So i've been driving the 99 DD trying to sort some stuff, and the last thing on the list for now (i hope) is a vibration it seems to have above 70mph. (With a small exception, will explain in a sec) Above 70mph, a vibration starts and increases in intensity the higher the speed.

Things i've done/changed/checked:
1) All wheel bearings show no sign of play, and definitely not enough to cause this. If one WAS bad enough for this, i'd have to imagine it would show up prior to 70mph, and even be heard if wheel spun on jackstands. Neither of these things are happening.
2) Wheels/tires were roadforce balanced. The worst one came in at 13lbs. The rest are under 10lbs.
3) Konig Hypergrams, thought i had a magic bullet with hub centering rings. I installed hub centering rings. No change.
4) Old diff was trashed. Installed new diff. No change.
5) Have had problems with motor mounts sitting header/oil pan on subframe. Installed new motor mounts on Tuesday that don't do this. No change.
6)Oem axles left and right, with no play or real visible damage. Passenger side was hammered fairly hard at some point to be removed, had to grind the nose a little bit to get a nut back on it.

Potential things in my brain:
1) A tumor? (it's not a tumor)
2) Driveshaft looks kinda gross for a car with only 56k miles on it. Are bent/out of balance driveshafts on these cars "a thing?"
3) General noise in car seems to change in 6th gear. If i'm cruising at 40mph in 6th gear, there's a different "noise" than it would be in 5th. Not just exhaust noise/drone (thanks Racing Beat!), but some other resonance that happens. I could ALMOST call it a vibration, but it's not near the intensity that it gets at 70mph. And it's like a lightswitch. Same intensity in 6th gear from 40-70mph. 71mph, gets much worse.
4) I've fought with PPF angle on this car. I'm still not super happy with where it is, but i only weigh so much so i can only bring it down so much. I think it's within spec right now, but barely.

Any ideas i've missed? Any of my potentials 1-4 seem more likely than others?

 

I think psyber had a similar problem due to PPF angle not too long ago. posts 1184-1195. A tumor could be possible if you are near California since everything is known to cause cancer in the state of California. The drive shaft could be a thing but i think with that it would be a steady increase not a light switch.

In for what you figure out though it sounds like a challenging witch hunt.

 

I had a bad driveshaft once that caused very similar symptoms.

 

Would start with PPF alignment since it's the easiest. Worn out CV joints would also do something similar, but there would be play in the axles in that case.

 

Danke, friends.

I have another driveshaft that visually looks better i can swap in. Just gotta find the time.

As for PPF alignment: How do i get this thing lower? Tuesday night i swapped motors mounts, loosened trans/ppf bolts (all three), and hung on it as much as i could with one hand while i tightened the bolts with the other. I got it to move a little bit, but i can see that the bolts are not lined up in the position that they appear to have been for most of the car's life. (Marks on PPF where the flanges were prior)

 

The very first time this happened to me, my OE driveshaft was shot. I could wiggle the shaft and feel it slapping around at the u-joint. Check your U-joints front/rear.


With or without drop mounts, I always found I had to jack the trans up, not lower, a bit before tightening the PPF/Trans bolts down. Bolting everything back up to normal, as it sat with bolt alignment, would often generate vibrations at speed. Loosen, jack the trans up a bit and tighten, it went away. YMMV depending on your mounts and driveshaft angle.

 

I have a very similar problem on my Lexus IS200 which is caused by dead U-joints in the propshaft. So another vote here for the PPF / prop shaft being the problem.

 

Gosh i sure hope so. Car is pretty much epic other than this and the obscenely boom-y exhaust, which boggles my mind.

 

Could also check the axle runout.
https://www.miataturbo.net/suspensio...t-video-96196/

 

Driveshaft u-joints go bad all the time, I usually find grease slung on the inside of the PPF, right in line with the CVs. Once it's off, you can feel how notchy they are. So check that on your spare before installing.

 

What about tires? My dunlops were great until 60mph then got hoppy. I ended up finding out they were not round. Kinda hard to diagnose because they balanced well. Do you have a different set of wheels/tires to throw on?

 

70mph is the speed MX5s with a powerplant frame which was done up in a low position at the front start to shake noticeably. Do the PPF measurement and it whould be ok. or it's the tailshaft but I bet 99% sure its the PPF alignment.

Dann

 

That would be a bummer, but possibru.

I have the lines of grease lining up with the CVs, which was part of my suspicions.

They looked round when they were on the road force balancer. I don't really have other wheels and tires for it. I have a set of stock wheels with OG Star Specs on them that the car sat on for 2-3 years. So... probably not round. And i have another set of VR1s, but the setup will not fit on this car.

PPF is on the high end of spec'd range. I would like it lower. To be clear, it's in range, but barely. It is high.

 

Man I've literally never done the "PPF alignment". I've always just threw the bolts in and chugga dugga them in place.

 

I've been fighting some challenges with the motor location that was changing the PPF alignment due to severity. Those are fixed now, so it was kinda second nature to re-check once fixed.

 

Try using a piece of wood (2 X 4) to give yourself some leverage to move the PPF into alignment. It will be easier with two people naturally but you might be able to finagle it on your own.

 

Wheel hub bent/starting to crack?

If not, for diagnostic purposes I'd put it up on jack stands and run it up to the problem speed and see if it is evident. It may be easier to locate the source.

Sometimes a toe issue can cause a vibration at certain speeds, fwiw. It is the same mechanism that causes the scalloping of the edges of tires. It can be rhythmic at certain speeds.

 

+? for PPF (pinion) angle and then driveshaft Ujoints worn.

I wouldn't call my pinion angle vibs 'light switch', more over the course of 5mph.

EDIT: easy way to check, put car on jack stands, remove rear wheels, run the car up to speed. That's how I balanced my driveshaft, and played with pinion angles.

 

I had a similar issue that I fixed by putting the car up on jackstands and unbolting/rotating the driveshaft 90deg on the pinion flange until the vibration went away. I forgot to mark the driveshaft/pinion flange bolt-hole alignment when I replaced the diff in the car and was chasing my tail with PPF alignment stuff. Another easy thing to try.

 

Checking in, haven't had time to do anything with it. I can say that alignment is dead on and that we didn't notice anything jumping/moving around when it was run up to 70mph on a lift, i didn't want to go faster though, because i was super sketched out.

I'll try to mess with the driveshaft this week but probably won't happen. OT again this week and gotta get ready for a trip to CO leaving this Saturday for 8 days.