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Pulley
Bicycles use pulleys both for chains and for cables.
Chain pulleys
- Most rear derailers use two pulleys in a spring-loaded cage to regulate the tension of the chain as it runs over different sized sprockets. The upper pulley is called the "jockey" pulley, the lower one is the "tension" pulley.
- Idler pulleys are used to:
- ...lead a chain around an obstruction (as on most SWB recumbents, where the front wheel gets in the way of a straight chain run.)
- ...adjust the tension on a chain (as with tandem synch chains.)
- ...apply extra tension and guidance to prevent accidental derailment on downhill bicycles.
Any idler adds friction, but in particular, an idler in the tensioned (pulling) run of a chain introduces nearly the same amount of friction even if it changes the angle of the chain only slightly. That is because the friction occurs when the chain pivots to engage and disengage the idler, not when the chain is passing around the idler. The friction introduced by the idler's bearing is small in comparison, as long as the bearing is of good quality. Whenever possible, an idler should be placed in the return run of the chain. Also, the larger the diameter of the idler, the less the friction -- both because the angle through which the chain pivots is smaller, and because the idler turns more slowly on its bearing. [This paragraph added by John Allen.]
Cable pulleys
- Pulleys have traditionally been used on 3-speeds to lead the cable where it crosses the seat tube, either at the top (following the top tube/seat stay) or the bottom (following the down tube/chain stay) of the seat tube.
- Many early '90s mountain bikes used a pulley at the bottom of the seat tube, to permit top-pull cable routing with bottom-pull front derailers. Since top-pull front derailers became readily available, this system has been obsolete.
- Accessory cable pulleys, such as the Avid Rollamajig ® are sometimes used in place of the traditional loop of cable housing where the cable enters the rear derailer.
- Pulleys are also used on brake cables:
- ...for centerpull/cantilever rear brakes on ladies' bicycles, where the cable has to make a sharp bend after it runs up the back of the seat tube.
- ...in some mountain-bike handlebar stems, where the housing enters in line with the stem extension, and the bare cable runs around a roller toward the cantilever. (Some people believe that this is dangerous, due to potentially higher risk of breaking the cable due to repeated flexing of the cable.)
- Eccentric pulleys are sometimes used to change the mechanical advantage of a cable. This is most commonly done to permit the use of direct-pull cantilevers with conventional levers.
Most pulleys used on bicycles are too small to avoid failure of cables due to fatigue as they wrap and unwrap. See material on sheaves and ropes in article by David Gordon Wilson. The indicator chain of classic Sturmey-Archer hubs avoids this problem. It would be good if more components, in particular shifters, used chains to wrap around their pulleys. [This paragraph added by John Allen.]
