OEM Bilstein dampers in JDM 1998 NB RS - what springs?

Junkwhale · Post by **Junkwhale** » Mon Dec 16, 2013 2:47 pm

Hi all.

I've just pulled the trigger and am now the owner of a shiny black JDM 1998 RS with OEM Bilsteins and what I'm assuming for now are the OEM springs.

The first thing I want to do is get the ride height down a bit as I'm not a fan of the tractor-look. I want to do some autocross/entry-level Motorsport with the car too, but it also needs to be liveable as it will be driven regularly on the road (not quite daily driver frequency though).

Ideally I'd like to reuse the bilsteins if I can but I can't seem to find much info on the dampers - the only things I've found that specifically mention the RS dampers state that they were an OEM only part (i.e. made by Bilstein for mazda for the mx5), and they're monotube gas type dampers. Is it just the valving that's specific for the mx5 or are they in some other way different?

What's the best way of getting some springs that are a) lower (how much lower is possible on an NB?) and b) have a spring rate that matches the OEM bilstein's valving?

marcellarius · Post by **marcellarius** » Mon Dec 16, 2013 4:38 pm

Congrats.

You might be able to get those dampers revalved for whatever spring rates you need; Racelign can probably do that.

If you're chasing performance/driveability then the right height is one that keeps the suspension in its optimal range of motion and retains as much travel as possible.

If you're looking to do this cheaply, you could probably get a set of lowering springs that will drop the car a tad without requiring damper upgrades.

How low can you go... hmm. Plenty of examples on Clubroadster

Junkwhale · Post by **Junkwhale** » Mon Dec 16, 2013 5:30 pm

Thanks, just about to go pick it up!

From what I understand typically aftermarket springs have much higher rates than stock shocks can control with the OEM rebound and compression damping so they end up ill matched (e.g. not enough rebound to control the springs decompression).

I guess my question is are there any obvious options/known lower springs with a rate that is a good match to how the NB Bilsteins come valved from factory or is revalving/buying matched shocks inevitable?

Angreal · Post by **Angreal** » Mon Dec 16, 2013 9:49 pm

I think off the top of my head they're 3.5 kg springs from standard at the front and something slightly less at the rear. A search on Google will tell you as I found the info once upon a time

Snapfrozen · Post by **Snapfrozen** » Mon Dec 16, 2013 10:06 pm

If you have a look at my MX5 it's on circlip adjustable bilsteins that were custom valved when I bought them

Junkwhale · Post by **Junkwhale** » Tue Dec 17, 2013 9:32 am

Snapfrozen wrote:If you have a look at my MX5 it's on circlip adjustable bilsteins that were custom valved when I bought them

thx, I take it those aren't the OEM bilsteins tho?

Angreal · Post by **Angreal** » Tue Dec 17, 2013 9:54 am

Snapfrozen wrote:If you have a look at my MX5 it's on circlip adjustable bilsteins that were custom valved when I bought them

Your ones I think are different. I used to have photos and know the spring ratings but it's been so long sorry

Junkwhale · Post by **Junkwhale** » Thu Dec 19, 2013 12:15 pm

So I've learnt a fair bit about suspension over the last few days (more interesting than the studying I should have been doing, hah!).

The JDM 1998 NB8C RS comes with Bilstein shocks and springs that are the same as the "Hard S" package Bilstein shocks and springs that are talked about on all the US Miata forums. I found a couple of sources that confirm this.

Can't post links yet as I'm still to new, but there's a really good overview on fatcatmotorsports dot com called "The Miata Suspension - A quest for truth".

Snapfrozen · Post by **Snapfrozen** » Thu Dec 19, 2013 9:44 pm

Talk to Racelign/autolign. If you need to ring around you can speak to Shane at the Silvia Park/Mt Wellington one in Auckland. He's the chap who did mine and knows his shit. I'm still running factory bumpstops, will be replacing them when I get my new tophats but don't see any point until then.

Junkwhale · Post by **Junkwhale** » Fri Dec 20, 2013 9:15 pm

Snapfrozen wrote:Talk to Racelign/autolign. If you need to ring around you can speak to Shane at the Silvia Park/Mt Wellington one in Auckland. He's the chap who did mine and knows his shit. I'm still running factory bumpstops, will be replacing them when I get my new tophats but don't see any point until then.

Thanks, helpful to have someone to ask after, will give him a call. I dropped into Autolign in Petone the other day to have a chat about options, and they quoted me ~$545 for some Tein S Techs as a first option for basic lowering (seems a bit expensive, can get the same springs for $230 inc delivery on ebay). The guy I talked to wasn't really familiar with mx5's though so I suspect it was just generic advice.

