Art Morrison 4 bar thread split from "Rear Suspension Questions" [Archive]

Norm Peterson

10-04-2005, 10:28 AM

. . . I keep hearing binding issues from past suspension posts about converging or triangulated 4 link rear suspensions . . .The answer seems to be "it depends". A word about the use of the term 'bind' - this does not necessarily imply that the suspension becomes locked solid in roll (although it would be damn close to that if you replaced all the OE rubber bushings with press-fitted cylindrical blocks of steel that were free to rotate only about the inner sleeves). It just means that it takes more lateral g's to achieve some given amount of roll and all of the associated additional lateral load transfer shows up at the rear wheels (read: handling can shift to anywhere between less understeer and scary-loose depending on just how much more 'bind' is introduced).

In normal applications involving rubber or other cylindrical bushing materials, yes, there is a finite amount of this 'bind'. The equivalent amount of added wheel rate is loosely tied to the stiffnesses of the bushings. But it's not a linear relationship. About all that can generally be added to that is that it's worse to stiffen some of the bushings (more specifically, certain directions of stiffness in them) or to stiffen some combinations of bushings than others. And that it's difficult to stiffen only the "good" directions (perhaps to reduce wheel hop and compliance steer) without stiffening the "bad" ones. The OE arrangement, softish bushings and all, works reasonably well under most circumstances, and although the Fox/SN95 Mustangs haven't been the top national rides in SCCA's Solo II F Stock (auto-x), they are at least reasonably competitive. Launching at your local dragstrip might be a little less tolerant of bushing softness.

In the all rod-ended application (which may be fine for circle track or drag racing, probably not truly streetable, and unknown for road course/auto-x/etc.) there's some current argument that there is no bind that shows up as added wheel rate, although there may be some lateral axle motion and pinion angle change that gets induced from pure roll that wouldn't be particularly desireable either. As a matter of pure coincidence, there's some ongoing analysis for this specific arrangement (not being done by me). FWIW, the lateral motion issue appears to affect clearance on the insides of rear tires.

. . . and I'm wondering if Morrison's rear follows suit. This is in no way insulting Art Morrison and company but wondering how one of their Max G chassis in a camaro will actually work compared to other systems.Geometrically, it has to behave a certain way, though this gets modified principally by bushing stiffnesses. I guess what this all adds up to is that how well the Max G chassis will work compared to other arrangements depends on the details of it vs the details in whatever you might be comparing it to. For handling, it will also depend on whether your driving really is intense enough to explore the range where the differences will show up enough to notice. If you're a reasonably sane driver but don't auto-x, open-track, or otherwise compete with your car in a cornering environment, you may well never notice. Ride comfort differences may be more noticeable, but that's such a fuzzy individual area (and not terribly important at ~0.8g, so I doubt that anybody is going to be evaluating both criteria simultaneously) that I'm just going to note it and leave it at that.

Norm

sinned

10-04-2005, 04:51 PM

In the all rod-ended application (which may be fine for circle track or drag racing, probably not truly streetable, and unknown for road course/auto-x/etc.) there's some current argument that there is no bind that shows up as added wheel rate, although there may be some lateral axle motion and pinion angle change that gets induced from pure roll that wouldn't be particularly desireable either. As a matter of pure coincidence, there's some ongoing analysis for this specific arrangement (not being done by me). FWIW, the lateral motion issue appears to affect clearance on the insides of rear tires.

NormI can offer this in reference to an all rod ended application, although I have no test data to contribute I run all rod ends on the semi-factory C4L arrangement with less than 1/4" of tire clearance on both the inside and outside of both rear fenders. After thousands of street miles and a few dozen track miles there is zero tire rub indicating virtually no lateral movement of the axle.