Pedal Precession
If you’ve ever had to pack up your bike for an airplane trip, you know that not every threaded part on a bicycle follows the “lefty-loosey” rule of thumb. The following parts of a standard bicycle are reverse or left-hand threaded – which is to say that they loosen clockwise and tighten counter-clockwise:
- The non drive-side pedal and the pedal hole of the crank arm
- The drive side or “fixed” bottom bracket bearing cup
- The cone on a freewheel (few people will have had reason to see these)
- The lockring on fixed-wheel hubs
Non drive-side pedals, fixed cups, and freewheel cones are reverse threaded for the reason of mechanical precession. Broadly, precession refers to a change in the direction of the axis of a rotating object. As a physical phenomenon, it influences the rotation of planetary bodies and toy tops.
For our purposes, precession could cause the first three parts listed above to unthread during riding if they were not reverse threaded.
The pedal is one of the unheralded bearing systems of a bicycle. Typically, it consists of a platform, with a set of ball bearings at either end, that rotates around an axle. This axle is threaded into a crank arm – clockwise on the drive side and counter-clockwise on the non-drive side. As you ride your bicycle, the non-drive side pedal’s platform rests on the same plane – horizontal, just like your foot – in relation to you. But, in relation to the crank arm to which the pedal’s axle is attached, the pedal’s platform is rotating constantly in a clockwise direction while the bicycle is being ridden.
Based on this, you would expect that, if anything, the pedal’s axle would tend to move in a clockwise direction in relation to the crank arm to which it is attached. Indeed, the movement of the bearings connecting the pedal’s platform to its axle would influence it to turn in this direction as well. But all these factors should be moot anyway – wouldn’t a decent bike builder make sure that the pedal’s axle was locked securely into the crank arm?
Firstly, though tightly fitted together, the pedal’s axle and the crank arm are not “locked” together. Despite being composed of metal, both of these parts have some degree of elasticity (as does all metal – think of a steel spring). This inherent elasticity means that there are micro vibrations and movements between the two parts – a phenomenon known as fretting – even though they appear locked together. Even the most perfectly fitted parts of a machine have some degree of fretting between them.
Secondly, if we look closely at the interaction between the pedal’s platform and the crank arm, it becomes apparent that the direction in which the platform is rotating is not the same direction in which its axle will move. As you press on the pedal, the pressure you are exerting is transferring through the platform, through the bearings, through the axle to which it is connected, and into the crank arm to which the pedal is attached. Following the rotation of the platform, we see that the pressure being transferred into the crank arm is moving in a clockwise motion. Precession, in this case, refers to the influence of friction (a static force) being exerted by the crank arm hole on the pedal axle which is threaded into it. Frictional force, by definition, being exerted by a surface in the direction opposite to its motion relative to the other surface, this frictional force is working in the opposite direction to the one in which the pedal’s axle is turning.
It is true that the rotation of the pedal’s platform is influencing the pedal’s axle to rotate in a clockwise direction but the frictional force of the precession effect is stronger than this influence. If you don’t believe me, build a left-hand drive fixed-wheel bike with normal cranks and test it out. Or, better yet, try this experiment.
Hold a pencil loosely in your right fist, with the longer end protruding from the bottom of your hand and the shorter end just poking out the top. Now, with your left hand, move the longer end of the pencil in an counter-clockwise direction. The pencil should be moving around your hand in an counter-clockwise direction – but have a look at the pencil itself: it should be rotating clockwise. This rotation is caused by the friction between the pencil and the inside of your hand, and is essentially the same thing that is happening to your pedal’s axle when you’re riding your bike. As your non-drive side crank turns counter-clockwise, your pedal’s platform is rotating clockwise in relation to the crank, and the pedal’s axle is being influenced by precession to rotate counter-clockwise – if it wasn’t reverse threaded, it would loosen and fall out. So all standard non-drive side cranks have reverse threaded pedal holes.
But what about Italian and French bottom brackets? As noted above, the fixed or drive side cup of these bottom brackets are threaded normally. But if they are subject to precession in the same way, why aren’t they constantly coming loose?
According to mechanical engineer Jobst Brandt, bottom brackets are not subject to nearly as much precession- inducing load as are pedals. As the cranks rotate during riding, the pressure (which would induce precession) being exerted by the act of pedaling is being counteracted by the tensions being exerted by the chain which is being pulled by the pedaling motion – the two loads all but cancel each other out. Thus the fixed cups of standard bottom brackets are reverse-threaded more on principle than for any practical purpose. ----------
