No no, I am not talking about truant students that flunk school. The dropout I am referring to exist in bicycle world and in this context, it means or refer to the end part of a slot in frame or fork where the axle of the wheel is attached.
Although there is a front dropout, dropouts are usually associated to the the rear end part of a bicycle frame and they come in two styles:
- Horizontal dropouts, which is a slot (1 to 3 inches long) that runs more or less horizontally at the end part of the frame for the rear axle to fit into. They permit the wheel to be placed in various positions front to rear. Horizontal dropouts are necessary for bicycles which don't have derailurs , because the axle must be movable to adjust the chain tension.
With horizontal dropouts, during installation, wheel need to be align carefully to the frame. Besides that, the axle nuts or quick release must be tightened quite securely, or the chain tension may pull the axle askew.
- The second is Vertical dropouts which obviously 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 derailleur 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 derailleur or pulley type chain tensioner.