I don't know if the pressure change is enough to influence the circumference enough, at least on a modern street tire. We used to change the stagger on race tires that way but they have a much different carcass. I always used very large radius curves in an interstate to test it. The speeds you see the problem occur also points to a tire/wheel out of round/out of balance problem.

You're problem starts here, I guarantee it:

"It also looked like the right rear axle was "jump-roping" just a bit, i.e., swinging around in a larger circle versus spinning perfectly on axis."

I have also seen wheels that were bent from the center, meaning the mounting face plane wasn't perpendicular to the axis of rotation of the barrel.

Take a careful look at the hub and disc. You can have a perfectly round and balanced tire (one that passes a balance and road force test off the car) but if when you mount the three components together the problem occurs. If the hub face isn't free of runout, or the disc doesn't seat properly, or has an issue, it can cause the tire/wheel assembly to not run true.

Pull the tire and check the mounting face of the hub and disc first.

I agree the mounts are likely transmitting more NVH, but I dont think they are the cause - - certainly not of a cyclical vibration. I don't remember whether I changed the mounts before or after the tires. Probably before - - but it was the changing of the tires that made me notice the vibration - - mostly because the new tires were so quiet and comfortable. I already tossed the old mounts or else I'd swap them in to test. Not sure I want to spend $250 to test the theory with new mounts, though admittedly, replacing motor mounts is a breeze relative to swapping wheel bearings and I suppose I could resell them - - people are always replacing 997 engine mounts.

I do love the 964 mounts - - at least I love them relative to the worn out factory mounts they replaced. I thought my rear suspension was worn out before swapping to the more solid mounts (which is a common upgrade on 997s and no one else reports a vibration issue) - - the soft/worn factory mounts allow the engine to move around a lot - - hit a big long bump/dip and there were two separately timed suspension events, the first was the wheels relative to the body, and the second was the engine moving around. The latter felt like worn out shocks until the mounts got replaced. Really tightened up the back end.

I plan on keeping this car, so I'd be tempted to do all new suspension arms to baseline things. Other than needing an alignment, it's not a lot more work to replace all the parts if I'm gonna tackle the wheel bearings anyway. Decisions...

Good idea - - especially since it still vibrated without the wheel mounted. Maybe a bent hub?

Could be a bent hub.
Could be a brake disc or wheel not seated properly.
Could be a disc with a thickness variation in the mounting face.

Anything that throws the rotational axis of the wheel off. That axis is supposed to be steady. If it wobbles around a point on one end, you'll get a vibration.

The fact that you can see a movement, a "jumping rope" effect, tells me that axis isn't steady. Find out what causes that movement.

Focus on that, everything else is a distraction. Find the problem, don't keep throwing parts at it.

I rigged up a measuring stand this afternoon (pics attached). Lateral wheel on both rear wheels mounted, inside and outside machined edge was less than. 010" - - that's about 0.25mm. Radial runout was more difficult to measure because of the tire in the way. Using a sharp pointer, my eye detected very, very little runout. I'll leave it to others to tell me whether that's acceptable or not.

While I was measuring lateral runout, I also did the 'ol wheel bearing check - - I could get the dial to move about 5 thou either direction (push/pull) as I spun the wheel 1/10th rotation at a time. I'd attribute the movement to the suspension bushings, not the bearing. The movement was consistent in all directions.

I spun the rear wheels while listening with a poor man's mechanic's stethoscope (12" socket extension) between the axle shaft and my ear. Heard nothing. I'm leaning away from wheel bearings - - don't think that's it.

Moving on, I checked the right rear disc rotor. Radial and lateral runout was only .001". I'd say the hub/rotor is not the problem.

I have wheel studs, which I removed on the right rear, and replaced with factory lug bolts just to see if it made a difference.

Took the car for another drive: no difference (of course, really only the lugs had changed).

