If you want low with Mopar SS springs you will need to have them dearched. In my mind a better way to go would be to spec a custom spring from Eaton Detroit (Pure Vision used them on Dust Ya' and Hammer) or your favorite spring supplier. They can build a spring that gets the back end sitting at the ride height you want and have the right spring rate for your setup.

All the Mopar SS springs are based on the shorter A-body units, so you will have to verify that you can use a stock B-body length spring with the relocation kit (I don't know for sure but I think you can) or tell your custom spring supplier that you are using that relocation kit and may need the A-body length spring.

I found this in a write up about Pure Visions "Hammer" 1970 Roadrunner:

"SUSPENSION, STEERING, AND CHASSIS WILL INSTALL CONFIDENCE. Modern K-frame from Reilly Motor Sports uses coil over shocks, tubular control arms, and Flaming River rack and pinion sterring. Red Zone Race Fabrication will be mounting a custom pair of leaf springs from Eaton Detroit Springs in the car using a Mopar Performance spring relocate kit, as well as intalling custom made sub-frame connectors."

I'd e-mail Steve Strope at Pure Vision and ask him some details about his springs. I've e-mailed him before and he seems like a good guy.

Hey! That blue Road Runner is my internet friend Pete's car. Nice guy, and a great car.

I would move with caution on any front end replacement kit on the market today, there seems to be some pretty significant issues with them from all that I have talked to that have direct experience.

It seems like we are not going to be ready for your project timeline for the front stuff in particular, but I have full confidence that our suspension systems will make that comment obsolete! Our shop car is expected to be complete and will be shown at the Mopar Spring Fling this coming year, I think it will change folk's impressions of what can be done to Mopars in terms of honestly good suspension setups.

Whatever you decide, have fun with your project, enjoy every second of the build and best of luck with it.

Mark

While it is true that most "drag racing style" four links (which are really five links when you count the panhard or track bar) are overconstrained in roll, that is not true of all four trailing link setups. Four trailing links, with one pair angled and no panhard, is kinematically free (Art Morrison uses this setup on many of their chassis). Also, although I've not yet fully worked out the kinematics in my head, I think the DSE four link is on the right track by introducing a rotational degree of freedom on roughly the roll axis. I'd bet that the DSE setup is very free in roll.

In other respects - roll center, instant center - four parallel trailing links are no different than three from a design perspective, so if the issues with binding in roll are sufficiently solved, it comes down to the specifics of design and packaging. It really comes down to the design goals for the system, and the competence of the designer in achieving those goals, which is where the most important differences between systems are really manifested.

Not sure what you are referring to? The bushing design? If so, this is not the solution to the constraint problem with a four link (parallel/semi-parallel). The issue is that one upper link needs to get a bit longer, or the other to get a bit shorter in roll in order to create a bind free system. Their bushing design is rotationally "free," at least statically (not sure how it will work when it is loaded in the actually suspension with a thrust load applied to the swivel thing they patented?), but this isn't where the binding issue comes into play. Think of it this way, a rod end has the same rotational freedom as their (unloaded) bushing, though it clearly can't rotate 360 degrees: it still has enough to accomodate the amount of roll angle induced by the axle in practical terms. If you agree, and look at the system with this in mind, you'll see what I am getting at. Again, the reason their system will work in roll is due to the compliance (radial, in this case) of the bushings used.

As far as the side view kinematics and rules, you are completely correct, the four link setup has no disadvantage, nor advantage over a three link, the same rules apply. Here again, the geometry of the system in side view is VERY important in determining the behavior of the vehicle, and it gets even more interesting when you look at how it behaves dynamically, it's not enough to say that the roll center height, IC, etc are "this" by design, one needs to consider the migration aspects of the system in practice to see how the car will behave in real life.