Snapfrozen · Post by **Snapfrozen** » Sat Dec 21, 2013 6:36 pm

Just a word of warning, the rear spring perch will hit the half shaft if lowered too far. You can avoid this by changing springs, tophats or using coilover springs

DEATH_INC. · Post by **DEATH_INC.** » Fri Apr 18, 2014 12:51 pm

Digressive damping (hard on slow bumps, soft on fast) is good for track use, not so good on the road. It lets the tyres stay in contact with the ground, while providing control, but it does give a firm ride on our cool roads....
Don't go blindly lowering it as much as you can without looking at what it does to the roll centers if you're serious about motorsport use...sometimes higher is actually better.
Firmer springs will help overcome that overly heavy rebound you're talking about too.
Tein are generally pretty good, and don't drop it on the ground either. I'd try 'em.

Junkwhale · Post by **Junkwhale** » Mon May 19, 2014 1:11 pm

Suspension: really f*ing complicated.

Revisiting this thread since I've yet to actually decide on a suspension setup, and seem to be going in circles.

I'm actually leaning towards Flyin' Miata springs now, as they seem like a nice compromise rate 318F/233R. Trying to figure out what shocks are best matched to those rates is surprisingly difficult.

Reading through this: http://farnorthracing.com/autocross_secrets19.html it seems relatively straightforward to figure out what damping you want:

Set the shocks at 65% critical damping between 0-3in/sec. Somewhere above that, digress the shock off to ~30% critical to handle the occasional high speed whack - the exact location of the knee in the force curve doesn't seem critical, as long as it doesn't happen during the chassis roll speed zone. And watch yourself go faster.

But when I try work out 65-70% of critical damping at various points on a shock dyno graph using numbers from Shaikh's FCM calculator (for motion ratio, bounce frequency, corner weights, unsprung weight), most of the recommended shocks are way over damped (esp on rebound).

Junkwhale · Post by **Junkwhale** » Mon May 19, 2014 1:55 pm

E.g. with FM springs, and assuming a weight of 2400lbs (prob on the light side) I end up with the following curves for 0.7 of critical damping up to 3 inch/sec shock velocity digressing to considerably less at higher velocities:

vs off the shelf Bilstein B8 shocks:

(x axis is 1 IPS increments)

So, the bilsteins actually seem like they provide plenty of damping for spring rates above the FM springs (assuming 0.7 critical damping is actually a good number). The problem with this is that it goes against everything that miata.net/FCM/US forums received wisdom that claim that the bilsteins stock valving isn't much good for anything over stock (168/130 or something) rates.

edit: note i gave in and just used imperial units everywhere, as any good reference stuff I can find is from the US so it's just easier (and making comparisons to what's posted on US forums)

Junkwhale · Post by **Junkwhale** » Mon May 19, 2014 2:01 pm

DEATH_INC. wrote:Digressive damping (hard on slow bumps, soft on fast) is good for track use, not so good on the road. It lets the tyres stay in contact with the ground, while providing control, but it does give a firm ride on our cool roads....

Digressive damping - i.e. firm damping for chassis control (0-3 or 0-5 IPS depending on the source) will feel firm, but won't be harsh on potholes, high speed hits to compression joints, bumps etc (5-25+ IPS impacts).

Obviously if you want a comfortable ride, soft damping and soft springs are your best bet, but it seems like a digressive profile is the best option for something you want to be fast (on a track, on a road, anywhere) while still not jarring your teeth out when you do hit a pot hole on your drive to work.

built4speed · Post by **built4speed** » Mon May 19, 2014 5:56 pm

Just get autolign to revalve your bilsteins to suit your spring rates if your fussy. My factory NA blisteins were valved adequately for 500F 350R

Junkwhale · Post by **Junkwhale** » Mon May 19, 2014 6:51 pm

built4speed wrote:Just get autolign to revalve your bilsteins to suit your spring rates if your fussy. My factory NA blisteins were valved adequately for 500F 350R

Yeah the NA bilsteins had a lot more rebound than the NB ones so can definitely handle much higher rates.

I guess I just want to understand what "suiting the spring rates" means in terms of actual damping as it seems that the answer varies dramatically depending on who you ask.

warrior · Post by **warrior** » Mon May 19, 2014 7:54 pm

In doing mods on cars you will get many opinions. You must decide what is best for you,

Timmo · Post by **Timmo** » Tue May 20, 2014 10:01 am

Be careful also that US market cars are likely different than the JDM counterparts so the Bilsteins may, in fact, be different even if they appear the same.

marcellarius · Post by **marcellarius** » Wed May 21, 2014 12:35 pm

I punched in numbers to the Autocross-to-win calculator that roughly correspond to my setup (NA, Tein Monoflex coilovers, 6 / 5kg springs). Comparing the shock curves suggested by that calculator to the graphs in the manual that came with my coilovers, even at minimum setting they provide about twice the damping that calculator suggests.

