A better rear axle for sports cars

[Moderator]

1984-5 Mazda RX7 GSL-SE rear axles came standard back-in-the-day with 4-on-4.5" hubs, a clutch type limited slip differential, and beefy disc brakes featuring vented rotors and integral (non-drum) parking brakes. They came with mounts for RX-7's four-link rear suspension which featured an axle-mounted Watts linkage, and also with an axle-mounted anti-sway bar. With bigger axle shafts and beefier bearings, the GSL-SE variant was notably stronger than regular RX-7 axle, so within the Mazda community wrecked GSL-SE's typically donated their axles to other RX7s. The Datsun community caught on. At least 80 GSL-SE axles have been installed in vintage Datsuns. I've heard of them holding up well in +300hp cars. But I'm not aware of anyone installing a GSL-SE axle in an MGB until now.

Back in the day, gear ratio options were the one downside of all RX-7 axles. That situation has changed because the pumpkins from Miatas and RX-7s are interchangeable. You can get a 3.636:1 gear ratio from Mazda if you know where to look and you can get an even lower ratio on the aftermarket.

Since you can bolt a Miata pumpkin right in, you can get a genuine Torsen (Type 1 or Type 2) limit slip differential in the bargain. Brilliant! But if you're determined to be different;) some more exotic differential builders cater to the Miata racing community.

The biggest advantage of this axle option: it's a real sports car axle. It's light! As configured for use in my MGB:

Axle Housing (narrowed & with mounts for 3-link plus Panhard rod): 28.7 pounds.
Miata 3rd Member (with modestly lightened carrier, 3.636 gears, Torsen T1 diff, & propshaft flange): 46.4 pounds.
Axle Shafts (with bearings, retainers, and lugs): 13.1 pounds each (x2).
Miata brake calipers: 6.1 pounds each (x2)
Volvo S40 brake rotors: 7.4 pounds each (x2)
---------------------------------------------
Grand total for RX7 GSL-SE axle, ready to install:  128.3#

Here are some preview pics. I'll send many more to Carl for upload to the MG Photo Gallery very soon.


(The original Miata driveshaft vibration dampener has been omitted because racecar.) 


(A Ford thread-in breather is shown here, in lieu of original plastic plug-in breather.)


(Miatas ring-and-pinion gears are super easy to set-up on the first try, without shims. Do it yourself and save.)


(More photos coming soon.)

[Scott Costanzo]

Curtis, very unique approach! I like it a lot! How far are you from a test drive?

[MGBV8]

Fantastic! Looking forward to more pics & details on the 3 link setup.

[Moderator]

Thank you both! Still need to plumb up and bleed brakes, install parking brake cables, and get an all-wheel alignment. I need to replace my tires and I want to replace my wheels too, but I don't plan to do either until the tariffs are struck down. 

[mgb260]

Curtis, looks good! Why switch from the RX7 brakes to Miata with Volvo rotor? I'd like to see your upper shock mount. There was also a Mazda Diesel pickup  that had a higher ratio, I think 3.31.
Parts interchange: 

https://fixjunk.com/solomiata/solomiata/Drivetrain.html

[Moderator]

Thanks, Jim! 

First reason: GSL-SE caliper weighs 7.6lb & rotor weighs 10.3 pounds, whereas Miata caliper weighs 6.1lb & Volvo S40 rotor weighs 7.4lb, so a total difference of 4.4 pounds (both sides). One of my main goals is to get my car under 2000 pounds, so I'm watching these things carefully.

Earlier I considered VW Jetta aluminum calipers. They only weigh 4.7 pounds each, but they project outward too far beyond the wheel mounting surface,

Second reason: it's hard to get all the parts to rebuild GSL-SE calipers anymore and brake pad options are limited too. By comparison, Miata calipers are cheap and readily available. I bought brand new ones (not rebuilt) from Rock Auto. They package well and I expect they'll work nicely. Miata NA 1.8L used 7/8" master cylinders, just like my 71 MGB, so brake bias should be in the right ballpark. (Not sure about GSL-SE.) I'll use CX-5 hoses (also from Rock Auto) instead of Miata hoses because they fit my installation better. I bought used Miata parking brake cables. They're longer than I need, and I've just begun to work on adapting them to work with an MGB lever, but I expect they'll work fine,

Third: vented rotors look silly on the rear of a 2000# sportscar, especially if it's far more likely to autocross or even hillclimb than endurance race.

I'm having trouble opening the link to other gear options. I know 3.636:1 gears were used outside of U.S.A. on MX-5s with automatic transmissions... and I was able to buy a genuine Mazda one brand-new online. Not sure how hard they are to find now.

I'll post a pic of upper shock brackets soon, but in a different thread;)

[mgb260]

Curtis, Another question. What front joints are you using on the 3 link? I always thought the greaseable Johnny joint would be better than Heims as they are rebuildable and use a poly insert.

[Moderator]

I totally agree! I'm using a product by Ballistic Fabrications that's very, very similar to a Johnny Joint. Some details are different. Until earlier this year, Ballistic used Nylatron GS inserts. Now their website says they've switched to poly for the inserts. I'm not sure if that's an upgrade or a cost reduction... but surely it doesn't matter because these parts seem absurdly heavy duty for our itty-bitty cars. I searched long and hard to find something similar to a Johnny Joint in a more modest size. 

[BlownMGB-V8]

So that's what those things are called. Didn't know that. Sounds like what I've got on my Bay car 4 link.

Jim

[MGBV8]

Johnny Joint?  I swear, I did not inhale.

[Moderator]

Yep. Registered trademark of John Currie of Currie Enterprises. Other companies sell their own versions.