The Stovebolt Site Forums The Shop Area The HiPo Shop Front Axle, How Low Can You Go?

Me again, I know I may have asked before but I thought some more input might help. I bought a 3 inch drop axle and mono leaf springs to install. The springs should give another 2 inch drop. I keep hearing about bumpsteer? Back in 1978 my uncle had a 1956 ford truck that everytime you hit a bump the wheels would shack back and forth until you slowed down. I took it to Vo-Tech and kept shimming the axle until it quit. Would this be the same thing you are all talking about. And if this happens take the same approach to correct it? I really want to keep the straight axle but want it to be pretty low. Thanks!

Are you taking the rear down too or creating a major rake?
Scott

Good article to read abut "BUMP STEER"

http://www.circletrack.com/chassistech/ctrp_1001_bump_steer_explained/index.html

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

I bought mono leaf springs for the back also. I have 285/60/15 for rear and 235/70/15 for the front. Not sure how much of a rake but I guess I'll find out. Hope to have a rolling chassis by end of summer. Maybe?

Read that article. Got some of it but alot was greek. How do I translate it to a straight axle aetup?

The article doesn't address straight axle set-ups. You can boil bump-steer on a straight axle down to one component - the drag link (assuming kingpins, wheel bearings, tie rod ends, and steering box are in good condition and adjusted properly). With the frame sitting as low as you will have with a 3" dropped axle and mono-leaf springs, the drag link will have a serious up-angle from the point that it connects to the pitman arm, forward to where it connects to the steering arm. To eliminate bump steer the drag link must be as close to level as possible.

I can't tell from your posts what year truck you have, but on some models a simple drag link "flip" at the steering arm will get you there. Others' have heated and bent the steering arm downward to get the drag link level (do your research on this one or have it done by a pro), you can also relocate your steering box higher on the frame with a simple bracket made from 1/4" plate. There are other ways as well, I would contact Sid, he's the expert on these set-ups, he can provide you with exactly what you need.

My 3" dropped axle and mono-leafs has zero bump steer, so I know it can be done.

You must get the drag link to be flat and not at any angle for it to steer correct which means the ball on the bracket needs to be flipped and the drag link will need to be split and tuned 180 degress to line up like what I had to do when I put the dropped axle in my 37.