BACKYARD MECHANIC: When tire wear is in the castors

Hey there, time traveller!
This article was published 05/04/2012 (4987 days ago), so information in it may no longer be current.

QUESTION: I have a 2003 BMW Z4 (E 85 roadster sports suspension) with 18-inch tires that are wearing on the inside edges. After having a four-wheel alignment, the report states excessive cross castor. The castor is non-adjustable and the car drives OK. Front camber is adjusted to maximum positive setting (factory camber pins removed).

Should I have to go to a camber kit or look elsewhere? Any suggestions would be appreciated. Terry

ANSWER: There are a couple potential problems that may be present in your vehicle.

Castor is used to make the steering tires tend to return to a straight-ahead position. By changing castor angles, you can make the car feel more or less stable. Manufacturers will set castor angles to complement the handling feel they want in the vehicle.

Castor angles are non-adjustable on your vehicle with the factory equipment, but the alignment printout states there is excessive cross castor. Normally, there will be about a 1/2-degree difference in castor from side to side, with the most positive castor on the driver’s side. This makes the car pull slightly to the driver’s side, which compensates for the crown of the road that tends to have the car pull the other way.

You say the car drives fine, so I wouldn’t worry too much about cross castor more than about one degree, as castor has no impact on tire wear. However, if cross castor is too high, it could indicate a bent suspension component or even a bent body structure. I’ve seen a couple cars that looked great, but the wheel base and castor was significantly different from side to side because the car had been in a serious impact and the inner structure was not repaired properly. Most alignment equipment will provide information to determine this, if the technician knows to look for these problems.

Camber or toe angles could be what are wearing your tires on the inside edges. If camber is already adjusted to its maximum range and it’s still outside of specifications, it usually indicates weak suspension springs — the car is sitting too low. Replacing the springs is the correct fix. Perhaps your car has already been lowered by installing shorter springs. In situations like this, installing aftermarket kits that allow additional camber adjustment will enable the car to be set back to correct camber and toe specs.

Finally, some high-performance sports cars such as Porsche, BMW, Ferrari, etc. use tires that specifically made for a particular car model. These vehicles have four-wheel independent suspensions and the tires sit normally with negative camber. To compensate for excessive tire wear on the inside edges, the specified tire for that vehicle has rubber with a harder compound on the inside edge than on the outside tread area. This evens out the tire wear across the tread. Using a non-specified replacement tire will create uneven tire tread wear.

 

QUESTION: I have a question regarding powder-coating my aluminum rims. Will it weaken them with the heat used in the process? Mike

ANSWER: Powder coating is used extensively today on everything from automobile oil filters to the colour on metal patio furniture. The heat from the powder-coating won’t harm the rims. During the powder-coating process, the object is cleaned of all rust, grease and previous coatings. This can be done mechanically, such as by sand blasting, or chemicallyusing an acid bath solution. Many commercial shops use a combination of the two cleaning methods.

With the metal clean, the part is connected to a ground clamp and a special powder-spraying machine blows a fine powder mist onto the metal. The powder sticks to the metal because of a static charge generated by the sprayer nozzle.

Next, the metal is heated in an oven to melt the powder and let it flow onto the part. The part is heated just enough to melt the “plastic” powder — in the 200- to 250-degree F range for about 20 to 25 minutes. If the part isn’t heated enough, the powder doesn’t melt and the part can be heated again. If the part is heated too much, the powder is damaged, so any problem would be visually obvious. Aluminum melts at around 700 degrees F, far below the powder-coating temperatures.

Most of the factory alloy wheels are finished already with a powder-coating process. Treat the finished wheel like it has paint on it for the best appearance.

Jim Kerr is an experienced mechanic, instructor and member of the Automobile Journalists’ Association of Canada.