XRAY - Model racing cars

Why do you put shims underneath the outer steering rod ends where they are mounted to the steering blocks?

Shims are used to get rid of the undesirable handling effect called "Bump Steer". This effect happens when a cars front toe changes, as its suspension compresses or rebounds, from the position that was pre-set using a setup station. The change can cause the front wheels to either increase or decrease in toe. To eliminate bump steer, it is necessary to change the angle of the steering rods using shims in between the outer steering rod ends and the steering blocks.

Here is a simple process to figure out if a car has bump steer and how to minimize or eliminate it:

1. Make sure the car is fully setup, has been de-tweaked, aligned and is race ready. Only the tires, body and clips should be removed.
2. Set up the car on a setup station
3. Press down and quickly release the front shock tower to get the suspension to settle
4. Use the setup stations toe gauge to check the front toe while the suspension is at rest and reset it to the desirable amount of front toe
5. Press down on the front shock tower until the front of the chassis is approximately 1 to 2 millimeter's above the surface and use the setup stations toe gauge to see if the front toe has changed
6. If the front toe has changed, then add or remove shims underneath the outer steering rod end in 1 millimeter increments
7. Repeat steps 5 and 6 until the front toe is the same with the suspension at rest or compressed. A barely noticeable change is acceptable, but you may need to use .5 millimeter increments to completely eliminate it.

Note: Bump steer is a suspension tuning option commonly used in off-road RC racing to change steering characteristics over rough and loose terrain. However, it is recommended to eliminate bump steer in touring car suspensions.