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11/10/2013 7:57 am  #1

IRS for '75 BMW 2002

I stumbled across this great site last night as I was browsing the internet for some ideas to use on my '75 BMW 2002. It's going to be seeing a lot more power than it used to via a turbo inline-6 (~450hp at the wheels), and so I've come to the conclusion that an updated IRS would really help keep the car in line compared with the old semi-trailing arm setup. That and I needed to narrow the hub to hub width by about 5 inches to get the wheels to fit under the bodywork. I'm still in the research stage of things, though I have started to model some of the components that I know I will be using in Solidworks (Wheels + Tires, Differential, Bearings etc). 

My original plan was to just swap in a late model E30 (late 80's 3-series) rear subframe assembly, which would give me upgraded brakes and wheel bearings along with a diff that could handle the increased power. However after fitting the assembly in place, I've realized it would be just as easy to make a custom setup that will give me much better geometry. I do still want to utilize the e30 LSD diff, hubs and wheel bearings. They can handle a lot of power and I already had them, so they're basically free.

My plan is to use a dual A-arm setup (I think that's the correct terminology). The area I have to work with is fairly small, like the rest of the car, and I think that will be the easiest setup to package while retaining close to ideal geometry. I also don't think I have the knowledge or skill to pull off a 5 or 6 link setup ala the newer M3's or Corvettes. I'm hoping to be able to keep track of my progress in this thread, as well as ask some of the questions that will inevitably come up during the process. For now I'll just leave you with some pictures of the car as she sits today. I've already sorted the front suspension (aside from sway bar end links). So once I can get the rear suspension figured out it will once again be a rolling chassis for the first time in 2.5 years. 

The Patient, lovingly named Fiona:

The new rear wheels compared to the original stock size:

Rough Wheel Placement:

Thanks for looking, I look forward to learning a lot from you guys!


11/10/2013 8:21 am  #2

Re: IRS for '75 BMW 2002

This is a little off topic, but where did you get the wheel flares and chin spoiler?
And welcome !! 

Last edited by Digz (11/10/2013 8:22 am)


11/10/2013 8:30 am  #3

Re: IRS for '75 BMW 2002


The 'body kit' is actually from the Turbo model, available in Europe for a few years. The parts are still available new from BMW for the most part, the darn front driver-side flare has been 'in-production' for the past 3 years with no telling when it will be available again. At this rate I'm either going to have to break down and get a reproduction piece or fabricate my own from sheet steel or aluminum.

     Thread Starter

11/10/2013 4:21 pm  #4

Re: IRS for '75 BMW 2002

Welcome Aboard!
I drove a BMW 2002 series when stationed  in Germany for a good number of years. Looks like a great project.  And in 2.5 years - wish you the Best - keep the "While-I'm-At-Its" at bay and it could be do-able.

I understand keeping costs down and free is always - GOOD.  What shape are the bearings in?  How do they mount in the uprights? How to tie the uprights into the dual wishbone?  (pics/solidworks renders please share)

As an alternative you might consider fabing up and new set, using C5 uprights and havng them machined for GM K6141 Ball Joints - with custom fabed A-Arms (Dual Wishbone - yes).   I'll be posting on this site as I complete that on my C3. [I've some crummy pics over on my thread to give you an idea] Right now,  I have the uprights done, the ball joints and tabs in hand - now to do some welding.  That whole deal will run a bit over a grand - unless you can machine yourself,, but that includes new SKF C5 racing bearings too.  Corvette and BMW wheels are a "near swap-out" so you wouldn't get costs there.

Be sure and check out some of the free and trail suspension software to help work your placements - with either approach you take.

WIth those "modest" flares and not the Box Style - it will be a bit of a sleeper I suppose, if you keep the paint job "conservative"..

Again - WELCOME and keep posting! 

Cheers - Jim

UNDERCONSTRUCTION! Highly Modified C3 Corvette
         Dual Wishbone IRS w Subframe + Custom Uprights

11/11/2013 9:06 am  #5

Re: IRS for '75 BMW 2002

What size are your new tires?  They look like they could eat your old ones!


11/11/2013 10:41 am  #6

Re: IRS for '75 BMW 2002


Thanks! It's crazy how many people have memories with a 2002 back in the day. Seemingly everyone I talk to either had one themselves or had a friend with one. 

I guess I should rephrase what parts I'd like to use from the e30 setup. My plan is to take the e30 trailing arms and cut off the 'knuckle'. This way I will be able to retain the brakes, parking brake, bearing, hub and wheel speed sensor (hoping to use that for a simple traction control system down the road). I've also already bought new bearings, brake consumables, and had the wheels machined to the proper center bore for the BMW hubs. I'd like to utilize as much of the stuff I already have as possible. 

That being said I'm not adverse to using C5 uprights or something else already designed for the job, I would just need to justify it as being less work in the long run. Which right now I don't see as being the case. I'll get some pictures of the parts in question and my model (very simple ATM) later this evening for you guys to look over. All opinions are welcome!

Funny you mention box flares, I've been on the fence about fabing some for the past year. I do like the sublteness of the arch flares though. I have a feeling I may end up somewhere in the middle with custom metal arches that are a bit larger than the stock pieces, but still mount to the body the same way. 


Haha, they are 265/45/16's. They are beefy to say the least. Sadly Kuhmo doesn't make them anymore so I'm not sure what I'm going to do when they are worn out. Might have to step up to 17" wheels, noone makes a good wide 16" tire anymore.

     Thread Starter

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