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Thread: Opinions RF floor reinforcement re: diff mount and deletions

  1. #1
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    Opinions RF floor reinforcement re: diff mount and deletions

    The time has come for me to do the kit. It's been on my parts shelf for a few years now. Ever since I installed the SC I've been keeping a close eye on things. I can see spot welds cracking in the hatch area and on the diff mount.

    I'm curious if going with the dual ear set-up is advantageous? I already have the dual ear cover to do the job, but if the single ear version will be just as good I don't see a reason for the hassle. The hassle in my case is that the spare tire will have to be permanently removed to do this. Which brings me to the "opinion" question: If you were buying a coupe, would you rather see that everything is intact as it was from the factory, or would you be willing to forgo that originality for the dual ear style reinforcement?

    I don't mind if the spare isn't there. I can load it in the back for longer trips, but if the dual ear version doesn't offer any advantages I'd assume keep it there for the next owner. Somehow, I'm guessing that it will have an advantage.

    Of course the SC is not a factory option, and the suspension as well as other components are aftermarket or not original, but I have retained every item that I have changed/removed.
    Last edited by Zoupe6; 02-16-2019 at 07:06 PM.

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    Who needs a spare--just join AAA Platinum and let them deal with it. I went Rogue Engineering duel ear figuring in for a dime in for dollar--I already had a Super Sprint exhaust. RF seems to believe the single ear will work, but I would be happier with a duel ear set up with supercharging (ok, I am happier with a duel ear set up and supercharging).

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    Better cooling and the dual mount which adds strength, something a SC car needs.

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    Quote Originally Posted by Zoupe6 View Post
    The time has come for me to do the kit. It's been on my parts shelf for a few years now. Ever since I installed the SC I've been keeping a close eye on things. I can see spot welds cracking in the hatch area and on the diff mount.

    I'm curious if going with the dual ear set-up is advantageous? I already have the dual ear cover to do the job, but if the single ear version will be just as good I don't see a reason for the hassle. The hassle in my case is that the spare tire will have to be permanently removed to do this. Which brings me to the "opinion" question: If you were buying a coupe, would you rather see that everything is intact as it was from the factory, or would you be willing to forgo that originality for the dual ear style reinforcement?

    I don't mind if the spare isn't there. I can load it in the back for longer trips, but if the dual ear version doesn't offer any advantages I'd assume keep it there for the next owner. Somehow, I'm guessing that it will have an advantage.

    Of course the SC is not a factory option, and the suspension as well as other components are aftermarket or not original, but I have retained every item that I have changed/removed.
    Hi Zoupe6, we have the same cars, just had the kit put in recently. I chose to keep the spare and original diff mount. The kit is designed to hold the "two floors" of the frame together, thats the most important part. The second part of it is keeping the diff mount from ripping off of the bottom floor. That can be done by reinforcing the stock mount.

    I was not sure if the diff cover with two mounts was available for our cars. The M diff is different, I believe. I could not find it. It might exist.

    When you do it this way you can keep your diff as is.

    If i can help with anything feel free to message me.

    You can also message Randy and get his opinion on it...

  5. #5
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    You have a beautiful coupe!! If I were looking to buy it, I would be much happier if it had the dual ear version (I have that on my roadster and keep spare in trunk). 'Originality' of some Z3 parts is a negative to me personally if they are design flaws (diff mount system, rear subframe bushings, rear shock mounts come to mind). I also think a properly done RF reinforcement should add a few thousand to the selling price.

    My thoughts for dual-ear: it keeps the differential supported better, keeps it from rotating, 4 points of attachment to frame (2 subframe, 2 diff mount) vs 3 for maybe better force distribution to the frame, you have more power with SC to worry about... (Of course poly subframe bushings are a necessity to keep subframe and diff from moving around as much in the first place...) Seems better in general to me, especially with Randy's sturdy reinforcement solution.

