Ehh, I mean at least it's just the driveshaft and I'm not having any of the other dozens of issues that can occur in a swap like this... right? RIGHT??

Called KPower this morning and they apologized for the inconvenience and said they'd see what they could do about getting me an outright refund. They told me they'd call me later, but I didn't hear anything else from them today.

I dropped the driveshaft off at a local shop today, Drivelines. They did my last driveshaft and make lots of shafts for trophy trucks and the local offroad upfitters around here. I trust their word. They took a look at the supplied driveshaft and told me that they can't modify or "fix" it (as would be expected) due to the nature of the thinner aluminum tubing used. They also told me they stay away from the type of slip joint my supplied driveshaft used due to their proclivity to wear and develop play quickly.
They quoted me somewhere in the $400 range to build a steel shaft using my current u-joints, or $630 to build a new shaft outright. Building one outright would allow me to use new flanges, and get rid of the heavy adapters bolted to the trans and diff. I'm leaning that direction but told them I'd sleep on it first. I'd like to get confirmation from KP that I can get some sort of reimbursement before pulling the trigger on another expensive driveshaft.

 

It's only a question of which option, isn't it? Not whether it will be replaced?

 

Yeah, I'm not considering trying to run the current shaft in any shape or form. Will probably have them make a shaft with new flanges so I can get rid of the adapters on each end. Those suckers have to weigh at least 4 lbs each.

 

Yes, good plan. One other up-side is that the local maker has dome the whole job, avoiding any issues with a patched in solution. That's more than beer money, but it isn't thousands either.

Good luck, surely this bumpy road has to end somewhere, maybe here even!?

Thinking about this issue, is there any likelihood that there is play in the gearbox output shaft? I am having difficulty getting my head around the notion that a driveline vibration can't be tracked down and fixed in a fairly straightforward manner. Obviously that driveshaft was a manufacturing fail, but it is not like it was a one-off occurrence.

 

Nice work getting the diff in, sorry to hear about the driveshaft. I had the same thought as Gee Emm, what if the trans itself is causing the issue? I've heard lots of complaints about the ZF being NVH'y in general lately, I.E. this post

Definitely interested to see where this road ends!

 

Yeah, exactly. The local maker will have done the whole job AND if I have an issue (not that I’m expecting one), I can take it back down to them instead of having to deal with another party first.

I’d sure love for this bumpy road to end here. It’s not money I want to spend, but it’s basically worth it to end this saga lol.

I haven’t checked the output shaft specifically, but don’t believe the gearbox to be the cause. Towards the beginning of this journey, but after replacing the first bad driveshaft I was sent, I had a coworker spin up the driveline with the car in a lift. I stethoscoped everywhere I could and felt for the vibration by hand. The noise and vibration could be felt and heard coming from the diff itself. Faint resonations could be heard in the trans bell housing and in the rear wheel knuckles but it was definitely coming from the diff. You could put your hand on it as the car approached 75mph and feel it start to vibrate violently, no such sensation anywhere else.

Of course, the vibration could be originating elsewhere and resonating through the diff for some reason. I don’t have much to back this up, but think there’s just something that causes the ZF trans to not play nice with the Miata diff. Possibly the vibration-prone transmission and heavier driveshaft start to warp the paper-thin Miata diff flange when past a certain velocity? I’m just spitballing here.

There’s many cases of this vibration happening with the ZF trans/miata rear end combo. SixShooter is actually the only person I know that hasn’t ran into the vibration issue. His driveshaft is allegedly lighter than the one that I had in my car, and our setups were otherwise identical. All the way down to the PPF being mounted to the transmission crossmember (although he reinforced his PPF with pieces from a second one).

 

Ha! Funny I was following that thread as well. Didn’t think there were many people watching it, with the topic being so niche and all haha.

I think the trans itself is a root cause of a lot of these issues, but am hesitant to replace it because it sounds like a common characteristic in all of them. Mine doesn’t seem excessively rattly, and doesn’t make any noteworthy noises (although it does rattle a little at idle, can hear the rattle go away with the clutch disengaged). In general, it shifts well and feels proper.

You’ve got me thinking, though. With the driveshaft out, I can turn the engine on and run through the gears to see if the transmission itself starts vibrating violently at a certain speed. I don’t believe it’s the root cause based on all my other diag but I might as well try that since the driveshaft is already out.

 

Lean on KPI and they will get you a full refund. They have some blanket policies they like to push, but keep pushing to get all your money back.

 

I ran two different drive shafts from kpi. The first one much heavier than the second one. Neither one had a vibration issue at speed.

 

Yeah, gonna do just that. First time around I didn’t push as hard as I should have. I just got off the phone with KP. I was told the owner of KP is in contact with the owner of the driveshaft manufacturer and I should get an update later today.

Ahh ok I’m making things up in my head now. I thought you only ever ran the second, lighter driveshaft.

 

The first one was very briefly if I remember correctly.

 

Was going to say, I thought in an older thread I read your post asking if KP would trade your driveshaft out for the new lighter one before you installed it. I can remember things like that but couldn't tell you what I ate for breakfast this morning. Something's definitely wrong with me.

I spoke to the driveshaft shop this morning. I'm going to pull the adapters off my transmission and diff tomorrow and bring them into the shop so they can ID the flanges and make an adapter-less driveshaft for me.

 

That sounds like a KPI problem, not yours. You should get your money back, let them fight their vendor for their money back.

 

Yeah, you're right. Thanks for the kick in the pants.

 

this sucks to see still, hoping the diff change fixed it, I’m still fighting the same crap with my tkx swap got it up to 95-100 a second ago and man it feels like something’s gone get launched into orbit… gona have to get under there again and try to pinpoint the diff or the driveshaft… and this was with the ppf braced to the a frame and the ppf connected to the crossmember… might get rid of my long ppf for my stubby one just to test but have a feeling it’s not gona make a difference

 

I’ll keep this thread posted for sure. I have a good feeling about the diff but my confidence has definitely been worn down throughout this whole process lol.

Are you running rubber diff bushings or poly? Swapping back to rubber bushings bumped my resonant frequency up to 110mph versus 75mph where it was beforehand. Def didn’t eliminate the root cause but at least more of the energy was being dissipated by the bushings as opposed to the rest of the driveline.

 

Maybe I didn't run the big driveshaft before he sent the small one. I really can't recall now. I can remember themes better than specifics at times.

 

I emailed DSS to see what they charge to high rpm balance a driveshaft if they will do 3rd party ones at high rpm, waiting to hear back... most normal places only do it to like 2-3k and really just depends how close they got it because any off at that rpm is greatly amplified at 10k and with 23" tires and a 4.10 diff that things spinning fast at 90+ Not saying it is that but at this point i want to make sure it isnt.

 

Yeah, the local shop I'm going through says they balance at 3.2k and generally don't need to go over that because any vibration present at higher rpm will show up by then. Whether that's the case in all scenarios, I don't know. This subject seems like one where five different people may give five different answers.

Did you just have a local shop make your driveshaft and think that it may be out of balance? Have you checked for runout too?

Also: my weight guesstimation regarding my driveshaft adapters wasn't too far off. 3.5lbs each. Yeah, I feel good about my decision to go with a whole new shaft now.

 

yep local place built it, i had issues and found a small dent on the side they went ahead and Re-tubed it for me and rebalanced and what not and the issues still there. If i recall correctly I don't think mine even balanced it to that Rpm i think it was high 2000's or they said 3 it was around there. I need to get back under there and check the runout, i checked it a long time ago on the first shaft but i cant recall the numbers and at this point they dont matter after rebuilding it anyhow. Might try to do that tonight if i get a chance.