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Steering problem

Old 04-24-05, 02:21 PM
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Steering problem

Hey, Im building a custom project that I have fabricated custom a-arms, kingpins and spindles. The problem that I am having is that when the a-arms are horizontal (parallel to the ground) the wheel points straight. When I lower the a-arms so they rotate on their natural arch and are say 30 degrees below the horizontal, the wheel turns outwards. This is happening on both sides... So, as I see it, if I hit bump, or under hard braking, the tires are going to either turn towards each other or turn away from each other... either way creating a lot of stress on the steering arms and kingpins.

If anyone knows what Im talking aboot lol and has any suggestions, please let me know...
Thanks!

Snuffs
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Old 04-24-05, 05:33 PM
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That's called bump steer. Designing a custom car is not a simple task.

I'd start out by reading all of Carroll Smith books if I were you.
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Old 04-24-05, 08:27 PM
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yes, bump steer indeed and this has plagued every car that has had moveable suspension. geometry is a bitch, good luck
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Old 04-24-05, 09:46 PM
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Your bump steer problem is caused because you made the tie rods pivot about much different paths than the spindle pivots about.

As you have just learned the hard way, setting up front suspension geometry is not a simple task because there are so many different things that you must take into account.

This is a 3 dimensional geometric problem that cannot be described or solved with just words posted on a forum. You may need to get professional help, or else your new front end will have some very bad manners. It probably has other problems (camber gain/loss, roll centers) besides the bump steer problem you described above; you just haven't gotten far enough to run into them yet.
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Old 04-24-05, 10:02 PM
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Hmmm.....

Recently, I helped my friend Robeast swap a scrounged S5 FC lower control arm with a good balljoint onto his S4 FC, which had lost a balljoint due to lots of salty winter driving and one really big pothole.

On the front of any FC, the wheel-hub-caliper assembly is attached to the lower A-arm via a small balljoint tucked close to the wheel. Standard issue car stuff.

The S4's lower control arm was 2 pieces, the arm proper and an extension bolted onto it with the balljoint bolted to that. The S5's arm was one clean casting with the balljoint right on it.

It struck me that the earlier 2-piece design must have been done for tuning. It would be much easier to change ball-joint location with that little extension piece than casting whole new arms every time a change was tried. Once they had it nailed down perfect, they made it permanent for the S5.

Point? Balljoint location is critical and subject to rigorous real-world testing. It's the pivot point that makes steering possible at all, makes up-down travel possible without swing-arm camber craziness, and is the point at which your unwanted toe-in/toe-out changes occur.

Just a thought. Have fun, and keep us posted!
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Old 04-25-05, 10:38 AM
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If these were British cars that would make sense. It was probably an issue of production costs. They probably figured out a cheaper way to mount the ball joint to the arm. When you see the computer power that goes into designing a car, an extra hour on S-snap or whatever program they are using for suspension, is where they would decide where to put the balljoint.
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Old 04-25-05, 10:09 PM
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well, probably what happened is these custom arms were modelled after a production piece...most cars are set up to toe in a bit on compression to help with stability, so the car doesnt dart about over bumps. this of course isnt always desirable on a race car
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Old 04-27-05, 12:38 AM
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Originally Posted by jgrewe
If these were British cars that would make sense. It was probably an issue of production costs. They probably figured out a cheaper way to mount the ball joint to the arm. When you see the computer power that goes into designing a car, an extra hour on S-snap or whatever program they are using for suspension, is where they would decide where to put the balljoint.
Well, remember they started selling the S4 in 1985-86, and it was being designed for a few years before that. There's probably more computing power in my office today than existed in the world at that time. (slight exaggeration, but not much)
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