Driving Under Pressure

Posted: Wednesday, December 21, 2005

SGT. DAVE STORTON
EVOC Contributor

Officer.com (Page is no longer maintained)

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.
--
Sgt. Dave Storton is the Director of the San Jose Police
Academy, and he holds a Master's Degree in Adult Education.
He is the lead instructor for the Emergency Vehicle Operations
Course (EVOC) at the San Jose Police Academy, and is a lead
instructor for the local regional academy. He teaches EVOC
instructor courses, advanced EVOC instructor courses, off road
EVOC, counter-terrorist / dignitary protection driving, and
motion picture stunt driving. Dave has trained over 3,500
drivers.