Diff bushings replace or not?

[wyatth]

Updated impressions on subframe mounts?

[VictorH]

Yes, again, all street use, now up to about 300 miles. Agree with all others who say the solid sub-frame bushing make no difference whatsoever on noise or vibration. The biggest difference has been the purple diff bushings. They are a bit noisy. I'm not sure I would change them if I had a chance to do them again. For most of my driving to and from work at speeds of low 50 mph or slowing down from 55 mph down to 40 mph (coasting) there is really quite a bit of diff whine. It's more than I would like to have.

I'm going to give the diff bushings a bit more time, but if I ever rebuild my diff, I think I would probably go back to stock diff bushings.

Also, had my rear end aligned. "What? you want just the rear-end aligned, no one does that!" Anyway, even with the sleeves I used inside the billet bushings it's still possible to get the rear a bit out of alignment. On my rear-end there was a bit more toe-IN on the right rear wheel (0.11 degrees IN) and just a touch of toe-OUT on the left rear (.02 degrees OUT). No change in camber as that wouldn't be expected anyway. Have it aligned now with 0.08 degrees IN on both wheels. Not a biggie and I couldn't feel it on the road.

Track impressions coming Sept 8th.

[VictorH]

Track is this coming Saturday, off by a week.
Also, was able to reduce the diff whine by re-tightening the 3rd diff bolt, the big one up top. Took about 20 ft lbs of torque out (now at around 110 ft lbs) and it has reduced the whine just a little bit.

[mtx450]

I love this progress! Happy to hear it's back on the road and doing well.

If you were to do it again, would you consider pulling the subframe and diff all together? I know it's a lot heavier to do it that way, but I'm wondering if it would be worth doing on this chassis?

I'm going to attempt this project on my E90 M3 over the winter, but I just did this on an old Nissan S14 I'm restoring and it was much easier to pull it all together.

[VictorH]

TRACK REPORT

Re the above. I don't think keeping the diff in would help much. The subframe is much larger and unwieldy than your picture, it's also moderately heavy depending on how much stuff you leave on (I took brakes and rotors off and it's just a big clunky piece). It's not a bad job, but I think if you do anything with the diff bushings just leave alone or replace with stock if they are worn.

Did one day event at Road Atlanta today with JZilla. Good guys, have run many times with them, maybe not the most organized, but they run a good program and they are flexible. For example the driver groups are on the car stickers rather than the armbands and I wanted to have my friend drive my car and I drive his. Was no sweat to get extra stickers for the switch, to put on each car.

So, the question that everyone is wondering (are you?) is what's the difference on the track with solid sub-frame bushings. First a bit about me so that you'll understand where the opinion is coming from. I have been tracking here in the Southeast for the past 10 years. Have run tracks from Road America to Roebling and a whole bunch in between. I've never raced, just DE, no time trials either. At events where I'm timed I'm never the fastest guy (okay, once at Ron Fellows Corvette Driving school in NV), but I'd say my strong point is I'm really consistent and able to read/feel what a car is doing really well. So, bottom line I'm not an "expert" or racer but think I have a pretty good feel for cars.

So, the "Do the subframe bushings make a difference on the track" question. in my car, NO. Now my baseline was bushings with 144,000 miles on them and many thousands of track miles, but they weren't really worn at all, and the rubber was still in good shape (mild climate down here). Some people say the car feels more planted, I can't detect any difference. Keep in mind the rest of the suspension in back is still on rubber bushings, so that's why it's not any noisier or more "planted." However, that might be why it also doesn't feel like a go-kart. I'm not disappointed, the car still is fantastic.

Gabe (Dogbone) and I were exchanging a few texts today and what other car can you drive to the track, flog like maniac all day long, then peel all the stickers, numbers and other stuff off, put the suspension to something more streetable and then set the cruise control, AC and turn on the stereo on the way home? As an all-around car it's unparalleled. I'm at 144K miles and no oil consumption, engine still makes good power and the handling for a 4-door sedan is pretty amazing.

What would I do differently. Unless you are having some sort of diff issue related to the stock bushings, I would suggest you leave those alone and be happy. If they need replacing and you still want a quiet street car, then replace the diff bushings with OEM BMW bushings.

[wyatth]

Thanks for the honest feedback. So many have said that one of their favorite mods is these subframe bushings but I definitely think placebo and cost justification plays a role (in any review). Thanks for enduring the PITA DIY and not letting that cloud your assessment.

