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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.

Derailleur Hanger
The dérailleur hanger or mech hanger is the part of the dropout that the rear dérailleur attaches to.

Most aluminum framed bikes have a removable mech hanger. These are intended as a cheap replaceable part so that in the event of an accident or mechanical problem that could damage the dérailleur or frame, the dérailleur hanger breaks or deforms instead. Sometimes a shear bolt, which is designed as a weak point, will also be used.

In general, steel framed bikes don't have this removable dérailleur hanger. Because steel is less likely to harden during deformation, it is generally easier to bend the hanger back into shape rather than bolt on a new part.

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 These have now been effectively superseded by the dérailleur hanger.

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 Forrester, 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. 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. Recently though, there has been some evidence, notably from James Annan, a British scientist working in Japan, suggesting that the movements 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 attributed to this cause. This is quite controversial and fork manufacturers have not admitted a fault, although there is some evidence that they may accept the principle, and thus advise on checking that the quick release tension has been strengthened.
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