Opinions RF floor reinforcement re: diff mount and deletions

[raubritter]

The big difference is that the 2-ear RF version uses much thicker steel mounting tabs that are connected to a thick steel beam that runs to the frame rails on each side, whereas the stock single ear setup uses 2 thin pieces of sheetmetal which in some cases have torn (sometimes the tabs themselves tear, sometimes they rip loose from the crossmember).

But if you're comparing the RF 2-ear version with a reinforced single-ear, then I think when Randy does those he sandwiches 2 stock single-ear ones together to make it stronger. I'm sure that's fine (he has one on his own FI car), so long as you address the subframe bushing issue. I definitely see the positive in being able to keep the spare tire carrier down below. But for me personally, I prefer the extra attachment point.

Also, about the spare tire-- I agree, especially as someone else mentioned recently the good roads tend to be far from populated areas. Although I no longer have the under-car storage location so I just keep it in the trunk.

But I checked my spare a few weeks ago and the valve stem had rotted/cracked and it wouldn't hold air. So just saying, if you have an original Z3 spare keep an eye on the valve stem after all these years... The tire itself looked fine, valve stem not so much. Got it replaced, but it would have been horribly ironic to have needed it and found it that way out in the boonies.

[crashgordan]

Good read! i need to send my car to RF, saving my pennies and parts... i have a 2000 M roadster

i assume the rear diff remains unchanged, but the cover can be changed? i see RF post #7, and i think i understand, but just for clarity...

QUESTION: What is the part number of the dual ear, or what year cars can it be taken from? watching fleabay for a dual ear rear diff cover...

(or is the rogue engineering the only solid option and do i need to modify anything inside the diff w/ /the E36 dual ear cover with either cover?)

[kornfeld]

The big difference is that the 2-ear RF version uses much thicker steel mounting tabs that are connected to a thick steel beam that runs to the frame rails on each side, whereas the stock single ear setup uses 2 thin pieces of sheetmetal which in some cases have torn (sometimes the tabs themselves tear, sometimes they rip loose from the crossmember).

But if you're comparing the RF 2-ear version with a reinforced single-ear, then I think when Randy does those he sandwiches 2 stock single-ear ones together to make it stronger. I'm sure that's fine (he has one on his own FI car), so long as you address the subframe bushing issue. I definitely see the positive in being able to keep the spare tire carrier down below. But for me personally, I prefer the extra attachment point.

Also, about the spare tire-- I agree, especially as someone else mentioned recently the good roads tend to be far from populated areas. Although I no longer have the under-car storage location so I just keep it in the trunk.

But I checked my spare a few weeks ago and the valve stem had rotted/cracked and it wouldn't hold air. So just saying, if you have an original Z3 spare keep an eye on the valve stem after all these years... The tire itself looked fine, valve stem not so much. Got it replaced, but it would have been horribly ironic to have needed it and found it that way out in the boonies.

On my car, they used the pendants from the dual ear kit to replace what comes on the car from the factory....so that's also an option:

P1010112.JPG

And the info about all of the rubber on the spare assembly going bad is totally valid....I actually couldn't even find a replacement for the tire that both fit the wheel and was in the ballpark of the correct diameter when I had the stock wheel/tire specs.

[Z-Thrizzle]

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.

here are some pics when prepping for RF kit. The original work done on the car did not reinforce the section of the diffmount/trunk floor that's facing the front of the car. This is where I had issues. To get the big picture...note that the IE subframe bushings were from that "bad batch" and broke to pieces.

[kornfeld]

Good read! i need to send my car to RF, saving my pennies and parts... i have a 2000 M roadster

i assume the rear diff remains unchanged, but the cover can be changed?

QUESTION: What is the part number of the dual ear, or what year cars can it be taken from? watching fleabay for a dual ear rear diff cover...

(or is the rogue engineering the only solid option)

PM'd you....

[crashgordan]

PM'd you....

excellent!

[raubritter]

Good read! i need to send my car to RF, saving my pennies and parts... i have a 2000 M roadster

i assume the rear diff remains unchanged, but the cover can be changed? i see RF post #7, and i think i understand, but just for clarity...

QUESTION: What is the part number of the dual ear, or what year cars can it be taken from? watching fleabay for a dual ear rear diff cover...

