Balancing auto tires simple concept

Hey there, time traveller!
This article was published 29/04/2005 (7581 days ago), so information in it may no longer be current.

HOW often do you need to balance your tires? If everything was perfect, only once — when they are first installed on the rims. However, life is seldom perfect and events occur that can cause problems. The first symptom will be a vibration you feel inside the car.

Balancing a tire is a simple concept. Almost all tires have a heavy spot in them. Placing a weight equal to the heavy spot on the opposite side of the wheel will balance the tire so it doesn’t bounce when it rolls down the road. This is called static balancing.

As wider tires became more common, dynamic balancing was required. Wide tires can have a heavy spot off to one edge of the tire. This causes the tire to wobble side to side as it rotates at higher speeds, so dynamic balancing places different size weights on the inside and outside edge of the wheel to balance the tire evenly from side to side as well as evenly around.

If you have your tires balanced, the shop will use an electronic balancer that dynamically balances the tire and shows what size and where the weights need to be installed.

Installing wheel weights used to be easy. There were weights for steel wheels and adhesive-backed weights for alloy wheels. With the many styles of alloy wheels now on vehicles, a variety of weights are required. Many wheel weights are now “coated” so that they don’t cause corrosion where they touch the wheel.

Weights typically attach to the wheel with a integral spring clip that must match the contour of the edge of the wheel. There are many different shapes so if the wrong weight is used, it can fall off while driving. Good repair shops will have a broad selection of weights for a correct fit.

Spring potholes or rubbing against a curb can sometimes knock a weight off. If you have a flat tire, it may also need to be rebalanced after the repair. Unbalanced tires will cause a vibration in the car that usually starts about 50 to 60 km-h and gets more pronounced as the vehicle speeds up.

Vibrations in the steering wheel may be from an unbalanced front tire, while rear tire vibrations are usually more apparent in the seat or body of the car. Most tires are balanced within 1/4 ounce or seven grams. Wheels and tires that rotate at higher speeds, such as race cars, may need even better

balance.

There are some vibrations that feel like a tire balance problem but are caused by other faults. Mud is one culprit. Mud sticking to the inside of a wheel will put it off balance. A trip to the wand car wash should fix this

problem.

Sometimes, tires will be out of round from sitting. This is often noticeable in the winter, but happens in summer too. This type of vibration will be noticed at low speeds (10 to 30 km-h) and should disappear after driving for a few blocks.

Other vibrations are caused by out of round tires, poorly positioned belts inside the tire, bent wheels, bent axles or out of balance brake rotors. Tire quality is very good so out of round tires or bad belts occur rarely. I often see vehicles driving with bent wheels, and hitting a curb or other solid object can bend an axle, too.

I had a truck with only .005 inch bend (about twice the thickness of a sheet of paper) in an axle and it was noticeable on smooth highway at 100 km-h.

One more tire problem can cause vibration. It’s called radial force variation. The tire’s sidewall rubber acts like a spring. If the springiness of the rubber is unequal around the tire, it causes the tire to bounce, just like it is out of balance. There are tire balancing machines that will test for radial force variation by spinning the tire with a rolling load placed on it. If the radial force is too uneven, the tire must be replaced.

If your car’s tires start doing the shake, rattle and roll, have them checked for balance again. Driving with an unbalanced tire places more load on suspensions and wheel bearings. It also puts uneven forces on the tire tread that will cause it to wear unevenly and unevenly worn tires will still vibrate even though they have been rebalanced.

Jim Kerr is an experienced mechanic, instructor of automotive technology and freelance journalist. You can e-mail questions to Jim at the address below.

kerr.jim@sasktel.net