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March 14, 2010 9:45 p.m. brianlloyd New Reader

OK, the '99 NB, the 'donor', is in the back of the hangar and the parts are starting to come off. The '90 NA, the 'recipient', is in the front of the hangar and is now safe (but not fun) to drive. (200,000 mi stock bushings, dampers, and springs do not a nimble car make.) Goal -- using parts from 'donor' and as little else as possible, turn 'recipient' into something fun to drive on the track.

The plan was to take all the suspension components and the diff from the NB and put them in the NA. That prompts the first question: what can I take off the NB and bolt onto the NA?

But as I started taking things apart I discovered that all was not well. When the NB fought the wall, the wall won. I thought that only the body took the damage but that left rear corner took more than just body damage. I have a bent wheel, a bent hub (upper control arm attach point), and a bent upper control arm. I did get one pleasant surprise; the stub axle is straight as is the lower control arm. Looks like no damage to the damper (new) or brake either.

So this prompts the second big question: which parts are just mix-n-match? For instance, can I use the NA upper rear control arm and hub with the other NB parts? Do I need (or want) to move the sub frames across too?

For now I will run it with the 1.6. It works fine. The plan is to take the 1.8 and overhaul it. The Frankenmiata wants to know what its newly-overhauled 1.8 will be like. Suggestions? I was thinking of hot-rodding the normally-aspirated 1.8 now and someday in the future going the LS3 route, bypassing the turbo stage along the way.

But all of that is secondary to suspension. I need a lesson on spring rates and how to match shocks and anti-sway bars. I plan to continue to run RA1s (205s, right off a SM). So far the advice I have gotten is, "get the SM suspension."

That is probably enough for now. I am waiting to learn.
March 14, 2010 10:44 p.m. Keith PowerDork

Mix and match - this can get deep :) The rear uprights are different between the NA and the NB, a matter of an extra 5mm of track. However, you can swap both NA uprights on to the other NB parts and you'll be good. In fact, this isn't a bad idea as you'll have a bit more room for tire. My Targa Miata has this setup - a complete NB rear with NA hubs/uprights. The rear upper control arms should be direct swaps.

I'd swap the entire front subframe, control arms, uprights, everything. You'll get a bit of improvement to the steering geometry and a more solidly mounted steering rack. It'll bolt right in.

I wouldn't jump on to SM suspension unless you're required to by rules - ie, if you're running SM. There are better setups out there, but SM racers simply don't have a choice. If you want, give me a call at Flyin' Miata and we can talk through some options.
March 15, 2010 3:10 p.m. brianlloyd New Reader

Keith wrote:

Mix and match - this can get deep :)
That is what I figured. That is also why I chose the subject line 'Frankenmiata'. ;-)

The rear uprights are different between the NA and the NB, a matter of an extra 5mm of track. However, you can swap both NA uprights on to the other NB parts and you'll be good. In fact, this isn't a bad idea as you'll have a bit more room for tire. My Targa Miata has this setup - a complete NB rear with NA hubs/uprights. The rear upper control arms should be direct swaps.

That is nice to know. If the NA uprights (OK, "uprights" not "hubs") are a direct swap, do I need to change the axles? Is the spacing between the wheel bearings the same or would I need to swap stub axles as well? (Spacers?)

For that matter, should I just prophylactically change the wheel bearings? (Shoot, I already did in the NB.)

I'd swap the entire front subframe, control arms, uprights, everything. You'll get a bit of improvement to the steering geometry and a more solidly mounted steering rack. It'll bolt right in.
OK, I was thinking that I was going to do that anyway but it is nice to have confirmation. I had recently replaced everything, e.g. bushings, bearings, ball-joints, springs, shocks, sway bars (got the FM state-2 suspension kit), etc. I would like to do what I can to preserve that investment.
I wouldn't jump on to SM suspension unless you're required to by rules - ie, if you're running SM. There are better setups out there, but SM racers simply don't have a choice. If you want, give me a call at Flyin' Miata and we can talk through some options.
OK, will do. I have a good background in physics and have a general understanding of what needs to be done but I can see how vehicle weight, tire/wheel weight, weight transfer, and speed (bump impulse in the time domain) all combine to define the proper spring rate(s). And spring rate defines the necessary damping factor (damper rate) to achieve critical damping. There's gotta be a way to figure out where one should start.

Thanks!
March 15, 2010 3:49 p.m. Keith PowerDork

The halfshafts can cope with the slight differences in track from the NA vs NB uprights, so go ahead and keep the NB parts if you want. In the rear, you can go with just about any combination of parts as long as they're all from a 1.8. The NB rear subframe has mounts for a very good rear brace, but your pre1994 NA isn't going to have the threaded mounting points on the chassis for the brace. The NB rear subframe also has an exhaust hanger or two that is designed for the NB exhaust, not the NA.

The SM spring rates are pretty stiff and have a surprising amount of front spring bias, but more importantly the damping isn't well matched to the springs. I suspect all the fast SM guys are running revalved shocks, even though that's not legal from what I understand.

