Cyclic vibration / Looking to remove front drive shaft
05-06-2019, 01:11 PM
#1
Rennlist Member
Thread Starter
Cyclic vibration / Looking to remove front drive shaft
SYMPTOMS:
Despite repeated road force rebalancing and even a new set of rear tires, I continue to have an annoying cyclic vibration at various highway speeds. While driving ~75mph I get a vibration through the body of the car (NOT in the steering wheel) that cycles in intensity about once every second or so. There is another resonant frequency at ~95mph, but with quicker cycles. It visibly shakes the empty passenger seat -- shake..still...shake..still...shake...still... (and so on). This happens irrespective of gear or clutch in/out (i.e., it's not the engine); it does not make any noise and happens regardless of accelerating, decelerating, coasting, turning right, and turning left (which would seem to rule out CVs and wheel bearings).
BACKGROUND / WHAT I'VE DONE:
- 2007 997TT with ~52.5k miles. I've owned the car for 1.5 years, and only put ~2k miles on. No material mods other than a Sachs 2.5 clutch and Cobb tune. Suspension is stock and alignment is fresh and per factory spec.
- Tires are new MP4S and have been road force balanced by a credible race shop who said "All zeros, and no runout on the wheels". Pressures are factory.
- Rebuilt front axles (outer CV boots were torn) -- front CVs looked good, no play in axles; no vibration through the steering wheel
- New rear Zimmerman brake rotors (made no difference)
- Spun the rear wheels (and later just the hubs) in the air and checked for play in wheel bearings: No play, no noise, feels smooth except for some mechanical feel of the open differential spinning the other hub in reverse
SUSPECTS:
- Driveshaft / Cardan shaft / flex disc to front diff: There's both a guibo/flex disc and a universal joint on this shaft. While no one seems to report having issues, my symptoms seem potentially consistent with a driveshaft with a bad u-joint (or maybe flex disc, not sure) -- apparently worn driveshafts are broadly notorious for cyclic vibrations (not Porsche specific) I've not had eyes on this driveshaft yet, but this is currently my leading suspect.
- Transmission mount: It seems possible that a bad transmission mount/bushing might allow the nose of the trans to drop and mess up the driveline angle for the cardan shaft. I need to check / replace.
- Rear CVs/Axles: Maybe a bent axle or worn CV? Most of my research suggests that CVs tend to vibrate only under acceleration, deceleration, or turning not all states or when cruising, so I'm leaning away from this. And they seem tight, but maybe there's a bend in the axle itself? I don't know how to test without swapping parts.
- Wheel bearings: My checks for play seem good, and there's no howling or noise, even when vibrating, so it makes me think this is not the issue.
- New 964 RS Engine mounts: These add a small bit of NVH, but mostly at idle, and it's not cyclical or annoying. Maybe the engine mounts are changing/magnifying the resonance of something else (like the driveshaft or transmission mount)?
QUESTION:
- If I remove the driveshaft / cardan shaft, am I safe to drive the car? This would allow me to quickly determine whether the driveshaft, flex disk, and trans mount were all culprits. I presume I'll get a christmas tree on the dash without the driveshaft, but I don't see what I'd be hurting. I'm assuming I'll need to stuff a rag in the receiving spline on the front diff unit to keep dirt out while driving. Anyone done this? Thoughts?
- Tips on identifying a bad wheel bearing, CV, or bent axle?
- Any other tips/suggestions for tracking down a cyclic vibration?
Thanks for the help.
Despite repeated road force rebalancing and even a new set of rear tires, I continue to have an annoying cyclic vibration at various highway speeds. While driving ~75mph I get a vibration through the body of the car (NOT in the steering wheel) that cycles in intensity about once every second or so. There is another resonant frequency at ~95mph, but with quicker cycles. It visibly shakes the empty passenger seat -- shake..still...shake..still...shake...still... (and so on). This happens irrespective of gear or clutch in/out (i.e., it's not the engine); it does not make any noise and happens regardless of accelerating, decelerating, coasting, turning right, and turning left (which would seem to rule out CVs and wheel bearings).
BACKGROUND / WHAT I'VE DONE:
- 2007 997TT with ~52.5k miles. I've owned the car for 1.5 years, and only put ~2k miles on. No material mods other than a Sachs 2.5 clutch and Cobb tune. Suspension is stock and alignment is fresh and per factory spec.
