Sep 2, 2016 update to the BMW M3 project car

Fixing the Gear in our M3’s Rear

After nearly two decades of faithful service, we needed to rejuvenate the rear end our M3. Rennie Bryant of Redline Performance guided us through this ordeal.
And to give everything a solid foundation, we also installed Active Autowerke’s differential support bracket.
Be very careful drilling around the differential.
A lift facilitated the job.
Our M3’s rear end needed new differential bushings. Powerflex had what we needed.


“I’m finding that the older I get, it’s not that I learn new things, it’s more like I find out how much of what I know is common knowledge.”
–Sam Lipsyte

We’ve owned our trusty 1997 BMW M3 for more than a decade, and during that time we’ve learned most of the E36 chassis’ common problems. Fortunately, expertise on the E36 M3 is abundant, with answers to nearly every malady as close as a Google search.

This online knowledge should be the first resource in any troubleshooting process; unfortunately, we learned this the hard way.

BMW was our featured marque at last year’s Speedfest at the Classic Motorsports Mitty, and we used the M3 as a chase car during the weekend’s parade and touring laps. Our task was simple: make sure that everyone played by the rules.

We had noticed an occasional rattling noise coming from the rear suspension, but when we inspected the car, nothing jumped out at us. The noise wasn’t severe, or predictable, so we chalked it up to worn subframe and differential bushings since the car was 18 years old, sported 169,000 miles, and still had the original bushings.

This is the point where we should have consulted the interwebs–but didn’t.

At the end of the Mitty weekend, we pointed the car south and headed home. We were about 30 miles away from home when the source of the rattle revealed itself–and left us stranded.

The rear differential on an E36-chassis BMW is tied to its subframe by just a single bolt. Over time this bolt has a tendency to back itself out, and if left unrepaired will shear. Once the bolt breaks, a violent vibration signals that the differential is now free to move, upsetting the driveshaft and differential alignment. Fortunately ours broke as we slowed for a gas stop; we are still grateful it didn’t let go at 70 mph.

Once we had the BMW towed home, we decided that it made sense to not only change and reinforce the diff bolt, but also to freshen our subframe and differential bushings. This is a major undertaking because the entire subframe needs to be removed. A lift would be a necessity, and expertise would come in handy, so we trailered the car to Pompano Beach, Florida, when we enlisted the help of Rennie Bryant, owner of Redline BMW Performance.

We arrived at Rennie’s with all the parts we thought we’d need, including new bushings. Since our BMW is predominantly a street car, solid metal pieces would be too extreme; instead we opted for more compliant Delrin bushings for the subframe, and slightly softer Powerflex urethane bushings for the rear diff.

Our car sees significant track time, so we went with Powerflex’s Black Series bushings, which are a bit stiffer than their standard poly units. Our Delrin pieces were chosen because we had previously installed Condor Speed Shop’s Delrin bushings in our rear control arms, and were very happy with their performance; a bit of stiffening for the rest of the rear suspension seemed in order. The Delrin and poly bushings would allow much less deflection than the stock rubber pieces, but wouldn’t be so stiff as to transmit excessive noise, vibration and harshness.

We pulled up to Redline BMW early on Saturday morning, ready to go. Redline BMW is an extremely busy shop, and lift space is at a premium, so we had to complete the installation by Monday morning in order to free up their valuable space. At the time this seemed like an easy proposition.

Once on the lift, the car needed to be disassembled. This included removing the exhaust, disassembling the rear brakes–including the emergency brake–and disconnecting the driveshaft and rear suspension.

Now the hard part: Once the mounting bolts were removed, the subframe assembly was ready to be released. Using a transmission jack as support, and with Rennie’s help, we carefully lowered the assembly onto a nearby workbench. This was done with care and deliberation, as the subframe assembly weighs nearly 200 pounds. Fortunately neither of us sneezed, and the subframe soon was resting safely on the bench.

Once our subframe was out of the car, we could easily inspect the mounts that attach it to the car’s unibody. We didn’t expect a problem, since BMW reinforced these mounting points on the M3 models. There have been rampant internet rumors of subframe mounting points ripping on the E36 chassis BMWs, and while this rarely occurs, it always makes good sense to inspect and evaluate. Fortunately, our mounting points were in perfect order.

Our next step was to remove the old bushings from the subframe and differential cover. We removed the subframe bushings with the use of a portable pneumatic press. Some prefer to burn out the old bushings and cut the remainder with a reciprocating saw, but we prefer to use a press. Once the old bushings were out, our subframe was ready for new ones.

This is where we ran into our first big hurdle. The E36 M3’s rear subframe bushings are slightly different than the fronts: The rear bushing sleeves are slightly wider to ease installation. Unfortunately we received four rear bushings and no fronts. We realize that mistakes happen, especially with parts that look so similar. But to further compound our problems, we cracked the differential cover while attempting to remove our old bushings on a hydraulic press.

It was now around 9 on Saturday night, and things were looking bleak. Somehow we needed to complete this job by the end of the next day, but there was a lot left to do. Instead of pushing our weary minds further, we settled in for a night of fitful sleep.

Sunday morning found us still doubting our odds of success. We needed new bushings and a replacement differential cover; we also had to extract the remainder of our broken differential bolt.

The bushing replacement was the most daunting problem, as most of the business world shuts down on Sunday. Fortunately Rennie had a solution: Redline BMW frequently uses Condor Speed Shop bushings, and Rennie just happened to have the owner’s personal cell phone number. A quick call to Carlos Mendez solved our biggest obstacle: Not only did he have the rear subframe bushings that we needed, he brought them over to Redline personally and offered to help.

