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Beating Bump Steerby Steve Grosekemper San Diego Region If you spend enough time at PCA events, you will eventually hear a multitude of strange new words that want to sneak into your daily vocabulary. These are words used by track and bench racers alike. They are best described as racer lingo, or “Porschese.” You may hear terms like apex, heel and toe, corner balance and bump steer. It is my goal to explain this sometimes-foreign tongue and translate its obscure lingo into plain , normally spoken English. Bump steer is one of the few Porschese terms that actually is what it says: If you hit a bump, the car steers to one side. On early Porsches, such as 356s, the tie rod ends were of two very different lengths. |
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356 example showing two uneven length tie rods. |
They were installed in the car at an angle. When the car hit a bump and lowered, the short tie rod would straighten more than the long one. This would increase the amount of toe-in at the wheel on the short tie-rod side, causing the car to steer to one side. Quite entertaining on a winding bumpy road!
Later on, the 911 addressed this problem by employing rack and pinion steering with equal length tie rods. In these cars the tie rods were parallel with the ground and caused very little toe change as the car went through bumps.
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This diagram shows the change made with introduction of the 911, which introduced rack and pinion steering with equal-length tie rods. |
As time went on, people discovered that if you lowered a 911, it would handle much better. However, this caused yet another problem: the tie rods were now at such an angle that a bump in the road would cause excessive toe change.
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On a lowered 911, spacers between the steering rack and front suspension member raise steering rack and keep tie rods equal. |
While this problem would not cause the car to pull to one side or another, it did create a very darty feeling.
The cure was to install spacers between the steering rack and the front suspension member to raise the steering rack. This levels out the angled tie rods causing less toe change. This is what is done when someone is said to "set the bump steer."
Steve Grosekemper is shop foreman at Dieter's Porsche/BMW Service in San Diego.
For previous tech articles with photos go to www.dietersmotorsports.com and click on Technical Articles
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