Rallycross Miata - The Supercharged NB Sequel

Since being bit by the miata bug and driven the rusty supercharged NA for a few events, I decided to find a nicer NB chassis to move up to.  Unfortunately, I chose the absolute worst time to do this, hardly any decent NB cars were listed.  The goal was to find a clean enough chassis to start with, something with at least working A/C and power steering.

https://grassrootsmotorsports.com/forum/build-projects-and-project-cars/rusty-rallycross-miata-the-supercharged-answer/177830/page1/

When Lof8 Andy posted this up, I could not resist.  Why not start with an incredily clean chassis from Florida and thrash it in Michigan's brutal rallycross circuit?  In seriousness, I won't be driving it in the winter or salty season and a trailer purchase for rallycross travel was in my plans already, so this might just work out.  I had a vacation already planned in Florida, and I worked out plans with Andy to pick it up during my trip and drive it home.  

https://grassrootsmotorsports.com/forum/cars-sale/super-clean-99-miata-for-sale/191602/page1/

Long-term Plans:

• Suspension
  • Bilstein w/ coilover conversion
  • OR FM Fox Coilovers
• Engine
  • Supercharger swap from the rusty NA (yes, it will require many new parts)
  • Megasquirt PNP
• Wheel/Tire setups:
  • 14" 7-spoke NA wheels, Alpha Eurocross 14" tires
  • 15" NB wheels with whatever winter tires I have on hand
• Skid plates and mud flaps
• Vinyl wrap for paint protection and cool points

The goal of the project is to build an enjoyable and reliable RWD rallycross toy for MR class, that can also be driven on the street during the summer months with a tire swap. 

Talking about suspension for a rallycross miata: it's been done before, many many times!  I'm definitely not trying to reinvent the wheel here.  The goal would be stiffer than OEM springs, with good damping tuning, and near OEM ride height.  With re-treads or winter tires being so squishy, we can probably get away with stiffer spring rates and not get beat up.

Short term idea: Koni sports w/ FM Springs.

For now the car is on Koni Sports and FM lowering springs, with FM sway bars.  For a short term solution, it might be possible to raise the trim height of the vehicle with perch spacers or higher circlip grooves (without encountering coil bind), and play with the koni damping adjustment.  The FM spring rates are 211% and 278% of the OEM spring rates, which might be a bit stiff but certainly better than stock.

Tried and True: Bilstein coilovers

From the GRM Miata Rallycross experts, kylini and EvanB, it looks a variety of spring rates with bilstein coilovers would work out.  Cost to convert would be roughly $500 without springs.  

https://grassrootsmotorsports.com/forum/grm/converting-my-str-miata-for-rallycross-its-already-bent-so-why-not/92163/page1/

https://grassrootsmotorsports.com/forum/grm/miata-suspension-for-rallycross/35901/page1/

From the GRM verbatim, and across the web, Dan Edmunds ran rallycross back in the day and a stage rally with very stiff re-valved bilstein and relatively soft springs (10" 225lb front, 12" 130lb rear).

https://www.miata.net/motm/2010/edmunds2.html

Wildcard idea: Flyin' Miata Fox coilovers

If I can make this work in the budget, it would be a really fun experiment to see how these hold up in a rallycross environment.  5" of travel (how much do the Bilstein B6 have?), offered with a 8" spring and helper spring for street use.  Standard spring is 550 front / 375 rear, which is a bit stiff for rallycross, would likely need different springs.

Reality Check: stock springs and whatever shocks are probably just as fast as any of this, but whats the fun in that?

I'm pretty excited to see my old car live on in a world of rally!  I know that's where I'd want to live if I was a miata!

Based off this Mazdamotorsports publication for spec miata (assuming the SM Bilstein are the same dimensionally as B6/HD) travel is listed at 4.6"

http://www.mazdamotorsports.com/wp-content/uploads/2019/01/SM-Dimensional-Explanation.pdf

Looks like fun! Moxnix on the forum here also Rallycrosses Miatas, very fast too. I'm not really sure what his setup is, but his main car is an NB in PR class last I knew. 

I think I recall that the konis may have 2 or 3 slots on the shock body where you can adjust the hight of the spring perch.  You may be able to get a bit more ride height out of the suspension that's on the car now (as a short term option). 

Interesting, from what I read only the NA koni have multiple spring perch grooves, so maybe it got NA shocks?  Either way, its not too hard to throw them on a lathe and cut a new groove.

NB koni

Vs. NA koni yellow

Aah, I did have an NA with konis prior.  Maybe I'm recalling the old car.

Oh man, I'll be eagerly following this thread.  I've got an NA that I'm setting up to use this year for rallycross, and the massive amount of suspension options has  given me some analysis paralysis. For now, I'm planning on putting MSM springs on a set of AGX's with NB tophats.  Mainly because it's all sitting in my garage and free.  

Looking forward to see how your build pans out!

You already have koni shocks.  Just get the coilover sleeves for those and put whatever you want on those?  The car I sold mazduce had I think 9" F350/R220 on NA koni's.

