is Bump Steer?
Bump Steer is when
your wheels steer themselves without input from the steering wheel.
The undesirable steering is caused by bumps in the track interacting
with improper length or angle of your suspension and steering
Most car builders
design their cars so that the effects of bump steer are minimal.
However, you must still take care to bolt on your suspension
carefully so as not to create unwanted bump steer. Make sure that
you are always using the correct components for a particular car.
Bump steer must be designed into the car and cannot be adjusted out
if improper parts are used or if pivot points are moved without
considering bump steer design principles.
In order to
accomplish zero bump the tie rod must fall between an imaginary line
that runs from the upper ball joint through the lower ball joint and
an imaginary line that runs through the upper a-arm pivot and the
lower control arm pivot. In addition, the centerline of the tie rod
must intersect with the instant center created by the upper a-arm
and the lower control arm (See diagram below).
The instant center is
an imaginary point that is created by drawing a line from the upper
a-arm ball joint through the a-arm pivot where it is intersected by
an imaginary line that extends from the lower ball joint through the
inner control arm pivot. Where the two imaginary lines intersect is
the instant center.
Really it is very simple. To achieve zero bump the front end must be
designed correctly. The tie rod must travel on the same arc as the
suspension when the car goes through travel. Simply matching lengths
and arcs to prevent any unwanted steering of the front tires.
To exaggerate, if the
tie rod were only 10" long and the suspension were 20"
long then when the suspension traveled the tie rod angle would
shorten much quicker than the suspension arc. In this scenario the
tie rod would shorten much quicker through travel than the
suspension and the car would toe in drastically over bumps. The
shorter arc of the tie rod would pull on the spindle and toe it in
Bump Simplified -
When designing a car, if the centerline of the outer tie rod lines
up with the centerline of the lower ball joint, and the inter tie
rod lines up with the lower pivot point then the length and angle of
the tie rod and suspension will be the same resulting in zero bump.
Most car builders design their cars in this fashion.
B. Preparing the Car
for Bump Steer Measurement
Your front suspension
must be complete and set for racetrack conditions before you can
measure the bump steer. All components must be tight and in proper
position and you will need a quality bump steer gauge.
- Set the car at ride height.
- Use the proper size tires and
- Caster must be set.
- Camber must be set.
- Toe in must be set.
- Tie rod lengths must be set.
- Steering should be centered
(tie rod ends centered on inner pivot points lower ball joints).
- Steering must be locked down.
- Measure from the ground to the
lower ball joint or other reliable reference point. Write the
- Remove springs and disconnect
the sway bar.
- Return the suspension to the
proper height by using your reference number to the ground.
- Obtain a supply of bump steer
- Bolt on the bump steer plate to
the hub. Level the plate and note where the dial indicator is on
the bump steer plate so that you can quickly return to the
correct ride height.
- Jack the suspension through
2"-3" of both compression and rebound travel and write
down your results.
- Shim as needed.
C. Making Bump Steer Corrections
Now that you have measured your
bump steer you will need to adjust, shim or relocate the suspension
components to get the exact reading that you desire. Below are some
tips that will quickly guide you through the corrective process for
cars with front steer style suspension.
||Symptom 1. Toes
out in compression and in on rebound all in one direction.
Decrease shim on outer tie rod or lower the inner tie rod.
2. Toes in on compression and out in rebound all in
More shim at outer tie rod or raise the inner tie rod.
3. Always toes in both compression and rebound.
3. Lengthen the tie rod as it is too short.
4. Always toes out on compression and rebound.
4. Shorten tie rod as it is too long.
5. Toes out on compression, then in on rebound and
then starts back towards out with more rebound travel.
5. Less shim at outer tie rod and shorten tie rod.
6. Toes in on compression, then moves out on rebound
and then starts back towards in with more rebound travel.
6. More shim at outer tie rod and lengthen tie rod.
D. Using the Bump Steer Gauge
Selecting a good bump steer gauge
makes the process easier. I like the bump steer gauges that utilize
only one dial indicator. One dial indicator bump gauges do the math
for you and you avoid having to watch two dial indicators move at
the same time. Sometimes when the bump is way out of adjustment it
takes two people to watch both of the indicators. The one indicator
design is much easier to use.
When you set up your bump steer
gauge with the car at the proper height set the dial indicator at
the center of the bump steer plate and be sure that the indicator is
set in the middle of its range. You want to avoid running out of
Once the indicator is set simply
jack the suspension through 2"-3" of compression. Stop at
each inch and record your reading. Repeat the process through
rebound and record those numbers at each 1-inch interval.
If the front of the bump steer
plate is moving towards the engine then you have a bump in
condition. If the front of the plate moves away from the engine then
you have bump out. The dial indicator will see small amounts so
watch it carefully and note your results.
E. How Much Bump Steer ?
Ideally you should run as little
bump steer as possible. Most of the tracks we see today are old and
bumpy. Bump steer on these rough surfaces causes the car to be
Some bump out can make the car
more stable on corner entry. Bump in is almost always undesirable.
Some people use small amounts of
bump out to create entry stability and an Ackerman type effect in
the center of the turn where as the bump setting causes the LF to
turn a bit farther than the RF as the RF compresses and the LF
My recommendation is to run .005
to .015 thousands of bump out but never allow the tires to bump in.
If you want Ackerman in the center
of the turn then add Ackerman while maintaining proper bump. If you
use bump to obtain some Ackerman effect the car will be unsettled as
it goes over each bump, which will break the contact patch from the
If the design of your car does not
allow for such precise bump adjustments then more bump out is better
than any bump in. However, strive to get the best bump numbers even
it if means replacing parts. Excessive bump over .050 can slow your
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