Vehicle Vibrations
by Jim Kerr

Rattles, creaks, and vibrations seem to be a part of older automobiles. These annoyances creep up on us with time and are not noticed until the problem becomes severe. Even then, we usually think these problems are part of driving an older vehicle. Actually, they may be warning you of expensive problems ahead. Investigating the source of a rattle, creak, or vibration may save you money.

One of the most common sources of automobile vibration is it's tires. Tires produced today are a tremendous improvement over the tires of fifteen years ago, but they still need balancing. Balancing involves placing a weight on the wheel opposite the heavy spot in the tire. Out of balance tires cause increased wear on all steering and suspension components.

Heavy spots in the centre of the tire tread cause it to bounce as it runs down the road. If the tire has a heavy spot off to one side of the tread then the tire is said to be out of balance "dynamically". This causes the tire to shimmy from side to side as it rolls. Tire balance equipment used in most repair shops are designed to measure the tire for all kinds of imbalance and weights are placed to correct for both bounce and shimmy.

Modern suspension systems are lighter and give the driver a better "feel" of road conditions, but they also transmit vibrations easier into the passenger compartment. Out of balance tires start to be noticeable around 50 kilometres per hour and will be most noticeable around 80 kilometres per hour. If the vibration seems to come from the steering wheel then the front tires should be checked. If the vibration is noticed more in the vehicle seats, then the rear tires should be balanced.

A defective tire belt beneath the tread can also cause a tire vibration. The vibration feels like an out of balance tire but the problem is only noticed at low speeds between 30 and 50 kph. At higher speeds the vibration lessens. The only cure for this problem is to replace the tire.

Tire radial force variation is the most difficult tire vibration to diagnose. Good tires have equal stiffness in the tire sidewall all the way around the tire. Some tires have a variation in stiffness that can cause a vibration. There are machines used in a few repair shops that can measure the stiffness as the tire rotates, but the machines cost about $15,000 so they are not in common use. Most radial force variation vibrations are diagnosed by eliminating everything else first! This can be a time consuming and expensive procedure. Replacing the tire is the only way of repairing this type of vibration.

Bent wheels or axles are another source of vibrations. Sliding into a curb on a slippery road may do no apparent damage but if you notice a vibration afterwards, the wheel should be checked. The balance weight may simply have been knocked off by the curb and the tire needs re-balancing, but a bent wheel or axle can be checked by a technician using a gauge to measure the amount of wobble or runnout. Most manufacturers allow no more than 1/32 of an inch runnout. Steel wheels can be straightened. Bent aluminum wheels and axles usually require replacement.

Vibrations that occur when the brake pedal is pressed indicate a warped brake drum or rotor. Often this vibration results from uneven tightening of the wheel lug nuts. The lug nuts should be loosened, and then torqued to factory specifications in a cross pattern. This simple step may cure the vibration. If the vibration still exists, then there could be a problem with the brake system. A binding brake caliper could overheat the brake rotors causing them to warp. The rotors or drums may require machining when the brake system is repaired.

Worn or loose universal joints on rear wheel drive vehicles can cause a vibration. Inspect the joint while turning the driveshaft and prying on the joint. If there is any looseness on a rear wheel drive vehicle the joint should be replaced. Continuing to drive with a worn universal joint could cause it to break. When a joint breaks it can damage the transmission or differential, and may even cause the driver to loose control of the vehicle. Don't take a chance with a worn universal joint.

Front wheel drive axle joints can also wear and cause a vibration. A clunk when shifting into gear with an automatic transmission can indicate a worn joint. A clicking noise from the front of the vehicle when turning a corner is another indicator of worn axle joints. The joint, or on some vehicles the whole axle, must be replaced to repair this problem.

There are many other causes of vehicle vibration. Mis-adjusted hood bumpers can cause the hood to flutter in the wind. This small vibration can sometimes be felt throughout the vehicle.

A broken exhaust system hanger can allow the exhaust system to bounce as the vehicle is driven. This low frequency vibration usually goes unnoticed but if left to continue the exhaust system will break.

Worn shock absorbers permit the tires and wheels to bounce uncontrolled. This vibration will wear tires in an uneven pattern and reduces driver control of the automobile.

Noticing vehicle vibrations and repairing them before they become serious will save the owner money. With the vibrations stopped, many of the other rattles and creaks will also disappear. Now your older vehicle drives more like the new vehicles sold today.

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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