Keeping up the Pressure
February 1, 2012
By Ed Trottier
Photos by Tom Morgan
It is always a good time to pay attention to your tire pressures, but now – with the change of seasons – perhaps the following general info is even more timely. Your recommended (COLD) tire pressures are found in a number of places; stickers in various places around the car and, of course, in your owner’s manual. (If you need to convert, as I do, 1 bar = 14.50 psi.) The significance of taking COLD tire pressure readings cannot be overstated. No “sunny side/shady side” readings. No adjustments (for regular road use) within 3 hrs after (significant) driving on the tires (more on that later).

Top: Keeping the pressure in your tires at their optimum means more than extending the life of expensive rubber. It also keeps the handling quality of your Porsche’s performance suspension at it’s best.

Above: Compared to street driving, pressures taken for race and track tires are done while hot due to the extreme “flexing” tires go endure while being used at their limits.
I think it was on a trip last year that I decided to do a little tire pressure “data gathering.” Of course, a new digital tire pressure gauge with display down to 0.1 increments (and backlighted!) in psi/bar/Kg/cm2/kPa was an attraction. (Slime Model 20071, $12.99 at AutoZone). I took cold readings (ambient temp 61°F) of 36psi F and 44psi R. We then headed off for a 3hr/180 mile run home, mostly in the 50 -70 mph range. On arrival, ambient temp was 54°F, with tire pressures 39psi F and 47.4psi R. Hmmm, ambient temperature down, tire pressures up by about 3 psi. It is this kind of typical, real world driving that car and tire manufacturers design for and specify with their COLD pressures. Basically, it is a “use/application/compromise” that takes into account variables such as tire width(s), aspect ratio(s), suspension design, handling, tire wear and ride comfort, for example.

So, what about the measured pressure increase from COLD? Tire folks talk about two significant heat inputs: road friction and deflection (“flexing”). Rolling, accelerating, braking and turning all create friction, which generates heat that shows itself as increased rubber temperature and tire pressure. But just as important is the “bulging and flattening” deflection that the tire undergoes as it accepts 1/4 of vehicle load at the bottom of every tire revolution. With steel and fabric belts, liner, sidewall, etc. flexing over and over, you get heat, which shows itself as rising tire temperature. Raise the temperature of a confined gas (OK, a mixture of gases) and voila’ pressure increase! And if you think about the amount of various materials in the tire, you can appreciate the three (3) hour wait to return to a stable and accurate COLD tire pressure reading.

Checking tire pressure regularly will help ensure a comfortable ride with good fuel mileage, traction and proper handling. Left unattended, tires will show you if they have been driven consistently under or over correct COLD pressure setting(s); over inflation shows more wear in center tread, under inflation shows more wear at inner and outer edges of the tread. Unfortunately, by then, you may have compromised tire life. You will not likely get the tire mileage life you expected. Check your COLD tire pressures frequently and consistently. (By the way, the Porsche factory manual for my car specifies tire pressure in “bar.” My $12.99 digital gauge gives me both bar and kPa (bar x 100).)