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Thread: Bump Steer!

  1. #1
    I wouldn't trust a... Conversion King Smokey228's Avatar
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    Default Bump Steer!

    Okay guys, I have been trying to workout the big fuss of bumpsteer... So i done alot of searching and have colaborated a bit of info on it... Happy Reading!
    Some useful sites are:
    http://www.racerpartswholesale.com/longtech3.htm
    http://www.bakerprecision.com/longacr17a.htm
    http://www.steeda.com/PR/Mustang/bum...bumpsteer.html

    And the most useful two were deffinetly:
    www.google.com
    www.wikipedia.com

    Bump Steer

    Definition: The tendency of a vehicle to suddenly veer or swerve to one side when hitting a bump or dip in the road. The condition is caused by uneven toe changes that occur as a result of the steering linkage or rack not being parallel with the road surface. This causes the wheels to change toe unevenly as the suspension undergoes bounce and rebound.

    Another 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/angle of your suspension and steering linkages.

    Okay, so if your getting the idea great, if your not, then heres an example! Lets say your driving through a parking lot. The right side of the car has a speed bump and you try to go around. You miss, the right front tire hits the speed bump and your car steers itself to the right even though you didnt adjust the steering wheel angle.
    So its not that bad, well we all know what body roll is! Now imagine you are going to turn a corner and you choose to go through the corner at a very high speed. As you approach the corner you turn the steering wheel in and the car begins to roll. As you have the steering wheel set in position to carry you through the turn, the car continues as the suspension compresses on the loaded side of the car. The front wheel turns in more than you had originally intended, causing the car to turn more sharply. You correct the steering wheel in the opposite direction pulling the car out of its roll and you miss your intended path through the turn.

    So, when the suspension travels up and down, the wheel moves up and down in an arc or half circle around a pivot point. This pivot point is not a physical location or suspension part but is an imaginary point on the chassis, similar to a vehicle's center of gravity. The distance from the pivot point to the wheel and spindle is equal at all points in the wheel's range of vertical movement. The tie rod and steering rack must be positioned so that as the wheel moves up and down, the tie rod follows an arc which is parallel to the arc followed by the spindle. If the steering rack or tie rod is not positioned correctly, the distance between the steering rack and wheel may differ from the distance between the pivot point to the wheel at different suspension heights. When this occurs the steering rack will push or pull the spindle as the suspension moves up or down thereby turning the wheel and causing a change in toe angle.

    The direction the front wheels are pointed is called toe angle. When both wheels are pointed parallel there is 0 degrees toe. When the front of the wheels are pointed inward, there would be positive degree toe or what is called toe in. When the front of the wheels are pointed outward there is negative degree toe or what is called toe out.

    Most car builders design their cars so that the effects of bump steer are minimal. However, you still gotta be careful when you bolt on your suspension, so as not to create unwanted bump steer. Make sure that you are always using the correct components for your 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.

    Okay, So that should clear that up... So checking bumpsteer...

    Checking Bump Steer

    For this your going to need:
    -some sort of block to rest the car on at 'ride height'.
    -a sheet of aluminium with your pcd drilled into it.
    -a Bump Steer Gauge.
    -a Car jack

    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.

    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.

    Okay, So firstly you must determine the position of the suspension system at ride height. This is a starting point as this is where normal toe-in is set. Later you are going to put the car on a ride height block and remove the front wheel and spring.

    1. There are two easy ways to determine the front suspension position
      1. Measure the length of the shock absorber.
      2. Measure the angle of the upper A frame.

