Originally Posted by
Bruce Livermore
Sorry Scott, I got sloppy and started subbing spring rate for wheel rate. And of course you are correct: the angle of the turkey arm does not change the torque /degree property of the spring itself. I've edited my post above.
My point is that changing the turkey leg angle DOES change the wheel rate and I should have been perfectly clear on that. I'd argue that wheel rate is what matters regarding handling, thus my focus on how the turkey leg angle affects it.
For a given spring, if the turkey leg starts pointed up at ride height (link pin center higher than its beam bushing center) the wheel rate will be higher than if the turkey leg is horizontal. The wheel rate will also go up as the car goes down (in jounce). I'd say that is "rising rate" since the force per inch (of wheel displacement) rises as the tire goes up (chassis down). In terms of motion ratio, in this case, the further the wheel goes up, the more it rotates the turkey leg (in degrees) per inch of vertical travel of the wheel, thus increasing the spring deflection (twist) per inch of vertical wheel travel. Is the "worse" or better as far as motion ratio? Depends on what worse means. The spring is getting more mechanical advantage (or the wheel less) which is what increases the wheel rate.
If on the other hand, if you mount the beam so high on the chassis that you start with the trailing arm pointing down (at your chosen ride height), the wheel rate will start higher than if the turkey leg is horizontal and the wheel rate will go down as the tire goes up (until the trailing arm becomes horizontal). This would be falling rate (until horizontal).
The most constant wheel rate (least wheel rate change per inch of wheel movement) will occur in the zone where the arm stays closest to horizontal. So if that's the goal one would design the mounting position and ride height to keep the trailing arm closest to level throughout normal suspension travel. And for a given spring, this would also give you the lowest average wheel rate (for better or worse).
I agree on the shock. The motion ratio goes down (less shock length change per inch of vertical wheel travel) as the turkey leg points up and this effect is even worse when using the offset shock adapters.
I hope I got it right this time!