From what I have read, bump steer has nothing to do with how low the truck sets or the axle, its a matter of geometry with the steering gear box and spring arc's.
The draglink is " X " distance long, steering arm to gear box. The axle is " X " distance from the rear spring hanger or front hanger depending on which end the shackle is on. The axle moves in a arc as goes up and down over bumps, pivoting around the solid spring hanger, the drag link follows the movement in a slightly different arc pivoting around the pitman arm end. Bump steer is a result of the two arcs being to different from one another, and as the axle moves up and down, it pulls the draglink causing the wheels to turn.
The more the axle is lowered or raised depending on how you look at it, the arc in which the axle rotates in gets more dramatic. Draw it out on paper and you will see what I mean.
My '37 axle is mounted solid to the frame at the back and the shackles are out front, this keeps the two steering arcs pretty close together. The steering box is with in a inch or so of the same distance the axle is in comparision to the spring hanger. I lower it about a inch and half last summer, this changed the center position of the steering wheel about 1/8 turn, so i had to make a adjustable draglink to shorten it up to keep the wheel on center. This tells me that even a little off from factory heigth will change it. I also had to add more caster to keep it with in spec's, I went from 2 to 3 degrees.
Joe
