Ordering Swift Springs for Koni Yellows? (2005 Rx8)

Hey guys, long time lurker here, but this is my first ever post. I've got a 2005 Rx-8 that I just purchased after selling my old NB in January (actually that car is now the "Hellcat Miata" haha). Anyhow, I love the way the car handles, but I think the limit right now on my car comes down to tires and suspension. I plan on Autocrossing and Tracking the car, sometimes taking it out on the weekend to rip some backroads. I've come to like how the car handles during high speed cornering (top of 3rd, 4th gear speeds), I just find it to feel a bit lazy during transitions and overall response, I'm also getting a lot of understeer mid-corner. It rotates adequately with some throttle, but I miss how balanced and ready that miata was to rotate off the brake pedal. 

So this week I bit the bullet and ordered in some Koni Sport Yellows to go on the car, they should be here in a week or 2. My problem really comes down to choosing springs. From what I can see online, nobody sells a set of lowering springs with any real spring rate increase intended for track use. Again, I'm not really going to be using the car for daily use at all, so I don't really care about ride harshness.

I'm looking to run in the neighborhood of 400lb front and 250lb rear (I'm open to suggestions for spring rate), more than double what most of the lowering kits from Eibach, TEIN, etc. feature. So I did some more research, and it seems a few of the guys doing STX are keen on Swift springs, I've heard the name mentioned a lot before in general racing chatter.

I've never ordered a set of custom springs, so I'm not sure exactly how to find out which size spring I need. On Swift's website, it mentions Inner Diameter and Spring Length. I've searched pretty extensively to find spring dimensions from OEM, Coil sets, and lowering spring kits to have something to base off of, and cant find any online. Do I need to just take out one of my OEM springs and measure it, and buy some slightly shorter ones? lol. I'm new to building a custom suspension setup, but if all goes well, I think I'll have a really good setup ready for some Star Specs (or those pretty new Nexen SUR4Gs!!) for under $1000. I'm going to be autocrossing the car in a week or two, and hopefully running TNIA at Palm Beach shortly after, so I should be able to get a better baseline for the stock performance. 

Thanks you guys, here's a pic of her.

I'm no suspension expert by any means, but I installed the eibach pro-kit lowering springs and hotchkis front and rear sway bar kit on my 2004 RX-8, and I've been thrilled with that combo.  I didn't even change the shocks/struts and it was a great upgrade, and the car feels perfect to me now. (I really only drive it on track).  

Perhaps my original components were worn out (car had about 100k on it when i got it) but I found it to have significant body roll in the corners with the stock set up.  It has virtually none now, and it easily shaved 2 sec/lap(around 2 min to 1:58 give or take) at my main track.  I can rotate the car with the throttle easily and consistently.  I'd say it's the best handling car I have!

I know that doesn't answer your question, but thought I'd chime in anyway.  Don't see too many rx-8's on track these days..

(I also run nexen sur4's, mostly cause they are always on sale at tire rack, and they are a great match for the car IMO)

Yeah, you're probably going to have to take off a spring (and subsequently the strut) to get the inner diameter and overall length.   Unless you can fannagle a way to measure the spring diameter with the strut in the way.

The formula involved with coil springs is pretty simple though, and should give you the info you need to play with the numbers until you find the one that best suits your needs.  Certain things are going to be relatively fixed (coil diameter, modulus of steel, the number 8) but certain things you can play with (how much lower you want to go, how thick the wire needs to be):

(Modulus of steel*diameter of wire)/(8*number of active coils*diameter of coils)

*active coils are coils that are not bound.  you should be able to see daylight between them.


Or you could just use an online calculator:  http://www.reliablespring.co.uk/calculator.htm

Not the same car, but I ran H&R sport springs on my 05 Legacy GT with the Koni sport shocks and they were fantastic. Firm enough to make quick transitions but not so much that it would launch the car into another lane if you hit a bump (or rumble strip) mid-corner. 

One thing to keep in mind is that Konis are usually designed to work well with spring rates that are pretty close to the stock spring rates. You may have to get them revalved if you're running much higher spring rates.

I'd be tempted to run the Konis with stock springs and change the sway bars rather than doing something more drastic.

BoxheadTim said:

One thing to keep in mind is that Konis are usually designed to work well with spring rates that are pretty close to the stock spring rates. You may have to get them revalved if you're running much higher spring rates.

I'd be tempted to run the Konis with stock springs and change the sway bars rather than doing something more drastic.

Unless Miata yellows are substantially different, I'd think they can handle plenty more rate pretty well. 

I thought 400/300 was excellent, but one needs to dial in the rebound to get that feel. 

Heck, I'm at 8k/6k (448lb/336lb)  now, and I find them not terrible, but bump stop selection and, again, fine tuning the rebound is really, really important to make them so. 

Get the rates for the springs you want to use and call Koni.   I had questions way back when and thought I had too much rate, they told me it was well within the dampers capability. 

They seem quite versatile, at least for Miata units. 

I have a set of Swift Springs on my Fiesta ST and I really like them. The ride is better than stock, and transitional response is excellent. They are linear as opposed to progressive, and they were designed to match the stock dampers so they work well with the Konis without any revalving needed.