Poll: Did Polyurethane Rear Subframe Bushings Prevent Diff Mount Failure?

[Blacklane]

Looking through the forum threads, it looks like around 2005, it became “common knowledge” to replace the OEM rubber subframe bushings with polyurethane ones to help prevent subframe movement that could lead to the differential mount failure.

It has been several years now and I’m wondering if the theory is holding up or if it is proving pointless. So I would like to hear from members who did this mod several years ago, whether or not they later experience the subframe failure, and whether they feel it was worthwhile (maybe you have a high-performance car and didn’t really expect it to help much).

Of course there are too many variables here to make a definite determination, but a few examples one way or the other would provide valuable insight.

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Update:
As of 28 Feb 2014, here is the tally from this thread:
Respondents who found a diff body mount failure after installing poly bushings: 0
Respondents who found a diff body mount failure after installing bushings, but didn't look before: 1
Respondents who had some damage but have seen no further damage since installing poly bushings: 4
Respondents who had no damage before installing poly bushings and still don't: 10
People who had damage with original rubber bushings installed: 75-80 (all Randy Forbes)
Respondents who a just happier with their handling after installing polyurethane bushings: 10

I'll still take updates.

[Randy Forbes]

I don't know if prevent was the popular term, but perhaps postpone would be more accurate.

Polyurethane subframe bushings could never be considered pointless, because they positively transform the handling of the car, with no other modifications__STILL the best $100.00 you can spend on one of these cars.

[jreifler]

Any suggestions on which brand has held up best over time for those who've switched? How involved is the installation? DIY capable, or something that most people look to their indy shop to do? Thanks.

[mcoupemindy]

Any suggestions on which brand has held up best over time for those who've switched? How involved is the installation? DIY capable, or something that most people look to their indy shop to do? Thanks.

AKG is now my go to choice. It is an in depth job, but nothing that the DIYer can't tackle with the right tools, some beers and patience. There are lots of DIY guides around.

[Bcar]

AKG is now my go to choice. It is an in depth job, but nothing that the DIYer can't tackle with the right tools, some beers and patience. There are lots of DIY guides around.

+1, I went with the AKGs and just finished my winter project. For sure DIY possible, get an impact wrench!
just be VERY careful of the "well, while im already in there..." bug... I ended up changing a lot more once the subframe was down...

[scrap]

AKG huh? I was thinking powerflex was the recomendation these days. This is my next project so current info is helpful.

[jajou318]

I would go with AKG. I had a diff support bushing from Powerflex go to mush on me in less than a month (this was the street version). They didn't cover it under warranty.

As for the subframe question, I noticed separation after new bushings but I didn't look that hard before.

[wazzuFreddo]

Would you guys recommend doing the sub-frame bushing and the differential bushings all at once?

I figure I'll do this when I am already back there replacing the suspension.

[z3papa]

I don't know if prevent was the popular term, but perhaps postpone would be more accurate.

Polyurethane subframe bushings could never be considered pointless, because they positively transform the handling of the car, with no other modifications__STILL the best $100.00 you can spend on one of these cars.

Even though it is you or more importantly it is because it is you, I'm going to give you crap for using this overused phrase. I'm surprised you didn't say "it makes the car handle like it's on rails". Back to my slumber. By the way, I'm heading your way but not quite as far south in a few days. Catch a little r and r.

[gmushial]

^ I think the choices are: do the RSFBs with the subframe still kind of in the car; or pull it out entirely and do all the bushings, with the luxury of easy access... which is what I did back in January, and basically it was cake-walk - because of schedule: did about 3 hrs a day over three days - one day to pull the subframe and do the diff bushing; next to do RTABs and RSFBs; and the 3rd to put it all back together. Went with IE green RSFBs and oem the rest. W/re the question of the thread: I had one weld gone already, which has been bolted, and two more looking like they want to fail - hence the motivation to do the work. The bolted weld has gone 2 yrs + 25k miles, but was hoping by doing the poly RSFBs I could preclude or at least defer the others from failing. So far, so go... but only have 3k miles on the IEs... time will tell.

[Flak]

I found one popped spot weld before poly bushings. Replaced with IE green poly first, then Powerflex purple. Still only that one popped weld. I also have poly trailing arm bushings and retained the stock diff mount bushing. It seems it postponed further failure.

[mcoupemindy]

I found one popped spot weld before poly bushings. Replaced with IE green poly first, then Powerflex purple. Still only that one popped weld. I also have poly trailing arm bushings and retained the stock diff mount bushing. It seems it postponed further failure.

As I've been saying for quite some time now, the "preventative" route is to put poly subframe bushings in and leave the differential cover the factory rubber. This allows for some vibration and torture damper on the subframe failure while shifting the movement load into the actual unibody of the car through the two mounting bolts.

[Telecomtodd]

As I've been saying for quite some time now, the "preventative" route is to put poly subframe bushings in and leave the differential cover the factory rubber. This allows for some vibration and torture damper on the subframe failure while shifting the movement load into the actual unibody of the car through the two mounting bolts.

