I called KW and they were perfectly willing to give me data on the front springs (61mm ID, 200 mm length, 80N/mm) but are very cagey about the rears. They told me that the ID is 61mm but couldn't give me the length or any data on the spring rate curve. I pressed the guy a bit about whether it was more of a dual-rate or smooth curve and he threw me a bone and said it was more of a curve.

I'm searching around for somebody nearby with a spring tester so I can figure out a starting point for the new spring rate.

In the mean time: Has anybody tried this yet?

 

I've got to ask, why? I've got KW V3's and they're very good both on the street and on track. Are you having handling problems that you want to remedy, or are you just weary of non-linear springs? If it's the latter, I say try them and then see if you want to change later, since you may be perfectly happy with them as-is.

 

I agree that they are good on the street but I didn't get them installed until after the last PDX in this region so no track experience.

I have delusions of being competitive in STX autocross and I want more consistent performance across a range of conditions.

I figure as long as the new linear spring rate is around the upper 3/4 of the progressive rate, the shocks shouldn't mind to much (I have no actual data to back this up but it sounds reasonable).

 

I've tracked mine a number of times with standard spring rates along with Hotchkis sway bars front and back on the middle settings. It has a very nice nearly neutral balance with just a touch of understeer that can be easily overcome in slower sections with throttle to get oversteer. I'm quite happy with this balance and don't want to change anything myself. Note that alignment, tire size and tire pressures will affect this. I'm running a pretty aggressive alignment at the moment with 255/40/17 NT01 track tires.

I've never felt it to be too soft, inconsistent or anything like that.

I'd say try it first, then change if you want. I recall reading that Tein taper springs work in the back of RX8's with the stock shock mounts, but I'm not sure if that was with KW's or another brand. Something to consider when/if you change.

 

I've autocrossed it several times since installing the KW's and Hotchkis miata front and rear bars (middle front, soft rear) and it's an inconsistent hand full. I started off with the swaybars soft both front and rear and it was impossible to trail brake without exiting corners backwards. Moving the front sway bar up to the middle helped but it's still pretty twitchy.

I'm probably going to hold off on making any suspension purchases for at least another 6 months as I'm hemorrhaging funds right now. My plan is to buy a spring rate tester in the spring and then go from there. I still have my OEM rear sway bar and a Hotchkis RX-8 front sway bar to fiddle with.

 

In my experience, this chassis wants very little rear bar, if you want the car to be neutral. Before exploring rear springs, I would put the OEM rear bar back on and see how that feels.

Three thoughts:

1. When I was running Bilstein PSS9s, which have progressive rear springs, I found the car would squat and set beyond the lighter portion of the rate curve quickly and predictably on the track. I quickly adjusted to it, and it did not bother me at all. AutoX is quicker and tighter, so it may be different.

2. When I was running those PSS9s with stiffer front springs, the front to rear spring rate ratio (1.63) was nearly identical to that of the KW V3s (1.60). In that setup, I was running a Progress front bar with the OEM 16mm rear bar, and the car was biased just a touch toward corner entry understeer. The matching Progress rear bar was way too much. My current setup has a higher spring rate ratio (2.00), and I am running the same front bar with a puny little 11mm NC Miata rear bar.

3. If the car is lowered, it is imperative that you replace the stock sway bar end links with adjustable units and set the sway bar preload properly. Otherwise, your bars will bind, and you will experience odd behavior like snap oversteer. I assume you know this and am just covering all the bases.

It is pretty common to run RX-8s and Miatas with a lot of front bar and little to no rear bar.

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