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BACKYARD MECHANIC: Unbalanced tires likely cause of flat spotting

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QUESTION: I have a 2004 Prius with 300,000 km on it and the problem is with the rear tires. Both tires have flat spots on them in a pattern all around the tread. The car rides and drives fine and the front tires are good, but if I switch a rear tire onto the front it shakes the steering and sounds like a bad bearing in the front end.

Different shops have told me I need a wheel alignment or suspension work or that the tires are faulty but, because I get different answers, I am not sure how to fix the problem. I took the car back to the tire shop where I bought the tires, thinking I may get warranty but the tire-shop staff tell me it isn't a tire problem. They are top-line Goodyear tires, so I know the problem isn't because of a cheap tire.

What do you think has happened and how can I prevent it from happening again?

ANSWER: When you get more than one tire with the same problem on a vehicle, it's seldom caused by a tire defect. Buying quality tires also minimizes the chance of getting a defective tire.

The flat spotting you describe on your tires is most likely caused by unbalanced tires. If the problem was with the wheel alignment, you would get a sideways scuffing on the tire tread all the way around, or wear on the inside or outside edges of the tread. Weak shock absorbers could allow the tire to bounce more and create wear as you describe, but you say the car rides and drives fine so I would think this isn't the primary cause of the problem.

Unbalanced tires can cause flat spots because every time the tire rotates, the heavy spot in the tire is forced against the road harder and creates a wear spot. This wear spot then causes the tire to bounce slightly, creating a pattern of wear spots around the tire. If the tire had been mounted on the front axle, you would have noticed a vibration in the steering. With it mounted on the rear axle, it isn't as noticeable.

Unfortunately, once a tire develops wear spots like this, the most economical repair is to replace the tire and have it balanced. Rotating the tires from the front to rear axles every oil change will help even out tire wear and also reduces the possibility of developing unusual wear patterns.

QUESTION: The battery has been going dead on my 1991 Chev pickup and I thought it was a battery problem, as I have had it in the truck for several years. Replacing the battery didn't help, so I also installed a new alternator. The battery still goes dead but I have noticed that when the engine is idling and I touch the battery cables near the battery, they are hot, almost to the point of burning my hand. What could be causing the battery to go dead, and are hot cables normal?

ANSWER: A hot electrical cable isn't normal in any vehicle and could likely be the cause of your battery problem. As electricity flows through a wire, there is some resistance to the flow of electrons. A large cable such as a battery cable has low resistance while a small wire will have a higher resistance, but it all depends on the amount of electricity flowing through the wire. It's sometimes easier to think of it in terms of water flowing through a hose: A bigger hose will allow more flow. Put a kink in it and the flow is less, but there's still pressure in the hose before the kink.

The same happens in an electrical wire. If you have a resistance in the wire (just like the water-hose kink), then there is a reduction of flow but the pressure goes up before the restriction. In wiring, a restriction can be caused by a corroded connection, loose terminals, broken wire strands or a wire that's too small. Heat will be generated wherever there is a restriction and the flow is decreased.

In your vehicle, the flow may be decreased to the point where the battery isn't receiving a full charge. If the hot area of the wiring is inside a cable, replace the cable. If it's at a connection, clean or replace the connector. That will likely solve your battery problems.

Jim Kerr is an experienced mechanic, instructor of automotive technology, freelance journalist and member of the Automobile Journalists' Association of Canada.

kerr.jim@sasktel.net

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