Bump Steer

Lets define bump steer as the following: An involentary movement of the wheels without direct input from the steering box.

Race cars are a funny breed, as the sport matures, so do designs, and integration of mixed parts to enhance handling, and how fast the suspension can swing left to right. The problem is with all the thought put into these designs, they overlook bump steering, also factor in damaged or worn components, or track bumps can cause the tires to toe in and toe out as the suspension moves up and down. The goal is to have little to no bump steer, the tires should hold the same toe measurement that is set when the race car is setup on the scales. A rule of thumb is to have a maximum of .050 for total bump steer movement.

To achieve Zero Bump the following has to happen, the tie rod must fall inline with the following: (draw an imaginary line between the upper ball joint and lower ball joint, one end of the tie rod pivot must be on this line, the inner end of the ball joint must fall on an imaginary line from the upper arm pivot to the lower control arm pivot, as long as the tie rod ends fall on these two lines it can be high or low, additionally the tie rod must fall on the instant center, which is an imaginary line drawn from the upper ball joint through the upper arm pivot, and a line drawn from the lower ball joint through the lower arm pivot, where the lines from the upper and lowr arms cross is called the instant center.) This is also similar to finding your roll center.....remember that section?

Proper Bump Steer happens when the tie rod falls with in our explaination from above and the movement matches that of the suspension. If the suspension measureed out from the pivots to the tie rod are 20 inches, and the tie rod is 18 inches, the tie rod is going to run out of movement before the suspension, this causes the tires to toe out or toe in as the suspension moves in extension or compression. Matching the suspension distance to the length of the tie rods reduces bump steer.

When looking for bump steer,

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 components should be centeredincluding 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 this number down.
Remove springs
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.

What to look for: Your saying "what can I do to find and correct bump steer?" Follow the procedure below and use the guide to help you restore Zero Bump Steer.

It is recommended that you use Bump steer plates. If you don't have bump steer plates you can use tape measures to calculate the differences , you must scribe the tires for accurate measurements. From the free standing height, jack the suspension up in inch intervals and take your measurements, follow the same procedure when lowering the suspension

For those of you that have bump steer plates, use your directions, there are several kinds available, I prefer the single dial caliper style, it is easier to keep track of one caliper then two, and it takes the guess work out of doing the math.......

First determine what direction the tire toe is following. Remember when you check bump steer, you move the suspension in extension and compression 3 inches from normal unassisted height. Are the tires toe in on extension? Compression? are they toe out on the opposite motion? are they staying toe out when extended and compressed?

	Problem	Cure
	1. Toes out in compression and in on rebound all in one direction.	1. Decrease shim on outer tie rod or lower the inner tie rod.

	2. Toes in on compression and out in rebound all in one direction.	2. More shim at outer tie rod or raise the inner tie rod.

	3. Always toes in both compression and rebound.	3. Lengthen the tie rod as it is too short.

	4. Always toes out on compression and rebound.	4. Shorten tie rod as it is too long

	5. Toes out on compression, then in on rebound and then starts back towards out with more rebound travel.	5. Less shim at outer tie rod and shorten tie rod.

	6. Toes in on compression, then moves out on rebound and then starts back towards in with more rebound travel.	6. More shim at outer tie rod and lengthen tie rod.

Remember that racing is a dangerous sport, use caution when working on your race car, always support your race car when its jacked up.