Originally Posted by
bikerdoc
It might be tyre related, - on many tyres there is often a solid white or yellow coloured dot on the outside wall of a tyre. This is used as a mounting indicator by skilled tyre techs to make sure that the dot location of the tyre is mounted closest to the valve stem on the rim. This indicates the point at which the tyre is the lightest (though there are red dots too but these are more commonly found on truck tyres and to a lesser extent car tyres). The valve stem represents the heaviest point of the wheel/rim. So by aligning the two close together, one is effectively given the best initial balance of the wheel. If the tyre has a red dot, ignore the yellow dot. Iif you have a steel wheel/rim, look for the low point dimple on the wheel/rim, and mount the tyre with the red dot next to the low point dimple. If the wheel is aluminium, or if it’s steel, but has no low point dimple, mount the tyre with the red dot next to the valve stem.
While steel wheels are forged, aluminium wheels are machined. For that reason, aluminium wheels are very uniform in their overall balance. And, if you look at an aluminium wheel, you notice that where a hole has been cut into the wheel/rim, an identical size and shape hole is cut at a location 180 degrees opposite it.
The result is that even with all their cut-outs and holes, aluminium wheels/rims are still very uniform in balance. Except for one place, where the valve stem is located. At that spot, the manufacturer has to bore a hole through the wheel/rim for the stem, removing metal from the wheel/rim.
That hole is about 9.7mm in diameter and the aluminium at that point is usually about 7.6mm thick. The aluminium weighs about 2.64 grams /cm3, therefore the aluminium removed to make the hole weighs about 0.52 ounces
The red dot is much more complicated than the yellow dot. It indicates the “radial force variation first harmonic maximum.” It’s a way of indicating where the centrifugal force tending to pull the rotating tyre away from the wheel is greatest. Another way of looking at it is that in a sense, if the tyre were out of round, the red dot would more or less correspond to the “high point” or place where radial run-out forces are greatest.
Matching the dots is no substitute for balancing tyre and wheel assemblies. What it does, however, is give you the best start, so you are more likely to use less total weight to bring an assembly into balance.
06-10-2014, 12:22 PM
