So, I finally had time to check out some of these suggestions.
I used my buddies tire machine and we found the balance of the wheels was a little off. Not massive, but a little. Corrected that.
The "road force" was 30 to 43 on the wheels and tires. I'm told that's not great.
The driveshaft has been balanced.
I measured the driveline angles and got the following results:
I measured the horizontal offset and did some math. It's only 1 inch over 4 feet. The engine and Trans is parallel to the center of the car. The diff is parallel to the center of the car.
I reinstalled the wheels and tires with the worst road force value tires on the front. The vibration *feels* less. It also doesn't seem to be affected by depressing the clutch or if you are on or off throttle. It seems the vibration starts at a higher speed as well (65mph).
So, I'm leaning towards a tire issue. OR a bad wheel bearing.
Next step is rigging up the vibration monitor in the car.
Whatchyall think?
Silly question but do you have a sacrificial go pro to stick somewhere under the car and take it for a ride?
So, I had a thought. My inlaws have a non vibrating mazda 6 with 18 inch wheels. I'm gonna steal their wheels and tires and drive the car and see if it IS a tire issue.
If the rx8 wheels fit the 6, I'll swap them on that and see if the issue follows.
Cheaper and easier than throwing parts at it...
I once thought that I had a bad wheel bearing. Replaced it. Noise didn’t go away. Swapped on tires from another car, and the noise was gone. (But, hey, fresh wheel bearings.)
What is your gear ratio?
2.5 degrees of U joint angle gets noticeable over 4000rpm. That'd be your engine speed in whatever gear is 1:1.
Pete. (l33t FS) said:
What is your gear ratio?
2.5 degrees of U joint angle gets noticeable over 4000rpm. That'd be your engine speed in whatever gear is 1:1.
4000 rpm in 4th gear (t5wc) with the 4.44 ratio is 70 mph.
That's close enough to have a look at it.
Any suggestions on fixing that if it is the issue? Point the Trans down a bit more and the diff up a bit more?
In reply to wvumtnbkr :
Yep. A little will go a long way since as you move the trans, it also moves the driveshaft, and likewise at the rear.
Pete. (l33t FS) said:
In reply to wvumtnbkr :
Yep. A little will go a long way since as you move the trans, it also moves the driveshaft, and likewise at the rear.
What number would you shoot for for driveline angles? I'm seeing 3 deg as a rule of thumb.
Obviously the tire switcheroo will happen first cause that is free and easy.
Under 1 degree u-joint angle, IMO.
Pete. (l33t FS) said:
Under 1 degree u-joint angle, IMO.
Wouldn't that mean that basically you can't have any offset?
I've seen recommendations of less than 1 degree for essentially parallelism. I'm well within that.
In other words, that would be MAJOR surgery to get below that. I'd like to rule everything else out before remounting the entire drivetrain. I could get to about 2.5 degree with just some shimming.
I want to be clear that I hear you and am not trying to discount your advice. It just seems shocking to me that a driveshaft would need to be that straight!
In reply to wvumtnbkr :
Shouldn't be major surgery. More like a few washers here and there.
Remember a change here makes a change there. Lowering the trans will change the U joint angles at BOTH ends and vice versa.
You are AT just under 2.5 degree U joint angles now. Drivetrain angles don't matter, just U joint angles.
You don't need to make it perfect, just better.
The reason it needs to be as straight as possible is because the more angle the U joints have, the more the driveshaft speeds up and down twice per revolution. And unless you are under a lot of load, that loooong driveshaft will be the elephant in the room and make the pinion gear and transmission output shaft accelerate and decelerate twice per revolution.
And people wonder why I appreciate Guibos... they make all that harmonic nonsense disappear 
Pete. (l33t FS) said:
In reply to wvumtnbkr :
Shouldn't be major surgery. More like a few washers here and there.
Remember a change here makes a change there. Lowering the trans will change the U joint angles at BOTH ends and vice versa.
You are AT just under 2.5 degree U joint angles now. Drivetrain angles don't matter, just U joint angles.
You don't need to make it perfect, just better.
The reason it needs to be as straight as possible is because the more angle the U joints have, the more the driveshaft speeds up and down twice per revolution. And unless you are under a lot of load, that loooong driveshaft will be the elephant in the room and make the pinion gear and transmission output shaft accelerate and decelerate twice per revolution.
And people wonder why I appreciate Guibos... they make all that harmonic nonsense disappear
This is a power plant frame car. It is literally moving every mount in the car and refabricating the power plant frame.
The only little bit of leeway I have is to shim where the ppf attaches to the diff.
I can't really move the engine anywhere at this point. It is 1/8 away from many things.
That's why I'm pushing back a bit. It would literally be starting over with the ppf fabrication.
Edit to say... I think I could shim the rear diff and get to many 2 degrees.
David S. Wallens said:
I once thought that I had a bad wheel bearing. Replaced it. Noise didn’t go away. Swapped on tires from another car, and the noise was gone. (But, hey, fresh wheel bearings.)
And that is exactly why I love my brand new Chassis Ears. Noise from the back of the car:
Diff? Replaced twice (or so)
Drivrshaft? Pulled that, too
Hmmm... what could it be?
