An Echo 4 bolt brake booster mounted on the back of a Zona Zip

A brake booster is a device shaped like a horse shoe used to reinforce the bike frame when using high-powered rim brakes. They are desirable because when the brakes are being used aggressively they may cause the frame to flex; the booster helps prevent this, thus increasing the brake effectiveness.

Brake boosters come in 3 varieties; 4 bolt, 2 bolt, and those for V-brakes.