Speed related vibration
 

Can't say I recall much talk of drive shafts being a weak link, especially on stock powered, lower mileage cars. Do they ever go bad that you have seen? Reason I ask is I am realizing a vibration I have been blaming on unbalanced tires seems to be speed related, and related to throttle input, not constant at speed. Seem to notice it at about 70mph, up to about 90, which is as high as I have tested it. It goes away while engine braking or giving very light throttle (enough to not engine brake, but not quite enough to maintain speed at interstate speeds), but with more throttle, it gets worse. Like I said, I thought it was a tire, very similar wobble/vibration. My only problem with this is at 70mph, the drive shaft should be spinning faster than the engine, if I am thinking correctly (.814:1 in 5th) and I'm not so sure the vibration is that high of a frequency. Feels much lower. What else could cause a vibration that gets worse with throttle and is speed related, not rpm? It seems to me that it's coming from the middle or maybe rear of the car. Could a wheel bearing do this? Or could it just be the 5th gear of my very poor condition trans which already sounds like it's a straight cut geared, beat up track monster. Help a brother out! This is my last real major annoyance with the car, since I solved my electrical gremlins today. Never dealt with either failing, so I'm lost on this one. My failures are always engine related.

This is what happens to a transmission when the front u-joint snaps...on stock power...
Attachment 197274
Used driveshafts are $50-$75

I have 3 miatas. I've replaced 2 driveshafts.

edit: driveshafts will usually be a faintly audible squeak at first. I've never driven a driveshaft to destruction, so I dont know what vibrations you would experience. After seeing the driveshaft that came from the above transmission, yes, it definitely would have been vibration.

Consider that the vibration frequency of a damaged driveshaft will be about four times as high as the vibration frequency of something post-differential.

Rear rotors.

I just had the same condition. Jack up the rear, put it on stands. Put it in fifth and let the wheels spin. With mine I could see the wheel wobbling. Just to be sure I swapped rotors left to right and the wheel wobble moved to the other side.

Have you tried on a set of wheels that you know are balanced (ie had them recently balanced/on another car and feel no vibration)? It may also be rotors...but I would think you would feel it when you are braking. Also check your tie rod ends, my Audi does similar and has a bad passenger side tie rod end.

Might be worth giving it a shot anyway. I will inspect it this weekend. I'm not so sure it could be rotors, as it only does it under load, which wouldn't make sense to me for the rotors. I'll still for sure give everything from the trans all the way back to the wheels a good inspection.

I had a similar feeling from around 50-80 mph. It was where the driveshaft meets the ring-and-pinion in the rear diff. There's a donut ('X' shaped) heavy rubber damper that dry-rotted. We removed the driveshaft, pulled off dry-rotted portion, re-attached driveshaft, and it's normal again. JUST HAPPENED around the same time I blew my 5-speed.

It sounded like I was driving on rumble strips they carve into the highway to wake up folks.

That's about how I would describe the feel, only under throttle. For sure be looking hard at that area.

To bring this back, what do you guys think about the transmission being misaligned, and/or driveshaft angle being "off". Would make sense considering one of the recent threads discussing PPF bolts and the transmission needing to be supported while tightening them. Otherwise you get increased noise from the transmission. This sag in the PPF area could also cause the driveshaft to be out of alignment couldn't it, which would increase vibration through the driveline. The thing that still throws me off is that it is throttle related. Which would seem to indicate something loose or worn. Maybe the PPF bolts have come completely loose and the movement from throttle application causes the transmission to move and create this vibration. Seems like it would be simple enough to get under there and find out, but damn it's so much work to get this low thing off the ground far enough to crawl under it. I wish I had a lift.

From more reading of other forums and whatnot, I have seen other cars with similar issues and they have been axle cv joint issues, being dry and whatnot. Sounds likely. This weekend I'm gonna try to look into it. Isn't the axle a bitch the break loose from the rear hub/bearing?

It can be. I had to pop 3 axles out of their hubs in like a week and only one of them came out easily. I tore up one axle beating on it and eventually took both the stuck axles to a machine shop to get pressed out. They were stuck like crazy, took the 20ton press and one hell of a BANG to get them out. Make sure you use anti seize when they go back in.
If you do take it to a machine shop have them support the hub with a sleive large enough to fit over the CV joint so that you are supporting the hub evenly while pressing it out, otherwhise you could mess up the hub in the process.

also look in to what tires you are running. I had some junk sunny tires that came with my wheels and no matter how perfect i got them to balance out i still had a nasty shake above 65mph. A set of falken rt615's later and my car is as smooth as can be.