Discussion only.

Has anybody read the factory manual section on the AWD system? Page 18 of Section 23E AWD General Information. Its parts of the transmission section.

The AWD's center differential is in the transmission, the front diff is in the transfer case.

So, is the front diff the LSD, and the center diff the open diff'?

I know the company line here "Front Diff = open, Center Diff = LSD and Rear Diff = LSD", but reading the manual suggests different, or, I just need somebody to explain how the two differentials work together.

Check it out, lets clear this up.

Backlash; Anybody checked their backlash? I checked mine the other day and have hit the panic button. I'm still to check it according to the manual, so crisis might be averted. In the meantime, anybody had backlash become an issue? Anybody had their LSD's rebuilt? How about center bearings... anybody had those replaced?

I know we all kick back lauding about how the AWD system is ex-EVO gear, but seriously - no failures? No issues? There's just a whole lot of moving parts in the system I figure faults/failures/fixes would be known, and that somebody has had some work done.

Here's an image of the two differentials overlaid to represent them in their normal positions;
http://i.imgur.com/q8y3Jd6.jpg

Disclaimer; I dont want to come across as I'm ragging on the AWD system, I just want to know more about it.

mmm? Anyone?

Front diff is open - located inside the transfer case. Should be able to swap in an Evo OEM front lsd or aftermarket helical lsd for more grip. (but don't know if anyone has tried this yet)

Centre diff is a viscous coupling LSD - located inside the gearbox.

Rear diff is a mechanical LSD.


If you look at the diagram and imagine right or left front axle losing grip - the right one sends it straight to the back wheels throught the gear tailshaft connection. If the left one spins the Centre diff LSD locks up and sends the power back through the gear to tailshaft again.

So if one front wheel slips, all it's power goes to rear wheels only. Then if a front LSD is added, it should send all power to the rear wheels, plus also the other front wheel:).

I'm 90% sure that's how it works...

Zero issues with mine with extra power from supercharger. Evo's run huge amounts of power with little issues, so should be all good for us too.

Hmmm nice read so far :)

Backlash; Anybody checked their backlash? I checked mine the other day and have hit the panic button. I'm still to check it according to the manual, so crisis might be averted. In the meantime, anybody had backlash become an issue? Anybody had their LSD's rebuilt? How about center bearings... anybody had those replaced?
What crisis are you talking about? I've had some mild "backlash" which presents as a minor thump when moving into reverse while cold. With the RPM a little higher there will sometimes be a minor thump as the reverse gear engages which has been diagnosed as small amount of slack in rear diff (but still in spec). but I havent thought about center bearings. Would be interested to know if they typically wear out and when.
Wouldnt they get noisy?

I don't understand where people get the idea that the rear diff is mechanical. All the literature I've ever seen says VCU. Did they change? Urban myth?

here is an extract from the shop manual:

https://lh5.googleusercontent.com/-ADr-3Fqwqss/VIFq177kYeI/AAAAAAAAH6U/RJAD0NEoS7M/s696/MagnaRearDiff.jpg

What crisis are you talking about? I've had some mild "backlash" which presents as a minor thump when moving into reverse while cold. With the RPM a little higher there will sometimes be a minor thump as the reverse gear engages which has been diagnosed as small amount of slack in rear diff (but still in spec). but I havent thought about center bearings. Would be interested to know if they typically wear out and when. Wouldnt they get noisy?

I still haven't double checked my backlash properly yet. Any time I find something a little out of spec' I hit the panic button. Most of the time it turns out to be nothing.

My centre bearing seem loose to me. In that I can shake the centre shaft around in the rubber mounts. I don't think I explained that well. Soft/loose is how I would describe the rubber that carriers the actual centre bearing of the centre bearing arrangement. The rubber isn't cracked, but it feels scaley.

I get the "clank" going 1st to reverse but it's not common, and a clunk sometimes if I do a full blown emergency stop. Aside from that, no noises.

If I get time, and some better photos, I'd like to try developing a basic 3D model of the front/centre differential set. Not with splines and gear teeth or anything, just enough of a model see the how the parts interact with each other.

Home sick, got bored, did some Googling.

