JL9000

I did what I was told to do. I don't have much more info on this, nor can I say the kit works at this point. For details you'll have to contact J's Racing directly.

I don't imagine the kit being too helpful unless it's slapped on a slammed vehicle.

No it isn't, but they do make bumpsteer kits as well.

Race Miata

I haven't had much experience with the S suspension yet but I'd like to mention about roll-center/roll-couple in general.

High roll center (or short roll-couple in particular) can work wonder sometimes. Remember suspension tuning is always a compromise. How much performance and how much ride-comfort do you want? Stiff springs reduce body roll but also add ride harshness. Stiff anti-rollbars reduce body roll more effectively than springs do but they also reduce independence between left and right suspension. High roll-center reduce body roll but doesn't have those side-effects. Sounds win-win? It is, at least for car manufacturers...

One problem with high roll-center is that when suspension compresses roll-couple increases. So, the result is that the harder you corner the lower the roll stiffness. It's like something sucking the side of the car to the ground when it leans. This behavior becomes more obvious on a slalom run. Every time you pull the car from leaning on one side to the other, it's like dragging a windsurfer sail out of the water (of course it's much exaggerated but you get the idea). Some real race cars run very low roll-center, and perhaps even below ground.

I remember my brother and I had some discussion before about how we define a car leaning worse than the other. Naturally, a car leaning the bigger angle suggests that it leans worse than the other car. However, I have a different opinion. I don't care if my car (e.g. my E36 when stock) leans (say) 5 degrees more than the other (e.g. my '90 miata when stock) as long as it takes less time to pull itself from leaning on one side to the other compare to the similar action on the other car.

So how high is roll-center too high? From my experience (or perhaps biased opinion), my '90 miata had too high roll-center for my taste when stock. My E36 felt alright when stock. My S seems alright but then the suspension is so much stiffer when stock it's hard to tell. So if there were an RCA kit for my '90 miata, I would definitely not bother with it. What about an RCA kit for an S? I can't give you a quick answer until I have some controlled experiments with it. Besides roll-couple arrangement, an RCA kit also affects camber curve of a given suspension. The -ve camber gain on any suspension increases exponentially, especially with in-wheel double wishbone like the S runs. That means on a lowered car without an RCA kit, the camber curve is already steeper than stock. This may or may not be advantageous since a properly lowered suspension should result in less body lean (i.e. less suspension compress). With an RCA kit, the camber curve will become even more steeper. Whether this is good or not there's only one way to find out, i.e. to try it out.

Race Miata

Another thing about this RCA kit is that unlike a strut-type suspension, our in-wheel double-wishbone design runs the spring/shock assemblies on the lower arms. That means you will get a lower ride height with the RCA kit even without changing the springs. If you don't have adjustable coilovers with enough height adjustment range, this could be a problem.

Race Miata

Actually, IMO the RCA kit should really include the bumpsteer kit as a full kit. Since "shimming" the lower arm may defeat the purpose of "shimming" the tie-rod end, so "shimming" the lower arm alone without "shimming" the tie-rod end could potentially increase bumpsteer than without the the RCA at all (hard to say, depending on geometry).

ImportSport

Tag for a very good explaination.