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    Thanks folks! I have the dual ear cover sitting in a box right now. Can't remember what it is from, but I was turned onto the part by forum members years ago. I've had the poly bushings in there for the last 40K miles. I ran without the spare for quite a while, until I had a nasty puncture/cut that I couldn't fix roadside. I can visualize how a dual ear would prevent rotational forces much better. Maybe Randy will chime in at some point. I also think the installation of the second mount is a little different for the non-M in addition to removing the spare; not sure on that one.

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    My black Rdstr was done with the single ear (my pieces to reinforce the trunk floor, and a 2nd sectioned mount to clad the original) and it held up through a couple of Vortech supercharger installations, and a brief foray into twinscrew territory.

    Really doesn't answer your question though, does it...?

    Installation-wise, and I'm assuming you're doing it yourself (anybody that can lace up wheels, well this should be pretty easy) one way isn't any harder or easier than the other. If anything, I think there's more work by doing the single ear (preparing the 2nd brkt and then dressing up the finished welds to make it look like stock).

    The differential cover referenced in the parts list is a standard E36 cover (fitted to all of them except the European M3 & the 318ti) and it will fit all 6-cylinder Z3s (2.3/2.5, 2.8, 3.0 & 3.2 engines).

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    Thanks, Randy! How did you know I could lace wheels? Hmmm, still no definitive answer, but physics suggests equal and opposite forces with rotation. One mount is bearing the brunt of both here. Why would they use two if it wasn't needed on the M models!? Hmmmm.....

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    Quote Originally Posted by Zoupe6 View Post
    Thanks, Randy! How did you know I could lace wheels? Hmmm, still no definitive answer, but physics suggests equal and opposite forces with rotation. One mount is bearing the brunt of both here. Why would they use two if it wasn't needed on the M models!? Hmmmm.....
    Long ago, and far away...

    You came to visit me in Toledo! I know we were still in Sue's condo back then, and it might've even been before all this madness with the trunkfloors began

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    Sorry Randy, but I've never been up there; only to Columbus (relatives) and a few other places. However, the previous owner was also here in this area and also knew how to lace a wheel. Unfortunately, he has passed and a good friend of mine inherited the car and all of his motorcycles. I purchased it from him as well as a number of his BMW motorcycles. Wish you still lived there so I could visit and learn a little somethin' somethin', and chit-chat.

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    Quote Originally Posted by Zoupe6 View Post
    Thanks, Randy! How did you know I could lace wheels? Hmmm, still no definitive answer, but physics suggests equal and opposite forces with rotation. One mount is bearing the brunt of both here. Why would they use two if it wasn't needed on the M models!? Hmmmm.....
    Maybe I misunderstood what you said here but all Z3 models, M or non-M, had a single diff ear mount. M's have a differently shaped trunkfloor though, to accommodate the dual exhaust, and as such did not come with a spare tire well from the factory.

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  12. #12
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    Ah, OK! I did not realize that. I guess that I was just focused on the reinforcement threads where the dual mount is commonly used for M models.

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    I found this video somewhat informative. It's pulling down on the floor, potentially causing the failure. I think the RF kit will mitigate this quite well in terms of floor issues single ear or dual. An "I" beam is immensely stronger than the stock configuration and transferring any additional forces to the longitudinal reinforcements. These seem to be essential.
    https://www.youtube.com/watch?v=trYjnmeFqZU

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    Quote Originally Posted by Zoupe6 View Post
    I found this video somewhat informative. It's pulling down on the floor, potentially causing the failure. I think the RF kit will mitigate this quite well in terms of floor issues single ear or dual. An "I" beam is immensely stronger than the stock configuration and transferring any additional forces to the longitudinal reinforcements. These seem to be essential.
    https://www.youtube.com/watch?v=trYjnmeFqZU
    sure does look like its pulling down...very interesting

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    Quote Originally Posted by Zoupe6 View Post
    I found this video somewhat informative. It's pulling down on the floor, potentially causing the failure. I think the RF kit will mitigate this quite well in terms of floor issues single ear or dual. An "I" beam is immensely stronger than the stock configuration and transferring any additional forces to the longitudinal reinforcements. These seem to be essential.
    https://www.youtube.com/watch?v=trYjnmeFqZU

    Yikes, that bolted on stuff... He should have used the Randy F method, especially with a V8... Anyway, that's why the more solid rear subframe bushings are very important (in addition to making a MASSIVE improvement in handling and feel).