[wyatth]

Hey Victor, is your opinion unchanged after another month and more street miles? Thanks

[gmx]

Interesting feedback. I don't have my solid subframe bushings in yet but I have felt this unnerving feeling with the rear end esp just before the car oversteers - lateral movement. It "shifts" before the rest of the chassis, hence the disconnected feeling between direction of front axle, chassis, and what the rear-end is doing. I was on soft/OEM suspension too so the load transfer is less resistant as a result and should be less obvious although slower.

[VictorH]

Originally Posted by gmx

Interesting feedback. I don't have my solid subframe bushings in yet but I have felt this unnerving feeling with the rear end esp just before the car oversteers - lateral movement. It "shifts" before the rest of the chassis, hence the disconnected feeling between direction of front axle, chassis, and what the rear-end is doing. I was on soft/OEM suspension too so the load transfer is less resistant as a result and should be less obvious although slower.

Couple of updates. My car of course is back on the road, doing daily driver duties. Regarding the movement some folks feel in the back, I'm not sure you can always attribute that to the rear sub-frame. Low tire pressure, worn-out shocks (probably not very likely, but I felt this in another car), trailing arm bushings that are worn, improper alignment and other factors can all affect rear grip and how the car feels when cornering.

Also, if you look at the design of the rear sub-frame and the front braces, essentially the sub-frame is basically fixed at 6-points. It's hard to imagine you could generate that much movement in the rear sub-frame. I think other factors are more likely.

Regarding the noise issue. Everything is good, the only regret if I have one is the rear differential bushings. I have about 1,000 miles on the set up now and the diff whine has not changed at all, that is it's not any quieter. It's noticeable and I don't really think it's that "cool" unless you just happen to like a noisy car. Having diff whine in back doesn't make a blue-tooth phone call or the stereo experience any better. I'm going to live with it but absent a big power adder, such as a supercharger, I'm not sure adding the diff bushings provide any advantage. I never had a broken diff bolt and my stock bushings looked okay too. Honestly, that is the only regret. I don't want to make it sound like it's really loud, it's not, but it's there, it's unmistakable and I get to hear it each and every time I drive the car.

[wyatth]

Thanks for the follow up. You actually raise a great point on the other factors, namely the alignment. There's usually other work done with the subframe that contributes to the changes, but I think comparing feedback can isolate some of that. But the alignment, you are right, that alone can transform the feel of the rear end. Surely we'll never get an alignment reading right before such work, while an alignment is always part of the job. I think you've convinced me to save my scheckles until it's actually maintenance work.

[M43S7RO]

Interesting to me that some people get increased diff whine. I went with the purple diff bushings and have a hard time hearing an increase in whine. Almost makes me wish I had just gone for solid, I opted for purple because of reports of whine I had read so wanted to play it safe. Are you manual? Maybe the gear box type makes a difference.

Originally Posted by VictorH

Couple of updates. My car of course is back on the road, doing daily driver duties. Regarding the movement some folks feel in the back, I'm not sure you can always attribute that to the rear sub-frame. Low tire pressure, worn-out shocks (probably not very likely, but I felt this in another car), trailing arm bushings that are worn, improper alignment and other factors can all affect rear grip and how the car feels when cornering.

Also, if you look at the design of the rear sub-frame and the front braces, essentially the sub-frame is basically fixed at 6-points. It's hard to imagine you could generate that much movement in the rear sub-frame. I think other factors are more likely.

Regarding the noise issue. Everything is good, the only regret if I have one is the rear differential bushings. I have about 1,000 miles on the set up now and the diff whine has not changed at all, that is it's not any quieter. It's noticeable and I don't really think it's that "cool" unless you just happen to like a noisy car. Having diff whine in back doesn't make a blue-tooth phone call or the stereo experience any better. I'm going to live with it but absent a big power adder, such as a supercharger, I'm not sure adding the diff bushings provide any advantage. I never had a broken diff bolt and my stock bushings looked okay too. Honestly, that is the only regret. I don't want to make it sound like it's really loud, it's not, but it's there, it's unmistakable and I get to hear it each and every time I drive the car.

[Bartledoo]

Originally Posted by VictorH

Couple of updates. My car of course is back on the road, doing daily driver duties. Regarding the movement some folks feel in the back, I'm not sure you can always attribute that to the rear sub-frame. Low tire pressure, worn-out shocks (probably not very likely, but I felt this in another car), trailing arm bushings that are worn, improper alignment and other factors can all affect rear grip and how the car feels when cornering.

Also, if you look at the design of the rear sub-frame and the front braces, essentially the sub-frame is basically fixed at 6-points. It's hard to imagine you could generate that much movement in the rear sub-frame. I think other factors are more likely.

