Input From Other ZF Transmission Users on 65mph Driveline Vibration?
 

Hey all. Zak here, back with another inquiry for all you ZF users out there.

Last weekend I installed a ZF6 transmission in my car via KMiata’s swap kit. Vehicle details are in my signature, I used an ACT 6 puck ZF clutch and the KMiata PPF delete brace as my clutch and diff brace solutions.


Has anyone else running a ZF transmission/Miata torsen rear end ran into a violent driveline vibration issue that begins at freeway speeds?

Since I performed the swap, my car now begins exhibiting a serious driveline vibration starting at 60-65mph. The car has only been driveable since Friday so I haven’t had much time to begin diagnosing it. So far, I’ve experimented with a whole slew of driveline angles, to no effect, and bolt checked everything accessible on the drivetrain. My original diff/trans angles were 2.8/2.3 respectively, and are now 2.0/2.1. I tried everything else in between and then some. My next move will be to get the driveshaft checked for balance at a local shop this week.

The only other case I’ve seen this was in Greg Peters’s car, which he eventually fixed by swapping in a Getrag diff after running the gauntlet trying to diagnose it for over a year. I know he’s not super popular on here, but I messaged him hoping he’d have some insight. He told me they never really figured out why the Getrag diff solved his issue, but it moved the vibration up to starting at 125mph, as opposed to the 60mph it used to start at. He also told me that while working with Kmiata to find a solution, he was told that there were multiple documented cases of this issue by other Kmiata ZF swap/torsen diff users.

I’ve messaged Kmiata already asking if they have a solution since it’s apparently a known problem, but figured I’d check on here for insight as well, since you all have been nothing but great to me.

At any rate, I’m gonna start doing what I can to diagnose the issue, but I have a race I’m signed up for in two weeks that I’d like to not lose my entry fee on (It’s Gridlife and I don’t think they give rain checks for next year. Also my parents wanted to come watch). If anyone has ran into this issue and had an “AHA” moment, your insight would be greatly appreciated.

Thanks as always.
-Zak

Hey Zak I would be careful about pushing your car too hard until you get that vibration figured out. I looked at the pics of the swap kit and gregs video. A few things about that swap kit that concern me based just off what I can see in pics and install video of Gregs. That kit relocates a good bit of rotational weight further away from the back of the crankshaft than original. Any imbalance that is moved further outboard of end of crankshaft support will tend to magnify flexing in the crankshaft. Another issue I question is the way they did the pilot bearing. It looks like the pilot bearing is registered in their machined flywheel. I cant really see from pics or gregs video, but are they registering their flywheel assembly in the center register of the crankshaft or just relying on bolt holes for centering? Is the pilot bearing they use a good, snug fit to the transmission pilot? My opinion is since this relocates a good amount of rotational mass further away from the back of the end of the crankshaft the likelyhood of induced harmonics into the crank from any imbalance or off centering is increased. The further away a heavy, rapidly spinning flywheel/ clutch assembly is from any main bearing support will tend to magnify strange vibrations and harmonics into the crankshaft if the flywheel assembly is off balanced or off center of the crankshaft. Any imbalance or off-centering of flywheel to crank or clutch disc to flywheel can very quickly wipe out main bearings, or damage crankshaft or mains. In my opinion thats why Greg failed main bearing caps on his prior engine. Of course this is just my opinion based off what little I can tell from pics and video of that particular setup. I could be totally off base LOL....

All of the above would assume more rpm related in any gear rather than just a fixed road speed though. Its just that large amount of rotational weight moved further away from crankshaft support is concerning to me.

Hey New, thanks for the response! I'm sure as heck not driving the car hard at all until this is ironed out. Haven't taken it over 80mph since the issue began.

The backspacing of all that rotating mass something I never considered. Since the swap, the car has incurred additional NVH across the board but I believe that to be due to the addition of a solid transmission mount. I noted more NVH toward redline but removed much of it by swapping from the supplied solid poly trans mount to a hollow rubber one.

The flywheel is centered via the crankshaft bolts and that's it. Pilot bearing was already installed in the flywheel when I got it. I didn't bother trying to pop it out or anything, but it felt snug, although I know that doesn't tell much.

That being said, the vibration is entirely driveline speed dependent. It happens at roughly 60-65mph regardless of gear/rpm. I've pulled the car up to redline in lower gears and the issue isn't present there. The other NVH isn't annoying to me. The car feels so much more solid until it hits freeway speeds.

In case anyone comes across this thread in the future, here's my driveshaft runout at the time of making this post.


Kpower is sending out a new driveshaft which should fix the issue. I'll update this thread for closure once it's in.

I am also going through this EXACT same thing right now. Gs6-37BZ & Miata msm torsen. I will read through this all and write a detailed response tonight.

