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Re: am I ruining my crank?
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Peter Cole
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Aug 08, 2006 04:24 PDT
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Alan Abrams wrote:
| | | | I grease the spindle and
*never* re-tighten.
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Hmmm .. . why _never_ re-tighten (which was _exactly_ what I was going
to do before I decided to post to Massbike first!!!).
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The reasons you don't re-tighten are twofold. First, if you tighten
correctly in the first place, it's entirely unnecessary. The second is
that you can "chase" the crank up the spindle.
When you tighten the crank bolt to a specific torque, there will be a
predictable force on the inside of the crank bore (square hole). As you
ride, the pedal forces act to actually move the crank slightly further
up the spindle, reducing the bolt tension but increasing the force
inside the bore. If you then re-tighten, the crank will squirm yet
higher, increasing the force inside the bore even more. When you pedal,
these "static" forces become superimposed on the dynamic forces from
pedaling, and the bore will only take so much force before it splits.
Catastrophic crank failures are very dangerous and, while not very
common, do happen. Cranks should be inspected for cracks every time you
clean your bike, especially if you're a large or "enthusiastic" cyclist.
| | Also, I thought there was a downside to greasing the spindle. Am I
wrong?
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In order to get a reliable mating force between crank and spindle, you
have to rely on the force you can measure (bolt torque) accurately
reflecting the one you can't (actual mating force). If there's an
unknown amount of friction, this relationship becomes unpredictable. By
greasing the spindle and bolt you get accurate tightening. If there's a
lot of friction, you are likely to under-tighten.
The claimed downside to greasing is that the cranks may become lose. I
have never had this happen, and there is no reason that it should, since
the mating forces are so high that the grease gets displaced from the
interface once it has done its job. The other fear is that grease makes
the cranks go further up the spindle "stretching" the bore. The mating
forces produced by specified bolt torques are too low for this to happen.
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