Regarding the noise issue. Everything is good, the only regret if I have one is the rear differential bushings. I have about 1,000 miles on the set up now and the diff whine has not changed at all, that is it's not any quieter. It's noticeable and I don't really think it's that "cool" unless you just happen to like a noisy car. Having diff whine in back doesn't make a blue-tooth phone call or the stereo experience any better. I'm going to live with it but absent a big power adder, such as a supercharger, I'm not sure adding the diff bushings provide any advantage. I never had a broken diff bolt and my stock bushings looked okay too. Honestly, that is the only regret. I don't want to make it sound like it's really loud, it's not, but it's there, it's unmistakable and I get to hear it each and every time I drive the car.

There is an underbody video somebody posted on the forum a long time ago showing the subframe movement, and it's much more than you'd expect.

[pbonsalb]

Originally Posted by M43S7RO

Interesting to me that some people get increased diff whine. I went with the purple diff bushings and have a hard time hearing an increase in whine. Almost makes me wish I had just gone for solid, I opted for purple because of reports of whine I had read so wanted to play it safe. Are you manual? Maybe the gear box type makes a difference.

I drove a convertible 6MT car with solid subframe and diff bushings and heard no whine while the top was down and a barely perceptible whine while the top was up. Could be either that convertibles have better sound deadening or that they are noisier and the existing noise drowns out the diff whine.

I run 75D subframe and 95A diff bushings in my 6MT E90 and I definitely hear diff whine. You cannot miss it in the speeds and gears it is present. I wish it was a little less audible. I also have a one piece carbon fiber driveshaft and maybe that makes a difference.

[M43S7RO]

I believe roastbeef posted a video of diff movement while going on/off throttle on oem diff bushings.

Originally Posted by Bartledoo

There is an underbody video somebody posted on the forum a long time ago showing the subframe movement, and it's much more than you'd expect.

[roastbeef]

Originally Posted by M43S7RO

I believe roastbeef posted a video of diff movement while going on/off throttle.

Angles are all jacked up because my camera scraped on something.

[VictorH]

Well, that's interesting, but it would be more interesting to know is what is the actual toe change with those deflections? I'm going to guess it's not that impressive, but it's also going to be dependent on the actual suspension geometry, and how much you are lowered.

Modern cars are pretty impressive. If you've ever driven one of those really scary older sports cars i.e. early '70s 911s or anything with a swing axle, the toe changes through a suspension movement is huge. I remember coming into a long sweeper in one of these cars, just a hair too hot, unconsciously I take maybe a 1/2" of throttle away and before you can say BOO! you're facing backwards and saying, "what just happened?"

[rmeitz167]

im wondering if the turner rear solid subframe bushings do have the properly machined portion to center the subframe

[tigermack]

Thanks for posting this! Lots of good info.

I wonder if it's worth a try to just swap the diff bushings out with the subframe still on the car. I will probably ghetto fab my own tool to remove it or just use the old fashion way with a 5lb hammer.

[VictorH]

Track Report #2- Roebling Road with Just Track It

Ran two day event with couple of friend, one with '17 Corvette (auto), other with '17 GT350 with really good alignment and Cup2s and then me in my E90 M3. I don't know that there's much more to add related to the rear sub-frame mounts and diff bushings. I've always felt my car was pretty solid and it's as good, maybe a touch better than before. At Roebling you are limited by rear grip but overall I like my set up.

Now for the "if you've ever done something boneheaded" report. It's Saturday AM and I've run session 1 no problem, sort of a warm up since I hadn't been to Roebling in a couple of years. It's a great track BTW, huge fun even though on paper it looks incredibly dull. if they ever repaved it, the track would be outstanding. Anyway, the Corvette guy is new to the track and after session one he asks if he can ride with me to see "the line." I say sure, so we go out session 2, lap #1 and corner #2, I spin!! Now corner two at Roebling is the fastest turn, when everything is good and warm, in my car, you enter at 90 mph and exit at 100. Or if you want you can enter at 95 but then you come out at about the same speed. Anyway, I remember thinking, "I don't need to brake for this one" and we're going probably 85 mph (best guess), I go to turn in and there is no rear grip whatsoever, the rear end just goes away. I put in as much correction as I can muster, think for a second that maybe I saved it, but no, get a really good tank slapper and swing around backwards probably going 40 mph at this point and still on track. Clamp on the brakes and stop just off track, perpendicular to the track surface. Motor's still running, let a couple cars pass and then head back to paddock for a quick check. Everything's okay, and head back out to finish the session. I've seen plenty of guys spin on the out-lap but never understood how you can do that. Now I know!

