Glossary

Aerodynamic grip is the extra adhesion provided by the additional downforce created by airflow over and under the wings and, in the case of modern-era Indycars, Formula One cars, and prototype sports cars, the body itself. The cars in GPL produce no aerodynamic downforce and therefore have no aerodynamic grip.

Aerodynamic drag is the friction produced by traveling through the air. It increases as the square of the velocity, and is the primary limiting factor in the car's top speed on long straights. The cars in GPL each have a different amount of aerodynamic drag. See the Drag chart and table for more details.

Apex refers to the part of the corner where the car comes closest to the inside edge of the track in its arc through the corner. An early apex means that the car nears the inside of the corner early in its travel through the corner, and then turns more sharply before accelerating away. A late apex means that the car turns more sharply in the first part of the corner, then passes near the inside edge of the track as it is accelerating away on a broadening arc. A late apex is preferable at most corners leading onto long straights, because it allows earlier acceleration, while an early apex may be preferable at corners at the end of long straights because it allows later braking. See also turning-in point and track-out point.

Bottoming. See How Do I Know when It's Bottoming?

Bump refers to upward movement of the wheel and tire with respect to the chassis, as occurs when it encounters a bump in the track surface. The opposite of rebound.

Bump steer refers to a change in toe-in as the wheel moves up or down. It's common to set up the rear suspension for a small amount of toe-in under bump to help make the more car stable when trail braking. GPL doesn't model bump steer.

Camber - wheel. With respect to a car's chassis, camber refers to the angle of the wheel and tire in relation to the car's vertical centerline. Negative camber means that the top of the wheel leans inward in relation to the centerline of the car; positive camber means the top of the wheel leans outward.

Camber - track. With respect to the track surface, camber refers to the angle of banking, or lateral slope of the track surface in relation to level ground.

Center of Gravity is the car's center of mass.

Cornering refers to the car's performance in terms of generating overall lateral grip. Not quite the same as handling.

CG. See Center of Gravity.

Cross-weight refers to diagonally asymmetric static loading of the tires. On real cars, diagonally asymmetric static load is induced by asymmetrical ride heights left to right, front to rear (also known as weight jacking), but GPL doesn't permit this. In GPL, asymmetrical diagonal loading can be accomplished by asymmetrical spring rates, such as soft left front and right rear springs and stiff right front and left rear springs. Diagonally asymmetric weight transfer can be induced dynamically by track surface features such as by transitioning a crown or running a wheel up on a curb.

Dampers are hydraulic devices which apply friction to dampen the vertical movement of the wheels with respect to the chassis. Dampers are also known (incorrectly) as shock absorbers.

Differential. See The Differential.

Dive refers to the chassis' rotation to the front around its lateral axis under deceleration. Under braking, the nose drops and the tail rises. The opposite of squat.

Drafting refers to following closely to another car at high speed, so that the fast-moving air in the leading car's slipstream presents less drag to the car behind. The draft behind GPL's cars is so strong that a car running very close behind another car can travel several miles per hour faster than it can running alone.

Drag refers to the total friction resisting the forward travel of the car. Factors include aerodynamic drag, friction produced by the rotation of the tires, friction in the wheel bearings and drivetrain, and internal friction and pumping losses in the engine. See the Drag table and chart for more details.

Friction circle is a concept intended to enhance our way of thinking about the forces produced by the tire. A tire can generate longitudinal grip (for acceleration or braking), or lateral grip (for cornering), or a combination of lateral and longitudinal grip. The total potential grip can be described as a circle, with the 12 o'clock position representing full acceleration, 6 o'clock representing full braking friction, and 3 and 9 o'clock representing full right and left cornering friction respectively. See pg. 81 of Going Faster by Carl Lopez and pg. 2-14 of Carroll Smith's Drive to Win for details.

Grip is the friction, or adhesion, provided by the tires in contact with the track. This adhesion is influenced by such factors as tire pressure, tire compound, as well as the angle of the tire in relation to the chassis (see camber). The percentage of the vehicle weight on a given tire is also a critical factor.

Handling refers to the car's behavior in corners. A good-handling car is predictable and forgiving. It will respond linearly to control inputs and will react in a smooth, controllable fashion to bumps, camber changes, and other irregularities in the track surface. A bad-handling car is unpredictable, nonlinear in its responses to control inputs and in its reactions to the track surface.

Lateral means to the side, or side to side. Lateral grip means grip produced in cornering. The car's lateral axis is the axis that could be drawn from one side through its center to the other side.

Left-foot braking refers to the practice of braking with the left foot, permitting simultaneous application of brake and throttle in midcorner to adjust the balance of the car. Not the same as Trail Braking.

