Bear with me while I share stupid ideas that will waste your time and don't affect whether or not you demand to get a new axle shaft, which you absolutely should do.

Any way you can verify the theory, without a new axle that will take a month to arrive? Maybe flat tow the thing with the driveshaft removed, in order to remove the driveshaft and the transmission from the equation entirely?

 

Great idea! I can roll it down the hill on the toll road next to my house and watch as it bursts into flames at the bottom.

Hahaha ok ok, that was too dark. I kid. But yeah, I've thought about doing something like that. When I had the driveshaft out of it last week, I revved the car up to 6k rpm in 6th gear and noted zero vibration, ruling out the trans itself as the issue. Didn't go any further though.

I think I could basically do what you're suggesting with the car on jacks or in a lift if I could find some plugs that would fit the axle shaft seals in the diff. Could pull the axles, plug the holes in the diff and at least rule those out and isolate the issue to the driveshaft/adapters and diff. I probably have something that would work sitting around the shop...

Great input tbh, thanks for the idea. I'm pretty busy today but will update the thread if I'm able to get the car in the air at some point.

 

Alright, I'm gonna have to postpone the axle-less test for a later date this week. I can definitively say, though, that the driveshaft runout is/was causing a decent bit of the vibration. Copy/pasted this from my build thread just now:

Got the car on a lift during lunch today. Was somewhat strapped for time again but I got the driveshaft just about perfectly centered on the trans adapter side. The adapter still needs to be centered but I'm assuming its runout is making less of a difference than the driveshaft's. And this was another "for science" test.
After aligning the front of the driveshaft, the vibration's resonant frequency rose another 4-500rpm in 6th gear. That's a win. Yesterday it started to become noticeable around 2,500rpm in 6th but now I can't feel it come on at all in the lift until around 3k rpm. It's also a touch less aggressive than before.

Generally speaking, the vibration won't be felt on the road until around 1,500-1,800rpm higher than it presents itself with the car in the air. It's also moderately less violent on the road than when I have the car in the air (as one would expect with tires, suspension, etc absorbing some of the energy). I'll have to take it out tonight and see how it behaves on the road. I have no doubt that the issue isn’t gone, but it's getting better bit by bit.

 

this is a good idea, not sure if i have a set but the stub shafts from the early na8 axles maybe stuck in there and run it up to test.. if it still does it it has to be the driveshaft no way it isnt

 

Dude that’s actually brilliant if you can just pop the stub shafts in there. I have an extra set. Want em? Lol

I got the car up to 100mph or so on my way home. The vibration is noticeably mellower now that the front of the driveshaft is centered up. Win.

Make sure your sh*t’s centered haha.

 

it didnt hit me till just now you dont have the stock diff anymore so cant do it that way.. anyhow, any real science to the whole hose clamp thing you did or did you kind of just take some guesses till it worked better or did you follow the lowest part of the runout? im pretty sure the weight on mine is at the front. Is the idea that the rear might also be out of balance causing it?

 

Yeah, I can't do it with the stub shafts but think I could still find some rubber/plastic plugs and do it that way.

No real science to the hose clamp theory. I just used a piece of chalk to mark the spot with the most runout and mounted the hose clamps so that the bolts were oriented opposite of that. From there, I rotated them all a quarter turn and retested the vibration from there. The front yoke of my driveshaft is pretty heavy, though, and I don't think that even 6 hose clamps were enough to counter balance it. Centering up the driveshaft made a bigger difference in the vibration.

I also used some hose clamps on the wobbly axle, and think that mellowed the vibration from the axle out a little bit.



Lol.

I'd still try the hose clamp technique and experiment with clamps in a few different positions on your driveshaft. It didn't make a difference for me, but I think that was because even a small amount of runout in my heavy-*** front yoke was going to need a lot of weight to offset it.

Also, I drove the car up to 100mph last night and the vibration is still there, but much better. It's almost acceptable now. I'm gonna have to center up the front adapter perfectly and see if that does the trick. A little less vibration and I'd feel comfortable tracking the car again at this point.

 

I might be wrong but the axle being so small and spinning at the same rpm as the wheel id think it would need a ton of weight to throw it off but if it made a positive difference than i cant argue with that. My driveshaft just has a yoke in one end that slides into the transmission and the other end has the centering ring that slides into the flange on the diff and has zero detectable play in it so i cant really move mine around. Juggling a few projects but hopefully ill get to mess with it in the next few days and start by removing the axles

 

Yeah, the difference in vibration from putting the hose clamps on it might just be in my head. I can't say for certain. All I know is that the car overall feels better than it did, vibration-wise.

Keep us posted when you get around to it. Have you checked for runout in the driveshaft with the car on jackstands and idling in 1st gear?

 

I just spent way too long reading through this.

What version of the trans mount is being discussed? The versions i see now have a loop for the exhaust.

Is there a TLDR for this so far?

No resolution?

 

TLDR is he bought an NC lol.



I suggest you look at his other build threads for the too short, want to read more.

 

Haha shucks I never posted closure to this thread. By trans mount, you mean the crossmember for the mount, right? They all come with a loop for the exhaust but the downpipe still needs to be lowered about an inch and a half to clear through the loop. I actually have my modified downpipe and need to sell it still.

TL;DR - I'm pretty sure I was getting close to solving the vibration and then the car broke and I finally threw in the towel.

Figured out that the adapters for the driveshaft on the trans and diff both were not self-centering perfectly. On top of that, the driveshaft wasn't self-centering onto the adapters perfectly either. Measured runout and adjusted everything by hand and got the vibration a lot better. It came on at about 100mph when I last touched it. The driveshaft still wasn't PERFECTLY centered. I was on track to get the alignment even closer but then I had a water pump bolt back out and take out my timing belt, bending the valves on my built interference motor. I ended up parting out the car because I hadn't been able to track it for four months and figured it'd be another four before I could track it again at this rate. I'm 90% sure the driveline vibration is what backed out the water pump bolt, as it was loctited in there pretty hard and I was unable to turn it by hand when I found it backed halfway out after the timing belt failure.

I sold the trans, diff, and adapter kit to a gentleman an hour north of me who owns a repair shop and knows a lot about Miatas. I told him everything I dealt with on the kit and he said he was confident taking on the project. The trans and diff are going to go into his MSM. Said he'd start it after Miata reunion and will let me know how it goes. I'll update this thread if/when I hear from him.