I’m on my third driveshaft from their vendor, last one measured .016” runout at the yoke, and can feel a ton of slop in that connection. They have all been like this.

Dude, good to hear from someone else going through the exact same concern currently. Thanks for sharing as well.

That number of driveshaft replacements is not comforting to hear. That checks out because when I talked to Jamie(?) on the phone this morning, the first thing he told me was to check driveshaft runout, so it sounds like they know that's an issue. This makes me want even more to have our esteemed local driveshaft shop just fab me up a driveshaft. They told me today that they can have one made with a one-day turnaround. I was quoted $650-750 though. I'm sure KPower isn't going to credit me that amount to get a driveshaft made.

I have the exact same setup, in a 2005 MSM. The car did not have any bad NVH issues beyond the normal poly bushing diff and such before the swap.

I have the same exact issue. Violent is a good way to describe it, the vibe is fairly high frequency, and LOUD. It happens when I'm over 65mph, and if the car is in gear, neutral, clutch in or out, engine on or off. It "feels" like it's coming right through my butt and over my right shoulder (I have a race seat mounted directly to the floor). I've described it to others as riding inside of a timpani drum.

My immediate thought was driveshaft, so I took shaft #1 to my local OG shop. They evaluated the shaft and told me that it was a very bottom market piece. No name brand UJoints, the yoke assembly also a no-name part which is not an indicator of quality. The first shaft had zero balance weights, and when my local guys spun it up, it was clear that the shaft had never been in the machine. They told me "maybe, no guarantees", threw some spicer U joints in. The thing had multiple stacks of balance weights all over it, which made no difference.

I sent it back and received another shaft on warranty, eating the cost of the attempted repair. K Miata also replaced the steel adapter because the fit between it's recess and the driveshaft centering flange feature was sloppy. Unclear if it was the adapter or the flange, but they sent a new one out with shaft #2.

The new adapter bolted on fine. The adapter has +/- .001" runout. The trans output feels nice and tight, I can not detect any play at all.

The car behaved the same with the second driveshaft. I noticed that there was a ton of slop in the slip yoke assembly. you could grab the yoke and displace it .040" up and down. Dynamically it was pretty close to true at the front of the yoke, but on the machined section it was clear that the shaft was "wobbling" in the slip joint. I still have this one on the bench.

The third shaft has .016" runout as measured on the machined (male) section of the yoke. Ideally you'd want that <.005", according to K Power and other sources. Local shop told me that in high shaft speed cars a driver will usually detect .003" - .004" "at speed." This was echoed by an American Lemans mechanic friend of mine.

This shaft also has slop that is detectable by hand, if you grab the shaft and yoke and push up and down, you can feel the slop. I put the yoke in a vice, to isolate the u-joints from the assembly. With the yoke completely slipped "together" i.e. best case, I can measure ~1cm of play at the other end of the shaft. I have no idea if that is normal.

The brand and design of the yoke changed from 1,2. I'll get that next time I get under the car.
Edit: Neapco brand on the yoke and u joints

I have also tried multiple diff/trans angles. My original install was with the trans ~2* down, diff ~2* nose up, parallel faces. This led to (what I was thinking was) a pretty shallow driveshaft angle of .5*, so I pushed the trans tail up, diff nose down for something more along the lines of ~1.4* down, diff 1.4* up, leading to driveshaft angle of ~1.5*. My theory is that the u-joints were not getting loaded correctly and causing a vibe. I've done some more reading and shallow driveshaft angles *shouldn't* lead to vibes, but accelerated wear to the u joints because the needle bearings do not spin, and will cause brunelling as the rollers wear the same spot on the race. At any rate it made little to no difference.


This brings up an interesting point. K Power said they stopped selling the kit that he had originally bought because it used the longest driveshaft. If the length of the driveshaft is causing issues, to me it says it could be driveshaft angle (unlikley?) or maybe the incorrect wall thickness of tubing, causing a resonance. Playing around with spicer's calculator I'm no closer to understanding if that is a valid theory.

K Power has repeated multiple times that they have "hundreds" of these out in the wild. Unclear if that is the zf6 to torsen, or shafts made by this vendor as part of any of their offerings. I'd lean toward maybe buying the latter.

They have been pretty OK about working with me, and as we continue without the problem solved, they have become more responsive. Calling them on the phone (somewhat persistently) is the fastest way, I've found. Clearly they have an issue here, I was starting to loose my mind thinking it was my install or the trans itself that was causing this vibe. I've been trying to sort this since last year. it's a bummer.