(or is the rogue engineering the only solid option and do i need to modify anything inside the diff w/ /the E36 dual ear cover with either cover?)

Yes, the differential itself is unchanged, just the rear cover and associated parts.

The base dual-ear option is from the E36 M3, which is very sleek and has no cooling fins. The Rogue cover has a bigger oil capacity and large cooling fins.

I initially got the Rogue cover with Randy's modification, but it just hung down too low for my comfort once I installed H&R springs (mistake!) and the car was so low that I was scraping on everything. I figured it was only a matter of time before it caught on something. So I replaced the Rogue cover with the E36 M3 one for better clearance. Then I finally got an adjustable height suspension so I could raise the car back up to a reasonable level. Still, I feel better not having the big Rogue fins hanging down.

Anyway even without cooling fins I figure if it's good enough for an M3 it's probably good enough for my car.

I will say that when I lived in Germany and originally got my Z3 (some years before getting Randy's work done), presumably from taking it on the autobahn for hours at a time at high speed/RPM, at some point diff fluid was coming up out of the vent hole. So I replaced that diff cover (when I cracked it, but that's another matter) with the Z3 M version with fins because I thought I might be able to use the extra cooling.

Now, in the US (I don't take my car on racetracks) I figure there's not really a need for cooling fins. Though the Rogue cover looks awesome!

[raubritter]

On my car, they used the pendants from the dual ear kit to replace what comes on the car from the factory....so that's also an option:

Nice!

Also, what did you end up getting for a spare replacement? I'll admit I only got the valve stem replaced for now, but I know I should look into replacing the tire too..

[raubritter]

here are some pics when prepping for RF kit. The original work done on the car did not reinforce the section of the diffmount/trunk floor that's facing the front of the car. This is where I had issues. To get the big picture...note that the IE subframe bushings were from that "bad batch" and broke to pieces.

I think this illustrates the problem pretty well... The stock system is weak link upon weak link. If you just reinforce the mount tabs-to-cross-member connection, then the hammering will just be transferred to the next weak link in the chain, the cross-member- to trunk floor connection (the infamous spot welds). And if you reinforce that, then the crossmember+trunkfloor to frame rail connection could be the next victim. Randy's reinforcement comprehensively addresses the whole path from diff mount to frame rails.

[Z-Thrizzle]

I think this illustrates the problem pretty well... The stock system is weak link upon weak link. If you just reinforce the mount tabs-to-cross-member connection, then the hammering will just be transferred to the next weak link in the chain, the cross-member- to trunk floor connection (the infamous spot welds). And if you reinforce that, then the crossmember+trunkfloor to frame rail connection could be the next victim. Randy's reinforcement comprehensively addresses the whole path from diff mount to frame rails.

Right! The force has to travel somewhere....
And the force that my car was getting was probably more than most others as my subframe bushings was disintegrating. The move the subframe moves, the harder it pulls on the diff mount.

[raubritter]

So I investigated my own question about the e30 setup (since our Z3s have the diff mount and rear subframe setup of the e30). My question is (assuming e30s don't have this problem, which I have not heard they do): what is different about the e30 vs our Z3s?

Granted a drawing is not super definitive, but this could be part of the answer:

e30_diffmnt.png

Notice how the e30 differential mount point is located on a cross frame rail that links the two side longitudinal frame rails. In the Z3, however, we have a 'cross member' made out of sheet metal and the trunk floor instead of a real frame rail serving that purpose.

I don't know how firm/sloppy the e30 subframe bushings are, but I think that if the cross frame rail is as beefy as the side rails (as appears to be the case on this drawing), then even with bad subframe bushings it's probably less likely to be a problem... The diff mount tabs themselves could be a weak point, I don't know.

Anyway, interesting...

[325bob]

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).

People who don't want to wait for a tow and then an overnight motel stay waiting for a tire delivery carry a spare that has me on my way in 5 minutes. T
he only coupe I've ever seen near me was on its jack,with a rear tire off, no one around?

[ihm96]

I just roll with fix a flat. Luckily I havent had any sidewall blowouts so thats always worked. AAA is my backup. I've never looked at my spare in the 3 years Ive owned it so I wouldnt trust it lol

People who don't want to wait for a tow and then an overnight motel stay waiting for a tire delivery carry a spare that has me on my way in 5 minutes. T
he only coupe I've ever seen near me was on its jack,with a rear tire off, no one around?