Did a lot of driving/vibration logging - - this time a little slower (80mph, so numbers are all a bit less). I'm coming to the conclusion that at least one of my wheels is still poorly balanced. My logs are almost all 1st order wheel speed vibration, i.e., wheel balance. I still feel it cyclically, but that's still most noticeable on grooved/smooth pavement.

I have a theory regarding smooth roads that I need to test: My aftermarket suspension controller is "active" - - it uses a g-sensor to determine how much damping to apply to the shocks. Throw the car into a corner and it will stiffen the outside and soften the inside. I'm wondering if there's not enough baseline damping on smooth roads and if an out of balance wheel is actually causing the shocks to act like they're bad/too soft allowing the wheel to bounce up and down causing the second order vibrations that are cyclic. It would work like this: When the road is smooth, the controller has the shocks go soft (for comfort), so the wheel imbalance causes the wheel to bounce on an under damped shock. The suspension controller then senses the vibration and increases the shock damping, eventually controlling the wheel bounce. It smooths out, the suspension controller goes soft, and the cycle repeats.... It would also explain why the cyclic vibration goes away on bumpy pavement (but the 1st order wheel vibration remains).

It's easy to test the above. I'll just put the factory controller back in. Tomorrow...

The root cause would still be a drive train imbalance (likely the wheels), but the suspension controller may actually make it worse in some circumstances. And my hard rubber engine mounts are likely passing more of the vibration into the cabin.

For my engine mounts, the 964 style uses dished washers. There were two schools of thought on how to install the washers - - I did it the more firm way. I'll also play with reversing the dished washers and see if that helps.

So there's the latest. Let me know what you think.

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The runout sounds OK. The .005” play in the wheel bearing also seems fine, I don’t know what the spec is on that car, but the first turbos had a spec of about that, so it doesn’t sound like an issue.

Time to revisit the balance on the wheel/tire assembly and make sure that it’s not an issue. In the old days, with a stubborn problem like this, one test was to swap wheels and tires with another car and see if the problem moved. Know anybody with a similar car?

Is the a wheel tight fit on the lip on the hub? No possibility of the attachment method affecting the concentricity?

So, there's nothing like back-to-back testing to kill a hypothesis dead. I swapped in the factory suspension controller and went for a drive. At first, I could feel the vibration, but it wasn't cycling. That got me pretty excited.

Swapped back to the aftermarket and drove some more. The vibration was cycling again. Ok, maybe I'm on to something.

Swapped to the factory controller again and damn: Cyclic vibration was back and bad as ever. Theory killed. Me bummed.

I don't think it's the hub fit: I would have seen that on the runout tests of the mounted wheels.

I need to get the wheels balanced again and see what's up. Something rotating at wheel speed is out of balance.

And I suspect there's two unbalanced wheels and they are occasionally resonating. Or maybe I also have something wrong with the suspension.

If it still vibrates I need to put the whole car on stands again and look for what's vibrating.

I know you've put on new tires, but, have you checked for a separated tread?

Reason I ask is this. I had a similar type of situation on my car. A vibration you sometimes could feel, sometimes not.
I took the car in for an alignment to correct another problem and the tech found I had a separated tread on one of the rears. Bubble wasn't visible from the side but it showed up when the tire was jacked up and rotated close to the ground so you could see the change in the distance tread-to-ground easily. Wasn't a very big bubble either, but it was there.

The road force balancer would have eliminated runout and bad tire (seam in tread) from a pothole.

If you don't see curb rash, then there is no point in checking lateral runout. You are looking for vertical runout anyway.

Swap the wheels over to see if the vertical play continues.

I'm suspecting the upright is not tight, or the control arm bushings are worn, or bolts not tight.

Can’t help but wonder if it still isn’t a tire issue. No noticeable problem in the old skins...noticeable problem on the new.

Any possible access to another set of tires/wheels to mount up and try?

I know you said you did new tires. Back 25 years ago I bought a new Ford Explorer 4wd. From new I would notice a vibration on smooth roads at lowish speeds. A couple trips to the dealer with "we didn't find anything". Finally get a service adviser to ride with me. He experiences it and agrees there is an issue. They proceed to replace most of the front end under warranty and proudly show me the invoice to FOMOCO for thousands. No change.