- Tires are new MP4S and have been road force balanced by a credible race shop who said "All zeros, and no runout on the wheels". Pressures are factory.
- Rebuilt front axles (outer CV boots were torn) -- front CVs looked good, no play in axles; no vibration through the steering wheel
- New rear Zimmerman brake rotors (made no difference)
- Spun the rear wheels (and later just the hubs) in the air and checked for play in wheel bearings: No play, no noise, feels smooth except for some mechanical feel of the open differential spinning the other hub in reverse
SUSPECTS:
- Driveshaft / Cardan shaft / flex disc to front diff: There's both a guibo/flex disc and a universal joint on this shaft. While no one seems to report having issues, my symptoms seem potentially consistent with a driveshaft with a bad u-joint (or maybe flex disc, not sure) -- apparently worn driveshafts are broadly notorious for cyclic vibrations (not Porsche specific) I've not had eyes on this driveshaft yet, but this is currently my leading suspect.
- Transmission mount: It seems possible that a bad transmission mount/bushing might allow the nose of the trans to drop and mess up the driveline angle for the cardan shaft. I need to check / replace.
- Rear CVs/Axles: Maybe a bent axle or worn CV? Most of my research suggests that CVs tend to vibrate only under acceleration, deceleration, or turning not all states or when cruising, so I'm leaning away from this. And they seem tight, but maybe there's a bend in the axle itself? I don't know how to test without swapping parts.
- Wheel bearings: My checks for play seem good, and there's no howling or noise, even when vibrating, so it makes me think this is not the issue.
- New 964 RS Engine mounts: These add a small bit of NVH, but mostly at idle, and it's not cyclical or annoying. Maybe the engine mounts are changing/magnifying the resonance of something else (like the driveshaft or transmission mount)?
QUESTION:
- If I remove the driveshaft / cardan shaft, am I safe to drive the car? This would allow me to quickly determine whether the driveshaft, flex disk, and trans mount were all culprits. I presume I'll get a christmas tree on the dash without the driveshaft, but I don't see what I'd be hurting. I'm assuming I'll need to stuff a rag in the receiving spline on the front diff unit to keep dirt out while driving. Anyone done this? Thoughts?
- Tips on identifying a bad wheel bearing, CV, or bent axle?
- Any other tips/suggestions for tracking down a cyclic vibration?
Thanks for the help.
05-06-2019, 02:14 PM
#2
Three Wheelin'
Hey Brainz,
I too have a vibration at higher speeds and in addition a soft clicking type noise that is constant and wheel speed dependent. Also changed out all of the CV axle boots and did the front diff fluids. Closer inspection of my front driveshaft showed a FUBARd flex disc with many cracks. I ordered a new flex disc and trans mount and will be replacing those hopefully this week. The flex disc was bad enough to scare me into parking the car until addressing it. Porsche officially wants you to replace the entire driveshaft because it comes pre-balanced. Others have replaced just the flex disc and I haven’t read of any issues with that.
My prior research indicates that you can drive just fine with the Cardan shaft removed but your dash will likely light up, especially with any aggressive driving where the computer expects compensation from the front wheels.
I too have a vibration at higher speeds and in addition a soft clicking type noise that is constant and wheel speed dependent. Also changed out all of the CV axle boots and did the front diff fluids. Closer inspection of my front driveshaft showed a FUBARd flex disc with many cracks. I ordered a new flex disc and trans mount and will be replacing those hopefully this week. The flex disc was bad enough to scare me into parking the car until addressing it. Porsche officially wants you to replace the entire driveshaft because it comes pre-balanced. Others have replaced just the flex disc and I haven’t read of any issues with that.
My prior research indicates that you can drive just fine with the Cardan shaft removed but your dash will likely light up, especially with any aggressive driving where the computer expects compensation from the front wheels.
05-06-2019, 03:48 PM
#3
Rennlist Member
Thread Starter
Thanks, Jeebus -- that all makes sense.. I'd seen your note on the flex disc and look forward to hearing what the result of that and the trans mount is. When you do the flex disc, you should pull the whole shaft and check the u-joint for play and smooth motion.
Is your vibration cyclic or steady? Did you have any driveline clunking or noise that would have otherwise suggested the flex disc was shot?
Regarding your clicking, my research suggests that could be a CV (most likely), but could also be an early wheel bearing or even the u-joint on the driveshaft.
