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
