Member
Hello Randy. Yes I had searched for it prior to you giving me the part number. They give you quite a bit of the stuff don't they? Well its all buttoned up. Anyone in Las Vegas need some let me know. Thanks for all your input and I will continue to monitor the spot welds.
99 M Coupe
99 996 Cabrio
86 Saleen Griggs Road Race Car
Member
You're welcome, glad I could help...Originally Posted by Curt C
Now I DON'T want to put the fear of God in you, but by only addressing the mounting bracket* you did the equivalent of replacing a 15-amp fuse that was blowing with one rated for 50 amps. The fuse won't blow any more, but the wiring__or in this example, the XMBR__is subject to a meltdown.
* the work on the bracket itself is the way I did them, so no complaints there (assuming you dressed up the welds a bit)
The pictures below are not mine, and at this point, I'm not even sure who sent them to me (except the first one, which is credited to Drew Poff).
These are a couple of results from treating only the symptoms, and not addressing the root cause:
While these are nuts, bolts & washers taking the place of the original spot welds, you can see how strengthening only the bracket led to the XMBR breaking away from the longitudinal rail on the LH side, now taking the floor with it. This would take more time/money to put right than if they had done nothing, and it still needs to be fixed/reinforced.
Another example of strengthening the bracket (and re-welding the popped spots/cracks) but not addressing the attachment of the xmbr to the side rails. Repairing this car will take twice the effort/money than if this fix had not been done.
12540951_10153410687026448_4887241401825184863_n.jpg thumbnail02.jpg
troll
In the video below with the broken diff bushing, it doesn't look like the differential is pushing down on the diff mount at all. It doesn't seem like it's possible to damage the subframe/trunk/diff mount like that.
It looks like the differential would not be moving very much if they had really stiff polyurethane subframe bushings.
I don't have a high HP/torque Z3 and I don't autocross or track it and I'm just wondering if I have stiff poly SFBs and I made a super-soft polyurethane diff bushing:
1. would that cause damage to drivetrain?
2. what undesirable driving characteristics/tradeoffs would I experience (clunking noise?)
3. why isn't that a budget-friendly way to prevent subframe damage (even if it isn't the best solution for a performance-oriented version of the Z3)?
4. how could you make a softer-than-stock diff bushing if you wanted to?
I get that it wouldn't be ideal, but buying a $45 diff bushing and drilling holes in it seems a lot cheaper and easier than cutting up my trunk and welding in a bunch of new metal.
troll
I was thinking that I could buy a purple Powerflex bushing and use it without the two large washers. If I drilled 1/2"-3/4" holes in the polyurethane it should hold the diff in place without straining the mount???
edit: Here's a bad photoshop with a basic idea of what I mean by drilling holes in the bushing.
Last edited by ragged325; 07-03-2022 at 09:37 PM.
Member
I'm not sure what the advantage of drilling poly bushings would be vs just installing a new OEM rubber bushing. Wouldn't you also need to be an engineer to know how many and what size holes to drill?
troll
I just installed a new stock diff bushing. I'm reading a little more and people are saying that using stiff poly SFBs and a new stock diff bushing will not keep my trunk floor from separating once it has started.
It seems to me that the diff is bolted to the subframe and if the subframe is held in place with stiff poly SFBs then the diff should not be able to move (and the diff mount is not really necessary anyway???)
Member
My understanding is that, yes, poly SFBs will limit/reduce the subframe's movement. How much will depend on the durometer rating of the bushes but it will still move some. That's where the relatively soft rubber diff mount lessens the strain and vibrational fatigue on the trunk floor.
Once the problem has started though, my thinking is that the only remedy then is a trunk floor reinforcement, the Randy Forbes kit being the gold standard.
I used extremely hard 75D AKG (black) for the SFBs. I love the plantedness of the rear end now but I do now hear some diff whine when decelerating at highway speeds. Mostly only with the top up which is not very often. I don't mind the trade off at all. I enjoy the more rigid rear end. Some folks are different. I'm not sure how far up the durometer scale you can go before the diff whine becomes audible.
Member
BMW CCA Member
Here is an article that Bimmer Breaker wrote some time ago about the Z3 trunk floor/subframe issue that I thought was insightful in case if you haven't seen it.
https://www.ascfabrics.com/post/trunkfloor
[edit] IMHO, the hardest durometer I would go for my z3 with the diff bushing would be 80a. I recently switched back to the stock bushing from the 80a.
Last edited by nevan; 07-07-2022 at 10:47 AM.
2000 Z3 M Titanium Silver / Imola Red+Black Nappa
2011 328i E92 Space Gray Metallic / Leder Dakota+Oyster
Since 1987 12 euros / 2 kdms / 2 jdms
- Zach
Member
The diff mount bushing needs the OEM rubber. The subframe bushing need replaced with Powerflex. This will take away the stress on the floor and handle better.
Member
Well, reading all of this is kind of frustrating! In an attempt to refresh my car and limit potential subframe damage I took it to a reputable shop here in Dallas that is known for focusing on just E36/7’s. Their recommendation was poly 75G for both the SFB and diff. I guess shame on me for not doing my research?!?
Glad I have a RF kit sitting in the house waiting for install.
Sent from my iPhone using Tapatalk
Last edited by hancod; 07-07-2022 at 11:32 PM.
2001 M Roadster
Member
(I searched the thread but havent' seen this question/answer, if it has been asked, please direct me..) - Can anyone recommend a shop in Western Canada (BC prefer) who can work on Z3 subframe reinforcement?
