By Eugene on Wednesday, August 15, 2001 - 07:19 am: Edit

About 5 degrees. Disco I. I found the following lengthy explanation ( which actaully was about a Jeep 2" lift). Could anybody, please, comment regarding how serious it is and whether any additional work (a longer pitman arm ? What is it? ) is required.

Thanks,
Eugene
//**************
The steering being off center gives you a clue that you changed the steering geometry. There is no such thing as a low buck lift. When you change the front springs and lift the front end you must consider the effects to the steering geometry. I will bet you that you also now have "Bump Steer". Simply explained when you hit a good size bump in the road and the springs compress you will turn either slightly or sometimes significantly to the left or right...

...Bump steer is caused by the change in geometry between the pitman arm on your steer box and the steering arm/ attachment point on your axle. If you look at most vehicles the drag link, which attaches the pitman arm and the steering arm on the axle, is usually horizontal meaning that the two end points are at the same height. The end of the pitman arm becomes the center point of rotation for the drag link when the springs are compressed or extended. The axle end of the pitman arm moves in an arc as the axle moves vertically up and down. Normal on road axle movement basically is around the center (O position) of the arc that the drag link makes. As the axle's movement get greater the axle end of the drag link starts moving in an arc while the axle is moving in a vertical plane. This causes the drag link to undergo tensional forces as the drag link "needs" to be longer to reach the axle during greater axle deflections. The drag link is not going to stretch so it pulls against the steering arm and the pitman arm. The steering arm has less resistance and the pull of the drag link slightly turns the wheels. Minor axle deflections that occur in normal driving generally are not generally large enough to cause the wheels to turn as described above (bump steer).

When you put a lift on a truck and do not reset the geometry you have relatively move the pitman arm out of the 0 or center position. So what happens. Effectively you have moved the pitman arm to one side of center by the number of inches of lift. Now what happens. Let's say that you have moved the pitman arm 2" inches off center in the direction of axle downward motion. Your going down I-90 and hit a bump where the pavement has settled away from a overpass giving you a good axle movement. As your axle move up it moves toward what is the 0 or center point of the drag link rotation in relation to the pitman arm. As the axle extend it starts 2" from the center of rotation then extends several more inches as the axle moves downward. Now it moves (deflects) far enough to get to the point where the pull from the drag link will start turning the wheels. IE bump steer. Which would not normally have happened but the lift moved the steering geometry to one side of center causing bump steer in that direction. Sometimes bump steer can be significant and can cause quite an exciting time.

The reason why I brought this up is that it was stated that the steering wheel was not centered after the lift. This would be expected behavior. When you move the geometry 2 inches to one side of center the drag link needs to be longer to reach the steering arm on the axle. Since it can not stretch you turn the steering wheel slightly to move the end of the drag link closer to the axle. That is why the steering wheel is off center.

Correction of bump steer is usually done by putting a longer pitman arm on which put it's end back into the 0 or centered position.
//**********************

By Ron on Wednesday, August 15, 2001 - 07:57 am: Edit

Blah blah blah.

Loosen clamps on the drag link, spin link until it lengthens enough to center the wheel. You will have no noticable bump steer. don't worry

Look at the allignment thread a ways down.

Ron