Originally Posted by MARTINSR
Now again, just throwing something out there I don't understand. Wouldn't more arch have more sideways movement? I would think the straight spring of a 69 F body would have less sideways movement than a large arched spring because that large arch can "swing" side to side at the bottom of the arch like a pendulum. Even if the spring is a mono leaf as I think it is on that 69 Firebird, wouldn't it being flat make all the difference as far as side movement of the rear axle?
I personally am not a fan of poly bushings because I like the cushy ride and lack of noise of the rubber. On my Gran Sport I have poly on one side of the sway bar links for instance so I get a little bit of stiff without making it totally stiff with the poly at both ends.
You are correct, as the spring moves through an arc the axle will move longitudinally and laterally . This effect can be utilized, as I stated in an earlier post, to your advantage. Primarily, the angle of the spring as it is mounted in the car (as viewed from the side) will dictate how the axle rotates in a bump. Most designs when the body rolls over in a corner will make the axle turn the rear of the car roll into the turn (roll understeer) rather than out of the turn (roll oversteer). That is why if you look at the mounting of the leaf spring from the side, the front eye is lower than the rear eye that is mounted in the shackle so the spring is angled down toward the front. You can also see this effect when you lower a leaf spring car, the wheel is frequently no longer centered in the wheel opening, it has moved forward.
Very rarly is a leaf spring designed to be flat when installed. If the spring is flat when installed, it has likely lost some of its ability to support the load and needs to be rearched or replaced. If it is flat at ride height, it will reverse arch in bump and can tend to make the rear axle steer outward on a turn (roll oversteer). I am making several assumptions in saying this, but it is generally true.
Another intersting feature of many Hotchkiss suspensions is that the axle is mounted forward of the center of the spring. That helps in roll understeer, and I also have to believe that it helps with wheel hop under braking and acceleration. The traction bar effect.
Lastly, it was discovered by GM back in the 60's that having one shock mounted forward of the axle and one rearward helped control wheel hop too!
Just like everything else in the car, there is usually more to it than meets the eye.