Mostly looking to service the VCU LSD's or replace them. At this point I'm 75% confident in saying the CP9A Lancer gear will fit the AWD Magna - this is the front, centre and rear LSD's being replaceable using Quaife units at least. This guess still needs confirming though, so don't go blowing your cash because some yutz (me) took a guess. Also, this is the LSD units only, not the actual "complete unit". As in, you cant just fit an EVO VII transfer case to your Magna AWD. The ratios are different from what I can tell.

As for how VCU's work I found this interesting article (http://forum.miata.net/vb/showpost.php?p=4267988&postcount=2) which plays true to how my car's LSD's function... based on a few rudimentary tests I've done.

By "test" I mean burnout attempts on grass, so no science at all. When cold the tracks left indicate a poorly functioning LSD, but when hot the tracks indicate a perfectly functioning LSD. In my case simply driving home from work doing a few quick "0-60's" will be enough to get the VCU up to operating temperature. Conversely, leaving the car sit for an hour is enough for the VCU to lose all that good heat.

Regardless of all my yammering, the article is informative in that it explains the other potential wear points in the VCU.

Anyway, here's the quote from the article;

Viscous couplings are generally not well understood and the Miata unit is no exception. First, it is thought to wear out. I think this concept is very overstated here. While there is no clutch plate contact, the usual notion is that the heat cooks the silicone oil and destroys it. This may be true and if so would be evident in demanding competitive applications. However, I suspect the opposite is true. The Miata VLSD doesn't get hot enough during an autocross to work effectively. Which brings us to the second point: the lock-up is lame anyway. Well, not if it's hot enough.

Historically, the technical papers that indicate the beginnings of understanding how VLSD work date to the late '80's and early '90's, precisely when Mazda came up with the VLSD in the Miata. And they were proud of it, sending their service technicians to a special course on the VLSD. I remember one dealer mechanic confiding in me, "They were so excited about the VLSD, I thought it would be on every Mazda car..."

Now, the alternating plates in the VLSD are toothed wheels with a couple dozen radial tabs. The reason for this is to eliminate warpage of the plates with temperature. Turns out there was some serendipity to this design. It has long been known that VLSD transmit more torque than estimated by shear forces alone, sometimes 10 or 20 times more torque! But sometimes NO more!(?) Engineers had been muddling along with empirical test results until about the '90's when several scholarly attempts were made to quantify and predict performance.

It turns out that the plate tabs twist and shift axially giving rise to all sorts of interesting phenomenon with fluid phase changes, bulk pressure, dissolved air, Coulomb friction, blah blah blah (see SAE 2004-01-0867 - Mohan). The short story is the VLSD differential enters an operating phase Mohan calls "Self-induced Torque Amplification" (STA). This is also described by engineers working with Nissan, Takemura and Niikura, although they call it "hump" (SAE 900558).

In any case, the VLSD has 3 phases. First is initial viscous dissipation where the oil heats and viscosity drops. The VLSD transmits less torque. This is the operation you can measure with a torque wrench as outlined in the Shop Manual and gives rise to the idea that the VLSD is lame. As the fluid heats, the transmitted torque levels and at last in the final phase, as the fluid temperature exceeds 200 C, the torque graph shoots up, with a very steep slope, to transmit 300-400 ft-lbs or more. The ability of the VLSD to transmit this torque will continue as long as the high shear rates are present. To achieve this temperature, the VLSD may have to be subjected to high shear rates for several minutes. Clearly, this is not accomplished at an autocross or even, perhaps, on the track. So, the poor reputation of the VLSD for typical Miata performance applications is supported by the scientific explanation. However, this continuous shearing may be experienced in off-road center differentials and, indeed, Mohan worked for New Venture Gear and viscous differential are still found on 4x4 vehicles here and there.

So, the Miata VLSD is probably not weak or broken, just poorly matched to its application. If it DOES break, it can be overheated oil, or broken tabs on the 'cluch' plates, or I suppose the fluid could leak out. In any case, the unit is not serviceable and one suspected of being defective is discarded.

You notice Mazda dropped it and moved on to the Torsen...

There are pictures of the various differential options in the Diff Spotters Guide in the Garage.

As for my "clank" I believe the thrust washers in my rear diff' are not so hot. Which results on the freeplay seen at the diff's input and end play on the output shafts. Again, I could be wrong as I'm no expert.