The issue IMO is that K Power does not QC these driveshafts. There isn't really a good way to do that short of installing them on an otherwise known-good car. Even the local driveshaft shop said they can only spin them so fast, and without torque applied it's really challenging to build a good high-rpm driveshaft. It could be that something with the driveshaft manufacture's setup has gone bad, their QC process has failed (if it ever existed, I got my first one with no attempt to balance) or they just don't have anyone with the skills to build it. I do know that the spicer u joints the local guys installed felt way "tighter", it actually took some force to move the U Joint around. They said this is preferable to the floppy boys that came with (all of) the shafts I've gotten.

Of course, it *could* be something else with the setup other than the driveshaft, but I've been over everything past the trans multiple times and I'm not coming up with anything fishy.

I had a few other issues with my kit, interested to hear if they are common.

-The selector rod was 1/2" too long, putting your fist into the radio in 1/3/5. I chopped it down 1/2" in the mill and welded it back together. This perplexed K Power. This was the case with OEM and aftermarket engine mounts.
-My driveshaft was ~1/2" too long, making it very hard to install into the car without unbolting the transmission mount and pushing the trans up. The slip joint was only extended ~3/16" installed. Not the end of the world, as the pinion angle is fixed in these cars. The giubo pilot on the BMW output flange is the real problem here. I've since cut it off.
-The clutch fork can bottom out onto the pressure plate. I had to build a little pedal stop and fiddle with the adjustment a bunch to get it so I could fully engage the clutch without that happening
-The bracket that bolts to the transmission needed some love to get bolted on, hole tolerance issue
-The crossmember had a poor design, with an upside down "V" notch that forces the exhaust to go under the crossmember. I cut that out and welded in a straight piece, so the exhaust can go over the crossmember. Otherwise the exhaust would have been dragging on the ground, a problem I think a lot of the K swap guys have.
-I swapped the (plastic) clutch fork pivot bushing for a steel one. It's noisy as heck with the clutch out, goes away with clutch in (throw out bearing engaged). Not K Miata's problem but I'll go back to a new plastic next time if this car stays on the street.

The car does feel amazing, and the transmission shifts with positive engagement. I'm very happy with the overall feel of the setup. The increased NVH and "rattleyness" of the ZF box is the tradeoff there.

I'm interested to try a less aggressive trans mount as you did Zak. I also put in a heavier weight oil in the box and it quieted down, with maybe a bit poorer shift performance. It's still awesome feeling, though.

My experience overall with this has been pretty frustrating. My first 6 speed box, I didn't inspect closely enough and it did not have neutral. I attempted to take this apart but failed and recycled the core. The 6 speeds are not really serviceable, I've tried everyone that used to and still does mess with e46 stuff, and they have all told me the same thing (just go get another box if it breaks)

Anyways, that's what I've got off the top of my head.

For me, the vibe we are talking about is completely independent of engine RPM.

I am curious now to check flywheel runout. I might be able to do this actually with the engine and trans in the car, I'll report back.

Edit: I was able to check runout with everything installed. Nothing concerning, +/- .001"

I put a new engine in the car with this driveline setup. The new engine def has way more vibration than my old. I had the crank and rods balanced by a reputable shop. Piston weights were all very very close. I did not have the flywheel checked for balance. The new engine has 2618 pistons with .0035" wall clearances and should be around 11:1. I was thinking this was just the nature of the beast with this build, against my old 9:1 super techs.

Rich, thanks so much for your detailed response. Hearing I'm not the only one dealing with this here is reassuring, and I'm appreciative of all the input you've provided.

To start with: I've run into a few of the same issues as you with the kit. Namely the noisy clutch fork pivot bushing (I went with steel as well, did that myself) and the tolerance issue with the trans mount bolt holes being way off. I ended up getting my exhaust rerouted around the brace as I didn't want to run it upside down (zero ground clearance) and didn't want to cut and weld it.

https://cimg3.ibsrv.net/gimg/www.mia...19a7d9a3f2.jpg

I'd definitely recommend swapping the poly mount for a rubber one. Less NVH and no notable downsides just yet.

My condolences for your experience with the swap and the first failed box. I'd quite possibly have burned my car to the ground at this point. Glad to hear K-Power is working with you on it, though.

Honestly, given the situation you're describing, I think I'm going to have my local driveshaft shop make a shaft for me. Probably a long shot, but maybe I can send the current driveshaft back and have KPower credit me for it. Damn shame you've gone through three already.

At this point, I'm hoping there's not some inherent issue that makes this setup super-prone to vibration, although I don't know enough to diagnose why that would be.

I wish I had more to add but all I've gone through so far is this driveshaft runout issue. I'll post more updates as I work things out, and will let you all know what KPower says. I'll check my slip joint play as well once the current shaft is off the car.

Both of my driveshafts were fine for balance, but I was an early adopter and the supplier may have changed.

I seem to recall setting pinion angle around 1.8.