[Randy Forbes]

I just roll with fix a flat. Luckily I havent had any sidewall blowouts so thats always worked. AAA is my backup. I've never looked at my spare in the 3 years Ive owned it so I wouldnt trust it lol

I can guarantee you that it's flat.

PSA: if you haven't already done so, drop the spare and discard the rubber hose that goes from the valve-stem to the access fitting on the basket; they're all dry-rotted and if there was any air in the spare, it allowed it to escape.

[raubritter]

I can guarantee you that it's flat.

PSA: if you haven't already done so, drop the spare and discard the rubber hose that goes from the valve-stem to the access fitting on the basket; they're all dry-rotted and if there was any air in the spare, it allowed it to escape.

PSA pt 2: I mentioned this earlier but my actual valve-stem was dry-rotted and cracked. I just checked the spare a few weeks ago and found out it was flat, and when it wouldn't air up, I found out why. Got the valve stem replaced.

[Randy Forbes]

I've had to replace the valve stem for people too

[Zoupe6]

Reviving this thread. I am going to do a variation of the theme. I am removing the rear sheet metal mount and using one of the pendants to go up through the beam so I can tie it into the heavy steal reinforcement as designed. I am also placing a steel piece inside so that I can weld the front support to this. This steel piece will also be tied into reinforcement. In addition, I am reinforcing the front support by welding an identical piece to it. I felt that I was loosing the real benefit of the kit by not tying in the diff mount to the heavy steel floor support.

[Randy Forbes]

Clearly, you're open to do whatever you want to, but the doubled up stock bracket has stood up to the hooning test for the several years my '99 M Rdstr was supercharged (first with a Dinan/Vortech and ultimately with a Eurosport Twinscrew supercharger). The ES/TS in particular, capable of delivering instantaneous boost (9/9-1/2 psi) and torque.

You'll have to mill (or cut) new slot(s) in the bottom plate, as the Z3 mount and E36 mount are not in the same location, fore/aft, left/right, and for that matter, elevation.

Reviving this thread. I am going to do a variation of the theme. I am removing the rear sheet metal mount and using one of the pendants to go up through the beam so I can tie it into the heavy steal reinforcement as designed. I am also placing a steel piece inside so that I can weld the front support to this. This steel piece will also be tied into reinforcement. In addition, I am reinforcing the front support by welding an identical piece to it. I felt that I was loosing the real benefit of the kit by not tying in the diff mount to the heavy steel floor support.

[Zoupe6]

Thanks for the input, Randy! Yes, I have the dual ear cover here for comparison, and have thought it out. However, your opinion is important as the ultimate designer and my 2.8 with 6psi (maybe 10psi in the future) doesn't quite compare, not to mention more simplified. I think you have talked me into it... I had one or two welds pop on the diff support and thought I might beef it up to the max.

[Randy Forbes]

Double up the OE mount, and then seal-weld it around the perimeter to the xmbr. You have the I-beam to support the xmbr.

Let's put it this way, I couldn't tear it out, and I was really trying to

Thanks for the input, Randy! Yes, I have the dual ear cover here for comparison, and have thought it out. However, your opinion is important as the ultimate designer and my 2.8 with 6psi (maybe 10psi in the future) doesn't quite compare, not to mention more simplified. I think you have talked me into it... I had one or two welds pop on the diff support and thought I might beef it up to the max.

[Zoupe6]

I know that I am being pedantic here, but here is what ended up doing. I welded in the unused pendant material (cut to fit of course) to the cross-member/lower floor support, drilled holes through the double plated diff supports and plug welded them to these additional pieces. I then cut slots in the upper plates so that I could weld them in like the center horizontal beam. It makes me feel good, at least.

[solimans]

I know that I am being pedantic here, but here is what ended up doing. I welded in the unused pendant material (cut to fit of course) to the cross-member/lower floor support, drilled holes through the double plated diff supports and plug welded them to these additional pieces. I then cut slots in the upper plates so that I could weld them in like the center horizontal beam. It makes me feel good, at least.

Solid


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