Note how the front of Terry Labontes Chevrolet is plastered to the track in this sho
Anti-dive is a suspension parameter that affects the amount of suspension deflection when the brakes are applied. When a car is decelerating due to braking there is a load transfer off the rear wheels and onto the front wheels proportional to the center of gravity height, the deceleration rate and inversely proportional to the wheelbase. If there is no anti-dive present, the vehicle suspension will deflect purely as a function of the wheel rate. This means only the spring rate is controlling this motion. As anti-dive is added, a portion of the load transfer is resisted by the suspension arms. The spring and the suspension arms are sharing the load in some proportion. If a point is reached called 100-percent anti-dive, all of the load transfer is resisted by the suspension arms and none is carried through the springs. When this happens there is no suspension deflection due to braking and no visible brake dive. There is still load transfer onto the wheels, but the chassis does not pitch nose down.
The method to achieve anti-dive is controlled by the upper and lower control-arm pivot points on the chassis. The exact determination is quite involved and is easily established with a good three-dimensional geometry computer program. In all suspensions there is a factor called the side view swing arm. This is a theoretical point of intersection of the arm planes projected into the fore-aft vertical plane through the wheel center. If this point is behind the wheel and above the ground, it will produce anti-dive. It will also provide anti-dive if it is below ground and ahead of the wheel. The other possible locations for this instant center are ahead and above ground as well as behind and below the ground. With these locations we have pro-dive. This means as the brakes are applied the suspension travels more than it would without any anti- features. The magnitude of the anti-feature, either pro or anti, is a function of how far the instant center is away from the ground. If it is on the ground, there is zero-percent anti-feature.
On passenger cars, anti-dive is added to make the pitch motions under braking more tolerable for the occupants because of the typical soft spring rates. The anti-dive is made the same for both sides of the car because most people brake in a straight line, and there is no directional preference on the street or in road racing for that matter. In circle-track racing it is common for racers to take any parameter to and beyond its limit.
In the evolution of the circle-track chassis, they have come to a solution that provides pro-dive on the left-front and anti-dive on the right-front suspension. The reasons for this are varied. For one thing, as the brakes are applied the car will deflect more on the left front than on the right. This will give a feeling of the front leading into the corner and not rolling out right away. This combination also provides for more of the load transfer to the front to be carried by the right-front wheel than the left front. The total transfer is the same; just the distribution is affected by the anti-feature in the geometry. More load transfer to the right front on corner entry is like adding wedge with the brakes on and is a stabilizing effect.
Anti-dive has an effect when there is a torque trying to rotate the spindle relative to the chassis. The typical case for this is under braking, but there is another case that can alter the loads and deflections of the wheels. This is the drag component of cornering load. When the wheels are turned and the tires are cornering hard there is a longitudinal load relative to the chassis centerline trying to rotate the spindle. To visualize this, think of the wheel turned 90 degrees!
Just like anti-dive in the front suspension, there can be anti-lift in the rear suspension that reduces rebound travel under braking. There is also anti-squat in the rear suspension under acceleration for rear-wheel-drive cars.
Circle Track Technical Council member Terry Satchell is a chassis specialist for Ford Motorsports based in Hendersonville, N.C.