Set Up with a Limited Slip

To set up a car with any one of the limited slip or torque biasing differentials described, some things need to be considered. Unlike an open diff, roll stiffness at the front may be reduced to perhaps 60 percent. If an under-steering condition on corner entry is a problem, the differential is most likely not unlocking sufficiently to allow enough wheel speed variation. Use less shim on a Quaife or Salisbury and a higher ramp angle (closer to 90 degrees) on the coast ramp of a ramp-equipped Salisbury. This recommendation assumes that corner weights are properly set and no stagger exists. If under-steer is encountered-beginning with power application in a turn, you need to use more rubber up front, more track width, some Ackerman steering, or toe out. Some teams that change differentials as part of a chassis set up use different wheel offsets or wider tires when going to a differential with more lockup. A shock problem could also account for this under-steer that only shows up when more differential lockup is used (as many Ralt RT-5 owners know). If under-steer occurs all the way through the corner, it is most likely corner-entry under-steer, of which the driver was not able to rid himself through his control inputs. Cure the corner entry under-steer and have a talk with the driver.

Differential-related over-steer is always on corner exit and usually a result of wheel spin. If you have a wheel spin problem in slow- to medium-speed corners) adjust the diff for more lockup. If you can go to a fully locked diff and still have wheel spin, you need less rear roll stiffness, more or softer rear rubber, or droop limiters on the front. Only when you have exhausted these possibilities would you start using traction control. Traction control works, but it works just like a driver being tamer with his right foot. Since the object is to set up the car so that the driver can keep the throttle open through as much of the lap as possible, it seems to be going backwards to install a device that makes him back off on corner exit. Traction control has its place, but only after the other factors at our control have been optimized.

Conclusion

To be really fast, we must allow the driving wheels to:

1.    uncouple when the throttle is lifted

2.    Lock together smoothly and progressively when it is reapplied

3.    Avoid under-steer when the throttle is wide open again.

The payoff will be a car that is more stable, responsive and easier to drive at a higher limit.

 

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