Mazda MX5 differential question

[Miatav8,MstrASE,A&P,F]

Slide under one at the dealership with a tape measure the spring length at ride height (spring seat to seat).
Use that dim to determine the coilover eye to eye length.
Make temporary braces to hold the suspension at ride height. These can be 1/8x 1-1/2 steel strip drilled to the same size as the original hardware. You can use the hardware with short bits of tube as spacers for the excess length.

Set the irs subframe height in the chassis so the rails will be at the desired gc with the unloaded tires minus 3/8" or so for tire loading.

The pinion angle may require adjusting in the subframe since the trans output angle and mounts are already fixed.

[Miatav8,MstrASE,A&P,F]

Now moot but you could have also looked up replacement axles.

if only somebody had done exactly that back on December 22nd., and posted the results. Oh wait...

[tibimakai]

I'm pretty sure that is not easy to measure like that, with the car on the ground.
The diff is mounted with that aluminum arm to the crossmember, but the input shaft side can swivel up and down freely at the moment.
I will set the right angle, lock the diff in that spot, and then move the whole assembly up, or down to reach the right height, but that is the big question, what is the right level.
I don't know anything about suspension, but what I have gathered in this forum, that the lower control arms should be level to the ground at ride height, right?
So, that control arm in the picture that I have posted above, should be parallel to the ground, no?

[Miatav8,MstrASE,A&P,F]

The pinion angle is set after the subframe height is set, not before.

No, I don't think the NC arm is level which has nothing to do with the reason a locost typically has a level LCA. You could measure from the subframe to the ground on an existing NC then set the tires at that height minus 3/8" for loading. Next you would set the subframe height in the chassis, then it would be a question of coilover upper seat mount placement in height and angle and spring rate adjustment to compress the damper to the right length relative to the suspension position, but yeah, it isn't very easy to stick one arm and a eyeball underneath the back of a sportscar at a dealership. They'll probably think you are borrowing a cat.

Just tell them you are deliberately homeless at the moment and moved in under there for a few minutes.

Or... don't tell them anything, ignore them for 45 seconds, and be done and gone.

[Driven5]

It's hard to tell for sure from that pic, but I agree that it doesn't quite look truly level to me either. Miatav8 has the right idea.

Just go take the nearest dealership-listed NC for a test drive. Wait... Is it not 'normal' for other people to crawl under the car and have a tape measure with them when car shopping?

Honestly though, unless this is for pointy end of the field competition use, it's going to be a case of 'close is good enough'. If anything it looks a little angled down from chassis to wheel, so setting it level would be little different than the many people who slightly lower their NC with no ill effects.

[tibimakai]

I've got the number from the Miata forum. Those people are awesome, they responded very fast.
Center of the crossmember to ground is exactly 8".

[tibimakai]

OK, I have aligned it and at the chassis side, it bottomed out at the bottom. Weird, because the other ozzy builder's mounting point seems much higher.
At this stage I'm at 8-3/8" with the bottom of the crossmember. I can't go any lower. The crossmember center hole, is off a bit(1/8"). I need to investigate further.
There is not much room for the nut/or bolt head. It is very close to the chassis tube. I had to push backwards the mounting points(the 1" square tubes are hanging out, they don't align properly with the frame's 1" tube) and I will strengthen those two spots afterwards.

[Miatav8,MstrASE,A&P,F]

Let's assume for a moment (because I don't know) the 442 will have a 7" GC to the lower rails. The rear tires are set on boards 7.375" below the 442 lower rails.

The bottom of the IRS subframe is jigged 8.375" above the tire boards / one inch higher than the bottom of the 442 lower rails. It looks like the subframe is about two inches too low.

IRS subframes usually require trimming off the mounting to maintain the scale to keep the wheel base and fenders in the correct location, with the sub integral to the chassis. However, you could trim and box the fwd mounts for more clearance and shift the sub aft an inch. It might be necessary anyway if large tires are installed, needing larger radius fenders that need to be far enough aft.

The load can be spread into the chassis with 1/8x2 or 3 inch strip using cardboard templates. Get it into final position without trimming and takes pics from various angles so we can draw attachment ideas.

