Bump Steer, How to tell...? Options

[jim_hoyland]

Is the test for bump steer driving over a speed bump straight on or at an angle. I don't get any noticable pulling when taking the speed bump straight on. Just want to get the test situation straight, any input will be appreciated.

Car is lowered about 50%

[stewteral]

Is the test for bump steer driving over a speed bump straight on or at an angle. I don't get any noticable pulling when taking the speed bump straight on. Just want to get the test situation straight, any input will be appreciated.

Car is lowered about 50%

Hi Jim:

Let's start with a definition: Bump Steer is the deflection of the Front AND rear wheels from their original path when the suspension encounters a BUMP or as a result of ROLLing in a corner (the latter is technically called Roll Steer).

I have a 914 street/track that I have been trying to get to handle correctly for quite a while. I have learned a number of things:
-The 914 was cleverly designed to be use AS DESIGNED. After lowering my car and making a HASH of the handling, I have raised back to almost STOCK ride height.
-When the car is lowered in the front, you CHANGE the range of suspension travel to a area of movement NOT intended. In this situation, you will be CREATING bump-steer issues as the Radius of travel of the suspension arm and that of the Tie-Rod become increasingly different. Thus and the suspension travels up (or down) the difference with the tie-rod will cause the wheel to DEFLECT from its intended path.
-In my car, I have 911 struts and have cut the shock tube to RAISE the spindles and lower the car. I took DAYS bump-steering and adjusting with a bump-steer tool that measures the deflection with a DIAL INDICATOR. After I heated and BENT the steering arms on the struts down, something like 3/4"--different for each car) I was able to get the front bump steer defection DOWN to about 0.010" at 2" of deflection. I made my gage, but one can be bought from STOCK CAR RACING outfits for something above $200.

I then bump-steered the REAR SUSPENSION and learned something about the design that really surprised me: As the LOADED rear suspension moves UP, it TOES-IN at about 0.200" in 2 inches of travel!!! At the same time the UNLOADED wheel moves DOWN and goes TOE-OUT about 0.200" in 2" of travel. The design goal became clear: Porsche did NOT want the 914 to oversteer. When the car loads into a corner, BOTH wheels deflect INTO the corner!
-I finally understood why I could NEVER stop the understeer on my first street 914 which ran on just 185 radials.
-I was at the Monterey Historic races a week ago and talked to a sharp Porsche specialist who prepared the 914-6 for one the of fat , old, rich guys who get to drive. He concurred on what I found with the rear suspension and told me that he usually sets up 914s at ZERO static toe-in and will go TOE-OUT on some cars, depending on the driver. The caution here is that TOE-OUT in the rear can make the car "tail-happy" and the driver must be VERY vigilant when driving the car. Oversteer can be a lot of fun in slow corners, SCARY in high-speed corners and TREACHEROUS in the wet!!!!! If you don't know now, due to the LOW POLAR MOMENT OF INERTIAL (that is to say all the WEIGHT in the middle of the car), 914s like to spin like a top when they get loose!!

I hope all my work can help you understand what is happening with the 914 suspension: I have to keep reminding myselft that the car was DESIGNED around 100 HP and 165 radials....ONLY. So the best tip I can give is to run the car at STOCK suspension ride-height and setting......and if you PLAN to be "Up on the Wheel" maybe 1/6- 1/8" TOE-OUT in the rear.

Best of luck,
Terry