Paul Kopit writes:
> Why do pedals have to be so tight in cranks?
> I finally got my first torque wrench, $9.99 at Harbor Freight, and I
> am playing with the new toy. The specification for a pedal into a
> crank is ~25 ft lbs. That seems high to me. I've been using a 5"
> hex key and getting the pedal in just beyond hand tight. I can
> remove the pedal with the same key and holding the arm with my hand
> and working the wrench with my fingers.
You probably can't get pedals up to 25ft lbs with a hex key, that
feature bing for quick assembly while the wrench flats for final
tightening. Just the same, all that tightness will not prevent your
pedals from working in the threads, the being no contact in that
interface to take up the load in compression.
The face of the pedal spindle is small, flat and at right angles to
pedaling forces while the threads are at 60 degrees, ideal of small
angular displacements of the spindle. beyond that, a left hand thread
is required to keep left pedals from falling out, regardless of how
tight you make them. Inspecting the contact face of a pedal crank
after significant use will reveal an eroded wear pattern, where the
spindle face bore, from which cracks radiate that can ultimately cause
the pedal eye to break out.
I have modified the contact shoulder on the pedal spindle to a 90
degree cone and have ended my stream of crank failures at that
interface. This solution was adopted by the auto industry for wheel
lug nuts nearly 100 years ago to keep left side wheels from unscrewing
and separating from cars. As with cars it would make left hand thread
unnecessary and put an end to crank failures at that junction.
Unfortunately bicycles, that have been around longer than cars, were
orphaned by engineering for their more powerful derivatives.
Jobst Brandt
[email protected]