4-link upper link question

[MoparCar]

Are there advantages to having the upper links on a triangulated 4-bar angled towards the center or towards the frame from the rear looking forward? I see various kits have different mounts. Some mount towards the center of the rear and angle to the frame and some mount towards the ends of the rear and angle to the center. Is it packaging or engineering or both? Usually the lowers are straight on both set ups.

Thanks

[greencactus3]

it changes the dynamics a lot. it will be a long explanation. picking up a good book might be the best way to figure out the differences.
or at least i dont have the capability to explain in a couple paragraphs

[MoparCar]

I have Adam's book but this doesn't really explain. I'll have to look at some additional info. Anyone else?

Thanks

[The WidowMaker]

im not an expert and i dont have any of the books, but here is a little to think about.....

the rear roll center is the pt at which the rear end rolls. its the pt at which one line passes through a seperate vertical line drawn through the rear axle centerline. the first line is drawn through the pts at which the uppers converge and where the lowers converge. therefore, if your uppers are pointed down from the axle to the frame (im not sure why they wouldnt be), and they are wide on the axle and narrow on the frame, then their convergence would be in front of the axle and lower than the frame mount. but if they converge behind the axle (narrow on the axle, wide on the frame), then the pt of convergence will be higher than the axle (read; high RRCH).

someone may have to correct me, but from a RRCH standpoint, it would make sense you have them wide on the axle and narrow on the frame.

now conversely, having them narrow on the axle would make me think it would act more like a three link or wishbone. you would still get the lateral stability, but you shouldnt get the binding of a traditional C4L. i am talking about really narrow though, maybe just inches apart.

or you can just run a parallel 4 link and watts and not worry about it at all.

Tim

[Mommy Wagon]

Traditional 4-link setups create lateral movement when the suspension is under load, and when under a state of unload. This is caused by a solid bar that is installed by attaching one end to the frame, and attaching the other end to laterally to the differential housing.

A triangulated 4-link allows vertical movement without lateral movement (if correctly set up). This also allows equal force dirtibution when cornering, instead of applying the lateral force to one small area of the differential housing.

Typically the lower bars (trailing arm) are parallel to the frame, and the upper bar mounts to the frame and to the top of the "center section area", creating the triangulation. Mounting the suspension points on avarious locations (wide lower, central upper) also allows more even distribution of forces when street maners and handling are the goal. If all you are concerned with is straightline acceleration then none of this will make a difference.

Basics:

Triangulation is always stronger than a boxed set up.

When proper bracing is performed, lighter materials can be used.

Triangulated 4-link suspensions also leave less room ofr exhaust, planning is more essention.

This is the basic explanation. There are many articles available on the subject, most of which dabble in the related physics of the quastion.

[MoparCar]

Thanks, I've been reading alot of the archived threads on suspension. None that I've found exactly explain the difference but I do like getting a better feel for the aspects. I look at the triangulated 4-links and each seems to do it a different way for the same basic purpose. air ride, Morrison, Chassisworks, etc. That's why I ask. All of these are very good systems and all of them know suspension. I've got an E-body mopar so I can't compare them side by side, but take a Camaro and all these companys make there suspensions a little different for the same car. Besides packaging, what would be the difference?

Thanks for the replies.

[Norm Peterson]

You'll want a book that describes in general how to determine things like roll centers and axle steer rather than one that kind of says "here's my answer". Fred Puhn's "How To Make Your Car Handle" is arguably the best of the softcover books in this respect. Better (and considerably more expensive) is the Millikens' "Race Car Vehicle Dynamics" (a thinking cap being a recommended and separately-sourced companion item ).

Anyway, differences among all of the various rear suspension arrangements that use a pair of uppers and a pair of lowers (at least one pair must be skewed in plan view) amount to what's reasonably possible with the specific arrangements in terms of roll center height, axle roll steer, and anti-squat. And locally, required details for strength.

For example, with the typical GM A/G-body and Fox/SN95 Mustang versions of the C4L (pumpkin-mounted uppers, slightly converging lowers) you can get reasonably low values of axle steer together with good to reasonably high percentages of anti-squat - but there isn't much you can do about lowering the high roll center height, which affects handling and ultimately your spring/sta-bar choices. Move the uppers out onto the axle tubes and you can get the roll center to drop, but might have to choose whether to favor less axle steer or more anti-squat.

Skewing the uppers inward from the axle ends instead will probably make the rear roll center migrate vertically as the suspension travels by more than the OE C4L (above). Not that that's necessarily a bad thing (it's not), just different. The other parameters (axle steer, anti-squat) also vary with suspension position and will also vary differently in this arrangement vs the OE-style C4L. This is sort of a Satchell link flipped upside down.

With any arrangement of triangulated 4-link, you still have the issue of chassis roll relative to the axle, and here's where all the discussions of bushing materials and designs, rod ends, and specialized links (aka "arms") comes in. Something has to "give", somewhere, to permit this relative angular motion either as bushing compliance or devices that allow rotation but not axial motion.


FWIW, the roll center is not a point about which the chassis rolls even though it's the height at which a lateral force produces no roll at all. Think of it mostly as being an analytical convenience that has other implications instead.


Norm

[MoparCar]

Norm,
Thanks for the details. I'm going to pick up Puhn's book. I have Herb Adams Chassis Engineering already, but I see I need more info.