Rennie had one more trick up his sleeve. He dove into the depths of his shop and emerged a while later, bloodied and greasy, with a replacement differential cover in nearly perfect shape. We heated the cover using a propane torch and easily pressed out the old bushings.

After a bit of cleaning, we installed the Powerflex Black Series differential bushings, which we sourced through BimmerWorld. These bushings would limit the movement and deflection of our differential, allowing for better launches, and improved handling. Fortunately the new bushings slid into place and fit perfectly.

Next on our list was the offending differential bolt. To our amazement, it was easily removed with the help of a reverse-thread drill bit, leaving our differential casing unharmed.

Now to make sure this problem never happened again. Our solution was Active Autowerke’s differential support bracket. Installation requires drilling two additional holes into the subframe to secure the beefy metal plate that sandwiches the bracket and subframe together for additional strength. The Active Autowerke kit also comes with a stronger differential bolt to help eliminate the possibility of this common failure.

As the sun set on Sunday, our BMW was finally back together. It had been a long and arduous process, but the results were worth it. The new bushings were a revelation, as our M3’s rear end feels much more planted than before. The car’s cornering ability has been increased exponentially with only minimal drawbacks: Our ride is slightly less compliant over sharp bumps, and there is a slight gear whine audible at certain engine speeds. Overall, however, we’ve been thrilled with the increase in performance and improved feel of the now-rejuvenated BMW.

If you are attempting this bushing replacement on your own, make sure you have a lift and an extra set of helping hands at your disposal. While this isn’t an expensive modification, it’s also not a one-person job. Fortunately it’s worth the effort.

Sources

Active Autowerke
activeautowerke.com
(305) 233-9300

BimmerWorld: Powerflex differential bushings ($144.99),
bimmerworld.com
(877) 639-9648

Condor Speed Shop: rear subframe bushings ($165)
condorspeedshop.com

Redline BMW Performance: tech help,
redlinebmwmini.com
(954) 783-7003

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Comments
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Turbine
Turbine New Reader
3/26/19 7:10 a.m.

Hey! I’m finally going to be tackling this job on my m3 sometime within the next month or two, and I was thinking about going with this same setup, but I’m a little nervous after reading about some of the issues people have had by running bushings with different stiffness ratings in the rear.  Have you had any issues with yours? What’s it like on the street? Thanks!

MTechnically
MTechnically New Reader
3/26/19 10:23 a.m.
Turbine said:

Hey! I’m finally going to be tackling this job on my m3 sometime within the next month or two, and I was thinking about going with this same setup, but I’m a little nervous after reading about some of the issues people have had by running bushings with different stiffness ratings in the rear.  Have you had any issues with yours? What’s it like on the street? Thanks!

I don't have an M3, but I somewhat recently rebuilt the entire rear subframe on my E34 touring. Now, I opted for Powerflex street (yellow) subframe bushings and OEM rubber for the differential and RTAB bushings, so it's not 100% comparable. I haven't noticed any negative effects of using harder bushing for subframe location in relation to their interaction with the rest of the rear bushings. I think a good rule of thumb is to keep parts of the same system a similar hardness. You don't want solid motor mounts with rubber transmission mounts, but I think its generally safe to run stiffer subframe bushings with softer differential bushings. Maybe someone with more experience will contradict me, but that's the assumption I work under and haven't had any bad results.

Turbine
Turbine New Reader
3/27/19 1:02 a.m.

In reply to MTechnically :

Alright awesome! Thanks for the info

Knurled.
Knurled. MegaDork
3/27/19 5:54 a.m.
MTechnically said:You don't want solid motor mounts with rubber transmission mounts,

 

...Yes you do!   Those mounts are 3 feet apart along the length of the car, across one of the main points of chassis twist and flex.  If you don't have some give in the trans mounts, WHEN (not if) the chassis flexes and twists, it will stress the trans case and cause weird shifting at best and broken transmission cases at worst.  The end of the transmission is not engineered to be a stressed member, the mounting point is just strong enough to hold the transmission off the ground and anything else is purely coincidental.

 

 

 

JBasham
JBasham HalfDork
3/28/19 3:20 p.m.

I have done this job in my home garage.  It's do-able, but my advice to anyone is assume you'll be at it for a few days before it's all said and done.  And always always go ahead and replace the BMW-specific mounting bolts that screw into the chassis before you re-mount the subframe.  If they're old and you break one, the whole unit needs to come off the car to replace it.

MTechnically
MTechnically New Reader
3/28/19 4:33 p.m.
Knurled. said:
MTechnically said:You don't want solid motor mounts with rubber transmission mounts,

 

...Yes you do!   Those mounts are 3 feet apart along the length of the car, across one of the main points of chassis twist and flex.  If you don't have some give in the trans mounts, WHEN (not if) the chassis flexes and twists, it will stress the trans case and cause weird shifting at best and broken transmission cases at worst.  The end of the transmission is not engineered to be a stressed member, the mounting point is just strong enough to hold the transmission off the ground and anything else is purely coincidental.

 

 

 

Fair point. I hadn't thought about the forces at play in that specific example. I've always heard that you do want to be careful using mounts of wildly different stiffness within the same system, because you will wear softer bushings much more quickly. Maybe I was misinformed about that. 

Tyler H
Tyler H UberDork
3/28/19 5:06 p.m.

Nice work! This job is in my M3's future.  Every other bushing on the car is original, and surprisingly not as horrible.  The subframe bushings look like crap, though.  I'll probably go with poly subframe bushings and OE diff bushing. 

One of these days, the car will be off jackstands. It's been in hovercraft mode for months.  Hopefully done by the Mitty, as an arbitrary goal.  We'll miss the GRM crew this year.

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