Is your vibration cyclic or steady? Did you have any driveline clunking or noise that would have otherwise suggested the flex disc was shot?
Regarding your clicking, my research suggests that could be a CV (most likely), but could also be an early wheel bearing or even the u-joint on the driveshaft.
05-06-2019, 05:56 PM
#4
Three Wheelin'
Thanks, Jeebus -- that all makes sense.. I'd seen your note on the flex disc and look forward to hearing what the result of that and the trans mount is. When you do the flex disc, you should pull the whole shaft and check the u-joint for play and smooth motion.
Is your vibration cyclic or steady? Did you have any driveline clunking or noise that would have otherwise suggested the flex disc was shot?
Regarding your clicking, my research suggests that could be a CV (most likely), but could also be an early wheel bearing or even the u-joint on the driveshaft.
Is your vibration cyclic or steady? Did you have any driveline clunking or noise that would have otherwise suggested the flex disc was shot?
Regarding your clicking, my research suggests that could be a CV (most likely), but could also be an early wheel bearing or even the u-joint on the driveshaft.
Thanks for the comment about clicking. I'm hoping it's NOT the CV since I paid someone $900 to ostensibly clean, inspect, and repack the joints. If they repacked a bad CV joint I will be unhappy. Could certainly be a wheel bearing, I guess time will tell. I'll sit on it for now and see if it gets worse. Doesn't seem to have any effect other than the soft clicking noise at this point.
I'll definitely take a bunch of photos once I get in there.
05-06-2019, 06:56 PM
#6
Burning Brakes
Hey Brainz,
I too have a vibration at higher speeds and in addition a soft clicking type noise that is constant and wheel speed dependent. Also changed out all of the CV axle boots and did the front diff fluids. Closer inspection of my front driveshaft showed a FUBARd flex disc with many cracks. I ordered a new flex disc and trans mount and will be replacing those hopefully this week. The flex disc was bad enough to scare me into parking the car until addressing it. Porsche officially wants you to replace the entire driveshaft because it comes pre-balanced. Others have replaced just the flex disc and I haven’t read of any issues with that.
My prior research indicates that you can drive just fine with the Cardan shaft removed but your dash will likely light up, especially with any aggressive driving where the computer expects compensation from the front wheels.
I too have a vibration at higher speeds and in addition a soft clicking type noise that is constant and wheel speed dependent. Also changed out all of the CV axle boots and did the front diff fluids. Closer inspection of my front driveshaft showed a FUBARd flex disc with many cracks. I ordered a new flex disc and trans mount and will be replacing those hopefully this week. The flex disc was bad enough to scare me into parking the car until addressing it. Porsche officially wants you to replace the entire driveshaft because it comes pre-balanced. Others have replaced just the flex disc and I haven’t read of any issues with that.
My prior research indicates that you can drive just fine with the Cardan shaft removed but your dash will likely light up, especially with any aggressive driving where the computer expects compensation from the front wheels.
If you replace the flex disc you can always have it re balanced if needed
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05-11-2019, 10:14 PM
#8
Three Wheelin'
Hey Brainz, I updated my DIY post on the 997 page after replacing the flex disc. It turns out that I DID have driveline clunking, which I originally mistook for the transmission noise. After replacement, the noise is 100% gone. The flex disc was totally shot.
Also, removing the driveshaft is relatively easy - once you remove the transmission mount bracket and the driveshaft bracket, and you remove the three bolts to the transmission output flange, you can push the driveshaft toward the differential to unseat it from the flange, and then it pulls right out very easily. I checked the U joint and it looked ok, and the differential housing bushing was good too. Let me know if you have any other q's I can answer, or if I can provide any help.
Also, removing the driveshaft is relatively easy - once you remove the transmission mount bracket and the driveshaft bracket, and you remove the three bolts to the transmission output flange, you can push the driveshaft toward the differential to unseat it from the flange, and then it pulls right out very easily. I checked the U joint and it looked ok, and the differential housing bushing was good too. Let me know if you have any other q's I can answer, or if I can provide any help.
05-18-2019, 11:14 PM
#10
Rennlist Member
Thread Starter
I got around to messing with the car today. I preemptively ordered a new guibo and trans mount. I'm jealous that Jeebus has access to a lift - - many hours of inching around under the car on one's back is quite tiring and I don't have room for a creeper.
