The brake caliper straddles the disc (rotor) and contains the hydraulic wheel cylinder(s). It is mounted firmly to the spindle, which allows it to deliver the torsional force of the wheel to the chassis via the control arms. Single piston, sliding, or "floating caliper" disc brakes have been used on the front wheels of passenger cars for many years. With standard single piston calipers, the caliper "floats", or slides on its mounting bolts or pins to apply both friction pads to the machined surfaces of the rotating disc. The caliper's piston seals are designed to retract the piston enough to allow the pads to lightly contact the rotor without any drag. Some calipers have as many as four pistons actuating the friction pads. These calipers are fixed in place; i.e., there is no lateral movement.