To maximize the performance of a given suspension system, it is necessary to optimize the geometry. Unfortunately, all suspension designs have built in compromises. These compromises are often due to restrictions within the regulations; in addition, even the most rigid of designs will have a certain amount of flex. It is a combination of these factors that leads to the phenomenon known as bump-steer.
Deviation Creates Bump-Steer
As covered in the article on race car suspension camber gain, most race cars have a certain amount of camber gain designed into their geometry. This camber change helps to keep more of the tires in contact with the track when cornering. However, the arc prescribed by the wheel’s vertical movement is different to that prescribed by the steering arm. It is this deviation that creates bump-steer.
So, what does bump-steer do to a car’s handling? There are a number of possibilities, depending on the severity of the problem in, including:
- Direction change under braking
- Shake in the steering system (particularly at the steering wheel)
- Sudden loss of grip during any part of the cornering process (entry, middle, or exit)
- The car pulls to one side at certain points on the track (can be on straights too)
All of the above are directly related to suspension movement at a time when the driver did not input any steering angle change.
Oval Rack Cars may Need Bump-Steer
But can bump-steer be adjusted out of a car’s suspension/steering system, and is it necessary to do so? Most race engineers will check and correct any bump-steer in their car periodically; however, some race series (and engineers therein) use bump-steer intentionally. A typical example of dialing in bump-steer is for oval track cars (NASCAR).
As circle track cars predominantly turn left, any assistance to help turn the cars is considered a positive. However, this must be done within reasonable limits because a driver may need to take evasive action on a straightaway. For instance, if a car crashes on a straightaway and stops in the middle or left side of the track, following drivers may need to steer right. If the car had been set-up to steer left when the driver lifted from the throttle (as in corner entry), he could find himself heading for the stationary car.
Rear Suspension Systems Too
Although bump-steer is predominantly a front end geometry problem, certain suspension systems can produce similar problems on the rear of a car: fully independent suspension systems, for instance.
In general, most high technology cars such as F1 and Indy cars, have very little bump-steer. The accuracy of the design and build process on these cars ensures the geometrical accuracy.
Measuring bump-steer is simply a case of measuring the deviation a wheel takes when the suspension is compressed (see photograph). Very accurate instruments can measure deviation as little as one thousandth of an inch (0.00004 mm’s). Changing the vertical position of the steering rack or rod-end pivots will change the bump-steer readings.
 |
 |