      Choose one and take the measurement with the car at ride height. Also measure the frame-to-ground clearance at this time.
    2. Make a ride height block from wood (or anything solid) the same as frame-to-ground to place under the frame when you take the front wheel off.
    3. Put the ride height block under the frame and remove the wheel and spring. Also disconnect the sway bar. Place a hydraulic (or floor) jack under the A frame and raise the suspension to ride height by measuring the shock length or A frame angle as before.
    4. Bolt the aluminum plate to the hub. If necessary you can drill other holes for your particular car. (After you're done, keep the bolts with the set for future use.) Rotate the plate so that the small level shows horizontal.
    5. Lean the bump steer gauge (tubular frame with dial indicator) against the plate at a slight angle (approx. 15) so that the upper tube is parallel with the plate and the dial indicator tip and roller bearing contact the plate. Loosen the knurled knobs adjust the height so the dial indicator tip and roller bearing are next to 0" on the plate scale. Rotate the dial on the dial indicator to 0".
    6. Using your hydraulic jack move the suspension system until the dial indicator is at the 1" mark. Note which direction the dial indicator moves to see if the wheel toes in or out. The reading you get on the dial indicator is the actual amount of bump steer. It's that simple. Record this and go onto to 2" and 3". If you want to make a more detailed measurement, you can take readings every 1/4" or 1/2" and plot a graph.Note: When you are taking measurements the steering must not move. If necessary, lock the steering in place. Also, the hub and plate must not rotate. If it does, turn it back to horizontal using the small level.
    7. Now go back to 0" and take similar measurements as the suspension systems moves down. Repeat this procedure on the other side of the car. Keep the readings as part of your records.
    8. To adjust bump steer refer to your setup sheet or manual, or there are several good books on the market that describe how to make adjustments. Or im sure someone on these forums will be of some help to atleast point you in the right direction!
    9. When you are done with the adjustments, re-check as before.


    Cheers all, hope it was as useful to you as it was to me!
    Jase
    JZA023
    11.51 @ 126MPH | 430RWHP @ 21PSI

    Quote Originally Posted by YLD-16L
    Cuzzo is such a premature ejaculator he fertilised the very egg that he grew from

  2. #2
    Forum Sponsor Carport Converter TurboRA28's Avatar
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    Default Re: Bump Steer!

    Get TA-022 to take a pic when he is using my homemade bump steer gauge
    1977 RA28 Celica - 1MZ-FE Members Rides
    1996 FZJ80 Landcruiser.
    Email : [email protected]

  3. #3
    Forum Sponsor Carport Converter TurboRA28's Avatar
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    Default Re: Bump Steer!

    Also that diagram while good for a double wishbone setup..

    If you are talking macpherson strut, then the most simple way for zero bump steer is to have the control arms and steering arms in a paralelogram. But thats only good if you can move components around , cutting & welding etc.

    Good info though, + rep for it
    1977 RA28 Celica - 1MZ-FE Members Rides
    1996 FZJ80 Landcruiser.
    Email : [email protected]

  4. #4
    I wouldn't trust a... Conversion King Smokey228's Avatar
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    Default Re: Bump Steer!

    Tah man, i borrowed it off a site jus for reference.. Im still trying to get my head around all this...
    and +rep for you too cause your wacky ideas got me into this, but i gotta share more love still..

    Jase
    JZA023
    11.51 @ 126MPH | 430RWHP @ 21PSI

    Quote Originally Posted by YLD-16L
    Cuzzo is such a premature ejaculator he fertilised the very egg that he grew from

  5. #5
    Junior Member Domestic Engineer
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    Default Re: Bump Steer!

    i am currently reading through 'how to make your car handle' by fred phun
    his method for measuring bumpsteer is as such...
    grab 2 pieces of wood and a piano hinge and connect them together.
    then on one of the boards connect two bolts to it that you can adjust so they are just sitting against the rim....pictures speak a thousand words soooo.....



    set up the board so the bolts are just resting on the rim with the suspension at the full bump position (fully lowered) then raise the chassis gradually through the range of suspension travel to the full droop positon. measure the distance that the bolts come away from the rim at each of these stages to get an idea of bumpsteer at any position of the chassis movement.
    Note that the wheels stay on the ground at all times, the only thing that you should notice is the board moving away from the wheel with a gap between one of the bolts at each movement. (indicating toe in and toe out at verious stages of suspension travel)

    ideally the bolts won't move throughout the range of travel indicating no toe change over the range of suspension travel, if it does then possible suggestions for fixing the issues including adjusting the height of the steering rack or changing the angles on the steering arms (done professionally to maintain strength in the part).

    note the easiest way to measure the full suspension travel is to remove the springs.

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