Bump steer is when the wheels of your car steer themselves without you touching the steering wheel, and the most common cause of bump steer is a suspension modification. If, for example, you lower your car, you may have an issue with bump steer that requires fixing. Here's what you need to know.
Suspension Design Is Complicated Stuff, But Here's The Simple Version
If you want to understand what causes bump steer, you need to dive into some suspension geometry "stuff." Rest assured, if your eyes glaze over, you can skip to the "Simply Stated" section and get the gist.
There is an imaginary line that runs from the upper ball joint all the way through the lower ball joint. There is also an imaginary line that runs through the lower control arm pivot and the upper A-arm pivot. In order to obtain a zero bump steer, your rod must fall between these two points, as well as have the centerline of the tie rod intersecting with the instant center that has been created by the lower control arm and the upper A-arm.
Instant center is the imaginary intersection point in the line between the upper A-arm ball joint through the A-arm pivot, and the lower ball joint through the inner control arm pivot on your vehicle.
Zero bump is relatively easy to achieve by remembering that the tie rod needs to travel on the same arc that the suspension travels. When you match the arcs and lengths, you can prevent unwanted steer.
When modifying your suspension system, line up the outer tie rod centerline with the lower ball joint centerline, and line up the inner tie rod with the lower pivot point. The result will be a zero bump because the length and angle of the tie rod and the suspension are the same.
Measuring for Bump Steer
You cannot measure bump steer until you have your front suspension done and set to track conditions. Every component needs to be in the proper position and tight before continuing. You also need a bump steer gauge. In order to measure, you must first put the car at the right ride height and make sure your tires are correct and at the right air pressure.
-Set the caster.
-Set the camber.
-Set the toe in.
-Set the tie rod lengths.
-Center the steering with the tie rod ends centered on the inner pivot points lower ball joints.
-Lock the steering down.
-From the ground, measure to the lower ball joint and write down the number.
-Take the springs off and disconnect the sway bar.
-Use the measurement from before and place the suspension to the right height.
-Gather some bump steer shims.
-Take the bump steer plate and bolt it to the hub. After you level the plate, take note of where the dial indicator is at on the plate. This will help you get to the right ride height.
-Jack your suspension through both compression and rebound travel by 2” – 3” and record your readings.
-Shim it where needed.
Correcting Bump Steer
Take the measurements you just recorded and use them to adjust, move the suspension or shim it in order to achieve the reading you want.
Here are some quick tips for correcting the bump steer on front steer style suspension.
Problem: Toes out while in compression and in when rebound and all one direction.
Solution: Lower the inner tie rod or reduce the shim that you have on the outer tie rod.
Problem: Toes in while in compression and out when in rebound and all one direction.
Solution: Raise the inner tie rod or add more shim at the outer tie rod.
Problem: Toes in while in rebound and compression.
Solution: The tie rod is too short. Lengthen it.
Problem: Toes out while in rebound and compression.
Solution: The tie rod is too long. Shorten it.
Problem: Toes in while in compression and out on rebound, but then as it travels in rebound, it begins to start back in.
Solution: Add more shim at the outer tie rod and add length to the tie rod.
How to Use the Bump Steer Gauge
A bump steer gauge should be easy to use, and a one dial indicator is recommended since you will not need to watch two gauges. It also does the math for you. A two dial indicator may take two people to watch the gauges in instances where the bump is completely out of adjustment.
When your car is at the proper height, you need to set the dial indicator at the bump steer plate’s center and the middle of its range. This way you will not run out of indicator travel.
When you have the indicator set, simply jack the suspension through compress of two to three inches. Record your readings at each inch interval. Do the same thing for the rebound.
Should you see the front of the bump steer plate moving in towards the engine, then you have bump in. If it goes the opposite direction, you have bump out. Watch the dial indicator carefully since it will pick up even the smallest amounts. Record all of your readings.
How Much Bump Steer
Since many of the tracks are old and rough, you want as little bump steer as you can get on your car. Track conditions can make cars unpredictable.
While bump in is almost always a bad thing, you want a little bump out to help stabilize the car on the corners. This small amount of bump out gives the car an Ackerman type effect when they are in the center of the turn. The bump out makes the left front turn further than the right front as the right front compresses and the left front extends.
It is recommended that you never allow your tires to bump in, but add .005 to 0.15 thousands for your bump out.
Add Ackerman while you maintain the proper bump if your want Ackerman in the turn. If you use bump to add the Ackerman effect, you will lose contact with the track since the car will become upset as it travels over the bumps.
If you cannot obtain precise bump adjustments on your vehicle, then remember that bump out is better than having bump in. You want to get the best bump numbers you can, even if you have to replace parts. Anything over .050 will slow your car down.