By Barry Spencer - Spencer's European
Chapman Report – May 1995

The reason for this article is to prevent people from setting their tire pressures to the numbers on the sidewalls. I have received a number of phone calls recently from customers complaining of their cars riding rough and vibrating. A number of things can cause this, such as: Shock absorbers, out-of-round or defective tires, out-of-round wheels, or incorrectly set tire pressure.

Many Lotuses have different tire/wheel combinations than stock. It is very important to get tire pressure correct to maximize tire wear, and improve handling and safety. The number on the sidewall is the maximum PSI for that tire, having nothing to do with the vehicle the tire has been fitted to. Please refer to the handbook to find the correct tire pressure. Handbook readings are cold tire measurements. Test readings will be approximate 4-6 PSI higher.

A tire Pyrometer is a very useful device used in racing to measure the temperature of the tread. The temperature is measured at 3 locations across the tread, 1) 1/2 from the inside edge of the tread; 2) Center of the tread; and, 3) 1/2 inch from the outside edge of the tread. At measurement, the tires need to be at operating temperature of approximately 15-20 miles of street driving. If the temperature in the center of the tread is too high, it nomally means the pressure is too high. Try to get all three readings within a degree or so of each other by adjusting tire pressure.

Also affecting temperature is camber. If the reading is too high on the inside tread, you may have too much negative camber. If the reading is too hight on the outside, you may not have enough negative camber.

Tires are becoming very expensive and are hard to find in some sizes, making it worth checking the tire pressures often. If you do have a different tire/wheel combination than stock, it is worth having your tire pressures set up professionally. Not having your tire pressure correct can be costly. Take the tire to get them correct. Your car will be much more enjoyable to drive.

(Editor's Comments: Barry is totally correct. It's been said, and it's true, racing is all about "where the rubber meets the road." A few additionaly comments. 1) Toe settings can also affect tire temps. Too much toe-in will cause the outer edge to read hotter. Too little will cause the inner edge to be hotter and the car will dart about, especially under braking. 2) In order to increase traction (friction between the road surface and tire) the rule is to lower tire pressure. This is why sand buggies and dragsters fun as little as 3-5 PSI. The problem is that the lower the tire pressure the greater the amount of tire distortion takes place. Too much tire distortion allows the contact patch (the actual contact area where the tire touches the tarmac) to become unstable and thus difficult for the driver to predict the amount of traction available.)