You reminded me that I flipped the crossmember over. Mine had a U shaped cutout for the exhaust rather than the V in your pictures. I think the U creates more space for exhaust pipe. My U hangs down and the pipe runs above it. I should take more pictures. Of course I needed to adjust the mounting points at the ends of the crossmember when it was flipped but I can't recall what I did there. I need to look. Haven't needed to touch it once it was done.

While I have zero experience with any kpower swap items, I have recently noticed my setup doing exactly as you all have described here. My driveline is all stock Miata stuff, MSM trans, stock driveshaft with OS Giken diff in stock housing. I have been daily driving the Miata for the last few days (hour each way to/from work) and have noticed that anything in the 60mph range creates a really aggressive vibration through the entire car. I only just noticed it over the last few days though and have done zero investigating.

I know this wasnt of much assistance, but figured a stock trans/setup sharing the same symptoms may help shed some light.

Perhaps it has something to do with the position of the Earth relative the Sun, lol.

That could be anything from a failed U-joint to a missing wheel weight.

I’ve read over the thread where you went back and forth with KPower regarding the crossmember positioning and looked at those pics you posted about a hundred times lol. Yeah, I think the V shaped crossmember relief definitely creates less room for the exhaust. I knew I was already getting my muffler inlet pipe modified though, so I’m not excessively mad about it.

Six, I seem to recall you mentioning something about keeping your PPF maybe? Did you adapt your PPF to work with the kit or did you go with the KPower delete brace?

@zak the thought of burning my car down has def gone through my mind. It also brings me some weird sense of relief that I’m not the only one with the issue.

I’d personally love to see if a shaft from some-other-shop works. Obviously my confidence in K Power’s supplier is near zero.

if I get another shaft from K Power, it will be a carbon or aluminum version with a different slip yoke assembly design (spline diameter close to shaft diameter)

Something to consider depending on your local guys capabilities. I’d be very interested to learn of all the details of the shaft build, including which slip yoke assembly, length, wall thickness, etc

it would be very cool if they could adapt the trans side directly to the three bolt output flange, to ditch the heavy metal adapter. I think godless commie here had this done in his swap.

Oh yeah, I also rotated tires to see maybe I lost a weight or tire went square or something.. no change.

Did KPower specifically say an aluminum or carbon driveshaft is what they’d send you next? That might help me reconsider what I was going to do. I just got off the phone with Chris and he said they were going to get in contact with the driveshaft supplier to get me sorted. Also mentioned another user was having issues that they were trying to get sorted (that’s you, I assume).

Following the call, I decided I was going to go to my local shop to have one fabbed anyways. My diesel mechanic buddy/coworker worked there for a couple years and said they know what they’re doing. I also kinda figured they’d be able to make me a lighter one, given I certainly never need/want this car to make 600lbs/ft of torque. I know nothing in reference to wall/shaft thickness,etc, of what goes into making a proper driveshaft for a given application, but hope the shop can steer me in the right direction. Also never considered they might be able to make one that bolts straight up to the BMW output flange. Wouldn’t mind ditching the adapter if that’s a sound idea. That f*cker’s heavy.

Bummer about the wheel weight haha. I’ve had my wheels so out of round/balance at different points on this car but it’s never produced a vibration even close to what I’m experiencing now, so I haven’t checked that yet.

I doubled up most of a PPF and had the two welded together. That reinforced it nicely. I had pictures in my build thread but I will look for them and post them here also. I bolted the front of the ppf to the top of the crossmember.

Ahh ok, I've seen these pics before. Forgot you were the one with the doubled up PPF. You just drilled another hole through the crossmember and bolted it straight in? Any bracketry/reinforcements on the crossmember side involved?

At that distance from the differential there isn't really a tremendous amount of vertical force trying to lift it off of the crossmember. I did use a large bolt and washers.

That's precisely what I figured and was hoping to hear, thanks.

I'm curious if your PPF configuration has anything to do with the fact that you haven't run into a driveline vibration. Just throwing sh*t at the wall right now, but based on the fact that Greg's vibration was fixed after the Cadillac diff conversion, and you've never run into the issue Rich and I are having, maybe the concern lies within our diff mounting solution. This fear is probably unfounded, but after reading/hearing a few stories, I'm a little scared that I'm going to get this bitchin' driveshaft made and then still have a vibration after.

I guess it won't be too hard to experiment with the setup you're running if I run into any issues after the driveshaft fix. I think I'm going to get a driveshaft made this week, throw it in, and then if I still have an issue after that, try mounting the PPF and see what happens then.

I admit, I probably need to slow down and take a breath.

@zak Any driveshaft other than the steel one will be at an additional cost to me. K Miata and I have not decided what we are going to do next, but they are working with me on this and have offered some credit for the steel one. I’ll let you talk to them about that, as we have not finalized any deal. They are checking time and cost for these other shafts, they come from a different vendor on the east coast and there may be some delays with parts availability and so on.

if it is the driveshaft, I feel a little slighted even if I get a cost break on whatever solution we arrive at because I’ve been doing a lot of QC work for them and their vendor. Any compensation for that is wishful thinking, but if I was paying shop rate for all that work I’d be deep in the hole.