Upon removal and inspection, the trans mount appears to have settled by at least an 1/8 inch, maybe 1/4 under load. And the guibo had one deep crack and some other shallow ones forming. They both probably had some miles left in them, but it's not worth redoing in the near future.
I pulled the drive shaft and checked the u joint for wear. It was fine - - smooth and no play. The spline looked brand new, though it had surprisingly little grease on it. I lightly lubed it up with the grease that BMW specs for their shaft-drive motorbikes.
There's also a bushing at the back of the extension on the front diff. It's not super stiff, but mine looked in good condition. The diff isn't terribly heavy. If anyone wants to replace this bushing, ideally you'll need a c-clamp press, as it's a lot more work to completely drop the front diff.
I took the car for a test drive without the driveshaft attached. No issues, though I didn't drive very aggressively as I didn't fully replace all the diff mount bolts as the plan was to put the shaft back in. As long as the front wheels are moving, there's no errors or issues with the front disconnected.
Unfortunately, I could still feel a vibration even without the driveshaft. It was reduced and very subtle, which is good. I attribute the improvement to the new trans mount.
I brought the car back and put it up on stands and ran it with the rear wheels in the air. The car freaked out initially and triggered the stability control and lit up the dash like Christmas, but after the 5 or 6 errors showed up, it runs fine with the front wheels stationary as all the nannies get turned off. A short drive with the wheels spinning will cancel the errors.
With the rear wheels in the air, my vibration issue appears to have become apparent: the right rear axle is "jump roping". I'm not sure if the CVs are damaged or if the axle itself is bent, but there is at least 1/8 inch of runout and it shook the whole wheel on that corner. The other side ran smoothly. So I'll be replacing the bad axle in the coming weeks. That's expensive, but I want it perfect.
I've got it all put back together with the driveshaft reconnected. I'll go for another test drive tomorrow just to see if the new guibo and reinstalled driveshaft feels different.
Stay tuned...
Upon removal and inspection, the trans mount appears to have settled by at least an 1/8 inch, maybe 1/4 under load. And the guibo had one deep crack and some other shallow ones forming. They both probably had some miles left in them, but it's not worth redoing in the near future.
I pulled the drive shaft and checked the u joint for wear. It was fine - - smooth and no play. The spline looked brand new, though it had surprisingly little grease on it. I lightly lubed it up with the grease that BMW specs for their shaft-drive motorbikes.
There's also a bushing at the back of the extension on the front diff. It's not super stiff, but mine looked in good condition. The diff isn't terribly heavy. If anyone wants to replace this bushing, ideally you'll need a c-clamp press, as it's a lot more work to completely drop the front diff.
I took the car for a test drive without the driveshaft attached. No issues, though I didn't drive very aggressively as I didn't fully replace all the diff mount bolts as the plan was to put the shaft back in. As long as the front wheels are moving, there's no errors or issues with the front disconnected.
Unfortunately, I could still feel a vibration even without the driveshaft. It was reduced and very subtle, which is good. I attribute the improvement to the new trans mount.
I brought the car back and put it up on stands and ran it with the rear wheels in the air. The car freaked out initially and triggered the stability control and lit up the dash like Christmas, but after the 5 or 6 errors showed up, it runs fine with the front wheels stationary as all the nannies get turned off. A short drive with the wheels spinning will cancel the errors.
With the rear wheels in the air, my vibration issue appears to have become apparent: the right rear axle is "jump roping". I'm not sure if the CVs are damaged or if the axle itself is bent, but there is at least 1/8 inch of runout and it shook the whole wheel on that corner. The other side ran smoothly. So I'll be replacing the bad axle in the coming weeks. That's expensive, but I want it perfect.
I've got it all put back together with the driveshaft reconnected. I'll go for another test drive tomorrow just to see if the new guibo and reinstalled driveshaft feels different.
Stay tuned...
05-19-2019, 03:04 PM
#11
Three Wheelin'
Good luck - hope it all goes well with the front drive shaft back in place. Interesting that the vibration you feel could be from a slightly bent axle. Would love to hear if there's a better test to evaluate the problem - with the suspension and the CV axles at the most extreme angles when running on jacks, maybe it would be better to stick a GoPro in the rear wheel well and point it at the axle, then go on a test drive? Just thinking out loud. New rear axles aren't cheap 
and I had a bad experience with "low mile used" axles from a breaker.
and I had a bad experience with "low mile used" axles from a breaker.
