Tire technology may change, but physics doesn't

by Jim Kerr . Apr 01 2016
Hasan Jamali / The Associated PressA Mercedes mechanic prepares tires ahead of the Formula 1 Grand Prix at the Bahrain International Circuit.

Hasan Jamali / The Associated Press

A Mercedes mechanic prepares tires ahead of the Formula 1 Grand Prix at the Bahrain International Circuit.

There are hundreds of different types of tires on the market; each designed for a particular purpose or vehicle. The possible combinations boggle the mind, so how can drivers choose new tires? It’s difficult. Even the big tire retailers can’t keep on top of all the combinations, but they are very knowledgeable and can provide a wealth of information.

Some information is found in the vehicle owner’s manual. Cars with ABS brakes should use replacement tires with the same size and traction rating as the OEM tires. Truck ABS computers are often reprogrammable for different tire sizes. Both are important for optimum operation of the ABS system.

Tire technology may be changing quickly, but the laws of physics aren’t. Even the latest run-flat tire designs still depend on traction to keep a vehicle on the road. Those four small tire contact patches rely on proper inflation pressure and sufficient tread depth for traction, so a pressure gauge and a tire-tread-depth gauge are cheap safety tools.

Wear bars in the tires start showing when there are two millimetres of tread left. These bars show up as solid bands of rubber across the face of the tire. If you have wear bars showing, your tires need replacing. Driving on wet or muddy roads requires more tread depth. New tires start with about 11 to 13 mm of tread, but should be replaced when they get down to about four mm. For driving in snow, the tire manufacturers often recommend about six mm of tread depth minimum. Inspect tires on a regular basis, looking for uneven wear caused by improper inflation or incorrect wheel alignment. Catching a problem early can increase tire life dramatically.

Pickups and sport utility vehicles with automatic-mode-transfer cases can be shifted into two-wheel drive, four-wheel drive high, four low, and automatic mode. Driving on hard surfaces in four-wheel drive causes driveline binding that can wear differentials, transfer cases and even tires. However, many drivers think nothing of cruising down the highway in automatic mode. This is when mismatched tires can cause a lot of damage. Slight variations in tire circumference are normal even for the same tire size, but automatic-transfer-case damage can occur if the difference is too great. The transfer-case computer compares front and rear axle speeds in automatic mode. If there is a difference in speed, such as would occur when tires spin or there is a difference in tire size, the computer tries to lock the front and rear axles together. This works fine if the tires are spinning on loose surfaces, but when caused by different tire circumferences, the transfer case is forced to slip and possibly burn out.

The same holds true for all-wheel drive passenger cars. The tires should all be equal in rolling circumference or it can cause excessive wear in the driveline parts. You should never install new tires on one end of the vehicle only. All-wheel drive vehicles should have all the tires replaced at the same time so the tread depths remain equal.

Frequent tire rotation helps keep tire wear even, but the faster the vehicle is driven, the more crucial it is to have tires evenly matched. For example, a difference in front to rear tread depth of only 2/32 inch won’t do any damage at low speeds, but can start to cause automatic-transfer-case binding at speeds above 110 km/h. A tread difference of 4/32 inch may cause transfer-case binding at speeds as low as 70 km/h. All the tires may need replacing if the difference is too much. This can be an expensive purchase when all you need to do is drive in two-wheel drive to prevent damage, but the traction advantages of automatic mode on slippery roads make installing matched tires desirable.

Finally, run-flat tires are increasingly popular as OEM installations, allowing manufacturers to remove spare tires and jacks and increase cargo volumes. Low tire-pressure warning systems in these vehicles notify drivers when tires are underinflated. Some systems monitor ABS wheel speed sensors to measure rotating rate differences of a low tire, but other systems use pressure-sensing transmitters mounted to the rim inside the tires. Care must be used when dismounting these tires, because it is easy to damage an expensive transmitter if the tire is forced against the unit. Information on sensor position is found in the vehicle owner’s manual, and let any tire-repair personnel know if you have pressure transmitters in the tires to avoid unnecessary repairs.

james.kerr@sasktel.net