Longitudinal means front to rear or rear to front. Longitudinal grip means grip produced in accelerating or braking. The car's longitudinal axis is the axis that could be drawn from front through its center to the rear.

Motion ratio refers to the ratio between spring movement and movement at the wheel dictated by the mechanical linkages in the suspension. A motion ratio of 4:1 would make a spring rate of 400 lb./in. produce a wheel rate of 100 lb./in.

Neutral throttle means holding the throttle at an opening that keeps the car at a constant speed, neither accelerating nor decelerating. It does not mean letting the throttle close, or trailing throttle.

Opposite lock means turning the wheel in the direction the car is sliding (your driving instructor may have called this "turning into the skid"). If you're in a right-hand turn, and the tail starts to come around - to your left - then you turn left to correct and, hopefully, prevent the car from spinning.

Oversteer is the condition in which the rear tires are sliding more than the front. The car's tail is coming out; if we don't correct with some opposite lock, the car will spin. This is also known as "loose". The opposite of understeer.

Rebound refers to downward movement of the wheel and tire with respect to the chassis. This is also known as "droop". The opposite of bump.

Ride Height. See Static Ride Height.

Roll refers to the chassis' rotation about its longitudinal axis, typically due to lateral forces generated in cornering.

Shock Absorbers are the same as dampers.

Short Gearing means numerically higher gear ratios. The engine must turn faster for a given speed over the road. Opposite of Tall Gearing.

Slip angle refers to the angle of the tire's travel in relation to its centerline. It's related to the tire's generation of lateral force.

Slip ratio refers to the ratio of the tire's longitudinal slippage under torque (power or brake) in relation to its travel over the track. It's a measure of the tire's longitudinal slippage, and is related to the longitudinal force being generated by the tire. The higher the slip ratio, the more the tire is spinning (under power) or sliding (under braking).

Snap oversteer refers to sudden oversteer which occurs when grip is suddenly lost at the rear. This can be due to large weight transfer away from the inside rear wheel due to bottoming the suspension, or sudden loss of grip on one or both rear wheels due to excessive application of power, or due to the rear wheel(s) locking under braking.

Spring rate refers to the stiffness of the springs in the suspension. With reference to GPL, most of the time when we say "spring rate" we're actually referring to wheel rate. We don't really know the actual spring rates in GPL, since all it provides us is wheel rate. See Spring Rates and Ride Height.

Static Ride Height refers to the height of the chassis above the track surface when the car is sitting still. See Spring Rates and Ride Height.

Squat refers to the chassis' rotation to the rear around its lateral axis under acceleration. Under power, the rear end drops and the nose rises. The opposite of dive.

Tall Gearing means numerically lower gear ratios. The engine must turn slower for a given speed over the road. Opposite of Short Gearing.

Toe-in refers to the angle of the wheel and tire in relation to the car's longitudinal centerline. Positive toe-in means that the front of the wheel points inward in relation to the centerline of the car. Negative toe-in, also called "toe out", means that the front of the wheel points outward.

Traction refers to the grip available during acceleration.

Traction circle is the same as friction circle.

Trail Braking is the technique of continuing to brake after turning into a corner, gradually easing off the brake before turning in and continuing to ease off - "trail off" - the brake as the car rotates toward the apex. The intent is to fill the friction circle, thus using as much of the tires' available grip at all times. Not the same as Left-Foot Braking, although the two are not mutually exclusive; you can trail brake with either foot. See Going Faster or Drive to Win for more details.

Track-out point is the point in the exit of a corner where the car gets closest to the outside edge of the track. See also apex.

Trailing throttle means holding the throttle at an opening that allows the car to decelerate, putting drag on the rear wheels from the engine and drivetrain. It does not necessarily mean letting the throttle close, although a closed throttle is trailing throttle. It's not the same as neutral throttle.

Trailing throttle oversteer or TTO refers to the oversteer induced when the throttle is closed or reduced in opening. If the throttle is closed abruptly, the car can spin due to TTO.

TTO - see trailing throttle oversteer.

Turning-in point is the point in the approach to a corner at which the driver initiates the turn, turning the wheel to make the car aim toward the apex.

Understeer is the condition in which the front tires are sliding more than the rear. The car is tending to go straight on, no matter how much we turn the wheel. It's also known as "tight" or "pushin'". The opposite of oversteer.

Weight jacking refers to diagonally asymmetric static loading of the tires. See cross-weight.

Wheel rate refers to the effective spring rate at the wheel. It's a product of the spring rate and the motion ratio.