Keeping tire pressure up for safe driving

By Barry Lake

HOW often do you check your tire pressures? Not often enough, if you're like most people.

Perhaps you drive a company car and the mechanics in the service garage look after it for you ... or do they?

Keeping a car's tires inflated to the correct pressures is far more important than many drivers realize.

You can have the best tires in the world on your car but, if they aren't inflated correctly, they could well perform like the worst.

And what, exactly, is the correct pressure?

The starting point is the figure recommended by the vehicle manufacturer.

There is a plaque or sticker on the car, sometimes inside a front door jamb, sometimes inside the glove box, which lists the various recommended pressures for different loads and speeds.

If you can't find the sticker on the car, the figures are repeated in the car's handbook.

Usually, manufacturer's recommendations for tire pressures tend to err on the low side.

The reason for this is that slightly softer tires give a far better ride, better absorption of small bumps, and less vibration from road irregularities. It makes the car feel less harsh, more luxurious.

But, while a soft-ish tire can give a nice soft ride, it is at a serious disadvantage when it comes to driving around corners or making an emergency braking maneuver.

Tires with low inflation pressures will also flex more and generate heat within the tire sidewalls which, in the extreme, can lead to tire failure.

Loaded weight of the vehicle also is important.

The greater the weight carried, the greater the tire pressures should be, in order to safely carry the load.

And, depending on the design of the car, this extra pressure can go to the rear tires only, or to both front and rear tires.

Again, reading the manufacturer's recommendations and using some plain old common sense will usually get it right.

Manufacturers always recommend higher pressure for higher loads in the vehicle, as well as for sustained high speed motoring.

But these usually still tend to err on the soft side, to ensure a nice ride for the occupants.

For those who rate safety more highly than that last degree or two of comfort, it is worth trying slightly higher pressures than those recommended.

Only go up by a small margin at a time (a couple of pounds per square inch in the old values), to avoid overdoing it and ruining the ride all together.

Advantages of higher pressures can include improved handling on corners and shorter stopping distances in emergency stopping situations.

A soft tire distorts under heavy cornering loads and can deform the tread so much that it no longer stays flat on the road, thereby losing grip.

Pumping the tire up harder will stop this distortion and tend to keep the tire tread flatter on the road when cornering harder than usual.

Extra pressure in the tires also makes the steering lighter, which can be an advantage when parking in tight spaces with cars that do not have power steering.

In hard braking situations, a soft tire will be "squashed" as the weight of the car bears down on the front wheels.

As the tire walls bow outwards, this lifts the center of the tread off the road, so that the tread takes a concave shape.

Now, only the outer edges of the tire are bearing on the road and grip is greatly reduced.

Take a look at some braking skid marks on a roadway and you will see the effect.

Instead of one flat rubber skid mark, usually there will be two parallel, narrow skid marks for each front wheel (which are the ones doing most of the work when braking).

Pumping the tires up harder limits this distortion and more rubber will be kept in contact with the road under heavy braking.

In the old days of cross-ply tires, pumping the tire up harder could cause the tread to take on a convex shape so that only the center of the tire was in contact with the road.

This could cause the tire tread center to wear more rapidly than the outer edges.

This is far less pronounced with modern radial-ply tires, which hold their shape very well, even with very high pressures.

Just as high tire pressures used to cause wear on the center of the tire tread, so too can too soft pressures cause wear on the outer edges of the tires -- and this one holds true for modern radial-ply tires as well as for the older cross-ply tires.

So the rules are clear:

Check your tire pressures regularly -- at least once a week is preferable.

Buy and use a good quality tire gauge that has been checked for accuracy (service station gauges suffer rough handling and often infrequent checking and can be wildly inaccurate).

Always take note of the recommended pressures for sustained high-speed motoring (usually when on country trips or running on motorways), and for carrying loads (most commonly when on holiday).

Consider running two or four pounds over the recommended pressures to increase cornering and braking performance at the expense of some ride comfort.