TBH I should have just had one made locally at this point and moved on with the project.

@sixshooter I am thinking about making a tubular steel PPF and getting rid of the K Miata delete kit. Good to hear you had decent results with that approach.

Rich, ok that's what I figured. I'm not in a position to say what's in the right or the wrong, as it's not KPower's own shafts. Like you said as well, they don't have a way of testing them before going out and have to trust the manufacturer's QC.

I feel you regarding being a QC mule, your situation is super frustrating and I can't imagine having the problem strung out as long as it has with no solution. I was thinking about it this morning, it's not as if something broke. You had a perfectly well-running car one day and then you spent $4k+ and then it didn't run right haha.

As far as compensation for my own diag and testing goes, I'm going to assume I'm on my own. I'll consider any credits/etc given by KPower to be icing on the cake. Frustrating, but in dealing with niche semi-one off racecar parts, I don't think I can expect everything to go off without a hitch. I just want to rule out the driveshaft at this point and get a move on with the diag. I can't stand my car not running, especially during our peak track season.

Second Sixshooter on this one. I had my OE driveshaft U joints go out, they started making an audible clunking going from reverse to first, etc. Replace it with a newer used driveshaft and was super impressed with how smooth the car was after that. The u joint failing was obviously getting worse and worse, slowly over time, and I never noticed the vibration until I replaced it with a lower mileage one and things got smoooooooooth.

Funny how many things degrade so slowly that you barely notice em until really late haha. Then you revive or replace whatever’s causing the issue and are like “Ahh, THIS is what it’s supposed to feel like!” :rofl:


https://cimg8.ibsrv.net/gimg/www.mia...db221bc6c.jpeg

Dropping the shaft off at Drivelines tomorrow to have another one made. I was told it’d be a 24-48 hour lead time. Will keep the thread updated when I get it back.

Excellent. Keep us posted!!

Driveshaft is dropped off at the shop. I put the thing on the counter and let the manager know about the runout issue, what I wanted, etc. Without looking up for more than a second, he goes "Yeah, it's all cheap Chinese sh*t too." :rofl: They said it'd be ready for pickup tomorrow, so hopefully I'll have it back in the car tomorrow night.

yeap my guy had the same reaction lol

i’m super excited to see how this goes :fingerscrossed:

New shaft procured. Spicer slip yoke and U-joints all around. Also notable, the slip yoke has no play when manipulated by hand, whereas the KPower shaft slip yoke has a detectable amount of play if twisted side to side. Both shafts weigh about the same. Wade at drivelines said they recommended against smaller diameter tubing given the application. I'm gonna pop it in my car tonight. Fingers crossed.

https://cimg7.ibsrv.net/gimg/www.mia...fc907cc5dc.jpg

Shop wall art.

https://cimg8.ibsrv.net/gimg/www.mia...5700832f24.jpg

Update: slightly better, but not fixed.

Got the new driveshaft in the car tonight and just took it out for a spin. The vibration now starts at 75mph as opposed to starting at around 60mph before. The vibration is also only present toward the rear end now as opposed to before when it was resonating from what seemed like the whole underside of the car. I need to stethoscope the underside again with the car in a lift and someone driving it.

I swapped to NB axles when I did the transmission swap for additional exhaust clearance. I’m gonna see if there’s room for my old stub-type axle on the left side and swap the original axles in. Once I’ve ruled those out, I’m going to remove my KPower diff brace and mount my PPF to the transmission crossmember like SixShooter and see if that makes a difference.


https://cimg2.ibsrv.net/gimg/www.mia...a7f0c9453.jpeg

Dang thats a bummer...
When you drive the car up to or past 75mph when the vibration is present if you push in the clutch to essentially bring the engine to idle but with the car in top gear at 75mph+ does the vibration remain or largely go away when you push in clutch? Maybe you already did this test and I just missed it?
If vibration goes away when doing above test can you run it up with out the driveshaft but with transmission in top gear to rule out flywheel imbalance, being out of true or clutch disc not centered well by the pilot? might be a waste of time but that simple test would help rule out any harmonics from the front of transmission somehow transmitting throughout driveline?

I have not tried disengaging the clutch while the vibration is active yet but will give it a go tomorrow. I still don’t believe it’s related to engine harmonics, though. The vibration occurs at ~5.5k in 4th, ~4.5k in 5th, ~3.7k in 6th; it follows vehicle speed perfectly. I did a few 2nd-4th gear full throttle pulls to redline and no vibration until ~75mph.

That being said, I’ll give it a try tomorrow just to rule it out as well.

