Vibration in 4th at hwy speeds - ensure proper alignment from engine to transmission?

[Maybe Magpie]

I was advised by a local driveshaft rebalancing shop that oftentimes the issue with the BMW two-part driveshafts of our type isn't actually the driveshaft, it's tired motor mounts and transmission mounts. I went with nice new transmission mounts from Rogue when I 5-speed swapped. My motor mounts are a bit saggy-looking though not cracked. Apparently a matter of a few millimeters of misalignment can cause noticeable vibration, I was told. Short of playing musical chairs with my engine mounts and shims and highway testing, is there a good way to get things as aligned as reasonable practicable? Pretty sure some misalignment and resonance is my culprit.

[82eye]

motor and trans mounts degrade and fail about the same time. diff mounts usually last a little longer. unfortunately the csb often goes with them. at some point all of them are due as simple maintenance wear items.

[Maybe Magpie]

motor and trans mounts degrade and fail about the same time. diff mounts usually last a little longer. unfortunately the csb often goes with them. at some point all of them are due as simple maintenance wear items.

Yup, that I know, but I have the E46-type mounts from Rogue (slightly stiffer than stock but still very streetable and rubber) which are slightly taller (I think) than the regular e30 ones, and I want to pick the right motor mounts and shims to ensure the motor is aligned right without playing guessing games. It's a technique question, I suppose?

[digger]

- With new stock engine and trans mount what is the thickness of the both the engine and trans mount after static deflection is taken into account (or measure the vertical height of front of engine(a particular feature) and then the rear of trans to some datum on the body chassis rail or trans tunnel etc) and replicate that. or

- Measure the stock trans mount and the rouge mount to get the delta and add the same delta to the engine (this is an approximate way to get the engine angled w.r.t body but there will still be a overall static offset)

is it only vibrating in 4th and no other gear? rpm dependent? speed dependent? vibration felt through wheel or body?

if it was me i would change the engine mounts and test, its easy to add a shim after the fact i doubt a few mm here or there matters based on having replaced worn mounts and observing any changes

[Maybe Magpie]

- With new stock engine and trans mount what is the thickness of the both the engine and trans mount after static deflection is taken into account (or measure the vertical height of front of engine(a particular feature) and then the rear of trans to some datum on the body chassis rail or trans tunnel etc) and replicate that. or

- Measure the stock trans mount and the rouge mount to get the delta and add the same delta to the engine (this is an approximate way to get the engine angled w.r.t body but there will still be a overall static offset)

is it only vibrating in 4th and no other gear? rpm dependent? speed dependent? vibration felt through wheel or body?

if it was me i would change the engine mounts and test, its easy to add a shim after the fact i doubt a few mm here or there matters based on having replaced worn mounts and observing any changes

In theory the former makes sense but I don't know the static deflection spec on fresh mounts, all else being equal. It's pretty much all in 4th at 40-80mph. Very slightly there in 5th at highway speeds but much less. 1-3 not at all. 4th is direct rather than OD, right? Most of the vibration is felt through the seat and tunnel, yeah. There's also this strange clunking like something's rebounding depending on the speed of clutch operation - the clutch has about 1500mi, as does the resurfaced OE flywheel. Pedals were refreshed too, primary and secondary cylinders both replaced, fresh lines (braided stainless). Mounting hardware is all there (5-speed swap done right).

Anyway I suppose I'll just replace the fuckers and see what happens if that seems like the best course, I just didn't want to act blindly without seeing if I was missing something. Thanks.

[82eye]

In theory the former makes sense but I don't know the static deflection spec on fresh mounts, all else being equal. It's pretty much all in 4th at 40-80mph. Very slightly there in 5th at highway speeds but much less. 1-3 not at all. 4th is direct rather than OD, right? Most of the vibration is felt through the seat and tunnel, yeah. There's also this strange clunking like something's rebounding depending on the speed of clutch operation - the clutch has about 1500mi, as does the resurfaced OE flywheel. Pedals were refreshed too, primary and secondary cylinders both replaced, fresh lines (braided stainless). Mounting hardware is all there (5-speed swap done right).

Anyway I suppose I'll just replace the fuckers and see what happens if that seems like the best course, I just didn't want to act blindly without seeing if I was missing something. Thanks.

i'm thinking the csb is gone or going as well. the vibration under the seat points to a possibly failing csb.

when i did my mounts replaced engine, trans, and diff at the same time. i used e30 engine mounts paired with e21 trans mounts, no shims or anything needed. i also did the guibo, all that rubber dies around the same time. i got lucky on the csb, but i'll wager it's on borrowed time.

edit: check or replace the dive stop when you do them. they are missing on most cars. the dive stop will save piles of damage if a mount breaks.