    Remember, the diff itself (not talking about the cover with the mount bushing, but the big metal case) is bolted to the subframe ('axle carrier'), which connects to the frame at the subframe bushings. So if those bushings are the floppy loose rubber stock BMW ones (which are like that to 'isolate' the driver from the noise and vibration), the subframe (and as a result) the diff can move around which is what you're seeing in that video. Taken to the opposite extreme, imagine if you just bolted/welded the subframe to the frame without a bushing at all. Could the diff still move around?

    The poly bushings are a compromise that are more solid, but not completely solid. You can actually get solid aluminum rear subframe bushings, but probably only tolerable for a race car...

    The more solid the bushings you use, the more noise and vibration will come into the car. It's a balancing act for a road car. I know my car was much more 'refined' when it was stock, but it handles and feels MUCH better now... The fact is that the diff is a collecting point for all sorts of road/drivetrain noise and its own gear noises. So if you firm up its connection to the car, you will get more vibration and noise in the car. But it's a small price to pay...

    Anyway having more solid rear subframe bushings should take a lot of the stress off the diff cover-to-trunkfloor mounts by keeping the subframe from moving much. But combined with Randy's method of installing a sturdy steel beam between the frame rails to hang sturdy diff cover mount points from ensures any remaining movement of the diff isn't going to hurt the car.

    So long story short, I think both the improved bushings and the improved mounting system are important.

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    Quote Originally Posted by Zoupe6 View Post
    Sorry Randy, but I've never been up there; only to Columbus (relatives) and a few other places. However, the previous owner was also here in this area and also knew how to lace a wheel. Unfortunately, he has passed and a good friend of mine inherited the car and all of his motorcycles. I purchased it from him as well as a number of his BMW motorcycles. Wish you still lived there so I could visit and learn a little somethin' somethin', and chit-chat.
    Huh, well shucky durn! That's a strange set of circumstances; bad for him, but good for you.

    When we used to travel between Sarasota & Toledo, when we made it to Lexington we'd have to decide if we were going to spend the night, or drive it the rest of the way home. Point is, not that much farther from Lexington to here than it would be to Toledo

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    I'm using a single ear RF reinforcement. The main reason for going that route was because the diff mount was previously reinforced by a shop. The reinforcement didn't address all areas of the trunk floor and I started getting some popped welds and separation. Thus I installed Randy's kit securing everything together. The car is supercharged and there hasn't been any issues since installing Randy's kit.
    -Phil

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    Hi gents, about the subframe bushings...would not the more flexible ones dampen the jarring action of the diff?


    Sent from my iPhone using Tapatalk

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    Well, from a handling perspective, having the rear wheels free to waggle around and point in a different direction than the car is pointing is not good... It makes a strange, disjointed feeling like the car is in separate pieces doing separate things (which it actually is). So I'd say for handling reasons, absolutely the less flexible rear subframe bushings the better.

    But if you really want flexible bushings, then you really need to have a super strong attachment at the rear of the diff (ie the RF mod). With the stock situation, you have thin sheetmetal rear mount tabs, attached to thin sheetmetal crossmember, attached with sometimes questionable spotwelds to the thin sheetmetal trunk floor, finally attached to the frame rails. The pounding from the movement and stress presumably eventually fatigues the sheetmetal and welds in some cars causing destruction.

    Randy's solution takes away the reliance on thin sheetmetal for supporting the rear of the differential, so it might allow you to get away with soft subframe bushings (so long as you're proactive about replacing subframe and diff mount bushings).

