Driving Under Pressure

SGT. STORTON suggests the max tire pressure written on the side wall, but is that when it is cool or hot? As you drive the temperature of the air increases which of course makes it expand increasing the air pressure.

Hi Clint. The sidewall max air pressure should be done when the tires are cold. (average air temp).

The manufacturer has taken the increase in pressure due to heat into consideration.

So, if the pressure goes up while driving on a real hot day, the tire is still well within it's safety limit.

If you looked up the"Burst pressure" for the average auto tire, I think you might be surprised how high it really is.

"The tires will not balloon out creating a peak in the center portion of the tread when tire pressure is above 35 psi. There is a steel belt that prevents this from happening."

"Also, a soft tire can be pushed in more by the pressure of the water on the center portion of the tread."

The truth is that tires DO change shape with pressure, t steel belt isexible, which is why cars are able to go over speed bumps, etc. You may need slightly more than 35 psi in the tires, and may see a performance advantage at 44 psi depending on the fully loaded weight. The best way to check what pressure is best for the tire is to find the "end of shoulder" band on the tire. It runs around the onside face of the tire. Put chalk on ts band and the side ofhe tire shoulder up onto the tread some, all the way around (or at least a good portion of the tire).

Inflate to 44 psi and make some driving maneauvers, obviously in a safe area like a training course, check the chalk, keep reducing pressure until you've reached 35 psi or the chalk is worn off to that "band." Many people refer to t as a performance band. Over inflating your tires does not necessarily increase performance.

Good article, though I feel too many generalizations were made here.

It's difficult to state inflation pressure in absolute terms, as it has a lot to do with the type of tire, including its sidewall strength. The standard police package tires, a special spec GY RS-A, has a fairly stiff sidewall; little is gained by going over 40 psi. A comfort oriented "touring tire" on the other hand (which I've seen some agencies use as replacements) has a very soft sidewall. Compared to the OEM RS-As, even at 50 psi they feel too soft during aggressive cornering.

Also, while higher pressures tend to make the vehicle FEEL more responsive, the actual ultimate grip or coefficient of friction is lost. Many roadracing or auto-crossing performance tire tests have comfirmed this. On the Bridgestone RE-01Rs for example, which is an ultra-aggressive performance tire with VERY STIFF sidewalls, the optimal grip (tested on a skidpad) is around 36 psi. On a heavy P71 (figure 4400 lbs patrol ready), and softer tires, you'll need more pressure, but 50 psi definitely puts it beyond the optimal grip pressure.

Another important consideration is wet weather traction. On my police package Intrepid, fitted with GY TripleTreds (a cross between touring and RS-A), wet traction is quickly diminished above 38 psi, due to the decreased contact patch. At 38 or below, I can almost do a full throttle launch on wet pavement and the most I get is a little squeel. At 44 psi, which feels much nicer during cornering, even a 1/4 throttle launch would spin the tires up to near redline in 1st gear.

Now I understand most patrol officers are not necessarily car enthusiasts that spend hours on end researching the perfect tire, and therefore adjusting the psi to perfection each time is a little too much. But still, it is important to correctly understand the effects of high and low pressure. I would say that with a pressure around 40 psi, a happy compromise between steering response, wet and dry traction, tire longetivity can be obtained. However, it's more accurate to actually drive the car and feel for yourself what is right.