My current car has Xida XL's but I forget the rates.

My Bilsteins are valved to the Dan Edmunds spec, I never really liked the harshness. My next plan whenever I get around to it is coilover sleeves on them and different spring rates and add a front swaybar back in. 

moxnix said:

You already have koni shocks.  Just get the coilover sleeves for those and put whatever you want on those?  The car I sold mazduce had I think 9" F350/R220 on NA koni's.

My current car has Xida XL's but I forget the rates.

That is also on option, probably the cheapest as well.  It looks like the allstar 5" coilover sleeve should fit Konis.

https://allstarperformance.com/alum-c-o-sleeve-5in-bilstein-koni-penske-all64162/

Can the Konis hold up to rallycross? At least on the DSM platform, the Koni shafts are quite bendy, though that may be application specific differences.  

EvanB said:

My Bilsteins are valved to the Dan Edmunds spec, I never really liked the harshness. My next plan whenever I get around to it is coilover sleeves on them and different spring rates and add a front swaybar back in. 

My take is that the damping may have been perfect for landing yumps but it was bone jarringly stiff.  They broke my GPS mount driving it on the street...

I always liked soft damping and moderately stiffer springs, and retain the stabilizer bar in the front.  Teh red RX-7 has front springs twice as stiff as stock, rear springs 50% stiffer, front stabilizer bar retained, and unrevalved Bilstein HDs, and it feels just about perfect.  My S40 had front springs twice as stiff as stock, adjustable rear dampers set as soft as they would go when rallycrossing, retained front and rear bars, and it felt really good for a front driver.

I plan on similar suspension tuning theory for the Mini.

I had Konis on both RX-7s and I really did not like the initial harshness, like they had digressive damping calibrated for "trap door".  Great for transitional response on smooth roads, not so much for following irregular surfaces.  KYB AGXs were better in every way.

One note: always check your data :) I don't know where you pulled up that spreadsheet with the FM spring rates, but it's very  wrong. Our springs had a 342 front rate before 2006, but the rear was actually softer than our current numbers at the time (update: it was 228 lb/in, so that 328 is probably an unfortunate typo of information that's 15 years out of date). I'd love to know where that data lives because it's probably leading a whole bunch of people down the wrong path. Probably FCM and Shaikh will get all wound up...

From the most credible source of information on FM parts, the Flyin' Miata website.

Front springs: 318 lb/in
Rear springs: 233 lb/in
 

engiekev said:

moxnix said:

You already have koni shocks.  Just get the coilover sleeves for those and put whatever you want on those?  The car I sold mazduce had I think 9" F350/R220 on NA koni's.

My current car has Xida XL's but I forget the rates.

That is also on option, probably the cheapest as well.  It looks like the allstar 5" coilover sleeve should fit Konis.

https://allstarperformance.com/alum-c-o-sleeve-5in-bilstein-koni-penske-all64162/

Can the Konis hold up to rallycross? At least on the DSM platform, the Koni shafts are quite bendy, though that may be application specific differences.  

One big difference: the Miata is a double wishbone suspension, so the shocks are just shocks that only see vertical loads. I'm pretty sure the DSM is struts, which means the shock has to take cornering loads. 

In reply to Keith Tanner :

Only in the front, on first (and third?) generation models.

That said, I have seen REAR shocks bend if the chassis is lowered to the point that the upper mount is off angle a lot.  Heck, this happened to the rear shocks of a certain rally E30 last year.

The only time I've seen bent Miata shocks there were accidents involved. And they always seem to involve V-Maxx shocks for some reason.

Keith Tanner said:

engiekev said:

moxnix said:

You already have koni shocks.  Just get the coilover sleeves for those and put whatever you want on those?  The car I sold mazduce had I think 9" F350/R220 on NA koni's.

My current car has Xida XL's but I forget the rates.

That is also on option, probably the cheapest as well.  It looks like the allstar 5" coilover sleeve should fit Konis.

https://allstarperformance.com/alum-c-o-sleeve-5in-bilstein-koni-penske-all64162/

Can the Konis hold up to rallycross? At least on the DSM platform, the Koni shafts are quite bendy, though that may be application specific differences.  

One big difference: the Miata is a double wishbone suspension, so the shocks are just shocks that only see vertical loads. I'm pretty sure the DSM is struts, which means the shock has to take cornering loads. 

Good point, depends on the DSM: 1g were McPherson strut and 2g were double wishbone.  I'm not sure about the shaft diameter on the 1g DSM koni, if it's comparable to kyb agx or the Evo Bilstein (really not many other options at all for 1G DSMs), and if that contributed to rumors about shaft bending. The 2g dsm had an Audi esque design (double-lower-balljoint "virtual steering pivot") , and it was critical to keep the spring coaxial to the shock shaft.

http://farnorthracing.com/autocross/konis.html

Keith Tanner said:

One note: always check your data :) I don't know where you pulled up that spreadsheet with the FM spring rates, but it's very  wrong. Our springs had a 342 front rate before 2006, but the rear was actually softer than our current numbers at the time (update: it was 228 lb/in, so that 328 is probably an unfortunate typo of information that's 15 years out of date). I'd love to know where that data lives because it's probably leading a whole bunch of people down the wrong path. Probably FCM and Shaikh will get all wound up...

