Truck-arm Angle?

[redss86]

Hey guys needing a little help here,

I am in the process of setting up my truck-arm rear suspension, but I am wandering which way you guys think I should set it up. The big question I have is how much of an angle, if any, should I have at ride height? The crossmember I am using is from Stock Car Products. It has two holes on the LH side for the arm and three on the RH, but I am going to cut the top hole off the RH side for extra floor clearance. I don't believe I would ever use it to begin with. Let me know what you think.

Here is what I have right now. At ride height w/ the arms in the bottom hole, they have a 1* up in the front. If I move them to the top hole then it increases to approximately 2.5-3*. The arms will be approximately 5-6" off the ground at the front mounts. The vehicle is an 86 Monte Carlo SS. The height of the car at the bottom edge of the rocker panel pinch weld is approximately 6.125". For the front suspension I plan on going with the G5 setup from SCandC.

Thanks for all your help.
jmarti81

[Norm Peterson]

As with the tuning most other rear suspension arrangements, you'll need to satisfy a few other things.

The height of the virtual intersection of the two arms should be kept lower than the height of the center of the PHB. Since the PHB is also the primary determinant of the rear RCH, the PHB height is going to depend on the front roll center height (which is about 2" for the G5, at least with the tire size used in its design).

I'd avoid even having the arm intersection height too close to the PHB height too. On trailing throttle, the rear will rise and the axle will steer slightly if you're also turning. Generally this will be a reduction in vehicle understeer, which can be useful in an auto-x. But if the heights are such that the axle steer goes past zero into vehicle oversteer (aka "loose steer"), it will feel "tricky" and nervous if you find yourself sharply backing out of the gas while turning (ref. trailing throttle oversteer / lift-throttle oversteer).

I'd guess that a level PHB that's in the neighborhood of 2" - 4" higher than the truckarm virtual intersection point and somewhat more than that higher than the 2" front RCH might define a reasonable starting range. Any firmer numbers on this difference in heights needs more data, as the rear steer variability effects depend on the distance between the rear axle and the virtual intersection of the arms and on how much suspension movement can be expected to occur (read: wheel rates).

Norm