a steering mechanism that consists of a gear in mesh with a toothed bar, called a "rack." The ends of the rack are linked to the steered wheels with tie rods. When the gear is rotated by the steering shaft, it moves the rack from side to side: turning the wheels.

rebound: the motion of a wheel that extends the suspension. The opposite of jounce.

recirculating-ball: a steering mechanism in which the steering shaft turns a worm gear that, in turn, causes a toothed metal block to move back and forth. Ball bearings in a recirculating track reduce friction between the worm gear and the block. As the block moves, its teeth rotate a gear connected to a steering arm, which then moves the steering linkage.

redline: the maximum recommended revolutions per minute for an engine. In cars equipped with a tachometer: an instrument that measures engine rpm: the redline is usually indicated by, surprisingly enough, a red line. Some tachometers mark the redline with a colored sector. Others have two lines: the lower one marking the maximum allowable sustained engine rpm, the higher line indicating the absolute maximum rpm.

ride height: a measurement between the ground and some fixed reference point on a car's body (the reference point varies according to the whims of the particular automaker). This dimension can be used to measure the amount of suspension deflection or the height of the body from the ground.

ride steer: a generally undesirable condition in which a wheel steers slightly as its suspension compresses or extends. Also called "bump steer."

rigid axle: a simple non-independent suspension, consisting of a rigid transverse member with wheel hubs solidly bolted to it. The axle can be attached to the body by leaf springs, or by a combination of suspension arms and links.

ring-and-pinion gear: any gearset consisting of a small gear (the pinion gear) which turns a large-diameter annular gear (the ring gear).

roadholding: the ability of a car to grip the pavement. Technically described as "lateral acceleration," because cornering is actually a continuous deviation from a straight path. Measured in gs.

road-load horsepower: the amount of power at the driving wheels needed to move a car down the road at a steady speed. This power varies according to the car's speed, aerodynamic drag, and mechanical friction, as well as the tires' rolling resistance.

Road-load horsepower is distinct from engine power because the output of the engine is sapped by various mechanical losses between the engine's output at its flywheel and the driving wheels.

roll: the rotation of a car's body about a longitudinal axis. Also less accurately called "sway" or "lean," it occurs in corners because the car's center of gravity is almost always higher than the axis about which it rotates.

rubber-isolated crossmember: a laterally aligned structural member that is attached to the body or the frame via vibration-absorbing rubber isolators. By bolting suspension or driveline components to such crossmembers, automotive engineers can reduce the transmission of noise and/or ride harshness to the body.