+1 Word

[Randy Forbes]

Even though it is you or more importantly it is because it is you, I'm going to give you crap for using this overused phrase. I'm surprised you didn't say "it makes the car handle like it's on rails". Back to my slumber. By the way, I'm heading your way but not quite as far south in a few days. Catch a little r and r.

Yes, but__I__don't say it that much...

On topic, how's this: for ALL the cars that I've personally repaired/reinforced (75 - 80?) only a couple (certainly LESS than five, maybe only 2 or 3) cars have had urethane subframe bushes installed upon arrival. ALL but one (1) car had them when they left.

I'm still using the green Ireland bushes, the ones installed in both of my own cars in 2005 are still holding up just fine. I am aware that they had a bad batch at some point, but in the cases that were brought to my attention, Ireland replaced them at no cost (and they're nowhere near as hard to replace as the originals were).

For anyone perpetuating the rumor that they're no longer available, I ordered four (4) car-sets yesterday (8 bushes). Just stick with the OE rubber bush in the differential cover/bodymount location.

Oh and Clark, it's just about lemonaide time down here...

[mpathic]

Came across this thread while looking for something else. The eight year and ~80k mile old Powerflex purple rear subframe mounts in my E36 //M3 starting getting creaky this year.
Outstanding improvement in rear suspension hookup when accelerating, with minimal added NVH, til this year when they started getting creaky. Not a fun job to drop and replace all the subframe bushings on an E36, that's where the simpler E30 style rear suspension makes life much easier.

[DekaB]

Any insight on the AKG subframe bushings that raise the carrier? Going to be doing the entire rear end and I have not found anything comprehensive as far as bushing choices.

[JWin]

Not often you hear Randy specifically endorse a given mod. Probably worth while to consider his opinion. My car has had them since 12k miles and now sits around 55k miles and I've not visited Mr. Forbes Garage as yet.

- - - Updated - - -

Any insight on the AKG subframe bushings that raise the carrier? Going to be doing the entire rear end and I have not found anything comprehensive as far as bushing choices.

Not sure what you mean by "raise the carrier" as I've just installed AKG street bushings and didn't notice anything out of the ordinary.

[mcoupemindy]

Not often you hear Randy specifically endorse a given mod. Probably worth while to consider his opinion. My car has had them since 12k miles and now sits around 55k miles and I've not visited Mr. Forbes Garage as yet.

- - - Updated - - -



Not sure what you mean by "raise the carrier" as I've just installed AKG street bushings and didn't notice anything out of the ordinary.

AKG makes a set of shorty subframe bushings and a bump up bushing for the differential that effectively moves the subframe about 1/4" up to the unibody. Is it to combat negative camber gain when lowering and keep the axles at optimal angle.

The disadvantage is that weld in adjusters couldn't be used as they would interfere with the gas tank.

It is an interesting idea.

[Z3 M Roadster]

As for what brand to buy, any of the 3 talked about here will do equally well. Any one can have a bad batch shit happens. I went with Powerflex for all my bushings except the one in the diff cover and after 5 years there have been no issues.

[IgotBMW]

Subscribed.

[DekaB]

I am still aiming towards a solid solution that helps to correct any geometry changes made by lowering the car.

[Codeman]

For anyone interested in doing this at home here's a thread about making your own bushing removal tool.

http://forums.bimmerforums.com/forum...or-uber-cheap!

[spacecowboy]

Yes, but__I__don't say it that much...
On topic, how's this: for ALL the cars that I've personally repaired/reinforced (75 - 80?) only a couple (certainly LESS than five, maybe only 2 or 3) cars have had urethane subframe bushes installed upon arrival. ALL but one (1) car had them when they left.

Is it possible that those 2 or 3 incurred damage before the rubber bushing were replaced with urethane?

[Randy Forbes]

Is it possible that those 2 or 3 incurred damage before the rubber bushing were replaced with urethane?

Good question; no answer. At this point, I don't even remember which cars they were, except for the first one, which was an AA Rotrex supercharged Coupe that belonged to John Corbishley.

Though it was quite a few years, and many, many Coupes ago, they say you always remember your first...

It was also the Coupe that I decided the access holes to adjust the rear shocks DID NOT need to be any larger than 1/2"!











Turns out it was also the first car I reinforced the rear sway bar mounts and trailing arm tabs; there was a lot of good reasons to remember that car!

[Ronan1]

I've the Randy Forbes dual ear diff mount, and I couldn't believe the difference that made to the overall handling and feeling of solidity once it was in place. It literally felt like a different car. Fast forward two a month ago, I finally got around to installing some aftermarket subframe bushings sourced by my mechanic, and the cars handling is transformed for the better once again.

The original bushings while still intact, had lost their rigidity. The rear end of the car now feels completely solid, and somehow attached to the steering inputs. The car flows solidly and fluidly around bends. It really is quite amazing the incremental handling improvements which can be made on these cars!