Chassis Ears says: Not the diff, driveshaft, not that wheel bearing, not that one, not the other two, but the transmission!
I'm still waiting on approval to borrow my companies vibration monitor.
The vibration would be fine for the challenge. However, getting there and back would be annoying!
I'm swapping wheels and tires on Saturday. If that doesn't solve it, I'm getting out some shims and probably the angle grinder. I might eliminate the ppf and go with a Trans cross member. That would be easiest and alleviate one of my concerns with breaking the tail housing off the trans.
When I had my Mazda pickup it developed a vibration that sounds exactly like what you're experiencing. Some googling told me that it was very common, and very difficult to fix. I changed everything in the driveline that I thought could be responsible and was never able to fix it. According to what I read back then it was also not uncommon in Miatas too.
I hope you have better luck than me.
OHSCrifle said:
In reply to P3PPY :
Had to Google that one..
https://www.amazon.com/Steelman-97202-Wireless-ChassisEAR-Diagnostic/dp/B00123J79O
If you get one, test it on like a ticking clock or something before mounting them on the various components of the car and driving around. I got some cheap knockoff with two bad sensors and one not great sensor. I could have been VERY mislead by those results.
I have the parts now to piece together my own, including using a different kind of microphone, they are more like guitar pick ups. They don't do air gap noise levels but straight vibrations. It was maybe $40 in parts from Amazon
Pete. I hate you.
Swapped wheels with my wife's Mazda 5. (Yrs, they physically fit).
Brand new tires.
Vibration significantly less. Still there. berkeley. E36 M3. Damn. berkeley.
Removing the ppf this weekend. Setting everything where it needs to be with less than 1 degree.
If this doesn't fix it, I'm driving to Ohio, and force feeding you vw quantum parts. Lolz.
Edit just to add that the first and last lines were jokes. Hope I didn't offend.
Well.... here we go.
Found 1 loose driveshaft bolt. No difference.
Tried shimming the diff a little up and a little down. No difference.
In order to really make a move, I need to remove the entire ppf and remake it. I'm not sure I have time for that.
I also had one more epiphany today. I think the offset of the engine is causing a greater driveshaft angle than the up and down from Trans to diff.
There are many space constraints to move the engine and Trans over. However, I think I can move the diff about 3/8" to get it more inline with the Trans output. Do you think 3/8" would work with the stock mazda cv joints?
Do yall think a double cardan joint driveshaft can solve this? Or, a different type of driveshaft? Is that even a thing?
Okay, I got everything to within about 1.2 degrees. I was able to shift EVERYTHING slightly.
Vibration didn't change. Ugh.
This driveshaft is straight from Trans to diff by sight. Only the angle finder sees a difference. About .2 degree difference in parallel from diff face to Trans output. The angle between the driveshaft and Trans is 1.2 degree. The angle from the diff to driveshaft is 1.0 degree.
Horizontal is basically straight on. I moved the diff a slight bit and moved the trans and engine. Removed the exhaust for the drive.
Literally no discernable difference.
Sometimes I hate projects.
When I did a similar swap on my rx7, I just worried about the shaft being parallel. It had way more angle than this shaft and didn't have any vibration issues. I'm thinking it's something else. Maybe the tires.
I might get another set of tires and try them. Wheels are cheap.
Oof. Maddening. I'm sorry...
Are there any bushings in the driveline that have to be shoved around to bolt things up?
I had a nasty vibration in an engine-swapped E30 because they just sort of mangled the trans crossmember until they could force the bushing close enough to bolt everything down, fixed by remaking the crossmember to avoid preload.
Not sure how that applies in PPF-land, but i assume there's isolation between the vibrating things and the people-carrier.
What's the state of the rear suspension bushings?
Sooooo.... I had a confuse. My angles looked good after shimming, but still had the vibration.
My lovely wife was helping me and drawing the angles on a white board. I had the driveshaft angle the wrong slope when I measured it. In other words, the driveshaft went uphill to the diff.
So, on Monday night, the week of the challenge, I cut the ppf in half. Moved everything where I think it should be and remeasured. I was able to move the engine mounts, diff mounts, and obviously disconnected the ppf (by cutting it.). The angles look much gooder now.
Tonight I verify that I haven't squished a brake line or smooshed a fuel line, and make sure that the new shifter location doesn't interfere with anything. If that all checks out and the angles still look good, I will tack weld the ppf back together tonight.
Tomorrow will be final welding, reassembly, and packing. Ooph. Let's hope this works!
Update. Got the ppf tacked together. Now I need to reinforce all of the welds and add some fishplates etc.
My angles are now all close, but not perfect.
Trans is down towards rear end at 89.9 degrees. Driveshaft goes uphill to rear at 91.2 degrees. Differential is front high at 90.6 degrees.
That working angle of the front shaft is a little worrisome. At over 1 degree (1.3 degree).
I did find out that my engine and Trans were skewed in the car (tough to measure!). I moved the tailshaft of the Trans about .5 inches to make the driveshaft as straight as possible.
Once I weld it up, I will go for a drive. If this doesn't work, I'm going to a separate diff mount and Trans mount (like all the other rx8 swapping people). That will be post challenge if this doesn't work.