I call the other dealer across town. The service manager asks first thing if it had the Goodyear whatever tires on it. Yep. He immediately says those have a TSB out on them. They came new flat spotted. New tires under warranty problem solved. I never returned to the original dealer.

I'll add one other thing. Some people don't know how to operate a balancer properly and this is made worse on really wide wheels, especially if they try yo hide all the weights on the inside. You have to read the fine print on the balancer instructions, as to how to set it given where you want to put the weights. I won't go into the details but some balancers just ask for wheel diameter and width and that's not enough info.

Try a different shop the next time you check the balance.

This is not likely but you have checked everything else, have the tire shop pop the rubber from the rim and check inside tire and possibly rotate rubber 90 deg on rim. Years ago I had a wheel doing strange vibration and there was some water inside.

Can you find some other used rubber to mount on the rear rims to try?

Thanks for all the great suggestions. I'm really suspecting/hoping it's a wheel/tire balance issue.

The car came with some UHP Hankook tires that were reasonably worn. They were hard and noisy, but had good grip in the dry -- but I don't remember any vibrations. Wet grip was not good. Code brown stuff when it rained. Needed new tires.

First Balance: So I bought Michelin PS4S tires off Tirerack and had my local Firestone install them.

- Went for a test drive: Vibration.

Second balance: Brought them back to Firestone

- Test drive: Vibration. I ask Firestone if they're doing road-force balance: No, they don't have a RF machine (facepalm)

Third balance: Take them to the local Discount Tire that has a roadforce machine. Sidenote: they didn't have a deepwell 19mm socket to remove lugnuts from my wheel studs -- had to lend them my $3 socket from Harbor Freight. More facepalm.

- Test drive: Vibration.

Fourth balance: At this point, I contact Tire Rack and say I've got an issue -- can't get it to balance. Feels like the back wheels -- not really feeling it through the steering wheel. "No problem -- we'll send you a new set of rear tires." We agree to have them shipped to a local independent mechanic (BMW specialist and race shop). These guys seem legit. Do the roadforce and the mechanic assures me "there all coming up zeros -- these wheels are balanced."

- Still dealing with a cyclic vibration.

That's when I started swapping parts:
- All four rotors (and brakes)
- New guibo
- New trans mount
- One rear axle
- Also had the car aligned (just in case -- new tires and all)

Here's what I need to do:

See if I can swap tires with a friend that's not having balance issues. I've got a couple Porsche buddies -- just need to make that happen.

That will inform whether I've got a wheel/tire problem, or something else.

At this point, I pray it's a wheel/tire problem, as I'm worried that I'll otherwise just be swapping parts. If I go down that route, I'll likely just do a full suspension refresh -- but hate to spend the money if it's not really needed right now.

Appreciate all the replies!

Now I'm glued to this thread, want to see where this goes......

I'm glued to this thread too... It's like my inner psyche has bought a twin turbo and now I can't figure out the problem. It's almost too stressful! :D I might tune in several weeks to learn what the answer was.

( I was actually going to suggest swapping tires with some friends- that should hone in on the culprit if it's in the wheels).
And whatever the solution is- I hope it is easy.

All I can add, is that I hope it's a tire issue too... My wife's car had (what from all objective observations) was a front passenger wheel bearing going out. In the end, it was a center propshaft bearing (yeah- I know, in your case, you took out the driveshaft earlier...so that's not the culprit) It's just amazing how this type of troubleshooting can become such a rabbithole, and end up in an entirely different place.


Good luck!
Ron

Suspension isn’t the issue. You need something to excite the vibration, that’s where an imbalance in a wheel and tire assembly comes into play. You could have worn suspension but until something starts a vibration, nothings going to happen. The fact that you’ve looked at the frequency and it coincides with tire rotation speeds should tell you that’s where the problem lies.