Member
I think you'd be better off starting a new thread with a title like 'Subframe reinforcement needed in western Canada'.
Member
Contact Randy Forbes for recommended installers. He recommended one for me where I live and it worked out great.
Member
Closest one on my list is a little south of Canada...
Flash's Auto Body & Paint
2417 N. Astor St.
Spokane WA 99207
BMW trained: Ryan Borth
ryan@flashsauto.com
https://spokaneautobodyrepairs.com/
The hardest part of the whole job seems to be convincing a shop owner that YOU WANT them to cut into your otherwise xlnt looking BMW. Once they get over that mental barrier, it's just a matter of following the instructions (provided with kit) and calling me if they have any questions.
Shops that have done a few cars are always appreciative when I send new leads their way.
Member
Is there a place in Florida, preferably close to Tampa that is recommended for reinforcing the Differential bracket and trunk floor?
Member
Port Richey close enough? Andrew at Tuning Dynamics has done several. https://www.tunedbytd.com/ Tell him I sent you
Had you wanted this done three (>3) or more years ago, I would've done it. Once I'd gotten past a hundred (104-106?) installations, combined with OHS/AVR, and my wife's concern (for my age & health) I had to stop taking in cars. Nowadays, I just produce the kits (and play with my own cars).
Thin Air
Randy, it is good to see you are still having fun playing with cars even if you have cut back a bit.
2002 M Roadster, Steel Gray Metailic, Gray Nappa Leather, Black Soft Top, Steel Gray Metailic Hard Top, TC Kline Double Adj Shocks with H&R Springs, Stromung Exhaust, SSR Type C Wheels. Looking for a new home.
2022 Z4 M40i, Misano Blue Metallic, Prem Pkg, Driver Assistance Pkg
2023 X5 M50, Phytonic Blue, Black Extended Merion Leather, Driving Asst Pro Pkg, Park Asst Pkg, Exec Pkg, Climate Comfort Pkg.
Member
BMW CCA Member
I'm a total newb to BMWs and trying my best to learn so forgive me if I ask stupid questions. Just got a 2001 S54 M Roadster (love it, BTW) and am looking into this RF Kit as a preventative measure. Spoke to a local BMW racing shop that is quite well respected. They said wait until you see a problem developing. They do not consider this a preventative upgrade. Said that although anti-rust welding practices have improved, they still consider welding to be a risk factor for future rust issues. Thus, although they definitely like Randy's kit, they do not recommend it if there is not yet any sign of a problem.
Is that a different perspective than what is commonly accepted wisdom here? Should I still consider this a preventative measure, an upgrade, even if there are no signs of a problem on my beautiful S54 M Roadster? Thanks in advance!
S54 M Roadster (Black Sapphire & Beige)
NA Miata has aquired a new disciple
RX-8 has gone to rotary heaven
Member
That's a different perspective for sure. Going with poly subframe bushings and Randy's kit is definitely an upgrade. Once the damage starts, you then have to get the panels back to predamage position, then do the RF upgrade. It's easier now while everything is in place.
Member
BMW CCA Member
Perhaps we might differentiate between the poly bushings as truly preventative, while holding off on welding until there are the first signs of a developing problem? Thoughts?
Last edited by Robrecht; 06-15-2023 at 09:56 PM.
S54 M Roadster (Black Sapphire & Beige)
NA Miata has aquired a new disciple
RX-8 has gone to rotary heaven
Member
I think as long as you keep an eye on it, you would be ok. It wouldn't hurt to go ahead and get the kit from Randy so that you would have it on hand when needed.
Member
Once you notice damage, the damage is done. Mine looked good until it didn't. I took it to VT Motorsports for the RF kit. Once they got everything off to begin the installation of the RF kit, Matt called saying my welds were ripped/popped/no longer attached from left to right on trunk floor. Everything else was intact and looked good making a good installation of the RF kit.
You can wait to see what happens or you can address the issue. Heads up, S54 if you drive it like you stole it, you'll rip your welds like I did.
I had poly bushing and still ripped the welds or was it Ebbetts Pass??
Bottom line, you're going to do it. The question is when.
Member
BMW CCA Member
Thanks for the feedback. Like I say, I'm no expert, just trying to make sense of the different opinions I hear, and I was surprised to hear the opinion of this racing shop. Would love to hear Randy's opinion on rust-prevention practices when welding.
S54 M Roadster (Black Sapphire & Beige)
NA Miata has aquired a new disciple
RX-8 has gone to rotary heaven
Grumpy
BMW CCA Member
What most point to as "the first signs" is only the first visible sign that your already done. The failure does not start at or near the diff mount. It starts over a ways were four or five structural members are all attached. That is the edge; those are the first welds to go. You just can't see it because the area is buried under a thick layer of seam sealer.
/.randy
Thin Air
I believe Randy's instructions include sealing and painting the effected areas as good as or better than factory so there should be no more of an issue than stock.
Maybe Randy can verify this.
2002 M Roadster, Steel Gray Metailic, Gray Nappa Leather, Black Soft Top, Steel Gray Metailic Hard Top, TC Kline Double Adj Shocks with H&R Springs, Stromung Exhaust, SSR Type C Wheels. Looking for a new home.
2022 Z4 M40i, Misano Blue Metallic, Prem Pkg, Driver Assistance Pkg
2023 X5 M50, Phytonic Blue, Black Extended Merion Leather, Driving Asst Pro Pkg, Park Asst Pkg, Exec Pkg, Climate Comfort Pkg.
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