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On a well balanced suspension/steering system a vehicle will "pull" or steer down the camber of the road. This has a lot more to do with gravity pulling the car down than any of the attributes we discussed earlier. As an aside at the Milford Proving Grounds we have a test track that is circular and banked. The way it is set up the slope of the bank increases as you move to the outside of the track. On of the interesting things about this setup is that you can go around the track with no steer input from the driver. If you accelerate the car will naturally move to the outside but the slope will be greater and it will find a new equilibrium. If you decelerate and you come down. It is pretty cool.
If that is what everyone is calling camber steer it is normal. In fact it is good. If it wasn't there it would imply that the suspension was not optimally tuned. HOWEVER I am not sure that is the problem with 9-3's. I own a 9-5 and it has a different problem. If I am driving on a road that is cambered with the left side down, the car will pull to the left. Driving on a road that is not cambered, the car pulls to the left less. Driving on a road with a camber that has the right side down my car goes straight. That is not how it should behave. For the same conditions it should go left, straight, right, respectively.
I have had it at the dealer twice so far for the problem. They have aligned it (reset the toe) multiple times and agree that it still pulls to the left. I switched the left and right front tires then the rears and to check if it was a tire conicity problem, but there was no difference. The mechanic says the car is perfectly aligned according to his machine. Although I have not put the car on an alignment rack myself I would guess that my toe (the only adjustment on the 9-5) is probaby set very close to ideal. I think, and this is pure speculation, that the camber setting of the one or more of the front wheels is off slightly. Incorrect camber will cause a drift very similar to what I am experiencing. I can not speak for all the posts on the board, but I am fairly sure that is what is wrong with my car.
OK enough about that. Bump steer is something different. Bump steer comes from the geometry of the suspension. In most common suspensions, the center of the wheel does not move up and down linearly. It also translates side to side. The tie rod is connected on one end to the rack and the other to the knuckle. So if the knuckle moves up, the end of the tie rod connected to it must move up. the other end of the tie rod stays stationary. As the tie rod moves up and down it traces out an arc with a radius of its length in a front view. The wheel will not trace out the same arc. Since the tie rod can not change length, steer will result when the two arcs are not on the same path. This is Bump steer. Bump steer is rarely a problem in modern cars. "Toe curves" toe vs wheel height are usually kept quite linear and cover less than 0.3 degrees over full travel. This effectively eliminates bump steer.
Hey if you are interested in cars and want to learn a little more I highly reccommend "Fundamentals of Vehicle Dynamics" by Thomas Gillespie. It is a great book for the engineer who is just getting started with vehicle dynamics, but is still understandable without an engineering background. You can get it directly from SAE or a bookstore might be able to order it for you. It is a great book. I am friends with the author, in fact I was an assistant for some classes he taught, so you may consider my opinion partial, but I loved the book before I met Tom. If you already have a more advanced understanding of vehicle dynamics "Race car Vehicle Dynamics" by Milliken is great, but MUCH harder to understand.
-Joe
, Sat, 2 Dec 2000 13:18:01
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