05-19-2019, 03:28 PM
#12
Rennlist Member
Thread Starter
Thanks for the wishes. Took it for a drive today with the driveshaft reconnected and it feels about the same as yesterday without the driveshaft. The good news, as stated previously, is that the vibration is considerably less noticeable - - trans mounts make a difference. Based on all the Uber cars I ride in (which all seem to vibrate, some badly), most probably wouldn't notice the minor vibration I'm now feeling.
I'm not excited to buy a rear axle, but based on what I observed yesterday when running on stands, the right rear has considerably more movement (with or without the wheels installed). I do get your point about extreme drive angles (not that it looked extreme), but I'd have guessed both sides would have acted the same and they didn't.
I could buy a dirt cheap (<$150) aftermarket axle for a Carrera and see how it runs/vibrates. But then you introduce another variable (questionable quality) and can't keep it installed (too weak), so you do the labor twice - - and frankly, as you know, it's getting too hot in Houston to spend my weekends in the garage.
As such, I'm just going to bite the bullet and install a new axle. If that fixes it, then job done and I trash the bad axle. But if it doesn't fix the vibration, then I'll rebuild the old axle and keep it as a spare, drink a beer and learn to live with a very mild vibration.
I'm not excited to buy a rear axle, but based on what I observed yesterday when running on stands, the right rear has considerably more movement (with or without the wheels installed). I do get your point about extreme drive angles (not that it looked extreme), but I'd have guessed both sides would have acted the same and they didn't.
I could buy a dirt cheap (<$150) aftermarket axle for a Carrera and see how it runs/vibrates. But then you introduce another variable (questionable quality) and can't keep it installed (too weak), so you do the labor twice - - and frankly, as you know, it's getting too hot in Houston to spend my weekends in the garage.
As such, I'm just going to bite the bullet and install a new axle. If that fixes it, then job done and I trash the bad axle. But if it doesn't fix the vibration, then I'll rebuild the old axle and keep it as a spare, drink a beer and learn to live with a very mild vibration.
09-18-2019, 12:21 AM
#14
Rennlist Member
Thread Starter
The short answer regarding identifying the root cause of my cyclic vibration is NO -- not definitively, but I'm 95% sure I've got a bad rear axle and/or maybe a bad rear wheel bearing. The key feature in my diagnosis is the right rear axle seems to "jump rope" rather than spin smoothly when the car is run on jack stands.
It got hot in Houston, and I had other projects to chase this summer. When it cools down, I'll give it another look and buy a new axle. For the moment, however, I've learned to live with a very minor annoyance at +/- 75 mph. I've had some [enthusiast] friends drive the car and they don't notice the vibration until I point it out -- it's subtle -- but once you know it's there, you notice it. I'll post back when I get around to it.
It got hot in Houston, and I had other projects to chase this summer. When it cools down, I'll give it another look and buy a new axle. For the moment, however, I've learned to live with a very minor annoyance at +/- 75 mph. I've had some [enthusiast] friends drive the car and they don't notice the vibration until I point it out -- it's subtle -- but once you know it's there, you notice it. I'll post back when I get around to it.
09-18-2019, 11:15 AM
#15
The short answer regarding identifying the root cause of my cyclic vibration is NO -- not definitively, but I'm 95% sure I've got a bad rear axle and/or maybe a bad rear wheel bearing. The key feature in my diagnosis is the right rear axle seems to "jump rope" rather than spin smoothly when the car is run on jack stands.
It got hot in Houston, and I had other projects to chase this summer. When it cools down, I'll give it another look and buy a new axle. For the moment, however, I've learned to live with a very minor annoyance at +/- 75 mph. I've had some [enthusiast] friends drive the car and they don't notice the vibration until I point it out -- it's subtle -- but once you know it's there, you notice it. I'll post back when I get around to it.
It got hot in Houston, and I had other projects to chase this summer. When it cools down, I'll give it another look and buy a new axle. For the moment, however, I've learned to live with a very minor annoyance at +/- 75 mph. I've had some [enthusiast] friends drive the car and they don't notice the vibration until I point it out -- it's subtle -- but once you know it's there, you notice it. I'll post back when I get around to it.
I know what you are talking about. Its like the little ding that only you notice and seems to be all you see.
Thanks for responding.