Bicycle rollers are a type of bicycle trainer that make it possible to ride a bicycle indoors without moving forward. However, unlike other types of bicycle trainers, rollers do not attach to the bicycle frame, and the rider must balance him or herself on the rollers while training. Bicycle rollers normally consist of three cylinders, drums, or "rollers" (two for the rear wheel and one for the front), on top of which the bicycle rides. A belt connects one of the rear rollers to the front roller, causing the front wheel of the bicycle to spin when the bicycle is pedaled. The spacing of bicycle rollers can usually be adjusted to match the bicycle's wheelbase. Generally, the front roller is adjusted to be slightly ahead of the hub of the front wheel.