Dropout (bicycle part)

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A horizontal dropout on a steel frame road bicycle converted to a single-speed. The non-replaceable derailleur hanger (below the axle) and an eyelet (above the axle) for mounting a fender or rack are now unused.

A vertical dropout on an aluminum frame mountain bike. The silver part is the replaceable derailleur hanger.

A bicycle dropout (or frame end and fork end) is a slot in a frame or fork where the axle of the wheel is attached.

On bicycles that do not have a derailleur or other chain tensioning device, rear horizontal dropouts allow adjustment of chain tension, and can accommodate a range of chain lengths or cog sizes. They were standard on most older derailleur bicycles from before the 1990s. They are also used by single-speed bicycles. An older multi-speed bicycle with horizontal dropouts can be readily converted into an fixed gear bicycle or fixed wheel bicycle. Horizontal dropout openings may face forwards or rearwards. Track bicycles traditionally have the opening facing rearwards (track ends) while road bicycles have forward facing dropouts.

Rear vertical dropouts have the slot facing downwards. The advantage is that the wheel axle cannot slip forward compared with horizontal dropouts. The disadvantage is that on a bicycle without a rear derailleur but with vertical dropouts, the chain tension cannot be adjusted by moving the wheel forwards or backwards, and needs another means of chain-tensioning, by a derailleur or chain tensioner. Fixed wheel bicycles cannot use any form of chain tensioning device.

In general, a modern bicycle frame intended for derailleur gears will have a vertical dropout, while one designed for singlespeed or hub gears will have horizontal dropouts.

1 Features
2 References

[edit] Features

[edit] Derailleur hanger

The derailleur hanger or mech hanger is the part of the dropout that the rear derailleur attaches to.

Most non-steel framed bikes have a separate removable derailleur hanger, generally made from aluminium which is bolted to the dropout. These are intended as a cheap replaceable part so that in the event of an accident or mechanical problem that could damage the derailleur or frame, the derailleur hanger breaks or deforms instead. Sometimes a shear bolt which is designed as a weak point will also be used. There are dozens of different derailleur hangers avialable and one model is seldomely interchangeable with another.

In general, steel-framed bikes do not have a removable derailleur hanger, because a steel dropout and hanger is stronger and therefore less likely to be damaged, and also it is more malleable and less likely to work harden during deformation and generally can be bent back into shape without breaking.

It is important for proper indexed shifting for the derailleur hanger to be properly aligned. The rear derailleur bolt hole must be close to parallel with the rear axle. If it is out of alignment, the rear derailleur will move not far enough, with respect to the rear sprockets, with each click. A special tool exists to detect and aid in correcting misalignment of the deraileur hanger.

On frames with no derailleur hanger, a direct mount derailleur or derailleur with a hanger plate is used. These only fit bikes with forward-facing horizontal dropouts, and are held in place by a small bolt and the rear wheel axle. These have now been effectively superseded by the derailleur hanger.

[edit] Lawyer lips

Lawyer tabs or lawyer lips (technically positive retention devices) are tabs fitted to the front fork dropouts of bicycles sold in some countries (particularly the U.S.) to prevent a wheel from leaving the fork if the quick release skewer comes undone. They were introduced in response to lawsuits supported by experts including John Forester, in cases where incorrectly adjusted quick release wheels came out of the forks. Lawyer tabs are designed to compensate for the fact that many riders do not know how to operate a quick release properly: some riders treat them as a folding wing nut, others do not do them up tightly enough for fear of snapping them or shearing the skewer (both are highly unlikely given the normal range of human strength).

A side-effect is that the quick release, which was developed to allow the wheel to be removed without having to unscrew any components, no longer works as designed: the skewer must be unscrewed in order to remove the wheel (although tools do remain unnecessary). This means that the tension on refitting must be adjusted again. Some cyclists file off the lawyer tabs so the quick release works as originally intended.

A correctly secured quick release is unlikely to be ejected from the dropout in normal use where rim brakes are in use, although recently there has been some evidence, notably from James Annan, a British scientist working in Japan, suggesting that the moments in disc brake systems can cause quick-release front wheels to be ejected past the lawyer tabs. A small number of serious crashes have been put down to this cause. This is controversial, and the fork manufacturers have not admitted a fault, although there is some evidence that they may accept the principle, and advice on checking quick release tension has been strengthened.^[1]^[2]

[edit] Eyelets

Dropouts may support one or more eyelets, small protruding tabs with holes, often threaded, to facilitate the mounting of racks and fenders.

[edit] Adjustment screws

Some rear, horizontal, forward-facing dropouts have small adjustment screws threaded through the very rear to aid in positioning the axle precisely. The screws may be fitted with springs to keep them from moving.^[3]

[edit] Chain tensioners

Some rear, horizontal, rearward-facing dropouts have threaded devices to aid in setting the proper chain tension. These may be integral to the dropout or separate items.^[4]