Need help with a strange cyclic car vibration

[brainz01]

My 2007 997 Turbo has an annoying vibration that I notice at speeds above 70mph. What’s strange is that the vibration is cyclic in intensity, alternating between a gentle shake and a stronger shake about once a second. Also, the vibration seems most noticeable when I’m driving on certain grooved stretches of road — Iike perhaps the grooves are contributing to a resonance.

The vibration is relatively mild — a non enthusiast might not notice — I feel it through the body of the car (not the steering wheel) and it visibly shakes the passenger seat (cyclically).

It’s a drivetrain vibration that’s speed dependent. Changing to a lower/higher gear and/or pressing the clutch doesn’t change it, nor does accelerating or decelerating. I don’t hear any noise, and driving in tight circles in either direction is quiet.

Things I’ve tried:
- New tires (MPS4S) that have been road force balanced three times, by different shops (no change)
- I pulled the drive shaft from the trans to the front diff to see if there was a bad u-joint (no change)
- New transmission mount and driveshaft guibo (no change)
- Played with tire pressures between 30psi to 50psi (no change)
- I can’t feel any play in the wheels or axles/CVs when checked by hand

I downloaded a vibration app last weekend and logged some readings. Each of the charts below was a 5 second log with me driving at ~85mph. Doing the math based on my tire size (305/30 R19) works out to about 18.5 wheel axle revolutions/second at that speed — and coincidentally, that’s my 1st order frequency. But I’m also getting 2nd, 3rd, 4th, and 8th(!) order vibrations too (with hints at the other missing orders too) which has got me scratching my head.

Also, FWIW, my engine RPMs for this test were around 2900rpm or 48hz — and interestingly there’s a peak (though smaller) for that.

Let me know what you think.





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[brainz01]

Here’s a few more vibration readings.

[sc_rufctr]

So you changed the transmission mount. What about the engine mounts?

[brainz01]

Engine mounts are new within the last year too: 964RS mounts which are stiffer than factory.

If it were engine mounts, I’d guess that the vibration would not be cyclic. And that it would be more engine speed dependent.


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[ninelevenick]

I know you didn't feel any play in the wheels, but don't rule out wheel bearing failure. I replaced a front wheel bearing on my 996 that was bad, but it still passed the old 9, 3 and 12, 6 wheel grab and shake test.

[brainz01]

Originally Posted by ninelevenick

I know you didn't feel any play in the wheels, but don't rule out wheel bearing failure. I replaced a front wheel bearing on my 996 that was bad, but it still passed the old 9, 3 and 12, 6 wheel grab and shake test.

Yeah, wheel bearing is my leading hypothesis. Maybe one is on it’s way out — has a pit in a race that’s giving all the multi-order vibrations?

What were your symptoms on your 996?

I’m kind of hoping an engineer/rotating-equipment mechanic will see my charts above and say “obvious wheel bearing issue”.

I forgot to mention. I also replaced brake rotors with new ones. No change.


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[Bill Douglas]

Wheel bent, tire not balanced?

[ninelevenick]

What were your symptoms on your 996?

I heard excessive road noise as if the tires were extremely worn. Additionally, I heard a constant wa wa wa sound like an out of balance tire.

[ninelevenick]

In your case, I'm thinking rear wheel bearing.

[LWJ]

No better idea here. I love this stuff on the forum. I bet you get a solution in a day.

[Chocaholic]

I vote for rear as well since no steering wheel feedback. A bearing would be the logical choice. How many miles are on the car?

[javadog]

Sounds like two vibrations are coming in and out of phase. I’d probably start by putting it on a rack with all four wheels off the ground, and get someone in the car to run it at the speed where you see the problem and walk around looking for any unusual signs. Wheel bearings usually do not cause vibration problems like this. You generally hear those going bad long before they get much play.

[rusnak]

maybe worn struts/ strut bushings. Actually you have at least two sources of vibrations. The lower frequency one(s) could be worn ball joints or worn rubber bushings.

[brainz01]

Some good suggestions and feedback, thank you.

The car has ~53k miles, but I've only owned it for the last 3k miles. The tires have maybe 1k miles on them. The old tires were worn out, noisy, and harsh - - so ironically, I think they masked the vibration. I really noticed the vibration after installing the new tires. After having the rears rebalanced twice, I had TireRack RMA the rear tires. My installer for the replacement tires is a well known race shop in town - - they said the new tires/wheels balanced up perfectly in the road force machine. The wheels are in nice condition and not obviously damaged but I've not specifically had them measured for run-out.

Some additional info I've been withholding so as not to overly bias the responses:

When I was changing the rotors (to rule them out as the vibration source), I ran the car on jack stands (at the jacking points with all 4 wheels in the air) with the tires/wheels mounted and not. The right rear wheel seems to have the most vibration irrespective of whether the tire/wheel was mounted. 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.

