Tires and ride quality
by Jim Kerr

There has been a lot of discussion in the media about tires and tire problems lately. There is a wide variety of tire designs on the market, and each is built for a specific purpose, but all passenger vehicle tires have one thing in common: they ride on a cushion of air.

There are solid rubber tires on the market. You can find them on some industrial vehicles such as forklifts, but you wouldn't want them on your car. The ride would shake your teeth out! While suspension design does provide improved ride qualities, it is the design of the tires and the air pressure inside them that has the most dramatic effect on ride.

Lets look at tire design first. While the number of belts and layers beneath the tread surface have some effect on ride, it is the tire sidewall that provides most of the ride. Radial tires have a very flexible sidewall. There are descriptors on the tire sidewall that shows us if the tire has a 2-ply, 4-ply, or higher rating.

Ply refers to the number of layers used in the sidewall construction, but a 4-ply rating doesn't necessarily indicate it actually has 4-ply construction. It just indicates it is as strong as a tire with 4-ply construction. Most passenger cars use 2 or 4 ply rated tires, while larger trucks may use 6 or more ply construction. The higher the rating, the stiffer the tire sidewall and the harsher ride it provides. The advantage of a higher ply rating is that it can carry a heavier load.

Sidewall height or aspect ratio is another factor in ride quality. The relatively high sidewall of a typical "70" series passenger tire ( ie: P215/70R-15) can flex substantially when the tire goes over a bump, so the passengers are cushioned from the jar. The low sidewalls of "50" series performance tire ( ie: P255/50R-17) flex very little when a bump is encountered. The short stiff sidewalls enhance handling because the tire treads react quicker to the inputs from the steering wall, but ride is not as smooth. Modern suspension designs have helped ride quality a lot, but performance tires still give a harsher ride.

Correct tires pressures are important for maximum tire life, good ride quality, and vehicle safety. Low tire pressures allow the tire sidewalls to flex much more, so a smoother ride is achieved, but low tire pressures can cause the sidewalls to flex too much. Heat builds up in the tire as the sidewalls flex and excessive heat causes tire failure. Sudden tire failure could cause a driver to lose control of the vehicle, so prevent it by keeping the proper pressures in your tires.

So what are the proper pressures? Well that depends upon what the tires are used for. A good starting point for tire pressures will be found in the vehicle's owners manual or on the tire pressure sticker on the vehicle. Most tire pressure information stickers are found on the driver's door by the latch mechanism or inside the glove box.

Never put less than the recommended pressure in your tires. The recommended tire pressures are designed for your vehicle to give a good ride while maintaining safe handling. Reduced tire pressures may cause unstable handling when the car is manoeuvred quickly.

Higher tire pressures should be used for special circumstances. If you are driving at high speeds, tire pressures should be increased. Driving a fully loaded vehicle causes the tires to flex more, so higher tire pressures should be used. Hard cornering or competition driving demands higher pressure to keep the sidewalls stiff and prevent the tire from rolling off the rim. Increase the tire pressures but never put more than the maximum tire pressure listed on the tire sidewall. Most passenger vehicle tires have maximum listed tire pressures in the 32 to 35 psi range, but large trucks may have maximum pressures listed at 45, 60, or even 75 psi!

Putting more pressure in a tire than the maximum listed on the sidewall can be very dangerous. Remember, pressure is measured in pounds per square inch, and there are a lot of square inches inside a tire. There can be tons of force pushing on the inside of a tire and even a few pounds excessive pressure could cause the tire to fail just like a balloon does when it gets blown up too much!

Measure tire pressures when the tire is cold. Driving even a few kilometres with low tire pressure causes heat inside the tire to expand the air and increase the pressure. Readings taken on a hot tire could be false because of the heated air inside. Tire pressures should be checked often. Once a month may be fine for slow speed driving, but once a week is better. If you are travel at higher speeds for long distances, check tire pressure every day. It can prevent having to change a tire on the side of the road, or may even save your life.

Any time there is a major change in ambient air temperature, tire pressures should be rechecked. Lower outside temperatures will cause tire pressures to be lower as well.

Any tire gauge is better than none. You can't tell tire pressures by looking at the tires; they must be measured. Buy one for each vehicle and keep it in the glove box. Most drivers use a pencil type tire gauge. They work reasonably well, but round "dial" type gauges are more accurate and they are also easier to read. Watch professional race teams and you will see them using dial type tire pressure gauges. You don't have to spend as much money on a gauge as race teams do to get a good gauge, but dial type gauges are worth the extra cost. Even more important is to use the gauge often.

Jim Kerr is a master automotive mechanic and teaches automotive technology. He has been writing automotive articles for fifteen years for newspapers and magazines in Canada and the United States, and is a member of the Automotive Journalist's Association of Canada (AJAC).

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