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Wednesday, December 2, 2020

Doug's Domain

Doug Vetter, ATP/CFI

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Thursday, May 27, 2010

Failed Trailing Arm Bushing Leads to Suspension Overhaul

I know, I know. You just read the title of this entry and are now wondering to yourself..."self, is this guy nuts? He was just preparing to do an accessory overhaul and now all of a sudden he's doing a rear suspension overhaul? What the hell happened to the accessory overhaul?" Allow me to explain.

For at least the last couple of years I've noticed a certain sloppiness in the rear end of the car. It hasn't felt as "planted" as it once did. I diagnosed that as worn trailing arm bushings and put those on my list of things to do. Times being what they are and my schedule being what it is, I put the job off and rationalized the delay with my desire to completely overhaul the rear end of the car...eventually. So that brings us to the present day.

While I was washing the car last weekend I sensed something was wrong with the left rear wheel. The ride height seemed lower than normal, almost as if someone had decided to slam it while I wasn't looking. I reached into the space between the top of the tire and the lip of the wheel well to test my theory and found my fingers were a tight fit. I stepped back and walked around the rear of the car to compare it to the other side. That's when I confirmed that the left side of the car was slightly lower than the right. This was made all the more apparent by the fact that BMWs of this vintage are supposed to be slightly HIGHER on the left than the right when the car is unloaded because the vehicle is designed to accommodate the weight of the driver.

(Image: New rear suspension parts including springs and shocks)I initially concluded that the spring had failed. This is not exactly an uncommon problem here in the northeast states where salt is laid liberally on the roads in the winter. The design of the control arm tends to keep the lower portion of the spring bathed in salt spray, so it's not surprising that the springs eventually corrode and fail, and in doing so, drop the car almost an inch. As I walked back and forth to compare the rear wheels I also realized that, in addition to the usual negative camber apparent on both rear wheels, the left wheel was noticeably toe-in. Since the BMW spec for rear wheel toe is a mere 0.3 degrees positive, I knew something was definitely wrong with the rear suspension.

First thing Monday morning I went to the dealer with a list of parts. I originally thought I'd pick up one M-sport package spring, but as it turns out BMW only sells them in pairs (BMW part number 33-53-9-059-403, $288). A earlier review of my maintenance schedule revealed that the shocks had been in service for exactly 100K miles (10K more than the last time I replaced them), so I added shocks (33-52-1-090-831, $268), shock mounts (33-52-1-092-362, $64), shock tower reinforcement plates (51-71-8-413-359, $41), and all the related hardware to the list before settling up with the parts guys for a cool $805 including a 20% discount and a $52 donation to the government.

If you're wondering why I ordered locally when Tischer would have been able to meet that discount level and help me avoid the tax, the reality is I wanted the option to bitch at someone face to face if the springs turned out to be the wrong version. This was a distinct possibility given that BMW does not publish the spring part numbers in the ETK and therefore make them easy to confirm with the BMW community.

As I left the dealer and drove to work, I thought about my plan of action and figured I'd do the springs and shocks and call it a long day. When I got out of the car at work, I once again attempted to check the gap between tire and wheel well just to make sure it wasn't getting worse. Surprisingly, the gap seemed to be a bit larger and the wheel didn't seem to have as much toe-in. And that's when it hit me. The problem wasn't the spring, but the trailing arm bushing, which was causing the entire arm to pivot left and right.

Since trailing arm bushings are a pain in the ass to replace on the car even if you do have the special pusher/puller tool to get the job done, I quickly found myself on a slippery slope. If the trailing arm bushings are shot, surely the other bushing and ball joint on the trailing arm couldn't be far behind. That and my desire to replace the right wheel bearing (with 193K original miles) would justify pulling the arms and having all new parts pressed in on a bench press rather than use lots of special tools I'd have to source myself or borrow from my technician.

Naturally, I continued, if the bushings on the trailing arm are sloppy no doubt the inner control arm bushings have been taking a beating for the last several years as well. Unfortunately, it's not possible to remove the inner bushings without moving the differential aft a few inches to gain access to the bolts. And if I need to disconnect the differential, I figured I might as well pull it for an overhaul (bearings, seals, reset the lash, repaint). Accelerating down the slope beyond Mach 1, I further rationalized that if the subframe were to be sans differential and control arms, surely it couldn't be that much more of an inconvenience to pull the entire subframe out of the car to clean off all the rust and powder coat it, while at the same time replace the subframe bushings with upgraded M3 parts.

Now beyond the area of maximum aerodynamic pressure at Mach 1.2 or so, I jumped to the conclusion that this would be a perfect opportunity to have a local welder stop by to install the M3 subframe reinforcement plates. I mean, heck...this car might make a mean track rat someday, and it would be stupid of me to track it without those reinforcements. Of course, since mobile welders don't come cheap (and the one closest to me charges a flat rate) I figured I might as well have him weld in both the swaybar tab and RTAB pocket reinforcement plates while he's at it.

I think you can see where this is going. The parts list is long and expensive and most of that list is on its way to me as I write this. The mobile welder is one phone call away. I've spoken to a powder coater about prepping and painting several of the rustier components while they're off the car, and I even managed to convince Dan at diffsonline.com to pick up my differential at the Bedford Airport when I fly it up there in the 172 on a post-annual test flight. There's no stopping now. It's going to be one hell of an adventure and the greatest test of my DIY skillset to date. The plan is to begin disassembly this weekend but holiday plans may have something to say about that. In any case, it appears that the E46 will get to flex its muscles for a couple weeks. I hope the E36 doesn't get jealous. Stay tuned!

Mileage: 193000, Parts $805, Parts Saved: $205