Auto-X / Roadrace rear suspension 1969 Valiant
Alright, I'll bite. If nothing other than to see some of the other responses and learn some more! Maybe @HemiDenny will chime in...
Your vote is tough, kinda needs a none of the above.
So in general, 3 links are best for road racing and other racing disciplines because of how adjustable they are. The problem with them is the location of the upper link. On an A-body if you run the upper link off the top of the housing it ends up being really short, or it ends up being inside the cabin space and running into the rear seat. If you're building a full on race car you're probably not worried about the rear seat or running structure back there to mount the upper link. But the XV kit is worried about that stuff, and the upper link is too short on account of it. If you think about how the axle will move, you see how the length of the upper link compared to the lower links will control how much pinion angle change there is. The XV set up will have a ton of pinion angle change, which isn't great. Here's an install on an E-body...
XV Level II Suspension Installation by jvike | plymouth | diys | DIY
You can offset the upper link to the right, and if you match the offset distance to your rear axle ratio you can get the tire balance equal on acceleration (counteracting the torque wrap). But you still might get a load balance issue in the back under braking. Regardless, that would mean a different upper link than what the XV kit has, which basically means you're designing and building your own 3 link. Which, if you're hell bent on the 3 link, is the way I think you'll have to do it. The XV kit isn't really a racing 3 link kit, if you want what I think you want you'll have to design and build it yourself.
The RMS Street Lynx is a triangulated 4 link, so, it actually doesn't have all that much adjustability. And because of the angles on the links and the relatively short upper links you can get binding with longer travel. I kinda think that's didactic, a lot of people run the RMS or versions of it that amount to a triangulated 4 link. But for all out racing, you lose adjustability compared to a 3 link and possibly even some suspension travel compared to a leaf spring set up. It works better than a 3 link with the stock chassis and floor locations, but it's still designed around the install and not cutting stuff up, and geometry is sacrificed for not cutting up the bodywork.
The fiberglass leafs with a panhard bar doesn't do much of anything different than a metal multileaf set up with a panhard. You lose some unsprung weight, but that's really the only difference and the axle housing itself is the lions share of the weight. Everything else will work almost exactly like a mulitleaf set up. Maybe a slightly faster spring reaction because you aren't sliding leafs on each other? Pretty theoretical at that point.
The Dan Gurney AAR's rear suspension ( to be taken with a grain of salt, because they run according to vintage race rules, not by what's "best")
And of course, Tom's car did very well with its Hotchkis rear leaf springs. He wasn't even running spring sliders in the back, which can help reduce a little of the resistance you get from the rear shackles and a little better side control too
Alright, I'll bite. If nothing other than to see some of the other responses and learn some more! Maybe @HemiDenny will chime in...
Your vote is tough, kinda needs a none of the above.
So in general, 3 links are best for road racing and other racing disciplines because of how adjustable they are. The problem with them is the location of the upper link. On an A-body if you run the upper link off the top of the housing it ends up being really short, or it ends up being inside the cabin space and running into the rear seat. If you're building a full on race car you're probably not worried about the rear seat or running structure back there to mount the upper link. But the XV kit is worried about that stuff, and the upper link is too short on account of it. If you think about how the axle will move, you see how the length of the upper link compared to the lower links will control how much pinion angle change there is. The XV set up will have a ton of pinion angle change, which isn't great. Here's an install on an E-body...
XV Level II Suspension Installation by jvike | plymouth | diys | DIY
You can offset the upper link to the right, and if you match the offset distance to your rear axle ratio you can get the tire balance equal on acceleration (counteracting the torque wrap). But you still might get a load balance issue in the back under braking. Regardless, that would mean a different upper link than what the XV kit has, which basically means you're designing and building your own 3 link. Which, if you're hell bent on the 3 link, is the way I think you'll have to do it. The XV kit isn't really a racing 3 link kit, if you want what I think you want you'll have to design and build it yourself.
The RMS Street Lynx is a triangulated 4 link, so, it actually doesn't have all that much adjustability. And because of the angles on the links and the relatively short upper links you can get binding with longer travel. I kinda think that's didactic, a lot of people run the RMS or versions of it that amount to a triangulated 4 link. But for all out racing, you lose adjustability compared to a 3 link and possibly even some suspension travel compared to a leaf spring set up. It works better than a 3 link with the stock chassis and floor locations, but it's still designed around the install and not cutting stuff up, and geometry is sacrificed for not cutting up the bodywork.
The fiberglass leafs with a panhard bar doesn't do much of anything different than a metal multileaf set up with a panhard. You lose some unsprung weight, but that's really the only difference and the axle housing itself is the lions share of the weight. Everything else will work almost exactly like a mulitleaf set up. Maybe a slightly faster spring reaction because you aren't sliding leafs on each other? Pretty theoretical at that point.
The Dan Gurney AAR's rear suspension ( to be taken with a grain of salt, because they run according to vintage race rules, not by what's "best")
And of course, Tom's car did very well with its Hotchkis rear leaf springs. He wasn't even running spring sliders in the back, which can help reduce a little of the resistance you get from the rear shackles and a little better side control too