Gracias, dawg. Yeah, I've heard similar things. Just assumed that it wasn't an issue with this swap since KPower made no mention of it. I wonder if I can get a rubber donut made for the shaft I already have.

Now that being said, I just got the car on a lift and checked it out again with a buddy driving it. At the resonant frequency, the vibration can be heard/felt coming only from the diff. The driveshaft is true and so is the output shaft adapter plate. I was suspecting maybe a CV joint on the new axles but I can't hear or feel anything coming through the knuckles. The vibration is very obviously felt through the diff/brace.

I'm gonna try mounting the PPF and see what happens.

Update. Mounted the PPF and set driveline angles to 2.2 trans and 2.1 diff. The vibration comes on earlier now. Presumably, the bushing in the PPF delete bracket was muting some of the vibe.

I need to set up a laser to confirm, but looking at the car from the rear, the diff seems to be offset a tiny bit to the right in comparison to the transmission. Otherwise, I've got nothing, no visible vibrations or sources of noise aside from a diff that starts vibrating violently at 3krpm in 6th gear.

I'm aware of the OEM BMW setup, but figured KPower had done their homework and confirmed that this didn't need to be replicated in a Miata before selling a $3.2k kit without it. I'm also still lost on how Six's car doesn't have the vibration but mine does with the same setup.

This has been a worse experience than blowing my engine.

Please check the left to right angles for the diff and the trans/engine combo. It can do the same thing as being out vertically. You've mentioned things not aligning.

I’ve ordered a line-style laser level off Amazon to confirm if/how much my trans is misaligned from my diff. Should be here today. Will update once I get it

@FNFMC totally agree that these boxes are buzzy without the mitigations bmw employed

But I still believe the problem Zack and I are experiencing is something dynamic “downstream” of the transmission. It’s not just some noise and buzz, this is a violent, repeatable issue, that is directly coupled to driveshaft RPM.

mine does not go away with the clutch pressed in, or with the car in neutral.

I’ve def considered building a BMW oem style two piece shaft, but without root causing this issue it may just be more wasted time and effort

@sixshooter I pulled a tape from trans flange to diff flange at 3 and 9 o clock, hoping to find some variance. They were within 1/16”, but I doubt the measurement is any good. I have an idea on how to get a more accurate comparison today, but need to pull the driveshaft back out etc

i had, perhaps erroneously, assumed that the centerline of the engine/trans,
in the fore/aft (bumper to bumper) axis would remain the same between zf & Miata, as it should be set by the engine/motor mounts. Obviously there is some tolerance here, you can pivot the engine/trans side to side (door to door?) at the output side of the transmission, but where it naturally wanted to rest lined up with the cross member mounting points, I initially did not investigate further. I guess with the PPF this movement is (kinda) constrained.

I don’t think that the diff being off center, if that is the case, is a deal breaker, but it will totally affect the driveshaft angle if it’s too far off. If I’m picturing this correctly, the flange faces can remain true to eachother in an arc that is constrained by your driveshaft angle

if I’m reading your comment correctly, you’re proposing that the nose of the diff is tilted up at the correct angle, but could be sitting off in the door to door axis? Or the same is true (and more likely) at the trans output shaft ? If the diff is not on the centerline of the car, we may have to do some math to determine if it’s misaligned.

I grabbed an alignment laser off Amazon and confirmed my trans seems to be off toward the driver side in comparison to the diff. I moved the front subframe maybe 4mm over to the passenger side. Just took the car on the road and got it up to 4.5k in 6th gear and the vibration is GREATLY reduced. Hard to tell if/when it’s coming on now but I didn’t get any jarring sensation in my ass like previously. Will drive it more later/tomorrow and report back.

that is good news

I was thinking about Six's post and if the diff and trans flanges are parallel, then the distance between the flanges @ 12, 3, 6, 9 o'clock should all be exactly the same

so I took two pieces of welding rod and zip-tied them together to make a telescoping gauge, and adjusted the transmission until the length was the same at each point

Had to push the ass end of the trans WAY towards driver door, to get the flanges to line up. Like, hogged the crossmember mounting holes out by 3-4mm, and used all the tolerance in the rest of the mounts

Unfortunately the vibe is the same.

But, This would also suggest that my transmission/engine are mounted biased towards the drivers side. I could try a subframe adjustment as you did and then readjust the trans side/side.

It would certainly give my exhaust a little more room, I moved the trans so much it hardly fits.

https://cimg5.ibsrv.net/gimg/www.mia...046c031f3.jpeg
Telescoping ruler

Haha the telescoping ruler is brilliant!

Bummed that the tail adjustment didn’t help out at all. The logic there seems solid! But I guess if the total distance between your output shaft flange and pinion is too great already, and the trans is positioned further to the driver side in comparison to the diff, it makes sense that the vibe didn’t change.