General rule of thumb: I've always found Miata seem to like around a 3:2 spring ratio - ie, 450 lb front and 300 rear. I've broken the rule a few times, but that's what makes things interesting
March 15, 2010 6:56 p.m. Marty! HalfDork

Here's a link from Miata.net that goes into interchangeability......

http://www.miata.net/garage/compatibility.html
March 15, 2010 10:21 p.m. Salanis UberDork

So... the NA and NB control arms are the same, but the uprights are different? The subframes are interchangeable but... are they different or not?

Sounds like the main difference is suspension geometry is taken from the uprights.
March 16, 2010 12:22 a.m. brianlloyd New Reader

Marty! wrote:

Here's a link from Miata.net that goes into interchangeability......

http://www.miata.net/garage/compatibility.html

Thanks! Just what I was looking for!
March 16, 2010 12:39 a.m. joey48442 SuperDork

Salanis wrote:

So... the NA and NB control arms are the same, but the uprights are different? The subframes are interchangeable but... are they different or not?

Sounds like the main difference is suspension geometry is taken from the uprights.

I know the fronts are different, but I'm not sure about the rear subframe.

Joey
March 16, 2010 9:42 a.m. Keith PowerDork

Salanis wrote:

So... the NA and NB control arms are the same, but the uprights are different? The subframes are interchangeable but... are they different or not?

Sounds like the main difference is suspension geometry is taken from the uprights.

I never said all the control arms are the same The rear uppers are.

The geometry of the lower control arms, front and rear, is the same between NA and NB. However, the sway bar end link bracket went from a double-shear design to a fat single-shear tab. If you try to buy a new lower control arm for your NA, Mazda will provide the NB version.

In the front, the NA subframe has relocated lower control arm mounting points. It also has a different steering rack design. The uprights have a steering arm that's about 7mm higher. The upper control arm has a different ball joint on it. So if you're going to swap them, you need to swap the subframe, upper control arm, steering rack and upright. There is no change in track.

The rear NB subframe has different mounting tabs for braces and exhaust hangers, but does not have different geometry. The uprights, as mentioned earlier, give an extra 5mm per side of track.

That Miata.net link isn't a definitive reference, there's some half-complete info in there. Just FYI.
March 16, 2010 2:40 p.m. brianlloyd New Reader

Keith wrote:

I never said all the control arms are the same The rear uppers are.
That is good to know as it is the left rear upper from the NB that got squashed. I can use the rear upper from the NA with impunity. (With new bushings to match everything else, of course.)
The geometry of the lower control arms, front and rear, is the same between NA and NB. However, the sway bar end link bracket went from a double-shear design to a fat single-shear tab. If you try to buy a new lower control arm for your NA, Mazda will provide the NB version.
Also good to know.
In the front, the NA subframe has relocated lower control arm mounting points.
How does that change the geometry? Different range of camber/caster? I know that I was trying to get more negative camber with the NB and was having trouble. I was planning to go to a higher spring rate anyway so I thought that I would lower it slightly at the same time. That should buy me a bit more -camber.
It also has a different steering rack design.
So I do need to move the steering rack. The steering rack on the NB is for power steering. The rack for the NA is not. Will I want to install the power steering pump or can I cap off the hydraulic power input and use it without hydraulic boost?
The uprights have a steering arm that's about 7mm higher. The upper control arm has a different ball joint on it. So if you're going to swap them, you need to swap the subframe, upper control arm, steering rack and upright. There is no change in track.
Hmm. I just put in new front uppers with new ball joints in the NB last summer as part of the total suspension rebuild. I guess that if they aren't damaged I will want to use them. I am guessing that I am going to need to unbolt the engine mounts. Don't they attach to the subframe?

I guess the bottom-line question is: just how hard is it to swap the front subframe?
The rear NB subframe has different mounting tabs for braces and exhaust hangers, but does not have different geometry. The uprights, as mentioned earlier, give an extra 5mm per side of track.
In that case, I guess the answer is to move the NB control arms to the rear (they have new Mazdaspeed bushings as of last summer) of the NA, keeping the NA subframe. I will need to rebush the LR upper from the NA to match everything else.

I did install the FM stage-2 suspension kit on the NB last summer. Now I will need to turn it into a stage-2.5 (modulo nothing being broken -- nothing obvious so far). Can I (should I) just take the stock upper shock mounts from the NB and put them on the NA?
That Miata.net link isn't a definitive reference, there's some half-complete info in there. Just FYI.
Ah. OK. As with any endeavor, there are a lot of sources of half-information. Knowing the real information is the trick. All this education! (Thank you, BTW.)
June 12, 2011 1:42 p.m. brianlloyd New Reader

OK, I am in the middle of the front subframe transplant. I am wondering if I should stick with the 1.6 in the NA or just move the 1.8 over while I am moving the subframe? I won't have to figure out how to mount the power steering pump on the 1.8.

I know, this probably seems like a silly project. But it is what I have and won't have to spend (much) money on to get done.

Brian
June 12, 2011 2:30 p.m. Duke UltraDork

If the 1.8 is solid, I would move it over whole you're already that deep into the car.