This is a video of an off road tire being trued on the vehicle taking into account everything that turns. The wheels are then marked (indexed, clocked) to make sure they are always mounted the same.

Nate Jones is the operator and designer of the machine being used. It's a high speed rotary razor. The other machine in house does tires off the car in a converted balancing machine. Nate balances land speed record cars and just about everything else, including LSR motorcycles. The first rule in his book is that the entire rotating assembly has to have 0 runout at the tread. Under normal conditions Nate trues and balances each tire/wheel ass'y off the car and then installs the wheels on the carm and trims out the balance with a strobe on-car balancer. He will cut more off the tire if needed but will advise if there is any runout at the hub and that should be corrected before any further work.

I guarantee you that on a smooth road your car will ride like silk, and with a proper wheel alignment will drive perfectly straight on a level road using no hands on the wheel.

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The Hunter Road Force machine takes in account the flex of the tire as it turns. All tires have a heavy spot that is at a fixed distance from the center of the wheel. That is until the load of the vehicle compresses this distance as the tire passes over the road at the contact patch. The faster you go the less this is true but the faster you go the more an out-of-round tire will throw a spot out further due to centrifugal force. The OOR from concentric and heavy spot are seldom in the same place.

Therefore, and according to Nate, every tire needs to be true and subsequently balanced, first off the car and then checked out on the car. Seems his method does relieve the need for the HRF method.

Well, the plot thickens, and I expect the pace of this drama is about to slow down, unfortunately:

A Porsche forum friend has been doing an engine out on his 997TT - - the coolant fittings on the water-cooled Mezger motors come loose with age/heat and require an engine drop to reglue/pin. Anyway, I checked to see if I could borrow his wheels. As luck would have it, he's not got the car back together yet and invited me over to try.

His wheels are like-new aftermarket Fuchs-style rims in the OEM size with fresh rubber. He said they were 4lbs heavier than stock, but otherwise ran true and vibration free on his car (pic of his wheels on my car attached).

I put his wheels on and went for a drive:
- They felt harder/stiffer/heavier than my wheels - - the TPMS locked on and indicated about 5psi higher than I usually run, so maybe that was part of it
- The car felt a little more loose - - not sure if was tread, compound, pressure or weight, but they felt different - - subjectively worse (though not drastically so)
- And the vibration was pretty much the same as with my wheels. I logged it to make sure I wasn't crazy (see first log).

As before, the vibration on rough pavement was harder to notice - - the car just feels "sports car jiggly" - - but the logs show it's there (first order vibration around 18.5hz). But where the vibration continues to really stands out (and cycle) is on the smooth grooved pavement, which is also where the 2nd order vibration peaks (see second log).

As JavaDog suggests, something is vibrating with wheel speed - - but it appears to not be an issue with the wheels.....

Here's my current suspects:
- Problem with the internal concentricity of a stub axle (manufacturing defect)? That would make a axle "jump rope" like I witnessed
- One of the other 3 axles is bad - - I already replaced the RR, maybe the LR? I have a rebuilt spare ready to go.
- Bad shock or worn suspension - - it's letting a wheel bounce?
- Wheel bearing?
- Please God, not something in the transmission
- Other?

The question is, how do I trouble shoot without parts swapping?

1) Run can on jackstands (remember AWD). It's kind of sketchy, and the transmission makes a lot of noise (gear lash on unloaded drivetrain). If I do this with the car suspended at the jack points (safer), I also get suspension droop - - not sure how that impacts things. I could support the car under the control arm (seems less safe), but I can only do that with the wheels off due to the width of the wheels.

2) Try to measure the CV/axle runout at the hub, the axle, and the trans. I'm gonna have to get creative with a dial attachment - - my mag bases don't stick well to aluminum

3) Buy some Chassis Ears and give each corner a good listen. I have no idea what that will tell me. I've got the odd clunk in the front suspension over hard bumps that sounds like a weak/failing top mount or sway bar link. That's new info for this thread - - I've ignored it because it's on the front, the knock is infrequent, and showed up long after the vibration.

What to do?

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