4/9/14 2:41 p.m.

I am trying to find an IRS for my 77 MGB. Can't find a Jag rear and was told they're heavy. Got some advice from a Cobra kit car manufacturer to try a BMW IRS. Seems plentiful...but wide! Stock rear track on the MGB is 49". The e46 BMW 3 series is "59. While I was going to flare the fenders anyway, I wasn't going to go 5" each side.

So, has anyone narrowed a BMW IRS? Also worried about the separate spring set up. Would rather have a coil-over. Since I am not actually installing into a BMW could I use the standard shock mount location of the BMW and just use coil-overs? That may also be beneficial since the spring caused a weird moment arm, from what I can tell since it's inboard of the shock and presses directly on the arm.

Would also need to modify the trailing arm since I think the attachment point would be outside of the MGB body. Not sure how that would affect the arm movement if I just made it straight instead of "C" shaped.


GameboyRMH MegaDork
4/9/14 2:44 p.m.

You can use the stock shock mount location for coilovers...for shortening the width, you'll need custom axles and custom arms. At that point you have to ask yourself if there's something closer to what you need or if you should just go full custom. What about a Miata IRS?

Giant Purple Snorklewacker
Giant Purple Snorklewacker MegaDork
4/9/14 2:50 p.m.

Ask him which BMW.

The 2002/E21/E30s had simple rear trailing arm setups.

The E28/E34/E39 cars had real coil overs on some models.

The E36/E46 cars have 5 links with heavy control arms linked by long upper/lower arms with the upper being the spring perch. They are very effective but you would struggle to narrow them and retain decent geometry.

Take a look at the Factory Five Cobra Roadsters with IRS. They use t-bird aluminum uprights and simple unequal length control arms with rod ends and a ford diff. It's expensive to buy from them but not technically difficult to fabricate. Here is one take on it...

The arms look like:

NOHOME SuperDork
4/9/14 4:50 p.m.

Use a Miata rear suspension. Going to be hard to beat $1500 as the cost but it will only need 3" flares per side and you might be able to get that to 2" with offset wheels.

Boss Frog sells axles for $1000 that will do the job wit the stock Miata spindles and a kit to convert the Miata sub-frame to T-bird 8.8. There are other kits available, such as Flying Miata that uses the Getrag differential but cost goes up. You weight the differences.

The FF axles are shortened to work with T bird uprights and are much cheaper than the T-bird/Miata hybrid options.

RossD PowerDork
4/10/14 8:54 a.m.

How wide is a XR4Ti? They use the usual Ford 7.5" right?

gravesgrg New Reader
4/16/14 12:41 p.m.

Couldn't find a Miata rear or a T-bird rear or a Jag rear...BMW rears are pleantiful and relatively cheap... the past couple of days I actually bought an E46 M3 IRS. Was going to go with a vanilla 3 series rear but the M3 already had the LSD. Looks like a fairly light rear end although I might end up only using the subframe and diff if I can't get the geometry correct. Want to shorten the axles 2-3" per side (unless BMW makes shorter axles for other cars that will work). Shortening the axles should also bring the trailing arm mounts inward but until the IRS arrives (being shipped) I can only imagine the mock-up of where it will land on the MG frame. screwy would the handling be if I straightened the trailing arm (from a "C" shape to a "/" shape)? Thought about having a machine shop make the LCA and make it adjustable (kind of like Ground Control's LCA) so I can control camber. Don't like the idea of cutting the UCA to make it shorter so I'll probably go with the LCA design for the UCA and move the coils to a coil-over (QA1's?) and mount them to the original hub mounting points. Then weld mounts (stud? T? sperical bearing? looking for suggestions) to the MG frame. Other than being a butt-ton of work, anyone see why this idea won't work?


turboswede UltimaDork
4/16/14 1:23 p.m.

Really? couldn't turn up any local yards that you could scoop an IRS unit from?

How about using some Miata uprights with custom built control arms ala BGKast's MidLana project? That will allow you to better fit it all under the sheet metal (though you'll need to play with the locating points since the geometry will be altered thanks to the changes in width)

TeamEvil Reader
4/16/14 1:35 p.m.

If you end up seriously considering a BMW rear pumpkin and IRS set-up you have a huge choice of axles, spacers, and CV joints to sort through. Even those from the Beetle will fit and are much narrower than the BMW items. Considering the width of the BMW pumpkin versus the VW Beetle transaxle, I would be surprised if you could mix and match those to a perfect track width.

