A useful design tool is antidive....
A useful design tool is antidive. Antidive keeps the front suspension from moving too quickly and too far. It only works while we are braking, and that is exactly when we need it to work. Antidive is accomplished by angling the control arms from a side view so that the front is higher than the rear. Slotted upper control arm mounts using slugs make setting antidive easy.
4. Rear Steer Some rear suspension systems can be adjusted for rear steer. If the adjustments are such that the rear end is made to move so that the RR wheel is farther to the rear than the LR wheel, we have rear steer to the right. This makes the car very loose and we need to adjust the suspension components, such as the trailing arm angles, so it will not steer in this way.
If the car is a dirt Late Model and the track is very dry-slick, a small amount of this type of rear steer can actually help the car keep grip with the rear tires. This is a special case, and in almost every other situation, we try not to steer the rear of the car to the right, be it dirt or asphalt.
A car that is tight on entry, once the middle phase has been sorted out, could be negatively influenced by conditions that are only caused by the motion and dynamics of braking. Braking can contribute to conditions such as excess camber change, improper load redistribution, excess dive (causing bumpsteer), or chassis compliance problems (bending of components) as well as the obvious brake bias adjustment.
1. Improper Shock Rates The shocks control the motion of the suspension during the transition phases. Corner entry is one of these phases, and as the car slows and turns left, the loads on the tires are redistributed.
If the car is tight on entry, the LR shock may be too soft in rebound or the RF shock may be too stiff in compression. Either will tend to load the RF and LR tires on initial entry under hard braking. The sum of the loads carried by these two corners make up the crossweight percentage, and anytime this percentage is increased, we tighten the car. To fix this, increase the rebound in the LR shock and/or soften the compression in the RF shock.
Chassis roll moves the upper...
Chassis roll moves the upper ball joints to the right, changing the front cambers so that the LF loses positive camber and the RF loses negative camber.
2. Spring Rates The spring rates in the car may contribute to a tight condition on entry. If the front spring rates are split so that the RF spring rate is higher than the LF spring rate, then an effect similar to that described above (with the shock rates) takes place while braking.
The stiffer RF spring causes the crossweight percentage to go up as we enter and brake into the corner and the front end dives. Reducing the spring split across the front to reduce the effect of increased crossweight percentage during braking on entry can make the car more neutral on entry.
3. Brake Bias Imbalance Again, make sure your brake bias is tuned correctly. If too much of the bias is on the front brakes, the car will push under heavy braking. If indeed brake bias is a problem, then lightly brake into the corner; it should get better.
4. Lack of Antidive If the car is diving excessively under heavy braking, the RF wheel will lose camber quickly and the tire will lose both footprint and traction. Antidive properties can help the situation. Although the camber change is quick and the wheel's camber settings return to normal a short time later, once a push starts, it is hard to stop it without slowing down.
Chassis dive pulls both top...
Chassis dive pulls both top ball joints toward the centerline of the car and changes the camber, with the LF losing positive camber and the RF gaining negative camber. The action of rolling and diving can cancel each other out at the RF wheel and result in a desired lack of change in camber through the turns.
For the BBSS asphalt setups, the LF wheel loses a good deal of camber and the RF wheel gains a lot of camber.
We want to eliminate as much of the camber change as possible by designing antidive into our front suspension. If we angle (from a side view) the upper control arm shaft that is mounted to the chassis so that the front is higher than the rear, we will have introduced some amount of antidive into the front suspension. We can also create more antidive by raising the rear chassis mounting point of the lower control arm, although this has less effect than angling the upper control arm shaft.
5. RF Camber Change The RF wheel will always experience movement as the car enters and rolls into the turns. If the front geometry is not designed correctly, then the RF camber relative to the racing surface will change as the car dives and rolls into the turns, and the tire will lose grip.
In testing over a five-year period, it was discovered that most tires "want" little or no change in RF camber relative to the racetrack surface as the car dives and rolls. We cannot read this type of camber change merely from bumping the wheel because the added effect of chassis roll on camber change is missing when we do that. Many of the geometry software programs on the market will allow you to simulate chassis movement and see the true camber change.
We have now tuned the car for the middle phase first, then the entry phase. This represents the most logical progression in refining the setup in our cars. The middle handling influences both entry and exit, and entry performance can influence midturn handling.
In Part II of "Basic Handling Solutions," we will explore the problems associated with the exit phase. Many racers believe the problems that occur here can be the most detrimental to performance in an actual race. That is because in practice, we never have to pass another car in "anger."
Problems involving the exit while accelerating can be managed better when we are by ourselves, but in the race, we need the car to turn well and to have good bite as we drive up and under another car. If the push or "loose-off" condition rears its ugly head, it will prevent us from moving up through the field.