    I guess if you want flexibility in rear subframe bushings then you're going to be allowing a lot of mass to move around with substantial force, so you need to make sure you damp it properly. Maybe the BMW stock bushings aren't physically able to (maybe it's too much to ask of a few small bushings to damp such forces?) Maybe the sheetmetal and welds at the mount were a foul-up. Maybe both. Maybe you can add some springs in there

    Again, though, in my opinion the penalty in handling for not locking down the rear subframe is large, if that matters to you.

  20. #20
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    Quote Originally Posted by raubritter View Post
    Well, from a handling perspective, having the rear wheels free to waggle around and point in a different direction than the car is pointing is not good... It makes a strange, disjointed feeling like the car is in separate pieces doing separate things (which it actually is). So I'd say for handling reasons, absolutely the less flexible rear subframe bushings the better.

    But if you really want flexible bushings, then you really need to have a super strong attachment at the rear of the diff (ie the RF mod). With the stock situation, you have thin sheetmetal rear mount tabs, attached to thin sheetmetal crossmember, attached with sometimes questionable spotwelds to the thin sheetmetal trunk floor, finally attached to the frame rails. The pounding from the movement and stress presumably eventually fatigues the sheetmetal and welds in some cars causing destruction.

    Randy's solution takes away the reliance on thin sheetmetal for supporting the rear of the differential, so it might allow you to get away with soft subframe bushings (so long as you're proactive about replacing subframe and diff mount bushings).

    I guess if you want flexibility in rear subframe bushings then you're going to be allowing a lot of mass to move around with substantial force, so you need to make sure you damp it properly. Maybe the BMW stock bushings aren't physically able to (maybe it's too much to ask of a few small bushings to damp such forces?) Maybe the sheetmetal and welds at the mount were a foul-up. Maybe both. Maybe you can add some springs in there

    Again, though, in my opinion the penalty in handling for not locking down the rear subframe is large, if that matters to you.
    Yes sir, granted, the solid bushings keep the rear in alignment...better performance by a degree. Good thing.

    My thought was that maybe the softer bushing dampen the hammering of the diff on the subframe and frame.

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    Yeah, I see what you're saying... makes sense. Although I would think the road could be sending impacts to the diff mount too, through the wheels->halfshafts->diff, and maybe also wheels->trailing arms->subframe->diff. Even the body of the car moving against the subframe in a corner or hard acceleration would be sending all sorts of forces to the subframe mounts and diff mount.

    Without some way of testing I can only speculate, but I think the bushings just aren't able to deal with the movement/forces. Or maybe they can for a while? Maybe they can, but the factory sheetmetal mounting system just fails over time or in some cases.

    The evidence has shown that sometimes the sheetmetal mount tabs for the diff tears or separates, sometimes the trunkfloor tears, sometimes the spotwelds holding it together fails. So what's clear is that whatever forces get through the damping of the stock rubber bushings, is too much for some stock diff mount sheetmetal systems.

    I wonder how the E30 3 series handled the rear diff mount and subframe mounts? The Z3 inherited the diff and diff mount setup from the E30 (and also rear suspension setup), and they don't have this problem (as far as I know). They might even use the same subframe system. Anyway I'm purely guessing at this point so I think I'll stop. This issue has been discussed massively in-depth for years on this forum

  22. #22
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    Quote Originally Posted by raubritter View Post
    Well, from a handling perspective, having the rear wheels free to waggle around and point in a different direction than the car is pointing is not good... It makes a strange, disjointed feeling like the car is in separate pieces doing separate things (which it actually is). So I'd say for handling reasons, absolutely the less flexible rear subframe bushings the better.

    But if you really want flexible bushings, then you really need to have a super strong attachment at the rear of the diff (ie the RF mod). With the stock situation, you have thin sheetmetal rear mount tabs, attached to thin sheetmetal crossmember, attached with sometimes questionable spotwelds to the thin sheetmetal trunk floor, finally attached to the frame rails. The pounding from the movement and stress presumably eventually fatigues the sheetmetal and welds in some cars causing destruction.

