Bicycle Mechanics - Wheel runs true, but differing pitches?
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07-29-09, 12:31 AM
I've been rebuilding the front wheel from a 1989 Fuji road bike. At the moment, the wheel runs about as true as I can possibly get it, though it's not at full tension yet. However, when I pluck the spokes on either side of the wheel, I get a lot of variation in pitch. I'm lucky enough to have near-perfect relative pitch, so I don't believe it's an error with my ear. Could it be that the rim is pretty old? I've heard of this causing problems, but there seems to be a rather large variation in pitch for something like this.
07-29-09, 12:41 AM
Probably just not enough tension to overcome a already-true rim. Bring it up to tension and see if it still behaves weirdly, I'd bet not.
07-29-09, 06:34 AM
Problem with relative pitch comparison is it still doesn't tell you actual tension. Can you hear the difference between 70 versus 100kgf? Or between +/- 5% or +/- 10% variations? As you bring wheel up to high-end of tension-range for the rim, the variations will decrease. But without knowing actual-tension, your overall tension could be too high or too low, even if the balance is fairly even.
You might just have a rim that's bent, so that the tensions need to be different to hold the rim straight. At low tension, that might result in a big difference in pitch, but you could still finish with a decent wheel.
Sometimes you can get a pattern of mismatched tension, with too loose leading spokes and too tight trailing spokes on one side matched by the opposite on the other side. That could affect all the spokes or just a few pairs. That should be easy to find by tone because a mismatched pair of crossing spokes will dampen the vibrations.
Bring it up to full tension. The worst case is that you have a bent rim that can't be built into a reasonable wheel, but you got nothing to lose by tensioning it now.
07-29-09, 11:33 AM
Trueness is only part of the story of wheel building. You cannot really do a complete job without dealing with the whole shebang. Tension balancing is one of those 'rest of the story' items.
07-29-09, 07:07 PM
You guys were right: Upon bringing it to full tension, the plucked pitches were extremely close!
Thanks for the urging advice =P
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