Bump Steer at the Front

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jdogma
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Bump Steer at the Front

Post by jdogma » 10 Sep 2012, 10:46

If your front, lower, inner A-arm pickups are directly behind the inner tie rod ball joint and the outer tie rod joint falls on the same line as the outer ball joint, and you do not have anti dive geometry, you will have 0 bump steer - when the wheels are pointed straight. There is no solution for 0 bump steer at all times at the front. That being said, slight bump steer at the front is not that much of an issue as the driver naturally corrects for it. If it is real bad there is a lot of steering kick over bumps. Many race cars have an arrangement like the Midlana, except with the rack mounted in front of the upper arm. I think I have seen the Midlana's solution before, but can't be sure, -not that that matters. I have done all suspensions this way when I design the front upright. I have also done 3 front suspensions using early C4 Corvette aluminum uprights, but they do not lend themselves to 0 bump rack placement. You can get a very acceptable compromise over most of the travel, with bump being more important. No production cars that I am aware of have 0 bump geometry.

A friend brought over one of those 2 seat Indy car things a few years back. It was so bad that when I stepped on the front of the frame with the car on the ground, the front wheels toed in and out over an inch! The rear suspension was not properly designed and I could grab a rear wheel and steer it by hand! And he said it handled pretty good except for a lot of understeer!!! -lol I would not drive it, although if I had a camera to mount to record rear wheel steer, I would have been tempted...

Midlana1
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Re: Bump Steer at the Front

Post by Midlana1 » 10 Sep 2012, 15:45

Agreed. I designed the suspension so that the inboard ends of the A-arms are spaced the same as the steering rack. However, when I sat the rack in place, it "felt like" (through the rubber boots, as if the rack pivots were further outboard than they should be. I'm going to have to pull the boots back and double-check, but at this point it is what it is. If it's wrong, the rack will have to be raised. Oh well.

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Re: Bump Steer at the Front

Post by jdogma » 10 Sep 2012, 21:27

I always mount the rack so that it can be shimmed up and down. Yours is the same as I remember... Front to rear adjustment only alters ackerman characteristics.
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