Re: Bump Steer

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Posted by Keith in Washington [] on Saturday, November 20, 2021 at 21:18:23 :

In Reply to: Bump Steer posted by John Malone [] on Saturday, November 20, 2021 at 11:13:19 :

You really haven’t gotten a good answer to question on what is bump steer. The article below doesn’t cover the type of suspension that our PWs have

Bump steer is pretty straight forward. Simply put, when the suspension is either compressed or extended the vehicle will turn to the left or right without any action by the driver. An example is this. My friend had a Bronco which would dive hard right every time he hit a large pothole or uneven bridge approach. I simply asked him if the truck was lifted and he said yes. I said that’s your cause as someone did not correct the steering geometry during the lift.

It’s a little difficult to explain without diagrams but here what’s up. In a leaf spring truck with a setup like the PW with a steering arm, drag link and pitman arm. When the truck came from factory the drag link is horizontal. This puts it in the center of its arc of movement when the springs compress and extend. With this factory geometry there will be only very minor bump steer which is not noticeable. If the truck is lifted with out restoring this geometry bump steer will occur. After the lift the drag link is no longer horizontal nor in the center of its swing arc. The result is as the springs compress, with the drag link not centered in its arc, the springs can’t compress because the drag link can’t swing with the springs so the drag link pushes on the pitman arm but can’t move it because of the gears in the steering box so it pushes and moves the steering arm on the knuckle which moves and turns the truck.

The fix is to restore the drag link to the center of its swing arc. This can be done with a longer pitman arm which restores the drag link to horizontal and centers it in its swing arc. However, with a longer pitman arm, you will be moving the wheels lock to lock much quicker and can exceed lock to lock distance.

Another solution is to have a Z shaped drag link which will keep both ends of the drag link horizontal. The Z is designed to lower the steering arm end of the drag link in relation to the pitman arm end thus negating the lift altered steering geometry.

The same also applies to a cross steer setup where the drag links connects to the passenger side steering knuckle. Z shaped drag links are available for these setups.

One final note the correcting drag links are longer than stock as the distance between the steering arm and pitman arm pivot points was increased by the lift. Without correcting the steering geometry the steering wheel will no longer be centered.

Hope this helps.

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