iflyM3
11-29-2006, 06:00 PM
What Exactly Is Bump Steer?
I used to get asked this one all of the time, and I never really could explain it well. However, I found this neat little article that explains it very well.
Table of Contents
Bump Steer Definition
Preparing the Car for Bump Steer Measurement
Making Bump Steer Corrections
Using the Bump Steer Gauge
How Much Bump Steer?
Diagram
A. Bump Steer Definition
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.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.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. 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.
I. 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 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.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.
SymptomCure
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.
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.
Symptom 3.
Always toes in both compression and rebound.
Cure 3.
Lengthen the tie rod as it is too short.
Symptom 4.
Always toes out on compression and rebound.
Cure 4.
Shorten tie rod as it is too long.
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.
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.
I used to get asked this one all of the time, and I never really could explain it well. However, I found this neat little article that explains it very well.
Table of Contents
Bump Steer Definition
Preparing the Car for Bump Steer Measurement
Making Bump Steer Corrections
Using the Bump Steer Gauge
How Much Bump Steer?
Diagram
A. Bump Steer Definition
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.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.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. 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.
I. 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 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.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.
SymptomCure
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.
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.
Symptom 3.
Always toes in both compression and rebound.
Cure 3.
Lengthen the tie rod as it is too short.
Symptom 4.
Always toes out on compression and rebound.
Cure 4.
Shorten tie rod as it is too long.
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.
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.