    Randy's solution takes away the reliance on thin sheetmetal for supporting the rear of the differential, so it might allow you to get away with soft subframe bushings (so long as you're proactive about replacing subframe and diff mount bushings).

    I guess if you want flexibility in rear subframe bushings then you're going to be allowing a lot of mass to move around with substantial force, so you need to make sure you damp it properly. Maybe the BMW stock bushings aren't physically able to (maybe it's too much to ask of a few small bushings to damp such forces?) Maybe the sheetmetal and welds at the mount were a foul-up. Maybe both. Maybe you can add some springs in there

    Again, though, in my opinion the penalty in handling for not locking down the rear subframe is large, if that matters to you.
    Quote Originally Posted by solimans View Post
    Yes sir, granted, the solid bushings keep the rear in alignment...better performance by a degree. Good thing.

    My thought was that maybe the softer bushing dampen the hammering of the diff on the subframe and frame.
    Taking Tom's XLNT description a step further, think of it as a slide-hammer; the further the mass can travel, the larger the impact.

    That's why I've always advocated for the urethane suspension carrier (aka subframe) mounting bushes to greatly reduce the amount the carrier can move about. The colossal improvement in handling and control__no doubt the primary motivation for the product's existence__makes them even more desirable.

    Because there will still be some movement, I prefer to retain the stock bushing(s) in the differential cover, regardless if you go (stock appearing and doubled-up) single mount or use the E36 differential cover with two (2) mounting points. That said, a couple years ago, all the aftermarket Rogue Engineering covers started coming with urethane mounts in them (the OEM mounts had gotten priced out of contention, just like the Z3 & M Rdstr/Coupe ones) but so far, I haven't had any problems reported. In fact, the first car I fitted one too is back here for some engine work, and I plan to look at the mounts/bushes carefully to make sure that all is well.




  23. #23
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    Randy, can you also let us know if there's noticeably more vibration and noise from the differential into the cabin with the poly diff bushings compared to the rubber ones? That's my concern with the poly ones...

    Also, is it me or does that RE poly diff bushing look similar to the subframe bushings? It would make sense at this point I guess to just go ahead and consider the diff mounts to actually be additional subframe mounts... so why not use the same bushings for all 4

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    Quote Originally Posted by raubritter View Post
    Randy, can you also let us know if there's noticeably more vibration and noise from the differential into the cabin with the poly diff bushings compared to the rubber ones? That's my concern with the poly ones...

    Also, is it me or does that RE poly diff bushing look similar to the subframe bushings? It would make sense at this point I guess to just go ahead and consider the diff mounts to actually be additional subframe mounts... so why not use the same bushings for all 4
    Diff bushings are a lot smaller and more shallow than subframe bushings

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    Quote Originally Posted by Mr Bingley View Post
    Who needs a spare--just join AAA Platinum and let them deal with it. I went Rogue Engineering duel ear figuring in for a dime in for dollar--I already had a Super Sprint exhaust. RF seems to believe the single ear will work, but I would be happier with a duel ear set up with supercharging (ok, I am happier with a duel ear set up and supercharging).
    Alternate perspective: people who are stranded out of cell range with a flat tire are people who need a spare tire. The day I bought my coupe, I came across a couple on a remote stretch of Hwy 1 late that night that were stranded with a flat tire that neither of them knew how to change. Fortunately they had a spare. This is something that is objectively better about the non-M models: you can choose whether to carry a spare under the car or not. On the M models, you don't have that luxury.

    And an alternate perspective on dual vs single ear: both options have two tabs of metal attached to the car. In the dual ear setup, it's one tab on either side; on the single ear setup, it's both tabs on one side. The single ear setup also has the bolt loaded in double shear instead of single shear. I don't know of any engineering analysis that has been done to compare the two, but I don't think it's a foregone conclusion that the dual ear is dramatically stronger (or even stronger at all).
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