The Stovebolt Site Forums The Shop Area The HiPo Shop Power Steering conversions and Bump Steer?

I'm considering a power steering conversion on my '54 GMC 1-ton. Since the kits are all for smaller trucks I know I'll be fabricating at least some of the pieces. I was looking at the frame and spring mounts and thinking about the conversion, and now I'm concerned about bump steer.

Most the conversions I've seen place the steering box ahead of the axle. The spring pivots at the rear and has a shackle at the front. In the original design getting the drag link to parallel the spring and arc the same as the spring would not be real difficult. Placing the box ahead of the axle, with the shackle as the spring pivot, do you have more bump steer than stock?

Steve,

Here is a simple thought

I have seen hydraulic cylinder assists on sweet and appleton racks for racing. They have a small servo unit that you could place in-line with your existing steering and mount a simple hydraulic cylinder to the tie bar.

other than that, yes, you will have a bit of bump steer the way you described, however I do not know how much more without the geometry to calculate it.

Scott

Thanks for the reply Scott.

Aside from the desire for the assist, one of the reasons for converting to power steering is that I plan to run a big block in this truck, and moving the steering box ahead of the axle the way the typical power steering converstion does seems like a way to both solve that problem.

An option that doesn't move the steering box leaves me with the interference problems.

I've been thinking I should draw the whole thing out to get the geometry sorted out, but haven't done so yet.

Steve two way to lessen the bump steer is one insure that the tie rod is parallel to the ground when at ride height and two use the longest tie rod possible. That is why I chose to use a crossover system where my tie rod goes from the driver side to the passenger side this makes for a tie rod that is a good twice as long as if it were to run along the frame.
If you are going to mount the box ahead of the axle put it as far forward as you can get it, if you can put it right behind the bumper. Another problem I ran into may not be of concern to you is when I tried mounting the box as you are thinking witch works great on the pickups but with the 10.00 22.5 tires on my 6100 the tire would hit the box and severely limit my right turns. If I were you I would try to mount it temporarily and then turn the steering from lock too lock and make sure everything clears.

Grant I'm trying to work out several issues at the same time, and getting the steering right seems one of the most crucial.

I'm still debating IFS versus straight axle, and had almost decided straight axle was the right answers, but now I'm reconsidering. Steering, disk brakes, and clearance for a big block, are the issues I'm trying to coordinate.

My approach to minimizing bump steer with the straight axle is to try to make sure the drag link or tie rod moves exactly parallel to the movement of the axle (horizontally and vertically).

If when going straight down the road the pitman arm was at the same distance from the steering rod as the spring mount is from the axle, and the line described by the drag link was both parallel and on the same plain as as one drawn from the spring pivot to the axle, then I should not have any bump steer, or at least very little.

The distance from the centerline of the axle where it meets the spring to the front shackle bolt is 19 inches. If I put the steering box 19 inches ahead of the pivot point on the stock steering arm, I'm mounting it the same place as the spring mount. Not insurmountable, but not as clean as I'd like.

But since the shackle is at the front it allows the position of one of the points on the plain to shift around, there doesn't seem to be a way to make that work.

I've almost convinced myself to go back to the IFS approach. I think that way at least I have no questions about the steering...

I don't think the movement of the shackle will cause you any problems, because the fore/aft position of the axle isn't going to change. The shackle simply lets the spring stretch or shrink in overall length as the axle moves in bump/rebound. The radius from the spring pivot to the centerline of the axle shouldn't change enough to induce a noticeable amount of bump steer, regardless of whether the steering gear is at the front or rear, IMHO.

Jerry

I agree with that. The spring does get longer as it flattens out, but you can't do anything about that. If the steering gear is in front of or behind the axle, the axle will move exactly the same amount; a negligible amount.
Aim to have the steering gear box close to one end or the other of the spring, and it will work just fine.

Or go with the crossover type linkage like Grant and I have, I can fit 9R22.5" tires, 38-39" OD, and the gear box is completely out of the way of the engine too. I have no steering issues with my setup, however one day I could see a track bar to lessen the side loading of the shackles, and if it is as long as the drag link then there will be absolutely no bump steer, not that there is any to start with..

Grigg