How to check and adjust Bump Steer

From HPBooks “How to Make Your Car Handle”
by Fred Puhn

Adjusting bump steer is an advanced suspension tuning technique, and often makes the difference between average and outstanding handling. Many elusive twitches and wiggles can be blamed on bump steer, and often it is the cause of high-speed stability problems. Bump steer is the change in toe setting as the wheels move through full travel. It can happen at front or rear. Ideally, you want zero bump steer-the toe setting should remain constant no matter where the suspension moves.

How to diagnose and adjust bump steer:

First you will need a tool to check bump steer. You could use a toe gauge, but a bump steer gauge is easy to construct, and often a lot easier to use. Below is a diagram of a home-built bump steer gauge.

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You can build this handy bump steer gauge out of two pieces of plywood, a piano hinge, two bolts and four nuts. The gauge is positioned on the floor, held by a heavy item such as a battery. The gauge can have several hole patterns for various size wheels. Adjust the bolts so they contact the wheel rim. If more accuracy is required, substitute a dial indicator for one of the bolts. This gauge measures changes in toe, not the actual toe setting.

Diagnosing bump steer

To check bump steer, remove the springs so the suspension may be moved through its full range of travel. With the tires resting on the garage floor, the car chassis is moved through its full range of travel with a jack under the frame. On some cars, it may be necessary to place blocks under the tires, so the frame can be lowered to its full bump position with the jack. Do not attempt to go beyond the normal range of suspension travel.

Start with the car at full bump position. Make sure the bump steer gauge has both bolts against the wheel rim. Raise the jack ½” and look at the bump steer gauge. If both bolts are still touching the wheel rim, there has been no change in toe. A change will show as a gap between one bolt and the wheel. If the gap is on the front bolt, the wheel has increased toe-in. A gap at the rear bolt indicates increased toe-out. Record the gap and the ride height.

Move the jack again, and take another reading. Record the toe change if any. Continue this process until you have measured the toe change throughout the full range of suspension travel, from bumpstop to full droop. Record the results.

If you are using a dial indicator, the gap shows up as a reading on the dial indicator. Start with the dial set to zero, so the reading is the change from the initial setting. This method is more accurate than measuring the gap between the bolt and the wheel rim.

Adjusting bump steer

If there is a significant toe change, an adjustment will be necessary. This will vary with the design of the car, and is different on the front and rear suspensions.

On the front suspension, the bump steer can be adjusted by changing the height of one end of the tie rod, assuming your car has a conventional steering system using equal-length tie rods. This type of steering system is almost universal with independent front suspension of the A-arm or MacPherson strut type. Other types of front suspension such as sliding pillar, trailing arm, or swing axle generally cannot be bump steered.

On rack and pinion steering suspensions, the rack may be lowered on its mounts. Installing shims under the rack brackets, or machining the bottom of the brackets will work for the adjustments. On cars with a steering box, the usual method of adjustment is to modify the steering arm. To avoid steering arm modifications, the steering system can be converted to spherical rod ends, in place of tie rod ends. This allows shims to be used between the steering arm and the rod end (for small bump steer adjustments). Large changes cannot be made this way, although it is a good way of making extremely precise adjustments.

The ideal adjustment translates to zero bump steer at the front. A car with bump steer in the front suspension will be unpredictable in a turn and unstable during braking. The car will also be very sensitive to toe-in changes. With bump steer, toe changes can happen with every dip in the road, or when the brakes are applied. Both toe-in and toe-out errors give terrible handling, and should be avoided in the front suspension.

Rear Suspension bump steer

Cars with independent rear suspension may also have bump steer at the rear. The adjustment is usually difficult or impossible. Stiffening the rear suspension is usually the only acceptable alternative. Many cars with semi-trailing arm rear suspension fall into this category. Race cars with fully adjustable rear suspensions have the advantage of being adjustable for bump steer. This is accomplished by changing the caster of the rear suspension upright (i.e. strut or coilover). This is also a trial and error procedure. Zero toe change is ideal, but if this is not possible, adjust for toe-in that increases as the wheel rises. Avoid rear toe-out adjustments completely.