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Tech Article #9 - BUMP STEER GAUGE INSTRUCTIONS

by Kevin Smith

A common front suspension problem in connection with the Bump Steer results when the steering tie rod moves in a path that is unlike the path in which what it is connected to is moving.  As an independent front suspension travels through the bump and rebound, a change can take place in the toe-in or toe-out.  During the bump check, measure for its existence and make corrections in the steering mounting points.  On the following pages, you will find the instructions for checking your bump steer.

CHASSIS SET-UP

BUMP STEER-As a front wheel moves up and down through its suspension travel, unless the steering box is directly connected to the A frame, the wheel will tend to turn either left or right when the steering wheel is held firmly in the same position.  This tendency of the front wheel to turn during suspension travel even though the steering wheel isn’t moving is called “bump steer”.  It is caused by the fact that the steering box is attached to the chassis and doesn’t move while its steering rods are connected to the steering arms, which do move up and down.  In some supermodifieds, attaching the steering box directly to the front axle eliminates bump steer.  In most other cars, bump steer is a fact of life. 

Most stock car chassis experts feel that the amount of bump steer in a racecar should be minimized.  However, another opinion has it that if the bump steer is in the correct direction (say the car has a tendency to turn left when the brakes are applied as on entry to the turn.  It’s a big help in handling.  If bump steer is in the incorrect direction (say the front wheels turn right when the brakes are applied on entry to the turn) the car is very difficult to drive.  Bump steer might fight everything that the driver is trying to accomplish with the steering wheel.  In general, the rougher the racetrack’s surface, the more important it is to minimize bump steer.  Just as the name implies, bump steer causes the car to turn itself when a wheel encounters unevenness. To measure bump steer, first set up the chassis with caster, camber, and toe-out all being correctly adjusted.  Then unhook the front shocks, springs and anti-roll bar.  Put the car up in the air on four jack stands.  Lock the steering wheel straight ahead.  Remove the tire and wheel and bolt a flat plate to the hub.

With the spindle about half an inch below its normal ride height, adjust the dial indicators on the right and left of the gauge so they are level.  Then measure the orientation of the plate with respect to the bump steer gauge by setting both dial indicators to zero.  This will be your baseline.  Jack the spindle and hub assembly up one inch and read the changes seen on the gauges.  The difference between the gauges is the measure of bump steer.  I suggest that with late models and modifieds, the left front should bump .030” out and the right front should bump .015” out in one inch of upward spindle travel.  I recommended that with cars using stock spindles, such as GN or NASCAR LMS cars, the left should bump .030” out and the right front should bump .015" out.  If the cars bump steer is off, it can be adjusted in most cases.  If you are working with fabricated spindles and a rack whose height can be moved up and down, adjusting the height of the rack with respect to the height of the spindles through the use of spacers is the solution.  If the steering box’s height can’t be adjusted (if it isn’t a rack, chances are that it can’t be adjusted) and if the steering rods join the steering arms with tapered rod ends, adjusting bump is very difficult, it can only be accomplished by heating and bending the car’s steering arms.  Even with that kind of effort, bump is still pretty tough to get right on the numbers with non-rack cars.

Adjusting bump is a matter of taking some time, yet it’s important.  No car should go onto the track until bump steer has been set.  Setting bump steer can’t be done in fifteen minutes.  In the worst of times, it’s an all day project to set bump.  But once the job has been finished, it doesn’t need to be done again until the car suffers damage to the front frame rails or spindle arms.

LATE MODEL AND SEMI-MODIFIED

11 STEPS:

  1. Set caster, camber, toe-in and check suspension ride height.  (Example:  upper A-arm angle or measurement between upper ball joint to fixed point on frame)
  2. Disconnect sway bar and shocks (springs?).
  3. Put your car on 4 jack stands 16” at side rails.  (Howe—Hamke—Port City—Barry Wright--Lefthander—etc.)
  4. Remove wheel and bolt plate to hub.  Lock steering straight ahead.
  5. Put gauge under A-frame with jack tube under ball joint and raise suspension back to original ride height.  (Refer back to step#1)
  6. Level plate and gauge.
  7. Adjust dial indicators on right and left at same height on tapes.
  8. Jack 1” travel.
  9. Right front should bump .015 out.  To adjust bump, shim tie rod up or down at steering arm. *
  10. Left front should bump .030 out.  To adjust bump, shim tie rod up or down at steering arm. *
  11. Reset toe-in.

*      This is the recommended setting.  You may want more or less to suit your driver, chassis or track.

NASCAR CUP BUMP STEER GAUGE

USE FOR: Conventional steering cars with Grand National Hubs

To Bump Steer a conventional steering car is very difficult.  Remember, a common rule is that front steer cars bump in and rear steer cars bump out.  (Grand National 5 on 5 Hubs may require using extensions on dial indicators or move bump steer assembly in or out so that dial indicators fit the plate.)

USE STEPS 1-7 FROM ABOVE, AND THEN PROCEED WITH THESE STEPS.

8. Jack 1” to 3” checking at 1” increments.

9. Left front should bump .030 out. *

10. Right front should bump .015 out. *

11. Reset toe-in.

THESE ARE SOME SUGGESTED METHODS TO HELP ADJUST YOUR BUMP.

A. Use drops made for NASCAR Cup cars.

B. Relocate tie rod location on drag link up or down.

C. Heat pitman arm and idle arm and bend up or down.

*These are recommended ranges. Use more or less to suit your driver, chassis or track.

This article was published on Tuesday 20 March, 2007.
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