Dropout

Sheldon Brown's Bicycle Glossary · letter D
← Drop Handlebar · Drum Brake →

A type of forkend that allows the rear wheel to be removed without derailing the chain first.

Track and BMX bicycles do not have rear dropouts; they use fork ends that open to the rear.

A current fad has led to the revival of this inferior forkend style for single speed bikes.

Rear dropouts come in two styles:

Horizontal dropouts have a longish slot for the rear axle to fit into, which runs more-or-less horizontally along the dropout. They permit the wheel to be placed in various positions front to rear. Horizontal dropouts are necessary for bicycles which don't have derailers, because the axle must be moveable to adjust the chain tension. With horizontal dropouts, it is possible to mis-align the wheel in the frame if it is installed carelessly. The axle nuts or quick-release must be tightened quite securely, or the chain tension may pull the axle askew.
Vertical dropouts have a vertical notch for the axle to go up into, and the axle's position is not adjustable. With vertical dropouts, the axle cannot be pulled out of position, even if it is not properly secured. Vertical dropouts are generally intended for use with derailer gearing, and do not permit any adjustment of the position of the rear wheel, so there's no way to regulate chain tension if you don't have a derailer or pulley type chain tensioner.

A few single-speed or internal-hub gear bikes use vertical dropouts with an eccentric bottom bracket to permit chain tension adjustment.

White Industries makes a special hub with an eccentric axle, also to permit vertical dropouts to be used with a straight chain run.

On derailer-equipped bicycles, the rear derailer is attached to the right rear dropout, either directly to a hanger that is part of the drop out, or by way of an adaptor claw.

Dropout spacing varies among different styles of bicycles

Forged Dropouts

Horizontal Dropout Campagnolo 1010	Horizontal Dropout Short	Semi-Vertical Dropout	Vertical Dropout

Stamped Dropouts			Investment Cast

With Hanger	Without Hanger	Raleigh 3-speed	Track Forkend Not a dropout!