Strap wrenches are designed to grip and turn odd-shaped objects.
The strap is capable of deforming around the object, providing a grip based on friction.
The availability of grippy synthetic rubber has helped popularize this type of wrench in recent years, but leather, metal, and chain strap wrenches have been on the market for decades.
While generally used when nothing else will turn an object, these wrenches have always been a widely used tool for bicycle repair.
Strap wrenches can be divided into three major styles: Friction Strap: Most strap wrenches use a high-friction strap made out of rubber or leather.
This strap is wrapped around the object that needs to be turned and through a slot in this handle.
The user then turns the handle so that the flat end at the top meets with the strap.
This pressure combined with pinching force at the channel opening holds the strap tight, putting pressure on the object.
This type of strap wrench can be adjusted to handle a wide range of sizes with the total size limit ranging several inches.
Friction wrenches designed for use with plastic pipes have an offset handle with pressure being applied to the side of the handle instead of the top.
Functionally there is no other difference between this type of strap wrench and one designed for general use.
Metal Strap: Instead of an adjustable channel, this type of wrench uses a fixed metal strap that is bent around the object when the handle is twisted.
This lack of adjustability means a single wrench can only handle objects within a very short range of sizes, typically with less than an inch between the largest and smallest object it can handle.
Chain strap: This type of strap wrench, sometimes called a chain whip, uses a piece of chain.
It has a handle that has a piece of chain attached to the end and a slot in the side to hold one of the chain links.
While adjustability is limited to the space between the links, the lower amount of surface area lets this type of wrench get a good bite on the part.
More force can be applied with a chain strap than a metal or rubber strap, but too much force can cause the links to dig in, damaging the part.
In most cases this type of wrench should only be used if the part will be disposed of once removed.
Chain whips are also useful for gripping sprockets: The chain fits between the teeth like a normal chain providing adequate grip without damaging the sprocket's surface.
Up until the early 1980s bicycles used threaded freewheels to attach the sprocket and hub to the rear wheel.
The only way to remove this type of hub is by turning the largest sprocket using a chain whip.
Modern hubs have gear cassettes attached with splines.