C2 - Bump steer tendency - caster angle?

[Timothy B.]

My 1967 C2 small block vert has what I think might be a bump steer tendency. How should a car of this vintage feel relative to bump steer? I have Duke recommended Pirelli P4000 tires on the factory wheels. To me, bump steer is the tendency for the car to "self steer" when going around slight curves as the tires hit bumps in the road. I had the car aligned twice early last year when I got the tires, and the car tracks straight and handles fairly well, with my only real complaint being this bump steer feel I get out of the car.

I have about 1.75 - 2.0 pos caster in the front (coulnd't get any more without going too negative in camber). I have about .75 deg negative camber in the front. Toein is set per Duke's recommendations. Rear camber is set 1 deg neg both sides with only slight toein total. I have lots of on center feel, and the tires return to center quickly. In fact, sometimes it seems I have to apply more force than I should have to when making slight curves left and right at road speed.

I was reading a post in this forum which referred to the link attached below for a "Wheel alignment short course" that is very interesting, but I noticed a comment in that article that I wonder about. Here is the text I am talking about:

"If the caster is equal but too negative, the steering will be light and the vehicle will wander and be difficult to keep in a straight line. If the caster is equal but too positive, the steering will be heavy and the steering wheel may kick when you hit a bump."

My car is a power steering car, so typically in this forum the recommended positive caster is around 2.5 deg, but based on the above comment should I consider trying less positive caster, and if so how much? Has anyone else experimented with slight changes in camber to get a feel for driving feel and/or bump steer?

http://www.familycar.com/alignment.htm

[Timothy B.]

Duke? Re: C2 - Bump steer tendency - caster angle?

Duke? Don't you want to jump into this with your suspension expertise? You have helped me tremendously so far...

Some more info for your consideration: In driving the car I noticed a slight tendency to drift to the left. Additionally, I noticed that the self centering force seemed slightly greater when returning to center from a slight right turn than from a left turn. Since, as described in the link to the article, "the car should pull to the side with the less positive caster", I decided to experiment. My actual parameters from my last alignment had the left side with less positive caster than the right. So, I added a 1/16" shim to the rear of the left (the front of the left has no shims) and did the same to the front of the passengers side. This should have slightly increased caster on the drivers side and reduced it on the right. I drove the car and it was a positive change. Car doesn't drift either way now, and centering effort seems the same. I can tell that the effort to initiate a slight curve to the left or right is now the same, when before it was greater to the right than the left. I will have to recheck toe.

I am discovering that slight changes left to right in caster CAN make a noticeable different in on center feel and effort to steer slight curves. It's in the details....:-)

Still wondering about bump steer...

[John H.]

Re: Duke? Re: C2 - Bump steer tendency - caster an

Tim -

"Bump steer" is the degree of change in toe-in with suspension vertical travel from jounce (compression) to rebound (extension); it's primarily the result of the difference in pivot locations between the inboard lower control arm bushings and the inboard tie rod end on each side. It's a geometry design issue driven by packaging, and there really isn't anything you can do about it with the production suspension and steering linkage.

More modern designs with rack & pinion steering frequently have much more favorable bump-steer characteristics, as the inner pivot points of the tie rod ends can be located closer to the lower control arm inner pivot points. On the Viper (see photo below), we actually shimmed the steering rack mounting points during the front wheel alignment process as the machine cycled the suspension up and down through its full travel to achieve the designed minimum bump-steer; the alignment computer screen displayed the design toe-in curve, and we shimmed the steering rack mountings until the actual alignment trace matched the design curve. This kind of attention to bump-steer is normally only given to race car chassis setup, and the only other production-car manufacturer (besides the Viper) that does it is Ferrari.