Him! It should be him! Because i already did. (My only dream in life is that someday, someone will actually read that original question instead of skimming two words out of it)

I have not verified that yet, though i would find it weird that that would be loose on two diffs. But it IS me, so i should just go ahead and expect lightning to strike twice.

 

Some dreams are best if they stay that way

 

But my insatiable curiousity!

(I suspect the real answer is "I don't know" but heaven forbid anyone post that on the internet. )

 

I can say in my case I did check that and it seemed to be tight to me. Just checked by hand, didn't whack it with a hammer or anything.

 

Yeah, my apologies, I knew you had a vibration, didn't realize I made it worse.

read below

Same here. Wiggled it by hand, it feels like it's "solidly" mounted on a small layer of rubber, which is what it's supposed to be as far as I know.

I think I'll put our intern in the car, try a PPF alignment, try rotating the current rebuilt driveshaft 180*, try the new rebuilt driveshaft we have, try your old driveshaft sitting in our scrap bin, and then finally we have a carbon fiber drive shaft to try as well. Hopefully one of those things fixes it.

 

Tell me more. I have 140hp and buying more things to not add power sounds fun.

 

I don't know much about it, I think it's this one?

https://www.rallysportdirect.com/par...hoCZA0QAvD_BwE

owner of the car bought it mostly because we get a fairly sizable discount afaik.

 

Bad driveshaft:



Good driveshaft:

 

Played with lots of dial gauge last night. Here's some general numbers. Wheel measurements are lateral runout, i did not have access to really check radial, though the one with the worst lateral looks pretty gross barrel-wise, radially.

Driver rear wheel in driver rear location: ~0.015"
Driver rear wheel in passenger rear location: ~0.012"
Passenger rear wheel in passenger rear location: ~0.005"
Passenger rear wheel in driver rear location: ~0.008"
Driver front wheel in driver rear location: ~0.008"

Driver CV: < 0.005"
Passenger CV: ~0.015"

Driveshaft: ~0.020"

The crappiest wheel is the one i'm 99% sure we had some issue getting to 13-14lbs road force, the other three we got in the 5-7lb range.

Driveshaft is pretty suspect, going from Curly's video above. I'll double check measurement by hand to make sure the base isn't moving at all (did it with car in gear in the air) before swapping driveshafts for funsies. I don't love the passenger CV either, but don't know what "spec" is.

 

Fix the driveshaft, the rest is probably fine. The driveshaft, it's really heavy, and it's weight is a lot further out from the centerline of rotation vs the axles, and the axles turn slower from the diff gearing. If an axle and driveshaft both had .020" runout, the driveshaft would be the source of probably 95-97% of the vibration.

 

Thanks Pat, that's what i was struggling with from that point, to determine if DS or CV was more badly-er-est. That makes perfect sense.

 

After watching your video on YouTube. It makes me wonder how much of that runout is just due to the rotor to wheel surface. How true is the hub and rotor? How much is it being amplified by the distance from center?

Need more data!

 

Yeah the lateral runout of the wheel is not the issue.

 

You gonna reply to me like we weren't sexting last night?

Put the shaft in yet?

 

Figured i'd head it off before someone else bit my head off.

Nah not yet. Sometime this week. Hard to get motivated to get under a car i can't drive right now after 70 hour weeks.

 

Bump

I'm having the same problem. I have replaced
The wheels and tires roadforce balanced to 10lbs
Axles
Diff (open to 3.9)
All the bushings
Alignment
Motor mounts (OEM)
And aligned the ppf.

The transmission is pretty quiet and shifts well and I have a small whine from the poly bushings at the diff. (normal)

My driveshaft feels good at the u joints but I have not checked run out. My car was in a small rear end accident that snaped the diff housing. I'm wondering if the PO tried to drive it with the diff hanging and it beat up the DS.

Did you ever replace your DS?

 

Its probably your driveshaft. Even the newest NA shafts (or NB for that matter) are 15-25 years old. Couple sources reference the stock ones not being balanced from the factory, so that plus 15+ years of use, coupled with the speeds they turn almost guarantees that's the issue. With the stock 5th gear overdrive of .89:1 the shaft is spinning roughly 10% faster than the engine, so cruising at ~3700rpm on the highway means that bad boi is chillen at 4000+rpm. Won't take much imbalance to have it vibe, which would be even more pronounced with poly diff bushings.

 

Hi, sorry. I did not replace my driveshaft yet. It's first up on the small laundry list of things to do when Spring cleaning hits. I had too many other irons in the fire to really give this car the attention it deserved this past year.

But like The Monkey Dude Himself, i'm pretty confident it's the driveshaft, which is why i'm going to take all 4 of mine to a shop in the next month or so and have them tell me which two are best and balance them.

 

I've been thinking about getting a shaftmaster drive shaft. The price is nice and I had thier alluminum one in my 6g 5.0. I'll probably go with a steel one for the Miata. With how much nvh the car has naturally I really would hate to get an alluminum ds and pick up more nvh. The drive shaft is pretty much is the last thing on my list of possible causes.

 

Something to think about is also a worn pilot bearing. If the input shaft isn't perfectly centered, it can cause vibrations that are not always the same intensity due to varying misalignment while the clutch is engaged. I'm a big +1 on likely driveshaft issues, from experience!