Driving Under Pressure

 I originally penned the following article in 2005. It prompted much discussion, including comments from some SCCA members that I was dead wrong about tire pressure and hydroplaning speed. I decided to update the article and share it again.  

 How many officers check the tire pressure on their patrol cars 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 a police vehicle is the maximum allowed pressure stamped on the sidewall of the tire. This pressure results in better performance, decreased tire wear, and it lessens your chance of hydroplaning at a given speed. The lower pressure listed on the door or in the owner’s manual is great for a nice comfortable ride, but you are not looking for a soft and cushy ride; you want performance. For some police vehicles, the owner’s manual is the same one used for the civilian version of the car. Your police vehicle is not a civilian car; it is a working police vehicle loaded with communications equipment and other police gear. 

The newer police vehicles on the road, such as the Ford Police Interceptor and the Ford Police Utility Vehicle, also handle better with the higher sidewall pressure. During the first Emergency Vehicle Operations Instructor Course in which we had a Utility Vehicle available, we tested it on the track with different tire pressures. Although the newer vehicles have electronic stability control and will not oversteer, they will understeer despite the computer trying to help you out. 

I took an experienced officer who was training to be an instructor and had him drive the track with standard tire pressure in the Ford Police Utility Vehicle. Then (after lunch and with the tires back to normal temperature), we inflated them to max sidewall pressure. He immediately noticed that the handling was better. Even with the new ESC systems, max sidewall pressure still results in the best performance. I encourage you to try this tire pressure experiment for yourself. Just be sure to let the tires cool before changing the pressure.

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 the tendency to understeer or oversteer is increased. This could spell disaster when negotiating a corner at high speed during a pursuit or a Code 3 run. Higher pressure reduces the tire deflection onto the sidewall, which keeps more of the treaded portion of the tire on the road.

As you inflate the tire, you get more stiffness and cornering ability. But you also get a smaller footprint, thus less traction. This means that at some point the trade-off between stiffness and traction reaches a breakeven point, so increasing tire pressure past that point will be counterproductive (NHTSA publication, Pneumatic Tire HS-810-561, p. 336). A study by the Society of Automotive Engineers in 2006 (Tire Asymmetries and Pressure Variations in the Radt/Milken Nondimensional Tire Model) concludes that tire pressure has a significant effect on tire performance when it comes to lateral force and slip angle. And if you reference the 2013MY Police Interceptor and Utility Modifier Guide published by Ford, standard tire pressures are listed. But it also reads, “Do not inflate tire higher than maximum pressure stamped on tire sidewall.” You will get markedly better performance when you inflate to the higher pressure stamped on the sidewall.

The recommendation to inflate the tires to maximum pressure stamped on the sidewall has been met with some resistance by fleet managers, because they are worried about liability. EVOC instructors are worried about an officer understeering a corner and crashing when it possibly could have been avoided. If you are not exceeding the maximum pressure stamped on the sidewall, you are within the manufacturer’s specification, and there should not be a liability issue. Some also worry that you risk tire failure due to high pressure. According to NHTSA (Pneumatic Tire HS-810-561, p. 221), Tires are designed to withstand six to ten times the recommended pressure, so you would have to inflate the tire to almost 200 pounds of pressure before failure may occur. 

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 surface. 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 coincidentally, the legal tread depth is 1/16th of an inch.

Tire manufacturers 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 (and even those who don’t), 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.

This fact about tire pressure and hydroplaning was disputed in many blogs and discussion forums when this article was originally posted. Some SCCA members said that I had no idea what I was talking about, because a larger tire footprint would obviously give you better traction. NASA and the NTSB have done extensive scientific studies on this matter; those who disagree are welcome to argue the math with a NASA scientist. If you get that opportunity, please send me an email and let me know how it went.

I have provided the formula below, which is from Joseph E. Badger, an internationally known accident reconstructionist and consultant with more than one hundred articles published in such periodicals as Law and Order magazine, Accident Reconstruction Journal, Accident Investigation Quarterly, and others. He retired after twenty years with the Indiana State Police.

For the mathematicians reading this article, the formula is:

V=7.95 √P(W/L)

V = Minimum dynamic hydroplaning speed in MPH

P = Tire inflation pressure in PSI

W = Tire footprint width in inches

L = Tire footprint length in inches

The expression W/L is the aspect ratio

Since most of us went to public school and have no hope of working this equation, you can see photos of a tire contact patch at various speeds and pressures, along with a summary of a study done by Michelin, at http://www.purigen98.com/Research.htm. 

Tire Wear

You not only get better handling performance, but you get better tire wear by maintaining proper pressure. Tires with low pressure will wear off the outside of the tread faster from deflection of the tire; the tires also 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. Back in 1999, the South Bay Regional Police Academy in California realized a significant cost savings by increasing the pressure in their training fleet’s tires.  

You are not overstressing the tire by using maximum allowed pressure, nor will the tire be forced off the rim as a result. 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, and the tires and rims are from a Crown Victoria. This is a dramatic example of how pressure holds the tire in shape and how much stress a tire can handle. (Anyone who wants advanced training should see BobbyOreMotorsports.com.) 

Some people think you will wear out the center section of the tire if you run it at maximum allowed pressure. This is not true. Overinflation can cause center tread wear, but running at the maximum allowed pressure is not overinflation. You can have center tread wear if you inflate the tire significantly above the maximum allowed pressure. Some people do this in a misguided attempt to increase mileage. Although higher pressure decreases rolling resistance and gives you better mileage, anything higher than maximum allowed pressure has little, if any, effect on mileage. 

The next time you inspect your vehicle, make sure you check your tire pressure; your performance drive ability 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, so don't wonder whether your tire pressure is correct when the pursuit starts. Check your tires routinely, just as you do with all other critical equipment.