From PSI Digest http://www.psitiredigest.com/tiredigestOct10.pdf

Air is what carries the load and as a
result the recommended tire pressure must
be based on the worst case load that the tire
will actually carry. The important piece of
the puzzle is that a fleet must determine the
actual worst case load per tire, not vehicle,
for steer, drive, and trailer positions.

Driving Under Pressure
Proper Tire Pressure Could Save Your Life

SGT. DAVE STORTON
EVOC Contributor
Officer.com


Courtesy Bobby Ore Motorsports

How many officers check the tire pressure on their patrol car on a regular basis? We all seem to be great at checking that the lights and siren work, because the time to find out they don't work is not when you get a Code 3 call. Likewise, the time to find out your tire pressure is too low is not when you are in a pursuit and trying to take a corner at high speed.

What is proper pressure?

The proper tire pressure for the Police Crown Victoria is 44 psi. If you look on the sidewall of the tire, you will see that it lists 44 psi max pressure. Regardless of what vehicle you have, use the maximum pressure listed on the sidewall. Higher pressure results in better performance, decreased tire wear, and it lessens your chance of hydroplaning at a given speed. This number on the sidewall lists the maximum amount of pressure you should ever put in the tire under normal driving conditions. Pursuits and Code 3 responses are not normal driving conditions. Many agencies maintain tire pressure at 35 psi since this is what is listed in the owner's manual and on the door placard. The reason the owner's manual lists 35 psi is because we get the same manual as the civilian version of the Crown Victoria. The police version, however, is fully loaded with communications equipment, a cage, and your gear. You are not looking for a soft and cushy ride, you want performance.

Myths about pressure

Let's put to rest some common misconceptions. 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, you are not overstressing the tire with higher pressure, and the tire will not be forced off the rim with higher pressure. The picture above is Bobby Ore of Bobby Ore Motorsports driving a Ford Ranger on two wheels. The tires on the left side have 100 psi in them, and they happen to be tires and rims from a 1999 Crown Victoria! This is a dramatic example of how pressure holds the tire in shape, and how much stress a tire can handle.

Performance

If you were able to watch a tire as it travels across the ground at high speed, you would see that it deflects to one side during cornering. The faster you are going through a corner, the more tire deflection you get. As the tire deflects over onto the sidewall, you get less traction and more of a tendency to understeer or oversteer. This could spell disaster when negotiating a corner at high speed during a pursuit or a Code 3 run. Higher pressure keeps the tire from deflecting onto the sidewall as much, which keeps more of the treaded portion on the road.

A good demonstration for EVOC instructors is to have students drive a high-speed course in a vehicle with 32 to 35 psi. Then have them run the same course with 44 to 50 psi in the tires. The student will experience a marked difference in performance. Having officers experience this difference in vehicle performance is much more effective than just telling them to check their tire pressure.

Hydroplaning

When a tire rolls across a road covered with water, the tire tread channels water away so the rubber remains in contact with the road. The factors that affect hydroplaning are speed, and water depth. Conventional wisdom says that vehicles will hydroplane in as little as 1/16th of an inch of water. Not so coincidentally, legal tread depth is 1/16th of an inch.

Tire manufactures and the Association of Law Enforcement Emergency Response Trainers International (ALERT) have shown that tires have more of a tendency to hydroplane when pressure is low. This happens because the tire footprint (the portion of the tire actually in contact with the road) is larger. For those of you who water ski, think of which is easier to get up on: a fat ski or a skinny ski. More tire surface in contact with the water makes it easier to hydroplane, just as it is easier to water ski on a fat ski. Also, a soft tire can be pushed in more by the pressure of the water on the center portion of the tread. This results in less rubber in contact with the road.

Tire wear

Much better tire wear results from maintaining proper pressure. Tires with lower pressure will wear off the outside of the tread faster from the deflection of the tire during cornering, and the tires will heat up more from increased road friction. This is one of the factors that caused the failure of a certain brand of tires on Ford Explorers some years ago. In 1999 the San Jose Police Department realized a significant cost savings by increasing the pressure in the training fleet to 50 psi. They soon followed up by increasing the pressure in the patrol fleet to 44 psi. For liability reasons, most agencies are reluctant to exceed the maximum pressure listed on the tire for actual patrol vehicles, but they reap the cost saving when going to 50 psi on training vehicles.

Next time you inspect your vehicle, make sure you check your tire pressure since your ability to performance drive is significantly affected by it. You are not driving to the store to get a loaf of bread! You may be called upon to chase a dangerous criminal or respond to assist another officer in trouble. You don't wonder whether or not your gun is loaded before you hit the street; don't wonder whether your tire pressure is correct once the pursuit starts. Check your tires routinely, just as you do with all other critical equipment.

http://nitrogentiremachine.com/prope..._inflation.htm

UNDER INFLATION
Under inflation is the most common cause of failures in any kind of tire and may result in severe cracking, component separation or "blowout," with unexpected loss of vehicle control and accident. Under inflation increases sidewall flexing and rolling resistance resulting in heat and mechanical damage. Under inflation can cause can cause many tire-related problems. Since a tire's load capacity is largely determined by its inflation pressure, under inflation results in an overloaded tire. An under inflated tire operates at high deflection resulting in decreased fuel economy, sluggish handling and may result in excessive mechanical flexing and heat buildup leading to catastrophic tire failure. Additionally, the tire's tread life could be reduced by as much as 25%. Lower inflation pressure will allow the tire to flex more as it rolls. This will build up internal heat, increase rolling resistance and cause a reduction in fuel economy of up to 5%. This also results in a significant loss of steering precision and cornering ability.

Wear On Both Edges: Cause... "UNDER INFLATION"

If your tire looks like this, it may be under inflated. Not having enough air in a tire is singly the worst thing you can do to a tire. Under inflation reduces tread life through increased tread wear on the outside edges (or shoulders) of the tire. It also generates excessive heat which reduces tire durability and can lead to tire failure. Finally, it reduces fuel economy through increased rolling resistance (soft tires make your vehicle work harder).

OVER INFLATION
If your tires are over inflated, they could be damaged more easily when running over potholes or debris in the road. Higher inflated tires cannot isolate road irregularities well, causing them to ride harsher. However, higher inflation pressures usually provide an improvement in steering response and cornering stability up to a point.

Tire footprint and traction are reduced when van, pickup, or RV tires are over inflated for the loads carried. In particular, tires with aggressive tread patterns may contribute to over steer or "road-walk" if inflated beyond the inflation pressure specified in the owner's manual and vehicle placard for standard or customary loads. Over inflation also increases the chances of bruise damage.

Wear In Center: Cause... "OVER INFLATION"

When a tire has too much air in it, the center of the tread bears most of the load and wears out faster than the outside edges. If a tire wears unevenly, the useful life is reduced and conversely the operating cost is increased. Additionally stopping distances will be increased because less tread area is contacting the road surface.