Rear Stiff Shock Absorber Set-Up Might Loosen Corner Entry and Tighten Corner Exit.
The idea is that with this one change we hope to
get the race car to turn in better and
also get the power down earlier on the corner exit. We are assuming a smooth road
surface, so slow speed settings on the shocks affect weight transfer. For the shocks
to wedge or de-wedge the car, we need the car to be moving in roll, or a combination of
roll and pitch. Where there is sufficient wheel travel happening, the shock force
add to roll and warp resistance from springs and anti-roll bars, and pitch resistance from
the springs. We look for what wheels are moving, and work out the effect of
the shock forces.
The change to the car has been to stiffen
slow speed damping at the rear, relative to
the front. Even if we do not have slow speed damping adjustment on our shocks, we
should still get good effect from this set up by increasing the rear bump settings and/or
decreasing front rebound. It is important that we don't go too stiff in rear bump,
because then we won't be able to get the power down because the rear tyres lose
proper contact with the road - quite aside from the weight transfer advantage we are
The progression in the corner, starting
from hard straight line acceleration is:
Hard Braking. Very large weight transfer
in pitch, rear to front.
Turn In. The driver modulates the brake
pedal and applies steering input at the same time. ie decreasing braking and
increasing steering input. As brake force is reduced, the car pitches towards the
rear. Race drivers will be familiar with the feeling of the outside front suspension
staying down (compressed) as you trail brake and turn in. So
the rearward pitch will be diagonal weight transfer from the inside front to outside
This will reduce inside rear percentage ie de-wedge the car. This movement of the
car is fairly momentary, so the stiff rear shock speeds up the weight transfer. A further
influence of the stiffer rear shocks is to add rear roll stiffness while roll is
building - helps de-wedge the car. ie to de-wedge (or loosen) the car on corner
Mid Corner. Steady state.
Shocks do not have any affect. The car is wedged a bit to allow some rear tyre
traction for the coming power application.
Applying the Power and Corner Exit.
The car is in cornering and wedged as the power comes on. The car pitches,
transferring weight front to rear as the rear tyre thrust increases. This weight
transfer further wedges the car as per the weight transfer example.
The rear stiff shocks will transfer the weight faster. ie wedge (tighten) the car on
corner exit. To get the most from this effect, rather than just rear stiff shocks,
attention should also be given to having less front slow speed rebound. Neil
(see shock tuning) describes how this works. As we
"let the car go" (reduce steering angle), the weight will come off the outside
front - this will probably be the shock with the greatest movement - and increase front
inside percentage (ie move towards oversteer). Less slow speed rebound will
slow down this weight transfer and contribute to understeer.
1. This effect only allows you to apply a little more power sooner. The
car should still move to oversteer with increasing rear tyre thrust as
explained here. yawaxis.html
2. This application for shock tuning
comes from Mark Oritz, writing in
Race Car Engineering. The above explanation is my own. He says
the oppostite set up is also valid. Moving to a front stiff shock set up
would tighten entry and loosen exit. He says the effect can be applied
to all cars, including performance road cars.
3. To understand some of the terms used (wedge, rear stiff etc) read