From the most credible source of information on FM parts, the Flyin' Miata website.

Front springs: 318 lb/in
Rear springs: 233 lb/in
 

Well that is embarrassing! I had a typo in my spreadsheet (hit 328 instead of 228 as noted) but it does appear this site is out of date.  I was trying to save time pulling from one source (its the 3rd result of searching "miata spring rates", after nebulous forum posts), those numbers are quite a bit different.

https://miatiniracing.files.wordpress.com/2018/06/mx-5-federraten.pdf

Is there way to tell the revision of FM springs other than purchase date?  I assume Lof8's NB is setup with post 2006 springs.

I fixed the spreadsheet image.

Every time I've swapped out Bilsteins, I've gotten improved ride quality.  In rallyX you want travel and compliance.  Bilsteins don't do that from what I've seen.  I'd be real curious about Fox since they earned their rep off-road.  They are pricey though.

I'm facing a similar question with my Subaru.   Koni's or someone's coilovers?  

AnthonyGS (Forum Supporter) said:

Every time I've swapped out Bilsteins, I've gotten improved ride quality.  In rallyX you want travel and compliance.  Bilsteins don't do that from what I've seen.  I'd be real curious about Fox since they earned their rep off-road.  They are pricey though.

I'm facing a similar question with my Subaru.   Koni's or someone's coilovers?  

What brand of coilovers? Almost everything out there is going to have terrible damper quality, other than Ohlins or similar, but I'm not familiar with the Subaru market.  Not a lot of current data but Dennis Grant's site still has some very valuable information.

http://farnorthracing.com/autocross_secrets18.html

Are inverted STi struts worth looking into (I thought those were a common swap for the rallycross subaru community)?

engiekev said:

Keith Tanner said:

One note: always check your data :) I don't know where you pulled up that spreadsheet with the FM spring rates, but it's very  wrong. Our springs had a 342 front rate before 2006, but the rear was actually softer than our current numbers at the time (update: it was 228 lb/in, so that 328 is probably an unfortunate typo of information that's 15 years out of date). I'd love to know where that data lives because it's probably leading a whole bunch of people down the wrong path. Probably FCM and Shaikh will get all wound up...

From the most credible source of information on FM parts, the Flyin' Miata website.

Front springs: 318 lb/in
Rear springs: 233 lb/in
 

Well that is embarrassing! I had a typo in my spreadsheet (hit 328 instead of 228 as noted) but it does appear this site is out of date.  I was trying to save time pulling from one source (its the 3rd result of searching "miata spring rates", after nebulous forum posts), those numbers are quite a bit different.

https://miatiniracing.files.wordpress.com/2018/06/mx-5-federraten.pdf

Is there way to tell the revision of FM springs other than purchase date?  I assume Lof8's NB is setup with post 2006 springs.

I fixed the spreadsheet image.

The old pre-2006 springs have (had) yellow tags on them with a four digit number and had a design height that was about 1/2" lower than the current ones. By this point, it's quite possible they'd be sitting even lower.

The current ones have printed numbers like FMS-1999-05REAR in white on the spring and a design ride height of 12.5" front and 13". They are less likely to drop with time. Made in the US!

About that Miatini site - the rates are also not accurate for the Ground Control setup depending on the shocks being used. And without ride height, that information is basically useless as a lot of soft springs will put you into the bumpstops continually, leading to a very high effective rate with a step change. So I'd confirm any information you get from that site from the source.

engiekev said:

AnthonyGS (Forum Supporter) said:

Every time I've swapped out Bilsteins, I've gotten improved ride quality.  In rallyX you want travel and compliance.  Bilsteins don't do that from what I've seen.  I'd be real curious about Fox since they earned their rep off-road.  They are pricey though.

I'm facing a similar question with my Subaru.   Koni's or someone's coilovers?  

What brand of coilovers? Almost everything out there is going to have terrible damper quality, other than Ohlins or similar, but I'm not familiar with the Subaru market.  Not a lot of current data but Dennis Grant's site still has some very valuable information.

http://farnorthracing.com/autocross_secrets18.html

Are inverted STi struts worth looking into (I thought those were a common swap for the rallycross subaru community)?

I suspect that inverted shocks in the rear would give you halfshaft clearance problems. In the front, it might be a little more likely but you'd be limited to spring lengths that were equal or less than the height of the knuckle as the shock goes through the upper wishbone and there's not enough room for a spring.

The FM springs on the car were purchased new from FM maybe 2 years ago. 

Paco Motorsports has rally-x specific coilovers for Miatas that I'm too much of a coward to buy for mine. 1" lift and a whopping 8.5" wheel travel all around. Buy them and let me live vicariously through you.