Miata spring rate advice sought

Greetings all; new guy here. I’m here to ask for educated opinions concerning spring rates for unimproved road driving with a first or second-generation Miatas. I recognize I’m more than a bit out of the central focus of this website, which I’ve been enjoying immensely these past two weeks, but as you can imagine I’m much, much, much further from the autocross, drift, and hard park crews. Hopefully you fellows and gals can help a semi-neophyte endurance rally driver out.

Down to business: My motivation is that I live and drive in the southwest, enjoy camping, and absolutely hate not being able to travel weather beaten dirt roads in my weapon of choice – first generation Miata’s. As a partial way to correct that deficiency, I’m assembling an adjustable coil-over setup using Bilstein B6 HD shocks made for second generation Miata. Assuming I choose the proper spring length, I should end up with just about 7” of clearance, and possibly a bit more depending on tire choice, while staying in the non-destructive articulation range of the Miata’s drive axles.

The spring lengths are easy enough to calculate given corner weights (610/575, lbs F/R), motion ratios (1.5/1.3, F/R), maximum and minimum confined spring lengths (to be determined upon receipt of Bilsteins, which are ordered but backordered into March), and desired bump/droop travel ratio (60/40, bump/droop). What I need the most help with is spring rates.

Stock Miata spring rates, model year dependent, are in the range of 154 lbs/in front, 97 lbs/in rear. If I were to take the advice of the vast majority of Miata enthusiasts, I’ve not got a “Big Boy” suspension until I’m in the 850/600 lb/in range. I’ll accept that I’m not man enough to handle a “Big Boy” suspension on Saline Road through Death Valley, and was thinking more in terms of 175-200 lbs/in front, and 125/150 lbs/in rear. I'd like my suspension to work for a living and earn it's keep.

How did I arrive at those beginning numbers? First off, because they give me a bit more head room for camping and self-rescue gear. Secondly, successful rallycross Miatas have run with 230/140 (F/R) rates, and since I’ll be driving at a much more leisurely pace I figured I don’t need as high a rate as folks who are in a big hurry. Admittedly not very scientific, but that’s all of the data points I’ve got. For no particular reason, other than it makes for pretty good bounce frequency ratios and front roll couples for the street without a rear sway bar, I’m leaning towards either 185/125 or 200/150 front/rear rates.

Thoughts?, and thanks



Edited 1 time(s). Last edit at 02/27/2018 03:50PM by Jim Lowe.

Hello, Jim. If the guys here cannot give you advice in the direction that you are looking for, I would advise you join the SCCA website, and ask the same questions of the Miata driving RallyCross competitors in the RX section of the SCCA forums.
The Miata, especially 1st/2nd Gen. have been the prevailing championship cars in many regions in both Prepared Rear and Modified Rear classes. Depending on Region, the track conditions vary immensely, but you should get enough feedback to create a good spreadsheet of spring rates vs. conditions, to be able to source what would work for you.
Good luck.

Thanks Daniel, that sounds like excellent advice, and I appreciate the reply.

A rallycross setup should be fine for you. the big boy stuff you mention is for autocross, i know a guy that runs 700/400 f/r and wins events but he says that's too stiff for road course and don't even think of it for anything else.

Use what others say as a reference but don't take it for cash because many win races despite their setup not because of it.



Edited 2 time(s). Last edit at 02/27/2018 06:02PM by Philippe Bellefleur.

Hi Jim,

Your numbers sound like you're in the right ballpark. I have a Miata tracktoy that I was going to turn into a Spec Miata about fifteen years ago, and I fiddled around with a bunch of suspension setups on it, including the Spec Miata one. The Spec Miata one is stupid hard - 700F/325R if I remember correctly, and it'll bounce the car even on small bumps. I ended up with a somewhat softer one - more like 550F/275R or so, with some Koni shocks to match. It ended up being a much better ride and I could more or less keep up with SM cars around THill. It's not too much of a tooth-rattler on the street but I wouldn't want to take it on stutterbumps. My point with all this is that the "Big Boy" guys may all be running their cars way too stiff and you shouldn't feel bad about considering softer rates.

My suggestion would be to try getting shocks that can handle a range and then try your 200/150 ones and see how you like them. Springs are relatively cheap (although it's kind of a bitch to replace the rears on a Miata).