After my incident, my friends still trusted me and I drove the Corvette and the GT350. The Corvette was pretty stock but with aftermarket anti-roll bars and Michelin 4S Tires. Car weight (they have scale at the track) is 3415 with 3/4 tank. It's pretty neutral, nice turn in, the motor is really nice, but I could do without the auto, the shifts in track mode are pretty jerky, but acceptable.

The GT350, I've driven a half dozen times at various tracks. The grip is tremendous. It's a big, somewhat heavy monster, but it has huge cornering grip and plenty of power. Plus if you like a big power manual trans car at the track this one is pretty fun. it's very neutral and it's easy to get up to the limit of adhesion and feel perfectly comfortable as the feedback is really good.

My car of course I know like the back of my hand. Funny thing, my GT350 friend said today (Sunday) "the track feels like a cheese grater to me." It's a pretty coarse surface but it sort of is what it is. The last session I normally don't push so hard but it was probably my quickest laps of the whole weekend ( I don't lap time). Mostly because there was a ZL-1 Camaro in front of me for a dozen laps before he finally relented and let me pass. Now granted it was being under-driven, but it was a fun cat and mouse. On the straights I would guess he's hitting 160+ and I'm in the low 150s. I could see he would coast just before the braking zone and then brake at the 5 marker. I'd come in full-power right up to the 3 1/2 (there's no 3.5 but half-way between the 4 and 3 marker) and make up a few car lengths right there, then in the corners he was okay and I was a bit better, but on the straights I'd get smoked over and over.

Was a great weekend, got home swapped out brake pads front and rear, changed the camber and toe, swapped tires and washed the car, 4 hour turn-around and off to work tomorrow. Going back to the cheese grater comment, my left-front tire is completely toasted. The outer 2/3 are completely bald and the inner third is still okay even though I'm running -3.5 camber up front. I think that track likes a bit more camber as for VIR and RA the camber is just right.

Even though it's a 10 year old car and just turned 147,200 miles it's still pretty incredible what a relatively modern car can do. Next up, a set of Michelin Cup2s.

[Redd]

Hi VictorH, thanks for the ongoing updates! I learned a lot from this thread.

I'd like to find out a bit more about your car - are you 6MT or DCT, and what exhaust do you have?

I'm facing the purple diff bushes dilemma and trying to determine how much diff whine is too much. I daily my M3 DCT.

Thanks in advance.

[VictorH]

Thank you for the nice comments.
The car:
2009 E90 M3 DCT, Nav, No sunroof, bought new by me with European delivery, July 2009
Miles: 147,700

Maintenance: New rod bearings at 90,000 miles, throttle actuators, gas tank breather, idle control valve, starter, thermostat, fuel pump and fuel pump controller (those items replaced due to failures, all by me). oil change every 7,500 miles irrespective of how many track days, DCT fluid changed x3 (DCT sump and side gasket replacement x1), rear diff fluid changed 4x, both front wheel bearings, left side x2

Mods
Engine- M24 oil cooler (added a long time ago), Do88 DCT cooler, CSF radiator (cooler engines are happier), note no small pulleys. BPM tune, BPM DCT tune, primary cat delete (otherwise stock exhaust). K&N air filter (not sure I like it that much, stock airbox).

Brakes-PFC Z-54 front BBK, Cobalt Friction XR-2 pads for track, PFC street for road, stock rear rotors and calipers, Cobalt friction XR-3 (more rear brake bias for track).

Suspension- Vorschlage camber plates, Turner Monoball front tension struts, BW sway bar drop links, Fall line rear toe arms, Fall line rear camber shims, BW billet rear sub-frame bushings, Purple diff bushings (wish I hadn't done that), BW race studs 75 mm, JRZ RS-Pro monotube shocks with reservoirs all around.

Alignment: Street: -1.5 degrees neg front camber, toe 1/16" in
Track: -3.5 degrees neg front camber, toe 1/16" out (tie rods all marked)
Rear: -1.75degrees neg camber, slight toe in (don't recall exact amount)

Tires: Track: Bridgestone RE-71R, 255/35/19 front, 275/35/19 rear for track
Street: Michelin 4S in stock sizes for street (get about 30,000 miles out of rears and 32,000 out of fronts).

No fancy wings, no splitters, no cosmetics other than front paint protection film replaced 2 years ago.

To me this is the ultimate dual-purpose street-track car.

[Redd]

Thanks for that. I guess your final comment on the purple diff bushings says it all. Lol.