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.
