As usual, 'Sheldon Brown'
(http://www.sheldonbrown.com/tooltips/cotterless.html) has some useful
advice.
Klaas Bil wrote:*... I've found that if I just tap
Quoted message said:the cranks on, then tighten the nuts more or less properly, then pound
the cranks as I described, I can tighten the nut more without using
more torque than in the first round. That makes me conclude that the
pounding is required for properly mounted cranks but maybe I am wrong?
Maybe I deform something??? *
Yes, that is what is happening. The shallow taper causes a huge
mechanical advantage. For every unit of force used to push the crank
onto the taper the square hole must react with a hundred times that
force in stretching tension. Hammering is, by definition, a large
impulsive force so in the instant of the hit the material around the
hole is stretched with perhaps a hundred tonnes and permanently
deformed.
This is bad because the deformations are concentrated at the corners of
the square hole. At best the material yeilds uniformly, but to me this
is highly unlikely. Rapid application of force causes ductile materials
to behave as if they were brittle, so what might have been a relatively
benign uniform stretch is probably a series of micro-cracks. This
significantly weakens the crank hole, and worse, makes the hole "softer"
and less able to resist pedaling forces. A crank so damaged is likely
to wobble and come off. (The band-aid fix for a crank damaged in this
way is called lock-tite.)
The best approach is to start with brand new, quality cranks and NEVER
pound on them. Bike mechanics routinely put cranks on that never come
off by slowly stretching the crank onto the taper with a calibrated
force. This is what the manufacturers recommend, and IMHO, it works
really well.
I use the approach the Campagnolo tech rep suggested - clean, dry
tapers, grease on the bolt threads, hand-placement of the cranks and a
slow torquing - with a torque wrench - to the recommended tightness. I
regularly apply FAR more pedaling pressure - both forwards and, in the
case of my fixed gear bikes, backwards - on long, high leverage bike
cranks than I ever would on my unicycle cranks and have never had a
problem. (Then again, I've never bent a crank on a big drop either.)
Klaas Bil wrote:*
Quoted message said:I don't have a torque wrench. At the few places I checked they were
horribly expensive. Specific buying recommendations made by USAans
(like you, recently) don't help for this European. I guesstimate the
exerted torque by multiplying the effective wrench length with a
guessed force - for the latter I try to think a %-age of my weight, a
bucket of water etc. Very imprecise but it's at least something.
*
This approach is better than nothing.
I sympathize, sort of - the German hardware stores I shopped at only
carried the best stuff. There were no racks of cheap Taiwanese imports
because the cultural norm in Germany is to only buy tools that you can
proudly hand down to your grandkids. That value isn't as strong over
here, so we have a market for disposable tools.
--
cyberbellum - Level 1.0 rider!
Optimists think the glass is half full. Pesimists think the glass is
half empty. Engineers think the glass is too big.
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