The buy-in for the Miata set-up is very tempting if you can live with the necessary flares or widened fenders, the BMW is really a reasonable alternative however.

Keith Tanner
Keith Tanner MegaDork
4/16/14 1:46 p.m.

Approximate fitment of the Miata rear under an MG body. This is a live axle, but it was sized to match the Miata width.

Basil Exposition
Basil Exposition Dork
4/16/14 1:55 p.m.

Surely you can find a Jaguar rear end, they were under a gazillion cars, a bunch of which are in the junkyards.

IIRC, there was a UK company building IRS for MGB's. Search around a bit and you might find it. Start with Brown & Gammons, they seem to have everything related to MG performance. Might be expensive, but all the work and sorting you're talking about won't be cheap...

Edit: Just googled "IRS for MGB" and got this thread:,458430

gravesgrg New Reader
4/16/14 4:59 p.m.

Wasn't familiar with But in my area (MD/VA/DC) none of the salvage yards had anything (Miata, BMW, Jag, Ford IRS...nothing). Nothing on eBay other than BMWs or really wide Jaguars and nothing inexpensive...unless I wasn't using the right search terms...happens to me sometimes.

I am familiar with Hoyle. And they wanted ~$5500 without brakes (was going to match the Wilwoods up front) and that was before shipping. Am planning to go with FastCars Inc up front when funding becomes available. With their +2" option they said it would be 53". Gonna go with fiberglass fenders front and rear.

Richland Motor does not do parts and I didn't want to tote (Southern term) the car to Michigan (11 hours).

I appreciate all the links. Think I am still going to plow forward with the M3 IRS. For what I have in that IRS I have a lot of money before I approach the cost of the Hoyle. I looked at a couple of the other links and Rebornco and CustomRodWorks don't seem to be in business anymore (webpages gone).

Was just hoping someone was as crazy as I am for wanting to mount the E46 IRS in an MG.

Giant Purple Snorklewacker
Giant Purple Snorklewacker MegaDork
4/16/14 5:23 p.m.

Oh 'cmon. Admit it you didn't even look for anything else . No one cares if you want to go about this all wrong. Seriously. Just look at our garages on here. They are full of all manner of retarded mistakes. Rock on with your bad 'ol E46 M3 IRS swap but ... just so you know that we know...

Brandywine Automotive in MD has 12 full rear Jag XJR suspensions at $400 each listed on page one of the results. There are 6 full pages of 50 per page listings for them in Mid-Atlantic alone.

gravesgrg New Reader
4/17/14 8:15 a.m.

OK. You got me...but I did reach out to local yards and they were unresponsive (maybe they don't know about email). Wish I had known a week earlier about Now that I am invested in the BMW IRS I want to make it work...Local German autoparts salvager wanted $500 more for an E36 M3 rear than I paid . The LSD was a big factor for me. I knew if I went with the Jag I would be replacing gears and popping in an LSD. I remember reading that the T-bird was way overweight with the IRS so I thought it was too beefy for my application. The guys at ERA actually suggested I try the BMW. I was all Jag-geeked before I talked to them. I had approached them regarding narrowing the rear. They thought it was overkill. Figured these guys make a living at this and they've probably tried everything to get the most performance in the Cobra kit car world.

I would almost be willing to trade the BMW IRS for an already narrowed Jag rear or beefed up Miata rear (I know, keep dreaming). And then there is just the challenge of making it work that makes me want to say...hell yeah, let's see if I can make it work...

gravesgrg New Reader
4/17/14 8:17 a.m.

In reply to TeamEvil:

Didn't even know about the Beetle connection. Do you have any more info or is it pretty straight forward?

TeamEvil Reader
4/17/14 4:14 p.m.

It looks to be a straight forward replacement. Select the spacers/axles (the choices here are pretty big) that put the track width where you want it and tread in the socket bolts. Double check everything and go on from there. You could also look to VW 911/912, Porsche, and VW Bus axles and CV joints as well, depending on if you want to do a bit of machining or look to the after market for bits and pieces to hook everything up.

There are a LOT of options out there due to the popularity of aircooled VW off-road racing/sand rails/drag racing. An untapped source of parts and information for sure.

amg_rx7 Dork
4/17/14 11:58 p.m.

I'll just leave this here:

Saab 96 with Miata subframes and drivetrain

10/16/18 7:55 p.m.

In reply to gravesgrg :

Any updates on the IRS for your MGB? I'd love to see what you have so far.


Fredericksburg VA

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