Z_WAAAAAZ

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.

Fireindc

iTrader: (13)

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.

Z_WAAAAAZ

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!”




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.

richbobby

Excellent. Keep us posted!!

Z_WAAAAAZ

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." They said it'd be ready for pickup tomorrow, so hopefully I'll have it back in the car tomorrow night.

richbobby

yeap my guy had the same reaction lol

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

Z_WAAAAAZ

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.



Shop wall art.

Z_WAAAAAZ

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.

Newaza

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?

Z_WAAAAAZ

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.

FNFMC

Not running a BMW gearbox swap, but from what have seen first hand, the problem is that the BMW gearbox has a lot of vibrations. The BMW crowd sometimes change the oem rubber donut for aluminium and quickly go back to OEM because of massive vibration increase. This is amplified in 6 gear (overdrive), spinning the shaft quicker. If we look to the OEM BMW setup, the shaft is split in 2, plus the rubber donut and a central suport bearing. I would replicate this setup.

Z_WAAAAAZ

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.

Z_WAAAAAZ

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.

sixshooter

iTrader: (12)

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.

Z_WAAAAAZ

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

richbobby

@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.

Z_WAAAAAZ

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 *** like previously. Will drive it more later/tomorrow and report back.

richbobby

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 *** 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.

richbobby

Telescoping ruler

Z_WAAAAAZ

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.