This could support what Shaikh and Dennis claim about lots of shocks being overdamped... or we aren't getting correct numbers from that calculator.

Junkwhale · Post by **Junkwhale** » Wed May 21, 2014 8:31 pm

marcellarius wrote:I punched in numbers to the Autocross-to-win calculator that roughly correspond to my setup (NA, Tein Monoflex coilovers, 6 / 5kg springs). Comparing the shock curves suggested by that calculator to the graphs in the manual that came with my coilovers, even at minimum setting they provide about twice the damping that calculator suggests.

This could support what Shaikh and Dennis claim about lots of shocks being overdamped... or we aren't getting correct numbers from that calculator.

I actually had to watch some youtube videos on sprung mass damper systems and the like to get my head around what critical damping was and why you'd want to be around 0.65-0.7 of critically damped. I tried working one out by hand and came out with a lb/(in/sec) figure that was similar to what the calculator produced (although i was using an equation* that ignored unsprung weight I found on one of the US forums). So I think the calculator isn't necessarily wrong.

I think it's more a case that the amount of overdamping or underdamping isn't linearly correlated with the resulting "suspension feels really bad" - you can get away with a whole lot more overdamping than underdamping before it starts to be a problem.

E.g if this oscillation for a mass-damper was the rebound of a damper in a car hitting a single bump (i.e. it has hit a bump, compressed to the top, then the spring is about to push wheel/chassis back to its resting state when this graph starts):

and the ideal damping ratio is 0.7 here (fastest return to equilibrium with minimal overshoot/oscillation) then being underdamped at a 0.4 damping ratio is going to be much more unsettling than being overdamped at a damping ratio of 1.0. So cars with a lot of rebound are in a lot of cases going to seem to be very well controlled even though they'd actually be providing more grip if they had less.

...that's my impression anyway, im still a newbie to all this stuff so i'm sure i'm missing a whole lot of the picture.

*equation i mentioned above: cd = 2 * sqrt [(CW)*(SR)/(MR)^2] where cd = critical damping {lb/(in/sec) or lb/ips}, CW = corner weight (lb), SR = spring rate (lb/in), MR = motion ratio

Junkwhale · Post by **Junkwhale** » Wed May 21, 2014 8:46 pm

Timmo wrote:Be careful also that US market cars are likely different than the JDM counterparts so the Bilsteins may, in fact, be different even if they appear the same.

Yeah, despite a lot of looking I couldn't find anything definitive on whether the bilsteins on JDM cars are the same.

Ive found a place in chch that has a shock dyno though, so I think I might actually get them checked/dynoed when I pull them out - will be interesting to see the results!

SLYDIT · Post by **SLYDIT** » Tue Jul 08, 2014 5:37 pm

dunno if you got any further on this but here is a GREAT article on making your bilsteins super awesome. get them revalved and add cheap adjustable perches and bumpstops...profit.

http://www.miataturbo.net/suspension-br ... ead-78451/

Junkwhale · Post by **Junkwhale** » Sat Aug 02, 2014 9:54 pm

SLYDIT wrote:dunno if you got any further on this but here is a GREAT article on making your bilsteins super awesome. get them revalved and add cheap adjustable perches and bumpstops...profit.

http://www.miataturbo.net/suspension-br ... ead-78451/

Thanks, have read the whole thread and the one on the same topic on miata.net.

I haven't got any further in terms of actually doing anything to the car, but I now have some FM springs which I'm planning on using.

Junkwhale · Post by **Junkwhale** » Sat Aug 02, 2014 10:12 pm

949 racing have released their much awaited (by some anyway) Tecna coilovers:

Their goal was basically to provide the best possible ride while being lowered at a liveable height. It's interesting to see that they've gone with 6kg/4kg spring rates (1.7hz/1.67hz) which is basically identical to Flyin Miata's NA/NB springs. A while ago emilio posted dyno curves for them on mt.net, so I thought it would be interesting to overlay them with the NB bilstein sports:

They are surprisingly similar, especially the compression damping, the roughly 1:2 compression to rebound ratio. The bilsteins do have a bit more low speed rebound than the tecna's (more of a knee) but then digress/blow-off to a flatter curve.

OEM Bilstein dampers in JDM 1998 NB RS - what springs?

OEM Bilstein dampers in JDM 1998 NB RS - what springs?

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