[Maybe Magpie]

i'm thinking the csb is gone or going as well. the vibration under the seat points to a possibly failing csb.

when i did my mounts replaced engine, trans, and diff at the same time. i used e30 engine mounts paired with e21 trans mounts, no shims or anything needed. i also did the guibo, all that rubber dies around the same time. i got lucky on the csb, but i'll wager it's on borrowed time.

edit: check or replace the dive stop when you do them. they are missing on most cars. the dive stop will save piles of damage if a mount breaks.

I compiled and/or pulled damn near every last part necessary for the swap myself but to save any headache of having to do it a second time I had an area shop (20+ years of 02s, e30s, e24s, etc) install it all that's done it many a time before. The new driveshaft included a new CSB+bracket. Apparently not even that rubber strip between the bracket and the chassis was worn enough to merit a replacement so now I have a spare, I suppose. I also have a spare SKF CSB in my parts pile now just in case, but I don't think it's the CSB. The shop tested the runout on the output shaft and it was within acceptable values.

I was very thorough on the parts diagrams for my car, the dive-stop is that funky thing that presses against the rear of the front subframe, right? I made sure to get one of those (and the correct manual to exhaust bracket as well). I did notice when doing my front suspension that there was wear on the subframe paint on its rear surface where the rubber buffer doohickey on the end of it has apparently been touching the subframe at times, is that not normal? If so, sounds like the motor mounts being tired might have something to do with it and I'll bump up the priority there. Diff mount looked older but not cracking or gnarly when I peeped it, though the CV boots are something I intend to replace when I do rear suspension. LSD breakaway torque was perfect factory spec when I tested it and it functions without issue, too.

To be clear it's not an "oh shit" level of vibration, just enough it makes my neurotic ass irritated that it's there more for worrying about the car than my own comfort. Chronic overthinker.

[82eye]

I was very thorough on the parts diagrams for my car, the dive-stop is that funky thing that presses against the rear of the front subframe, right?

yes

I did notice when doing my front suspension that there was wear on the subframe paint on its rear surface where the rubber buffer doohickey on the end of it has apparently been touching the subframe at times, is that not normal?

no it is not normal. it should only ever touch the subframe as a last ditch resort before consumate disaster. shows the mounts have had it.

if the driveshaft is even minimally out of phase it can cause the issue as well. e30 driveshafts are notorious for it, and many shops can't / won't balance it out correct, even driveline specialty shops. there are numerous threads on it.

the problem with a little vibration is it always works into a big one.

edit: also some shops forget to pre-stress the csb when installing, which can also cause vibration.

[Maybe Magpie]

yes



no it is not normal. it should only ever touch the subframe as a last ditch resort before consumate disaster. shows the mounts have had it.

if the driveshaft is even minimally out of phase it can cause the issue as well. e30 driveshafts are notorious for it, and many shops can't / won't balance it out correct, even driveline specialty shops. there are numerous threads on it.

the problem with a little vibration is it always works into a big one.

edit: also some shops forget to pre-stress the csb when installing, which can also cause vibration.

Right then, I'll bump the motor mounts up to top priority. Thanks for the info. Do you know of any OEM+ style alternatives of the E21 type beefier motor mounts? They doubled in price recently, which is a bummer, otherwise I would just roll with those. The Group N rubber stuff is NLA now, too.

[digger]

Right then, I'll bump the motor mounts up to top priority. Thanks for the info. Do you know of any OEM+ style alternatives of the E21 type beefier motor mounts? They doubled in price recently, which is a bummer, otherwise I would just roll with those. The Group N rubber stuff is NLA now, too.

motor mounts? 11811132322 from old 6 series seem to be available again

[Maybe Magpie]

motor mounts? 11811132322 from old 6 series seem to be available again

Yes, but they've gone from $100 a set to $200 a set, and I am tapped out pretty hard right now after the suspension refresh up front and am not sure I can swing that - but I have the beefier Rogue transmission mounts and don't want to mismatch the stiffnesses too badly either, something of a conundrum. Unlike the regular type, there don't seem to be OEM/non-genuine-but-still-quality versions of that.

[82eye]

Right then, I'll bump the motor mounts up to top priority. Thanks for the info. Do you know of any OEM+ style alternatives of the E21 type beefier motor mounts? They doubled in price recently, which is a bummer, otherwise I would just roll with those. The Group N rubber stuff is NLA now, too.

anything bmw is gonna be super expensive under the new american tariff dictatorship. just bite the bullet and get the real thing. there aren't really any decent alternatives. the e30 engine mount and e21 trans mount is a tried and proven combination.

[Maybe Magpie]

Ended up getting the Ireland Engineering reinf mounts because fast (close) and affordable. To my chagrin: they're UroParts, but at least made in India, which I can live with. Significant reduction in shifting clunks, drivetrain slop, and vibration. Still has a li'l bit of 4th vibration, but it solved like 90% of the issue and some bonuses on top, an insanely annoying headliner buzz/flap is gone now. Case closed, I suppose.