Precession (mechanical)

Precession (mechanical)

Precession is the name given to the process of one part rotating with respect to another due to fretting between the two. It is described as "precession, in which a round object rolling in a circular ring in one direction will itself turn in the opposite direction." "In machinery, fretting is the micro-motion of tightly fitting parts that superficially appear immobile with respect to each other."cite web
url =
title = Fretting damage in Bicycle Mechanics
accessdate = 2006-12-26

"For a pedal, a rotating load arises from downward pedaling force on a spindle rotating with its crank making the predominantly downward force effectively rotate about the pedal spindle. What may be less evident is that even tightly fitting parts have relative clearance due to their elasticity, metals not being rigid materials as is evident from steel springs. Under load, micro deformations, enough to cause motion, occur in such joints. This can be seen from wear marks where pedal spindles seat on crank faces."cite web
url =
title = Left Hand Threads
last = Brandt
first = Jobst
year = 2004/5
accessdate = 2006-12-26

Fretting-induced precession is a purely mechanical process which does not depend on inertia and is not inversely proportional to spin rate. It is completely unrelated to torque-free and torque-induced precession.


Precession caused by fretting can cause fastenings under large torque loads to unscrew themselves.

Automobile lug nuts

Automobiles have also used left-threaded lug nuts on left-side wheels, but now commonly use tapered lug nuts, which do not fret.

Bicycle pedals

Bicycle pedals are left-threaded on the left-hand crank so that precession tightens the pedal rather than loosening it. Jobst Brandt, an engineer (author of "The Bicycle Wheel") and cyclist, has suggested a tapered seat for bicycle pedals similar to lug nuts to prevent the damage that fretting does to soft aluminium crankarms, but no major manufacturer has taken up the idea (probably because it requires compatible pedals and crankarms, thus a new standard).

Bicycle bottom brackets

English threaded bicycle bottom brackets are left-threaded on the right-hand (usually drive) side into the bottom bracket shell. This is the opposite of pedals into cranks because the sense of the relative motion between the parts is opposite. (Italian and French threaded bottom brackets have right-hand threading on both sides.)

See also

* Fretting


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