And regarding the AME stuff, the suspension engineer on my team happens to have a few years of direct experience with that setup. ;)

Mark

You're right, introducing an axial rotation DOF in each link will still leave the parallel 4 link overconstrained, but it will remove one additional overconstraint in the case of poly bushings at both link ends, without the NVH and wear issues of heim joints. From that perspective, a three link has the same issues. Just how much remaining deflection is required to accomodate roll in the normal plus/minus few degrees range is the question I don't have the answer to, but if the resulting roll stiffness introduced by the deflection is a low percentage of the total roll stiffness created by springs and sway bar, it will not be a significant factor in the handling. Engineering is the art of informed compromise, and what appears to be a compromise may be well-informed or not.

I completely agree that roll center, roll axis, roll steer, and instant center, and the variance of those parameters through the suspension range, are the primary determinants of the handling characteristics of a rear suspension design. For consumers of these products, if the vendor can't answer what those parameters are for a given design, and why those parameters were chosen, it is a complete crapshoot whether the design will "improve" handling. And to make the point I was originally trying to make, you can't determine these things just by counting links.

The good news for PT guys on a budget is that plain old multi-leaf springs can do a OK job on almost all fronts, with the primary compromises being static friction (which can be reduced by careful design) and unsprung weight (which can't be fixed, unless you go fiberglass, which has its own issues).

Yeah, Katz did some pretty good work at AME. Most of those old-line chassis makers have no serious engineering in their suspension designs, but the AME stuff is pretty good. As it happens, I just picked up the Tri5 Morrison chassis for my wife's 55 BelAir a couple of days ago. It'll never see the track, but it will drive a lot better on the street!

Good luck on your new company, sounds like you guys are pretty jazzed.

How do you intend to "fix" the issues of bind in roll on a parallel 4-link? Since the issues bind cannot be solved using 4 parallel links there is actually a great deal of difference between a 3-link and parallel 4-link.

If you run a great deal of anti squat the "parallel" 4 link will bind more quickly in roll but still slightly less than a converging 4 link with the same amount of anti squat. But even so we`re only dealing with maybe 3* of roll here and we`re not class racing these cars for a living. I haven`t worked with a drag race type 4 link yet that wouldn`t easily move through more than that with no noticable binding at all (unless it used a diag. track bar). So while it may seem like an issue on paper or the computer screen it`s not a problem in real life on a street car IMO. I prefer a good 3 link on a car intended for serious handling or we wouldn`t have built our own system but a decent parallel 4 link can still work pretty well. We`ve converted a couple former drag cars with 4 link into street cars intended to handle well with good results. The 4 link was the least of their problems. ;) Marcus SC&C

Marcus, in my limited track experience I hit 3* of roll easy. I realize a lot of the cars here are pro-bling but some will actually get pushed hard occasionally. Saying the parallel 4-link is not a problem in real life may a bit misleading. If you are going through the trouble of fabricating a rear suspension, why compromise anything?

If you read my post carefully, I did not say that you "fix" "binding" in a four link, I said that if the roll stiffness introduced by such binding is not large compared to springs and sway bar, it will not have a significant impact on handling. That is simple physics. Think of it this way: a sway bar works by introducing a massive amount of "binding" in roll. If the four link moves freely enough that you can lift one side of the axle housing several inches without the other side lifting, then the roll stiffness introduced by the suspension links is negligible.

Make no mistake, all suspension designs are "compromised" from the start. Three link designs are popular in competition because they are easily adjustable, but they have issues with tradeoffs between the most desirable instant center and the most desirable roll steer (same with four links). Also, the change in suspension parameters through suspension travel can be significant if the links are short (race cars tend to use very long links, because they don't worry about packaging in a rear seat area). Torque arm style three links have less of the issues mentioned above, but are less adjustable in some ways, and are heavier. And so on.

I stand by my point - it is possible to make a very good handling car, or a very evil handling car, with three links, four links, torque arm, truck arm, leaf springs, or what have you. It all depends on how well you manage the compromises associated with each to achieve the results you want. It would be great if we could have some of the kinds of discussions here about the effect of various suspension parameters (roll center, roll steer, instant center, camber gain, scrub radius, etc) and their subjective effects on handling, and how to achieve them. There are lots of such discussions on corner-carvers.com, but the atmosphere tends far too much toward the hostile.