[tibimakai]

I'm new to this, so I don't understand some of the abbreviations.
Could you please explain these things, in a more common language, at least until I pick up these. I know some of it, like LCA, CG, but not all of them.
Treat me as a newbie.
I'm planning a GC of 6.5" for street use, and I'm planning to use 245/45R17 wheels/tire combo= dia. 25.7".
Do you see any issues running narrower tires at the front?
To tell you the truth, I did not understand much from your explanation.
English is not my primary language either, I'm Hungarian.
What the tire board means?
At this point, I have no idea what my wheel base will be. I assumed, that the best way to start is to finish the rear end, and then work on the front, to know the wheel track width.
"It looks like the subframe is about two inches too low." This sounds about right, if I compare it with the other builder.
I will trim some of the mounts, close to the mounting hole/bushing and I will be boxing those in.
You are saying that the 8 3/8" should I increase it to 10 3/8"?

[Miatav8,MstrASE,A&P,F]

The drawing assumes the original NC tire diameter is used.
I believe the original NC miata tire size is 205-45-17 for an outside diameter (OD) of 24.26".
A 245-45-17 has an OD of 25.68"; A difference of 1.42"/2= 0.71".
So for the larger tire, the IRS GC would be 8.375+0.71= 9.085" above the table/board. This is the right side measurement in the picture.
The wheel hubs are 0.71" higher.

The 442 chassis GC+0.375" would stay the same even though the tire is bigger. This is the left side measurement in the picture.

The board/build table is a way to support/jig the tire to the right height.

[tibimakai]

Thank you very much for all that, it is a great explanation. This I understand clearly.
So, by lifting the crossmember and inch higher, I will have to pull back and up the front mounting point, I understand now.
That would be better for the mounting bolt/nut, would give me more clearance as well.
Do you think that 6.5" GC is OK? I don't want to scrape the engine on bumps.
Engine is 1" lower, so it will be at 5.5" from ground.
Do you think that would look weird if I would use 215/50 R17 at front? 50 profile compared to 45 in the rear?
I believe it would look better, and it would be easier to steer as well.
I'm having a hard time deciding on the steering rack as well. That is another subject on it's own. Trying to plan ahead.
By the way, what is you name? I would hate to keep calling you by the forum name.
My name is Tibor(Teebor).
All this is greatly appreciated. Now I can go and measure and cut. Thanks for that.

[Miatav8,MstrASE,A&P,F]

Hello Tibor. I'm another Steve but MV8 is fine also.

I think it would look fine. A 215-50 has a slightly thinner sidewall than a 245-45.
5.5" GC to the oil pan is reasonable.

[tibimakai]

Hi Steve(Istvan in Hungarian).
I have added a 1" square tube to each point and it is very close to your numbers. In stead of 9.46 it is 9.5".
I have already cut the "frame" with the 17 degree angles at the ends and added the bushings in the middle of the two side tubes.
I just have to weld everything together. Another day.
Thank you very much for your help, I really need it, because I don't really know what I'm doing.

[Driven5]

One thing you look to have possibly missed in the replies to your Miata forum question, is that the response you got was from somebody with a lowered car. They were running coilovers at 13 inch wheel center to fender lip. Stock is 14.5-15 inches. So when they said 8 inches to center of the subframe, for stock geometry that would put you at more like 9.5-10 inches. It also looks like that car has 215/45R17 instead of the stock 205/45R17, which is another 3/8" difference.

Perhaps try mocking it up with the diff in place to get a measure on the height at the center of the outputs.

[Miatav8,MstrASE,A&P,F]

Justin makes a great point. I assumed the measurement was taken from an unmodified NC so the numbers will require adjustment to target the correct height.

The point of being as accurate as practical in targeting any parts location is that nothing we fabricate is 100% accurate to the target numbers. The closer you can get, the better the end result will be. It does not have to be perfect. No car is.

215-45 has 3.81 sidewall and the 205-45 has 3.63 for a difference of 0.018" added to the subframe height.
If the stock fender height to wheel center is 14.5-15.0, I'd go for the middle with 14.75 minus 13.0 for 1.75" added to the subframe height.
8+ 1.75 + 0.018 + 0.375 for 10.143". I'd shoot for that and keep moving forward on the build.