[MNBoxster]

Originally Posted by markk

I know what understeer, oversteer, camber, caster, toe-in, toe-out, etc., etc. is but what is BUMPSTEER ????????

Mark.

Hi,

Here goes... What is Bumpsteer?

The technical definition of Bumpsteer is a change in Toe Angle caused by the Suspension moving up or down. Bumpsteer is built into the geometry of the Suspension and Steering system, and has nothing to do with turning the Steering Wheel. The effect of Bumpsteer is for the Wheel to toe-in or toe-out when the Suspension moves up or down. This toe change or steering occurs any time the Suspension moves, whether it is from Body Roll, Brake-Dive, or hitting a Bump in the Road. Bumpsteer is undesirable because the Suspension , not the Driver, is steering the Car. This effect can range from barely noticable to severe negative feedback to the Driver through the Steering Wheel. While only an annoyance in Normal Driving, in High Speed or Spirited Driving, the Car, which may already be at the Limit, could hit a Bump and go completely Off-Balance, which may result in a Loss of Control by the Driver


What Causes Bumpsteer?

The Front Wheels do not move directly straight up or down when the Car hits a Bump. Instead, the Wheel follows an arc, or curving path, that pushes the Wheel slightly inward (towards the centerline of the car) or outward (away from the car) in response to vertical Wheel movement. The Outer Tie-Rod (which connects the Steering Rack to the Wheel) also moves in-and-out in an arc as it moves up and down. If the rate which the Outer Tie-Rod arcs in or out does not match the rate the Wheel moves in or out, the Wheel will be turned by the Tie-Rod. This is Bumpsteer. The center point of the arc traveled by the Wheel (known as the instant-center) is controlled by the location and angle of the moving Suspension Links and the Center of the Tire's Contact Patch. This point moves as the Ride Height or Tire Width changes (since Side Wall height is determined by the Aspect Ratio, increasing the Width of the Tire also slightly raises the Side Wall height and therefore the Ride Height or more accurately, the Instant Center) . In contrast, the arc of the Outer Tie-Rod is controlled by the position of the Steering Rack, which is fixed.


To eliminate Bumpsteer, both the length and the center point of the two arcs must be the same. But, since the instant center moves with Ride Height, Bumpsteer cannot be totally eliminated throughout the entire range of Suspension travel. Consequently, Suspension Designers concentrate on minimizing Bumpsteer within the range of movement closest to Factory Specs, especially Ride Height and Tire Width. Changing the Ride Height, Tire Width, or other Suspension components may move the Suspension outside this narrow optimized window.

To fix a Bumpsteer problem, you need to alter the height of Outer Tie-Rod relative to the Steering Rack or change the Instant Center. Small changes in this relationship can be made with Offset Rack Bushings. Making big changes requires Adjustable Tie-Rod Ends, often known as a Bumpsteer Kit. Many Aftermarket Companies offer Bumpsteer Kits such as Baer Brakes, Steeda Autosports, Competition Engineering (although I don't know if a Kit is available for the Boxster). When installing a Bumpsteer Kit, it is highly-recommended that you have a Suspension Shop install the kit. It is advisable that an Alignment Rack be used in the installation of the Bumpsteer Kit. Using an Alignment Rack allows the Technician to monitor the changes in Toe with Suspension Travel and make changes in the Bumpsteer Kit’s settings to properly adjust it.

Hope this helps...

Happy Motoring!...Jim'99