a removable-roof body style popularized by Porsche that is similar to a convertible except that it incorporates a fixed, roll-bar-like structure running from side to side behind the front seats.

throttle-body: a housing containing a valve to regulate the airflow through the intake manifold. The throttle-body is usually located between the air cleaner and the intake plenum.

throttle-body fuel injection: a form of fuel injection in which the injectors are located at the engine's throttle-body, thereby feeding fuel to more than one cylinder. Such an arrangement saves money by using fewer injectors; but because it routes both fuel and air through the intake manifold, it eliminates some of the tuning possibilities offered by port fuel injection.

toe-control link: a lateral link in a multilink suspension designed to control a wheel's direction as the suspension moves up and down.

toe-in: the intentional nonparallel orientation of opposite wheels. Toe-in is measured by subtracting the distance between the front edges of a pair of tires from the distance between the rear edges of the same pair of tires. The toe-in dimension is positive when the fronts of the tires are turned toward the center of the car.

toe steer: the changes in the direction of a wheel that occur without driver steering input. Toe steer can be caused by ride steer or by deflections in suspension components caused by the stresses of cornering, accelerating, and/or braking on smooth and bumpy roads.

torque: the rotational equivalent of force, measured in pound-feet.

torque converter: a particular kind of fluid coupling with a third element added to the usual input and output turbines. Called "the stator," this additional element redirects the churning fluid against the output turbine, increasing torque. This torque increase, however, is achieved at the expense of rpm and efficiency.

torque steer: a tendency for a car to turn in a particular direction when power is applied. Torque steer is common in front-drive cars because reaction forces created in the half-shafts can generate uneven steering forces in the front tires.

torsion bar: a spring consisting of a long solid or tubular rod with one end fixed to the chassis and the other twisted by a lever connected to the suspension.

traction control: an electronic control system that prevents wheelspin by detecting when a driven wheel is about to break traction, and then reducing engine power and/or applying the appropriate brakes to prevent it.

trail-braking: a driving technique in which the driver begins to brake before entering a turn and then continues to brake as he eases into the corner. As cornering forces build, the driver gradually feathers off the brakes: trading braking power for cornering grip. By increasing the vertical loading : and thus the traction: at the front tires, trail-braking can improve a car's turn-in.

trailing arm: a suspension element consisting of a longitudinal member that pivots from the body at its forward end and has a wheel hub rigidly attached to its trailing end. A sufficiently rigid trailing arm can provide all of a wheel's location. In that case it is similar to a semi-trailing arm, except that its pivot axis is exactly perpendicular to the car's longitudinal center line.

trailing link: a suspension link that is aligned to resist longitudinal motions in a wheel; it is mounted to the chassis ahead of the wheel.

transaxle: A transmission and a differential combined in one integrated assembly.

transmission: a gearbox with a number of selectable ratios, used to match the engine's rpm and torque to differing vehicle requirements.

tread squirm: the flexibility in the tire tread between the surface of the tread and the tire carcass. Snow tires, with their small, deep, unsupported tread blocks, have a large amount of tread squirm. Slick racing tires, which have no tread pattern, have very little squirm.

tube frame: a car frame made up of rigid tubing welded together. Tube frames are easier to manufacture in small quantities than unitized frames.

tumblehome: the term that describes the convex curvature on the side of a car body.

tuned intake and exhaust systems: intake and exhaust systems that, by harnessing the pressure pulses and resonances inside the various passages and chambers of the intake and exhaust manifolds, increase the flow of intake charge into and out of the combustion chambers.

turbocharger: a supercharger powered by an exhaust-driven turbine. Turbochargers always use centrifugal-flow compressors, which operate efficiently at the high rotational speeds produced by the exhaust turbine.

turbo lag: within a turbocharger's operating range, lag is the delay between the instant a car's accelerator is depressed and the time the turbocharged engine develops a large fraction of the power available at that point in the engine's power curve.

turn-in: the moment of transition between driving straight ahead and cornering.