By Jim Kerr

My truck began to shake last week. It started out as a minor tremor but quickly became more pronounced over the next couple days. Around the block it wasn’t bad. At about 45 km/h it started to become fairly noticeable and continued to increase as speed went up to about 100 km/h. Above that, the shake actually started to smooth out.

Initially, I thought perhaps I had some mud in a wheel – I had been out to the farm and took a tour of the crops. A quick stop at the local coin-op car wash cleaned the wheels out, but the shake remained. Then I thought I had lost a wheel weight and a tire was out of balance. Normally an out of balance tire vibrates more as speed increases, but sometimes they produce sympathetic vibrations in the suspension that only become noticeable at certain speeds. So it was off to the shop to balance my tires.

There are two types of balance to deal with when balancing tires. One is static balance. This means weight is added to the wheel opposite the heavy spot on the tire. The other is dynamic balance. As tires became wider, it was important to not only place the weight opposite the heavy spot but also on the same side of the wheel. Otherwise, you could set up a lateral vibration as the heavy spots, often near the sides of the tires tried to move to the centerline of the wheel as it drives down the road. To better understand dynamic balance, think of spinning a pail of water around in the air. If you always keep it at the same level, it spins around fairly smoothly, but start to move the pail up and down as it spins and the oscillations can rapidly move you away from where you are standing. The same happens in a tire when the heavy spots are not in the centre of the tire. Fortunately, the tire balancing equipment used today measures both static and dynamic balance at the same time and tells you where to add the weights.

One tire was a little out of balance but not very much. After reinstalling them back on the truck, I took it out for another drive. The shake was still there, so back to the shop. A bent wheel or axle shaft can also cause a vibration. I hadn’t hit anything but even a very bad pothole can bend parts, especially if you have the brakes applied when you hit it. After hooking a dial indicator to the suspension, I measured the run out on the wheels as I rotated them. Everything was in good order, so the problem must be in a tire.

Tires don’t often suddenly cause shakes or vibrations but it can happen. The most common cause of this is a tread or ply separation. The tire is made up of several layers of reinforced rubber and another layer for the tread. When manufactured, it looks like a smooth donut, but after being baked in the mould, the tread pattern is present and all the layers are bonded together. Occasionally, the bond may not be perfect and if air leaks through the soft rubber sealing layer inside the tire and get between the tire layers, you get a tire separation.

I couldn’t see any problems on the tires, so it was time to start rotating them. Using the spare tire, I replaced one wheel and tire at a time, starting with the rear tires and then drove the truck. I started at the rear because the shake was throughout the truck. If the shake had been more in the steering wheel, I would have started my swapping at the front. The first tire swap didn’t fix the problem, but after swapping the other rear tire, my truck was back to its usual smooth operation.
Even knowing which tire was causing the problem, I still couldn’t see anything wrong with it, but as the tread was 70% worn, it was time for a new tire. It is never advisable to replace just one tire on a drive axle as different diameter tires can cause wear in the differential. The other rear tire was equally worn so after installing and balancing a pair of new tires, my truck was ready for the road again.

Smooth running tires are important for more than just passenger comfort. A vibrating tire wears shock absorbers, suspension and steering parts much faster. Diagnosing a tire problem can take time, but the smooth ride is worth it.

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