Once thing you might want to consider is replacing the plastic under-engine cover with a metal one, or at least protecting the rack better than it is.

---

Self-righteous douche canoe

Thanks for the advice, Daniel, Philippe, and Michel, I really appreciate the help. I've got two other 1995 Miatas; one with 336/224 springs, the other with stock springs; both with re-valved Bilsteins. Great on what passes for paved back roads around here, but not enough clearance or travel for dirt, rocks, and gravel.

Nobody's mentioned the 60/40 bump/droop target, so I'm thinking I'm in the ball park there. That should put me at about a 14.5"/15" front/rear fender lip to hub center, and roughly 6-3/4" sump clearance.

A sump plate is definitely tops on the list, though I'm going to try to preserve or at least mimic the existing plastic under shield as much as possible as it helps with cooling and provides a lot of protection from splash during stream crossings.

I've also got a tasty set of 15x6 Braid Fullrace TA wheels on the way, but they're on a slow boat from Barcelona, and won't arrive until early to mid summer. The big question is what tires I'm going to mount on them? Nothing reasonably sized short of rally tires fit under the Miata, and rally tires aren't made for 8,000 plus mile road trips, so it's probably going to be something like all-season Altimax RT43's in 185/60-15 or 185/65-15, or something similarly compromised. Oh well...

If you want to grass-o-cross with tires on the cheap shouldn't you think of snow tires instead of all seasons?

And that boat probably got requisitioned by Rajoy to transport brown shirts from Madrid to Barcelona...

Quote
Philippe Bellefleur
If you want to grass-o-cross with tires on the cheap shouldn't you think of snow tires instead of all seasons?

And that boat probably got requisitioned by Rajoy to transport brown shirts from Madrid to Barcelona...

Snows for sure for rally cross, but I don't think they'll hold up in the Mohave in August.

Fortunately for my boat, Madrid is landlocked. I spent several summers in Catalonia and in the Basque lands when Franco was still in charge. That was a weird experience for a naïve 16 year old wet behind the ears gringo.



Edited 1 time(s). Last edit at 03/01/2018 05:35PM by Jim Lowe.

While not particularly relevant to my project, and perhaps more trivia than anything of practical value here in the States, there's a gentleman in Deutschland with what I believe is a stage rally NB series Miata. He's running 7 kg/mm front and 5 kg/mm rear springs on re-valved Bilstein B6 HD's, though I understand he now recommends 9/7 front/rear rates.

I'm a bit surprised at how high a rate he's using, but I suppose that's at least partially a function of the relatively low travel available with the B6 Bilsteins and the need to prevent bottoming the suspension.

Seems like tarmac rates to me.

Ok, I finally went out and looked - I'm running 450/375 and really should soften up the rear to about 325 or 350. It'll chirp and skitter a little if the road's anything but smooth and I have it hammered in hard - like coming out of turn 5 at Thunderhill.

As for the guy in Germany:

http://mx5-racing.de/

und

https://www.facebook.com/rallyemiata

Looks like mostly tarmac, some smooth dirt sections. I can see how he might be getting away with 7/5's (392/280 in lbs/in). Going to 9/7 (504/392 lbs/in) seems harsh.


On the whole...are you sure you wouldn't like a nice Golf instead of a Miata for desert trekking? The horsepower/weight's about the same, the suspension's more suitable, and you can carry a lot more crap.

---

Self-righteous douche canoe

Quote
hoche
On the whole...are you sure you wouldn't like a nice Golf instead of a Miata for desert trekking? The horsepower/weight's about the same, the suspension's more suitable, and you can carry a lot more crap.

Einen Golf? Ich glaube nicht!

Certainly a Golf would be a much more sensible choice. Better yet would be a Forester or Outback, raised an inch or so. Almost anything would be more sensible than a Miata for desert trekking. That I'm here posting on Rally Anarchy at all is manifest proof that sensible isn't my strong suit.

Quote
Jim Lowe
That I'm here posting on Rally Anarchy at all is manifest proof that sensible isn't my strong suit.

Best post on here in a while!

Quote
Mad Matt F

Quote
Jim Lowe
That I'm here posting on Rally Anarchy at all is manifest proof that sensible isn't my strong suit.

Best post on here in a while!

The dude's got a point.

---

Self-righteous douche canoe