Lemme know if you shift your subframe over and if it makes a difference. I’m gonna take my car out for a longer spin tomorrow and potentially shift my rear subframe over depending on results and how much energy I have.

Update: I shifted the rear subframe toward the driver’s side as well, although it only had maybe 2mm of adjustment in it. Drove the car on the freeway again. The vibration doesn’t come on until ~5.2k rpm in 6th now (~110 mph?) and is no longer super jarring. I’m going to reinstall the ppf brace this week as that seemed to help mute it a bit. Will try to align the trans tailshaft left to right as well using Rich’s technique.

You've had the superfecta of issues keeping things buzzing.

I’m surprised you can get your subframe to shift, mine has little to no play at all

i followed your lead and got a laser

if you make a centerline of the car by splitting the distance between the pinch welds f/r, it looks like my engine and trans are almost exactly on the centerline.

the diff is shifted 16-17mm over to passenger side. The trig puts the driveshaft at .9* side angle. That matches my up down angle.

I’m no closer. Either my driveshaft is still just trash (I know that it is off) or I’m missing something. I just can’t accept that all the shafts both you and I have tried, even your nice one, are the only thing causing my vibe.

I may have cheated a bit with the subframe. Had the car on a lift and a powertrain jack supporting the subframe, then took a big pry bar and wedged it between the subframe and frame horn. It definitely moved over 1/4" or so. Can see an outline where the bolts and nuts were clamping it before. I think that's all the adjustment I have, though.

I'll have to get the centerline measurements for my trans/diff like you did. I'm spitballing here, but I want to say my total offset looked like less than 16-17mm. I didn't measure it, though, so I could be wrong.

I'm with you on the driveshaft bit. I was surprised to find that the new shaft only moved my resonant frequency up ~15mph, and my old shaft was f*cked.

Going to try and put the PPF delete brace back in my car today and see what effect that has. Swapping from the brace back to the PPF caused the vibration to come on quite a bit earlier, so hopefully I can alleviate some additional vibration by reverting back to the original setup.

I drove the car a LOT yesterday, and the vibration seems to be present starting at a similar speed as before (maybe 75mph), but is 80% less robust now. It's almost undetectable until you start getting north of 100mph, and even there it's still not nearly as violent as it used to be at 70mph. If I weren't looking for it acutely, I probably wouldn't suspect anything was wrong driving it at regular highway speeds.

So pardon my ignorance here, but is there a reason a CV joint style driveshaft could not be used? Something less sensitive to driveshaft angle and phasing than a cross style u-joint.

I see no reason why it couldn't. Honestly, given my current situation finding that decreasing the total necessary driveline angle helped the problem, I'm starting to think it'd help. FNFMC made a good point, the stock BMW setup incorporated the giubo and a two piece driveshaft with a center support bearing.

Maybe I'm just holding out false hope, but I don't see why my car would need a setup like BMW's when Six's car doesn't. I'm sure chassis wear and other structural factors will cause different resonant frequencies in each of our cars, but I still can't help but feel like there's something I'm missing that would make my current setup work.

I pulled out my PPF and reinstalled the KPower brace on my lunch break today. Vibration doesn't feel like it changed much on the lift, but then again, I didn't feel the improvement from the subframe adjustment until taking it on the road. While I had the car in the air, I took it up to 6k in 6th gear just for sh*ts. The vibration goes away at about 5.8k rpm. Not sure what good that knowledge does me, but it's data nonetheless.

Just spoke to Jamie at KPower. We chatted about a couple things with the swap. It was suggested that I confirm my current driveshaft was balanced to a high enough speed. Jamie said the shop that was originally making driveshafts for one of their 86 kits didn't balance the shafts at a high enough rpm and they ran into vibration issues until they specified they needed the shafts balanced at said higher speed.

I still have some more diag to do, but I was told KPower is switching suppliers and will be using more expensive driveshafts in their ZF kit from here on out. I believe Rich is getting his first. Seeing as I already got a shaft made, Jamie was kind enough to welcome my request to return my supplied driveshaft for credit towards a Getrag swap kit. Sounds like the new shafts are probably landing at KPower sometime next week. I told Jamie I'd call back Monday and get my shaft sent back to them. Will keep this thread updated.

Yeah, that is accurate. We’re trying an aluminum driveshaft with a different style yoke. I have not seen the design yet but the k power folks have described the yoke as being similar diameter to the driveshaft, with little to no slop.

Aluminum has 1/3 the stiffness as steel, so its resonance frequency will be different. We’ll see if this has any effect, we’re not really isolating variables here, and certainly will not care if this thing runs smooth.

Keep me posted nonetheless! I'd imagine the weight will have an effect on the vibration as well. Maybe less weight being thrown around will help out.

