Jockey Wheels or Pulleys?

There seems to be some confusion over whether the correct term for those small plastic, carbon, titanium or alloy wheels in a derailleur should be called jockey wheels or chain pulleys.

Technically speaking the majority of rear derailleurs use two pulleys in sprung cage to control the tension of the chain as it moves over different sized sprockets. The lower pulley is known as the tension pulley and the upper one known as the jockey pulley.

In addition to these there also exists ‘idler’ pulleys that are used on downhill mtbs to keep the chain firmly on the front chainrings or on tandems and recumbents where an extra guide is required to control long stretches of chain to stop it sagging or to keep it in line.

An idler or pulley increases friction on the chain meaning that the riders effort will be spent unnecessarily. This can occur through the bearings but more so if the direction of the pulley is not completely in line with the direction of the chain, such as when guiding the chain onto sprockets. This is because the friction occurs when the chain pivots to engage and disengage with the pulley, not when the chain is passing around the pulley. For this reason the larger the pulley the less friction because the angle the chain pivots is smaller and the slower it rotates on its bearings for a given chain length. Hence the move to custom fitting larger, lighter, smoother jockey wheels to derailliuers to increase rider speed.

Returing to the main question of whether to use the term jockey wheel or pulley. Either will do…pulley is technically correct but go into any bike shop and jockey wheel will be the term that gets used most.