Clear communication between driver and machine is a fundamental requirement for speed, safety, confidence and predictability on the racetrack. The ideal relationship between you and your race car is you knowing exactly what your car will do at every inch around the track, every lap. The car’s response to your control inputs, and its response to pavement ripples, should be the same every time. Ideally, the car will respond to your inputs exactly the same way every time, no matter where you are on the track, and its response to pavement irregularities will be minimal, as if the track were perfectly smooth.
Of course, there is no such thing as a smooth race track, so part of the challenge of improving your racecar’s performance involves dealing with the reality of rough pavement. If you succeed in that effort, you will create a competitive advantage for yourself. Your car also will be safer and more predictable on rough pavement.
GREMLINS AT THE WHEEL
Steering, power application and braking are the three primary inputs that you have some control over. But there are many influences that affect those inputs that you cannot control from the driver’s seat. For example, anything that alters the direction any one of your four tires is aimed will steer the car just as if you turned the steering wheel. That kind of alteration of your control inputs significantly degrades the clarity and consistency of your communication with the car. Those outside influences make your car less predictable, less controllable and less safe.
Imagine what it would be like if a gremlin were in the car with you, randomly sawing away at a second steering wheel that altered your steering inputs. Eradicating that gremlin would suddenly be your highest priority, right? Bump steer, when combined with rough pavement, is that gremlin. You can’t eliminate pavement roughness, but you can minimize the amount of bump steer in your suspension.
THE GEOMETRY OF BUMP STEER
One corner of a car with double wishbone suspension will be the example we discuss here. As the suspension moves up and down, the ball joint on each wishbone swings in an arc that is centered on its frame-end pivot points. The toe link, also called a track rod or tie rod, follows an arc centered on a pivot at its inboard end. The upper ball joint, the lower ball joint, and the toe link outboard pivot are the three pivots that control the location and motion of the spindle, and therefore the wheel and tire at each corner of the car. As those three pivot points swing in their three arcs, the location of the spindle follows, usually changing the camber angle and the steer angle of the spindle and tire as the suspension moves up or down.
Unfortunately, the location, the bushing stiffness, and the load on every suspension pivot point affects the bump steer characteristics of that corner of the car. Another unfortunate reality is that most road-going cars are designed with intentional bump steer to ensure understeer in all possible conditions. Racers are focused on maximum controllability instead, so eliminating the interaction between vertical suspension travel and the steer angle of each wheel is a primary goal of racing suspension development. Theories about intentional use of bump steer on race cars are occasionally proposed, but maximizing controllability by minimizing detrimental handling interactions provides a much larger benefit than any of those theories can hope to produce. Zero bump steer is best.
Roll steer is closely related to bump steer, and it can affect live axle suspension even though bump steer does not. Again, zero roll steer is best.
Both the design and the alignment of a suspension assembly determine how much bump steer that assembly has, and how the steering angle changes with vertical suspension travel. There is such a huge variety of suspension designs that there are no universal rules of thumb about which way to adjust anything to reduce bump steer. Measuring the actual behavior of the suspension on your car is the best way to find out how much bump steer it has.
MEASURING BUMP STEER
This project takes some quality time in the shop, some prep work on the car, some basic and cheap measurement equipment, and about a dozen very precise measurements. It is worth the effort to make your measurement equipment consistent and quick to set up, because you will need to repeat this process every time you change your suspension geometry or alignment.
Start by measuring the static dimension from the top of each wheel to the fender edge. Then, with the car up on four jack stands, remove the springs and anti-roll bar links at all four corners. Reinstall the wheels and tires, and mark a very thin line all the way around each tire tread that you can accurately measure to. Firmly restrain the steering wheel in the straight-ahead position. Raise one suspension corner to its static position using a floor jack under the lower wishbone. Set up a toe measurement string at the static axle height along the side of the car. The string should extend from bumper to bumper so that you can measure both the front and rear bump steer with the same string.
Now that you have completed the setup for measuring bump steer, grab a note pad and pencil to record all of the measurements that you are about to make. Measure from the string to the line on the tread at the front side of the tire, then measure to the line at the back side of the tire. The actual measurements don’t matter, but the difference between them does. The difference in those two measurements is the static toe dimension, measured at the tire tread diameter.
Lower the wheel 1” and measure the toe again. Now raise the wheel to 1” above static and measure the toe a third time. Any difference in those three toe measurements means that there is some bump steer in that suspension assembly.
UP OR DOWN TO ZERO BUMP STEER?
Let’s say that the toe link is located forward of the axle center. Here are some constants:
- If the toe measurements change toward toe-in as the suspension moves in the bump direction, the outboard end of the toe link is too high, or the inboard end is too low.
- If the toe measurements change toward toe-out as the suspension moves in the bump direction, the outboard end of the toe link is too low.
- If the toe link is located aft of the axle center, the opposite is true. In that case:
- If the toe measurements change toward toe-in as the suspension moves in the bump direction, the outboard end of the toe link is too low.
- If the toe measurements change toward toe-out as the suspension moves in the bump direction, the outboard end of the toe link is too high.
If two of the three measurements are similar, but the third measurement is different, then in addition to a toe link pivot height problem, the toe link is the wrong length for zero bump steer of that suspension assembly. That requires significant modification to correct. In that case, it is worth taking more measurements at different wheel heights to verify the issue. If the toe measurements change toward toe-in with both bump and droop, the toe link is too short. The opposite is also true.
The bump steer measurement procedure described in this article does not include the effects of bushing deflections due to cornering loads. Stiffer bushings reduce the magnitude of this effect. Replacing the bushings with spherical bearings further reduce it to near zero. The only way to fully account for bushing deflection is to measure it on a suspension kinematics and compliance rig, but very few amateur racers can afford that sort of testing. Stiff bushings are much cheaper.
When you have measured the bump steer characteristic of all four corners of your race car, and determined which way each outboard toe link pivot needs to move to reduce bump steer, it’s time to figure out how to implement those changes. Unless you have a lot of experience making suspension modifications, get some expert guidance on how to move pivot points safely. The easiest way to raise or lower the outboard end of the toe link is to change the caster setting, but you might have a very good reason to keep the caster you have now. The next easiest way to alter bump steer is to add shims to the steering rack mounts to move the rack up or down.
It is a good idea to provide easy pivot-point-height adjustability with shims or a range of spacer heights. That is because it will take at least a few tries to home in on the height for minimum bump steer. Make a height change, measure the bump steer, and repeat until you have converged on the optimum height. Plan on spending an hour or more while adjusting away the bump steer gremlin at each corner. Fine adjustment increments are helpful when you are near the optimum pivot point height, but you won’t need an increment that is finer than one-sixteenth of an inch.
At the end of your bump steer project, your car won’t look any different than it did before, but it will behave better. All of the steering inputs that the tires see will come from you, and none of them will come from the suspension steering itself over the bumps and dips. That will reduce your workload since you won’t have to change your steering input to compensate for the car steering itself. The clarity and consistency of your driver-machine interface will be improved. The theory that a car communicates with its driver will become reality, making the car safer and easier to drive at the limit, and perhaps more fun to drive.