If the alu shaft fixes your issue, I'll just run one of the new shafts. Hope it makes a difference.

Assuming it is something like this:

https://www.actionmachineinc.com/col...ip-spline-kits

did you validate the left/right flange parallelism? curious as you've seemed to reduce the vibe quite a bit but not rid the car of the issue, even at the lower speeds. wondering if there is still an alignment issue.

I also don't have a great understanding how much tolerance there is for misalignment

I haven’t had a chance to dig into the left/right parallelism yet. Had about 20 minutes free before the shop doors closed yesterday so I just experimented with the tailshaft positioning before I left. I’ll have to check left/right angles like you did when I have some more time.

I did, however, have a potential “aha” moment after work today, though. I completely forgot I have a random passenger side poly motor mount that’s shorter than the ones currently installed on my car. I used to run a hybrid Mazdacomp/poly motor mount setup, then got this mount from a friend and ran it for a while (four months), not noticing that it made my motor/trans lean/shift a bit towards the passenger side until I took a close look while it was on the lift one day.

The vibration seemed to lessen after moving my motor/trans toward the passenger side of the vehicle. I’ve got my front subframe shifted full right, which lessened the severity of the vibration. I don’t think it’s a stellar idea to run long term, but I can install this mount to get the powertrain to sit even further right and see if that has any effect on the vibration.

I also don’t know how much tolerance there is for misalignment. Kinda just bumblef*cking my way through this, but this seems like it may confirm or deny my convictions about the driveline angle being too great due to excessive lateral distance between the motor/trans and the diff.

https://cimg0.ibsrv.net/gimg/www.mia...9772cdfaf.jpeg

Pic I took when I went to pull my motor back in August. Trans situated pretty far to the passenger side of the car with that mount setup.

Dropped in that other passenger side motor mount. Had to hog the crossmember bolt holes out more to allow the bolts to reach the trans mount, so it moved the trans over to the passenger side a decent bit. Maybe a half inch or so. No change in the vibration. This leads me to think that if there's an alignment issue, it's on the diff side. Or I'm way off in the weeds. Probably the latter.

well, it takes two to tango. The alignment is just a relative thing between the two flanges.

Should have the new shaft in hand this week, hoping for the best. I am also going to take some pics this evening when I get back in the shop of what I've found with my car.

which trans mount did you end up with?

Yeah, true. At some point, the alignment is just gonna be within some acceptable range or it’s not. I think it can now be safely assumed that mine is within that acceptable range.

Anxiously awaiting the result with that fancy alu shaft. That’ll be the deciding factor for my next move. Interested to see what else you’ve played with on your car too.

I just grabbed a cheap rubber mount off Amazon. Mainly for the next day shipping. Seems like it’ll hold up, though, and it definitely helped with overall NVH. Link here:

I'm still amazed at the problem. I really don't know why you would have an issue and I don't. But you're finding changes that make a difference so you are obviously landing rounds on the target even if you haven't hit the bullseye yet.

My general M-O is to just fire enough rounds that at least a few of them land. Hopefully I can keep coming up with ideas on this.

Ever since swapping out the bad driveshaft, the vibration can be felt exclusively in the diff. It doesn’t seem super likely to me, but I wonder if the vibration/original driveshaft with tons of runout damaged or accelerated wear in my diff. The two month-old fluid I drained last Friday came out black, although I couldn’t find any particles in it. I’m gonna drain it again this week and see what I find. The pinion still spins freely by hand and doesn’t grind or exhibit any serious drag, though, so maybe it still is a balance issue.

Will continue throwing darts at the board until hearing back from Rich.

I
https://cimg5.ibsrv.net/gimg/www.mia...480e10166.jpegI was a little more precise with my measurements this time around, but got generally the same result.

The engine and transmission lay nearly on the centerline of the car.

with the trans on this centerline, the side to side flange measurement is <1mm off, left to right.

The diff is offset to passenger about 15mm (K Miata echoed this)

my north/south alignment required an 1/8” spacer under the trans mount to raise the tail of the trans up. It is also <1mm difference top to bottom.

any play on in your pinion? I’ll check that, I don’t think it’s out of the realm of possibility that the vibe could have damaged the bearings/races.

I prob have 8 or so test drives, each one getting well into the vibe zone.

I can also dump fluid, mine is fairly fresh, miles wise.

Dang, proper stuff on those measurements man. I guess with that considered, it’s pretty safe to say that your driveline alignment is within spec, or that one/some of the previous alignment settings have been within spec.

I haven’t felt any play in my pinion but haven’t really got in there and manipulated it by hand. I’ll try this week.

If you wanna try dumping fluid, I can send you a couple quarts of Motorcraft 75W-85. I got a major surplus of the stuff.

@richbobby you got a timeframe on the new shaft from KPower?