Bump Steer Explained
02-20-07, 10:20 AM
#1
Bump Steer Explained
Been doing a little reading and I found a nice summary of bump steer; just thought I would share.
Bump Steer :
A. 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.
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
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.
1. Set the car at ride height.
2. Use the proper size tires and air pressures.
3. Caster must be set.
4. Camber must be set.
5. Toe in must be set.
6. Tie rod lengths must be set.
7. Steering should be centered (tie rod ends centered on inner pivot points lower ball joints).
8. Steering must be locked down.
9. Measure from the ground to the lower ball joint or other reliable reference point. Write the number down.
10. Remove springs and disconnect the sway bar.
11. Return the suspension to the proper height by using your reference number to the ground.
12. Obtain a supply of bump steer shims.
13. 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.
14. Jack the suspension through 2"-3" of both compression and rebound travel and write down your results.
15. 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 Cure
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.
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 indicator travel.
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 unpredictable.
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 extends.
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 racing surface.
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.
F. Diagram
Bump Steer :
A. 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.
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
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.
1. Set the car at ride height.
2. Use the proper size tires and air pressures.
3. Caster must be set.
4. Camber must be set.
5. Toe in must be set.
6. Tie rod lengths must be set.
7. Steering should be centered (tie rod ends centered on inner pivot points lower ball joints).
8. Steering must be locked down.
9. Measure from the ground to the lower ball joint or other reliable reference point. Write the number down.
10. Remove springs and disconnect the sway bar.
11. Return the suspension to the proper height by using your reference number to the ground.
12. Obtain a supply of bump steer shims.
13. 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.
14. Jack the suspension through 2"-3" of both compression and rebound travel and write down your results.
15. 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 Cure
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.
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 indicator travel.
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 unpredictable.
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 extends.
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 racing surface.
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.
F. Diagram
02-20-07, 06:36 PM
#2
Here's my uderstanding as steering geometry relates to the FD. Essentially you have the upper arm pivot points (blue) and lower arm pivot points (green) and there coinciding ball joints. You essentially draw an imaginary line from the center of the ball joints through the A-arm attachment point on the car for each A-arm and where these lines intersect is the "instant center." Your steering rack should be mounted so that the inner pivot of the tie rod end lies in line with the line intersecting the upper and lower pivot points and the tie rod end should be attached in line with the line intersecting the upper and lower ball joints. Additionally, the tie rod should be angled in such a way so that an imaginary line drawn through it will intersect with the "instant center."
As long as all of the prerequisites are met it seems that the arc of the tie rod end will be in phase with the arc or the suspension components during bound and rebound.
Given that the inner pivots cannot be adjusted on the tie rods, the relocation of the steering rack, for a motor swap for example, necesitates that extensions be used to compensate for the shifting of the inner pivot point of the tie rod. If a steering rack is lowered, such as in the case of an LS1 swap, the tie rods must be extended both outwards and down so that the tie rod angles will intersect the imaginary instant center and therefore maintain proper arc.
This is my very basic understanding, though I'm sure that there are others who can touch a bit deeper on the subject. I would be interested to read more about the Supra7 to understand why they were never able to correct the steering geometry of the car.
As long as all of the prerequisites are met it seems that the arc of the tie rod end will be in phase with the arc or the suspension components during bound and rebound.
Given that the inner pivots cannot be adjusted on the tie rods, the relocation of the steering rack, for a motor swap for example, necesitates that extensions be used to compensate for the shifting of the inner pivot point of the tie rod. If a steering rack is lowered, such as in the case of an LS1 swap, the tie rods must be extended both outwards and down so that the tie rod angles will intersect the imaginary instant center and therefore maintain proper arc.
This is my very basic understanding, though I'm sure that there are others who can touch a bit deeper on the subject. I would be interested to read more about the Supra7 to understand why they were never able to correct the steering geometry of the car.
02-20-07, 07:17 PM
#4
I know it's difficult to tell by the pictures, but the Supras rack does not look significantly lower than the stock rack. A good way to gauge this is to follow the shape of the suframe with your eye and just transpose the stock subframe down over the Supra & SF with your imagination. They are really quite similar.
02-20-07, 08:44 PM
#6
NP ;o)
Here's a little more info for ya on a Jpeg that I made:
In order to prove the relation of the arms to the instant center I made a model out of paper creating the proper pivot points and fixed distances between ball joints and pivoting points. Since the steering tie rod end is fixed to the wheel up right it's distance from the hub is therefore fixed and it will always point towards the instant center. If you don't believe me you can try it for yourself
Here's a little more info for ya on a Jpeg that I made:
In order to prove the relation of the arms to the instant center I made a model out of paper creating the proper pivot points and fixed distances between ball joints and pivoting points. Since the steering tie rod end is fixed to the wheel up right it's distance from the hub is therefore fixed and it will always point towards the instant center. If you don't believe me you can try it for yourself
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02-21-07, 10:55 AM
#9
corrected version
So the above gave a pretty decent description of straight-ahead bump steer. The next thing to think about is how the steering geometry behaves during a turn. Even with a car that is engineered to have 0 bump steer when the wheels are pointed straight ahead, during a turn the inner pivot points of the tie rods are shifted (since they are tied to the rack) thus the tie rod arcs will not match the suspension arcs. It only makes sense that a toe out condition would occur at the outer wheel (during suspension compression) whose spindle is being pulled towards the rack while the inner wheel would toe in slightly (though not as much since most weight is transferred to the outer wheel during a turn). In fact, if enough weight was transferred off of the inner wheel it would toe out as well since the susension would be expanding rather than compressing. Ofcourse this is just my interpretation, I'll have to do more reading to confirm, but it seems intuitive for this to be the case.
I verified this inner pivot shift / toe in-out relationship by shifting the pivot point of pencil relative to a book and watching as the pencil reached further across the books spine when its pivot came inward and vice-versa when the pivot was shifted in the opposite direction.
I verified this inner pivot shift / toe in-out relationship by shifting the pivot point of pencil relative to a book and watching as the pencil reached further across the books spine when its pivot came inward and vice-versa when the pivot was shifted in the opposite direction.
Last edited by wanklin; 02-21-07 at 11:03 AM.
02-26-07, 08:58 PM
#10
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as always, steering geometry is a compromise. designing the right steering arm location involves not only bump steer, but also ackermann geometry (which can be simplified by viewing from the top). this usually means its not ideal to put the outer point directly in-line with the LCA pivot point as it usually means parallel steer (especially with cars with the steering rack in front of the steering axis where it would be easier to get reverse ackermann than pro-ackermann)
03-02-07, 05:05 PM
#11
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Not an RX7 but this shows the spindles arc travel with unequil length A-arms.
http://i22.photobucket.com/albums/b3...k/chassis2.swf
Great find Rob, thanks for sharing.
http://www.moddedmustangs.com/forums...ed-vt875.html?
http://i22.photobucket.com/albums/b3...k/chassis2.swf
Great find Rob, thanks for sharing.
http://www.moddedmustangs.com/forums...ed-vt875.html?
03-03-07, 02:08 PM
#12
Actually got it from a suspension site, but that´s pretty much it :o)
beautiful arc link Steve; a nice visual to show how camber changes as the suspension moves through its range of motion. If i was a computer wiz i´d whip one up to show the relationship between the steering arc and the A-arms as above.
beautiful arc link Steve; a nice visual to show how camber changes as the suspension moves through its range of motion. If i was a computer wiz i´d whip one up to show the relationship between the steering arc and the A-arms as above.
