Re: Bump Steer

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Posted by Keith in Washington [] on Sunday, November 21, 2021 at 12:34:37 :

In Reply to: Re: Bump Steer posted by Keith in Washington [] on Saturday, November 20, 2021 at 21:18:23 :

I should add this to my comments above. The factors in the steering geometry that setup bump steer are simple if you understand the basic movements involved.

First the springs on the axle basically restrict the axle movement to a vertical up/down plane. The steering arm which is attached to the axle has the same movement up/down.

The drag link pivots on the pitman arm. When the drag link pivots, not from steering input but axle movement, the steering arm end of the drag link moves in a circular arc.

So now you have a steering arm that only moves up/down is connected to the drag link that moves in an arc. So as the springs/steering arm and drag link move there is a conflict in motion which becomes more severe with increasing spring flex. The drag link which can only swing in an arc starts pulling on the steering arm and the wheels will start to turn as the only thing that can move to solve the conflict between the springs and drag link is the steering arm which is connected to the knuckle.

The reason that the drag link needs to be horizontal is that at rest the steering arm is located at the center of the drag links arc. So traveling down the road the spring deflection only moves the drag link several degrees which results in little conflict of motion. However, the greater the spring deflection the greater conflict. Take your arm and hold your forearm level, then rotate your arm up and down at the elbow and watch the motion of your finger tips. You will notice very little horizontal movement of your fingers when the arm is horizontal but as the arm approaches vertical it’s almost all horizontal movement. The same is true for the drag link.

The bottom line is as long as the drag link is near horizontal there is minimal bump steer.

I mentioned that doing a lift will cause bump steer if the drag link is not corrected to horizontal. If there is a 20 degree tilt to the drag link after the lift, the drag link is now in a position in its arc where there is significant horizontal movement at it’s steering arm end. So now if you drive down the road and hit a good bump that compresses the springs the conflict in motion between the steering arm and drag link will cause what is called “bump steer”. The name is really appropriate.

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