Bump Steer

Defining Bump Steer

Simply put, bump steer is when the front wheels move up and down, causing the wheels to toe in and out, causing the car to steer itself. In other words, bump steer is the term for the tendency of a wheel to steer itself when going through a suspension stroke. To visualize bump steer, imagine the left wheel rolls over a bump, compressing the suspension on that corner and causing the wheel to automatically rotate to the left (toe out), causing the car to turn itself left for a moment without any actual steering wheel input. Alternatively, when the vehicle becomes airborne their front wheels will toe in because of bump steer.

Why Having Bump Steer is Bad

First off, bump steer can waste a lot of energy since the suspension is constantly going through small up and down movements as the car travels down the road. It can also cause the tires to be improperly aligned. If such an energy loss is not minimized, the solar vehicle will have high rolling resistance, making a less competitive car. Additionally, bump steer can cause increased tire wear and tear, make the vehicle more difficult to control (especially on rough roads), and make it more dangerous since it can cause the car to turn without the driver’s input.

Bump Steer Causes & Solutions

Ideally a vehicle has near zero bump steer since most cars will never be able to achieve absolute zero bump steer. Acceptable amounts of bump steer can vary depending on the application but a good rule of thumb is an upper limit of 0.030-inch. Many cars try to achieve under 0.010-inch when possible. It’s most important to have minimal bump steer though when turning. Also take into account that when you change your anti-dive (a suspension parameter that controls how much a vehicle's suspension deflects when the brakes are applied) or or moment center, bump steer characteristics can change.

In general, bump steer is caused by the interaction between bumps in the road and length or angle of the vehicle’s suspension and steering linkages. In particular, bump steer is caused by incorrect geometry between the suspension and the pivot point on the steering rack interacting with the road. Therefore, there are certain elements of construction or criteria that needs to be meet when designing car geometry in order to achieve minimal bump steer:

For near zero bump steer, tie rods on each side of the instant center (IC) need to point toward the IC for its side
- Note: the IC is the point where a line extending through the center of rotation of the ball joints and inner mounts of each lower and upper control arm intersect
Tie rod must be a specific length. I.e., tie rod length must equal the distance formed by 1) a line extending through the centers of rotation of the tie=end ends, and 2) the tie rod line intersection with
- a) lines extending through both upper and lower ball joints
- b) the plane that passes through the inner chassis mounts

Bump steer happens when the rod is not aligned with the IC/is the wrong length. If the tie rod line passes below the IC, wheel will bump in (towards the car’s centerline) as the wheel travels up and bump out when the wheel travels down. Vice versa/bump in if the tie rod line over the IC.

If the tie rod is too short, bump steer in when wheel travels in both directions from right height. Too long, wheel would bump out. These observations thus indicate if we have a tie rod alignment problem or a tie rod length problem. To determine which, record each inch for several inches of travel in both directions from static rid height and note the tendencies. Bump steer can then also be adjusted by moving any of the front suspension components pickup points up, down, in, or out (e.g. the inner tie rod mounting point can be moved up by moving the rack. This then requires a tie rod length change for proper alignment).

Overall design changes can also affect bump steer. This is true even if the car was near perfect bump steer before any design changes. For instance, if the car height is lowered or lifted, wheel toe setting change, requiring realignment/adjustment of the steering tie rod length to avoid excessive tire wear. With tie rod changes, however, bump steer values will also change and have to be treated.

See reference 1/2 below for some examples as well as tips for measuring bump steer through the best methods. To measure bump steer, most commonly the tool called the bump steer gauge is used. This tool is bolted to the wheel hub and has a dial indicator. Other methods include the double dial caliper system, single dial system, laser system, etc.

Some Last Notes

Ultimately, minimizing bump steer will make the suspension and overall car more energy efficient. Longer lower wishbones will help reduce bump steer. Maximizing the distance between the upper and lower wish bones is also good as this reduces the loads on the upper wishbone and minimizes bump steer over a larger wheel bump travel distance. Tie rod length and steering knuckle location also affects both bump steer and Ackerman steering. An iterative solution must be used to find the optimal tie rod length and knuckle location. Remember that if the tie rods are too long/short the wheel with turn (toe in or toe out) as the wheel bumps up and down. Bump steer wears the tires and waste energy, making finding the proper tie rod size crucial. Ackerman steering can be checked after this bump steer minimizing step. After steering knuckle and tie rod have been altered to achieve Ackerman steering, bump steer will be present. Hence, the process of zeroing bump steer must be repeated. This iterative process of achieving Ackerman, zeroing bump steer should end with zeroing bump steer because it affects the car at all times while Ackerman affects the car only when turning. Both are important but bump steer is a little bit more important.

(See “The Winning Solar Car”, book, pages 261-273, for additional info).

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