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Bump Steer |
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| Table of
Contents |
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- Bump Steer Definition
- Preparing the Car for Bump Steer
Measurement
- Making Bump Steer Corrections
- Using the Bump Steer Gauge
- How Much Bump Steer?
- Diagram
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| A. Bump Steer Definition |
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| 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 linkages. |
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| 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. |
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| 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). |
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| 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. |
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| Sounds
complicated? 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. |
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| 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 through travel. |
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I. Bump Simplified |
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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. |
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| B.
Preparing the Car for Bump Steer Measurement |
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| 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. |
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- Set the car at ride height.
- Use the proper size tires and air
pressures.
- 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 number down.
- 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 shims.
- 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.
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| C. Making
Bump Steer Corrections |
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| 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. |
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Symptom |
Cure |
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Symptom 1. Toes out
in compression and in on rebound all in one direction. |
Cure 1. Decrease
shim on outer tie rod or lower the inner tie rod. |
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Symptom 2. Toes
in on compression and out in rebound all in one direction. |
Cure 2. More
shim at outer tie rod or raise the inner tie rod. |
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Symptom 3. Always
toes in both compression and rebound. |
Cure
3. Lengthen the tie rod as it is too short. |
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Symptom 4. Always
toes out on compression and rebound. |
Cure
4. Shorten tie rod as it is too long. |
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Symptom 5. Toes
out on compression, then in on rebound and then starts back towards out
with more rebound travel. |
Cure
5. Less shim at outer tie rod and shorten tie rod. |
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Symptom 6. Toes
in on compression, then moves out on rebound and then starts back
towards in with more rebound travel. |
Cure
6. More shim at outer tie rod and lengthen tie rod. |
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| D. Using
the Bump Steer Gauge |
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| 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. |
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| 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 indicator travel. |
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| 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. |
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| 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. |
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| E. How
Much Bump Steer? |
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| 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 unpredictable. |
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| Some bump out
can make the car more stable on corner entry. Bump in is almost always
undesirable. |
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| 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
extends. |
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| My
recommendation is to run .005 to .015 thousands of bump out but never
allow the tires to bump in. |
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| 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 racing surface. |
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| 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 car down. |
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| F. Diagram |
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