Randy Forbes Subframe Reinforcement

[CLM21]

So I am in the middle of doing an LS swap, and looking at ways I can reinforce the rear diff as well as that bar that runs along the truck floor which holds the diff mount. I ran across Randy Forbes on bimmerforums who builds reinforcements for the z3ms, which looks like it uses the exact same subframe and diff mount. Does anyone know if this is the case?

Randy said that he had sold a few of the z3m reinforcement kits to e30 guys, but he wasn't sure exactly what they had done to get them to fit. I did some searching but I couldn't find the people who had bought them. Wondering if anyone knows who the people are that installed them.

Anyways, heres some pics of the z3m stuff...

Heres the rear subframe. Looks exactly like the e30 subframe.



Heres the entire rear end. Once again, looks to be the exact same from the diff, diff mount, and RTAs. I do know that the rtas are slightly different, as some people upgrade to z3m arms so they must be reinforced. But they are still the same exact shape and mount the same....



And here is the rail that runs along the underside of the trunk. I guess the real question is whether or not this is the same, as this is the rail that I am looking at reinforcing in order to step up to a twin ear diff mount and make sure it doesnt rip off the trunk floor... it also looks the same. Does anyone know if thats the case?



And finally here is the CAD model of the reinforced rail.

[kronus]

the RF kits are built to address a issue specific to Z3 (M) Coupes and Roadsters where the torque pulls (poorly done) spot welds out of the trunk floor and detaches the brace.

AFAIK the E30 doesn't have this issue even with higher-horsepower builds. You'll probably be fine taking off the stock single ear mount and welding (with gussets) two ears to mount the new diff cover.

If you want to go crazy, the RF kit as is won't be super useful as a starting point due to differences in trunk floor width. You could make the same thing more economically with flat stock and a U channel.

[CLM21]

If the e30 and the z3m have essentially the same rear end, why does only one have an issue with torque ripping through the trunk floor? I guess I may just add the second ear to mount and see how it holds up.....

[kronus]

they have the same subframe arrangement, which mounts to pins that come out of the frame rails. they also have the same diff, and thus the diff mount ear is in the same position. they are not the same chassis, and so do not have the same rear sheet metal configuration for the diff mount ear to tie into, which is where the problem is.

[CLM21]

Ok, that makes sense. I saw the trunk floor of a z3m... or maybe it was a e36 now that I think about it... but it seemed to look very similar to the e30. I'm not sure how that z3m ear pictured above is attached to that beam, but it looks like it may just be welded to the bottom of it. Definitely makes sense that they would have an issue with tearing that off the trunk floor. Haven't really looked that close at the e30 ear.

Thanks for the reply.

[kronus]

this has some detailed info about the Z3M failures.

[DrLeadFoot]

Randy Forbes Subframe Reinforcement

Here is a link that has a bunch of DTM chassis pics.

http://www.s14.net/jakesdtmtours/suspension.html

[stonea]

I'm here just to to speak highly of Randy, he's a huge help and a great guy over on bimmerforums with the z3 community, one day I'll eventually be putting his kit in m z3.