With the above additional information, Google searches suggested that bad CV joints could cause strange vibrations/axle alignment issues, so I thought maybe that was my problem. I bought a pair of low-mile used axles and rebuilt/greased/booted them. So far I swapped the right rear axle and there was no change. Maybe my issue is left axle (TBD), but I was pretty bummed when swapping the right one didn't fix it given the jump roping issue I observed above.

Regarding the "beating" vibration suggestion - - two vibrations that come in and out of phase causing the vibration to cycle - - that does make sense. I'm open to suggestions on how to test that. But it doesn't explain why the vibration is more pronounced on grooved pavement.

Google searches suggest that the wheel bearings on the water cooled Porsches (996/7s and Boxster/Caymans all share the same bearings) do fail. But most cite a noisy growling noise, which I'm not hearing. Many have reported that it's difficult to feel play until it gets really bad with the bearing type used. There's only a handful of reports of bearing issues without noise, but there were a few report that also had weird handling issues. A bad bearing in the front appears to be more obvious (and frequent). Rear failures seem harder to identify.

I've been thinking wheel bearings based on my vibration readings, but I don't want to blindly throw parts/time at it.

Suggestions appreciated.

[brainz01]

Originally Posted by rusnak

maybe worn struts/ strut bushings. Actually you have at least two sources of vibrations. The lower frequency one(s) could be worn ball joints or worn rubber bushings.

I've wondered about the suspension. It's original, so 13+ years old, but seems in good condition.

In order to replace the rear axle, I've undone many of the ball joints. They weren't obviously bad.

Maybe a failing rear shock? It's not oily or obviously damaged, but failures are not unheard of, though I wouldn't say I've noticed any handling issues.

The shocks on the 997 have electronic damping adjustment. I have an aftermarket suspension controller that can change the damping rate - - maybe I run with shocks on a firmer setting and see if that makes a difference?

[javadog]

If you see some motion that isn't perfectly concentric and round, time to mount a dial indicator and check for runout. Start with the wheel, then the brake disc, then the hub, etc.

For phase changes in vibrations, look to the other wheels. Two possibilities: two wheels of the same diameter turning at different speeds (like left and right side tires in a gentle turn) or two tires of slightly different diameters (worn vs. new, or front versus rear when different sizes are run front to back.)

I don't think this is down to the pavement. You have something that isn't round and probably also something that isn't balanced.

Make sure you look at the backsides of the wheels when they turn. Most runout from impact damage is on the backside.

[bkreigsr]

Are all four of your tread patterns orientated correctly?

Speed-related harmonic imbalance can occur when you have one tire with the 'outside' marking on the inside, or one of the 'directional arrows' is pointing the wrong direction.

...and a bunch of other stuff....

Bill K

[HardDrive]

While I think suspension/tire issue is most likely cause, I have to wonder about the stiffer motor mounts. Did the issue pre-date you installing the motor mounts?

[brainz01]

Originally Posted by javadog

If you see some motion that isn't perfectly concentric and round, time to mount a dial indicator and check for runout. Start with the wheel, then the brake disc, then the hub, etc.

For phase changes in vibrations, look to the other wheels. Two possibilities: two wheels of the same diameter turning at different speeds (like left and right side tires in a gentle turn) or two tires of slightly different diameters (worn vs. new, or front versus rear when different sizes are run front to back.)

I don't think this is down to the pavement. You have something that isn't round and probably also something that isn't balanced.

Make sure you look at the backsides of the wheels when they turn. Most runout from impact damage is on the backside.

I'll have to noodle on how to get a dial indicator positioned so that I can measure the wheels on the car. Open to all ideas - - I have indicators and mag bases. Brake disc and hubs seem more manageable to measure

Regarding rotational size difference - - I did sort of test that - - maybe. When I mentioned that I played with tire pressures, I did one side at 30psi and one side at 50psi. I expected maybe the cyclic rate would change as the tires now rotated in and out of phase at different speeds. But I sensed no difference in vibration.

[brainz01]

Originally Posted by bkreigsr

Are all four of your tread patterns orientated correctly?



Speed-related harmonic imbalance can occur when you have one tire with the 'outside' marking on the inside, or one of the 'directional arrows' is pointing the wrong direction.



...and a bunch of other stuff....



Bill K

Just checked. All outside edges on the outside. MPS4S are not directional.

Good idea, though. Reminds me: I once had an XC90 that was pulling slightly to the left under acceleration. Turned out it had a replacement tire on the front right that was a different brand. Car was used and I hadn't noticed when I bought it.