Posted by Keith in Washington on Tuesday, August 02, 2005 at 6:38PM :
In Reply to: spring blocks posted by Dana in Ma on Monday, August 01, 2005 at 5:11PM :
Yea and I bet everytime you hit a good bump the truck will steer to left (if it is a ff PW) Bump steer is caused by the pitman arm not being lengthened to accomadate the lift. The steering geometry is not correct if the pitman arm is not changed with the lift. If the drag link is level, and the springs compress the drag link simply piviots at the pitman end and no problems occur. If the Pitman arm is short and the drag link slopes down to the steering arm there will be problems. In this case when the springs compress the spring move in a vertical up ward motion. However when the drag link rotates upward it pushes the steering arm forward as it must rotate in an arc. This in effect removes the slope on the drag link and as the drag link becomes more level it increases the distance between the pitman arm and the steering arm this is what forces the steering arm forward (which is the easiest thing for the drag link to move). You may also feel it a little in your steering wheel but it is much easier for the drag link to move the steering arm forward than it is for it to move the pitman arm which would need to rotate the gears in the steering box to get any movement. And of course the springs can not be move forward.
Obviously the higher the lift or the more slope on the drag link the greater the bump steer will be. I had this problem with my carryall as someone rearched the spring 3" and did not change the pitman. It was really bad. Also since they just rearched the springs they were then too short between the eyes and the shackles ended up in direct line with the springs (not vertical so they could swing) which resulted in the eyes in the spring being broken. Glad it was the spring eyes and not the frame or spring mounts.
Follow Ups: