Mazda MX5 differential question

[tibimakai]

I'm planning to use an NC Miata diff in my car, and I don't have it yet, so I could use some help here.
It would be much appreciated, if anybody could help.
I'm trying to advance with the transmission tunnel, but I don't know where it is located the diff input shaft, in relation to the center of the car.
It's in the center, or it is offset?
Second question that I have, is referring to the pinion angle.
The up and down angle gets added up with the left and right angle, or not to get the total pinion angle?
I'm planning to use the whole rear end of an NC Miata.

[Driven5]

As far as I'm aware, the differential is centered at the axles. However, it does have an offset pinion.



I don't think it's set up to have any appreciable lateral angle.

[tibimakai]

I have assumed, that since has that offset, the whole drivetrain is offset to one side, but after making the motor/trans mounts, I'm starting to realize, that it may not be like that.
I'm looking at the subframe, and it seems like the diff "mounting crossmember" is mounted offset in the subframe.
Most likely I will have to modify the mounts. I better stop and wait until I have all the needed parts.
Thank you for trying to help.

[rx7locost]

It shouldn't make much of a difference where the diff is located (within reason). It might just mean that you have 2 different lengths of axles. Certainly, it can have an effect on the tunnel. I would wait until you have the diff in hand. You will want to make the tunnel as small as possible in order to maximize the cockpit space.

[tibimakai]

There are two different length shafts for sure. Checked it on Rockauto.
Right now, I have the engine/trans/tunnel offset to the passenger side, by ~.812". Tunnel would be this way 7" wide. I would have 19" for myself, 18" for the passenger, which is not to bad. Most likely would look ugly with that wide tunnel. It would be nice, if I could make the top rounded, more like a Caterham.

[RTz]

Tunnel would be this way 7" wide. I would have 19" for myself, 18" for the passenger, which is not to bad. Most likely would look ugly with that wide tunnel.

I'm guessing you are using 1" x 1" tubes for the tunnel. If you change those to 1" x 1/2" tubing you'd reduce the tunnel width by an inch. Also, for reference, I'm running NA Miata drive-train and my tunnel is 5" wide...

[Driven5]

Apparently I might be misinformed above. I was aware of people swapping L/R axles interchangeably on NC's, but it looks like they are indeed different lengths by ~1.6" according to Autozone. So perhaps it is offset to fully center the pinion and minimize tunnel interference. Agree it's best to just have the parts in hand before actually committing anything to metal.

[tibimakai]

The problem with the NC transmission is, that there are some bulges on one side, and the clutch slave cylinder on the other side. I have already cut off some chunks from it.
I will reduce the tunnel as much as possible, but for the first try I wanted to be sure that the tunnel won't interfere with the driveshaft.
Even like this, I still have 19", respectively 18" for the other side, of seating width.
I'm home for another week and a half and it is hard not to do anything on the car.
I may start modifying stuff, to bring everything to the center.
Per Rockauto, the difference between the two shaft is 1.562", but as I have just noticed on the subframe, those mounting points are offset, so I have to assume, that the diff is offset in the subframe, but central to the car.

[tibimakai]

Answering to my own question. It is at the center.

[Miatav8,MstrASE,A&P,F]

Looks centered to me too.

Now moot but you could have also looked up replacement axles. If the left and right are the same part number, the pinion will be offset from the cl. Although if they are different part numbers, the pinion could still be offset but probably centered.

[tibimakai]

The axles have different lengths. I have checked.

[Driven5]

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 have got the the parts, so this weekend I will confirm for sure.
Got the diff, crossmember and the rear suspension. I'm only missing the shafts.
The challenge will be, to place all these to the chassis, in the air. I don't have the table anymore.
I have a questions to the pros, can I use the center of the diff's output shaft holes, as the center of the wheels at ride height?
This is what I'm trying to replicate and I don't have any info about it. The person is old and in wheelchair, and he build this 10 years ago.

[Driven5]

I have a questions to the pros, can I use the center of the diff's output shaft holes, as the center of the wheels at ride height?

For a suspension that moves, it's reasonable starting point if slightly less than 'ideal'. There are a couple of small benefits to setting the diff outputs slightly higher than the wheel center. One is that both strength and efficiency are technically maximized at zero angle, which would be most important during the squat from maximum acceleration. The other is that not enough relative movement between the parts over time can actually accelerate wear between the mating surfaces there, but between the lower weight and low miles these cars see, the parts should still generally last 'forever'.

If you can get a measure the relative positions that Mazda intended at ride height on a stock NC, that would also make for a good starting point.

[tibimakai]

Found this picture(Youtube video) that shows, the rear control arm horizontal to the ground.
Should I use that as a reference?