, so you can create a little bit of understeer by running lower in the front or oversteer by running lower in the rear. My car is AWD and tends to understeer at the limits so I run the rear tires around 5PSI lower than the front. The effect is not dramatic but at the limits every little bit helps.
When you are deflating the tires for the drive home you should aim for a few PSI above the recommended level, since tire pressure will be higher while they’re still hot, and check them again once they’ve had a few hours to cool down. Don’t drive home with the tires still inflated to autocross levels — the road noise will annoy you, every bump will jostle you, the car will try to follow every track in the road, and road hazards will pose a much greater risk of puncture. It would not be a pleasant drive.
Tools of the Trade
Having an air pump in your trunk is a good idea in general. I recommend buying the cheapest one that you can find because there is no such thing as a good lighter-socket powered air pump. You should not need to use your own pump at most autocross events, any sponsoring club in good enough financial shape to have a laptop for timing ought to have a proper 110v or gas-powered air compressor for everyone to use. In my area it’s the Porsche club that has no compressor and records times with pencil and paper, go figure.
A good tire gauge is, of course, absolutely essential. Never trust an air pump or those “pop-up stick” type gauges to provide an accurate reading, my pump reads about +5 at 40PSI. I received a digital gauge as a give-away item from a local car dealer — Rick Case Acura I think, the printing wore off — that feels like it cost about a quarter to manufacture but is accurate enough and easy to use. The disadvantage to digital is that most will display the highest pressure reading for several seconds — if you are deflating a tire or moving on to another then between readings you must wait until it resets itself. I’m an impatient man so I picked up an analog gauge this week similar to the one below for $12 at my neighborhood auto parts store. The bleeder valve makes deflating tires much easier and faster.
