Mechanically Speaking

Mechanically Speaking: More reasons behind all that shaking

"That low speed shake or wobble could be the onset of a wheel about to fall off."

Two weeks ago I wrote about shaking problems while driving down the road. You might feel it as wobbling through the steering wheel or a shaking feeling through the seat of your pants.

If this has started to occur in the early spring it is attributable in many cases to a uneven buildup of dirt in your winter wheels. As I suggested a thorough cleaning of the inside of your wheels will likely alleviate this problem

Now I have a different shaking scenario that happens all too frequently just after your winters were removed from your vehicle and your summers installed. Loose wheel nuts! Yes, that low speed shake or wobble could be the onset of a wheel about to fall off. This is a very dangerous scenario. Usually the wobble is serious enough that the driver seeks a solution before the eventual exit of a wheel.

After installing your summer tires most tire shops and mechanic shops put a note on the bottom of your invoice or they hang something on your rearview mirror that suggests you return within 50 to 100 kilometers or so to have your wheel nuts retorqued. Do not ignore this request.

If you do your own tire changeovers make sure you abide by the retorque rule as well. If you are doing this job yourself there are more things to concern yourself about than just retorquing.

Why do wheel nuts come loose? Why do they come loose more often in the spring? The answer is a bit scientific but the problem can be mostly overcome simply.

Dirt, mud and rust accumulations are a big part of winter driving. The interface between your wheel and the steel surface (hub) that your wheel is bolted to has been subject to at least six months of salt, sand and water.

There has likely been a buildup of rust and mud in this area. There is a circular portion in the middle of the hub surface that locates the centre of the wheels.

Many people buy winter wheels for their winter tires that are ‘generic’ so to speak. In the interest of cost saving these wheels centre themselves only by the bolts that are used to attach them. These wheels typically have centre openings that are bigger than the raised portion at the centre of the hub surface.

The portion of the hub surface not covered by the winter wheel has built up an accumulation of rust and mud. The winter wheel is removed and this buildup gets left on the hub. The summer wheel which typically is not generic and made to fit the vehicle exactly centres itself tightly around the raised portion of the hub and is expediently bolted down. Unfortunately it is being bolted down on to rust and mud and not a flat exact fitting surface.

The wheel installer has now made a sandwich of rust and mud instead of a solidly clamped surface. In quick order, under the clamping force of the wheel nuts, the sand and rust will wiggle its way out leaving space and thus the wheel nuts will be loose and no longer holding the wheel tight.

If you are lucky and you head back for retorquing all the sand and loose rust will have worked its way out and that retorque will create a solidly clamped joint. In most cases this is what will result. Sometimes the nuts will come loose again and you know the result.

This can all be avoided by careful wheel replacing. When you remove the winter wheel the hub surface must be cleaned completely. Also check the summer wheel that you are putting on. The surface that clamps to the hub must also be clean and free of loose rust and dirt. Also check the surfaces of the nuts or bolts and the wheel surface that the nut or bolts tighten against that they are all clean. Install the nuts and cinch them up in a diagonal pattern and then tighten them to specification with a torque wrench. Done correctly it is very unlikely that anything will come loose but it doesn’t hurt to check them again a few kilometres down the road.

Trail’s Ron Nutini is a licensed automotive technician and graduate of mechanical engineering from UBC. E-mail: nutechauto@telus.net

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