Rear derailleur: The guide pulley feeds the chain to the selected rear wheel sprocket. The tension pulley uses a spring to keep the entire chain assembly tight. Freewheels: These allow the bike to roll forward without the cyclist pedaling. When the bike is pedaled forward, a ratchet engages to drive the rear wheel. Chain: The chain needs to be the correct width to operate smoothly over the sprockets and chainwheels, and the correct length for optimum shifting performance. Front derailleur: The chain passes through this cage as it feeds onto the front chainwheels. By moving from side to side, the front derailleur is able to shift the chain among the chainwheels. Spider: The spider is the multi-armed part of the crank. Chainwheels: These are also called chainrings or front sprockets. The larger the chainwheel, the higher the gear is. Bicycle gear ratios: The lowest gear ratio on the bike might be a front chainwheel with 13 teeth and a rear gear with 52 teeth. In this case, the rear wheel will turn once for every four turns of the chainwheel. Working up through the gears gradually changes this ratio, allowing the bike to travel farther with every turn of the pedals. Gearless bikes: Before the bicycle chain was invented, bicycle pedals were fixed to the axle of the front wheel. Today’s track racers are also gearless (and brakeless), but only for the sake of fairness. This means that all of the bikes have the same mechanical advantage.