High speed vibration - give me some ideas
04-05-2011, 01:25 PM
#31
Burning Brakes
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For over a year I have had a vibration that can shake the whole car. Characteristics:
1. Starts almost always at 76 MPH. Below 76, smooth as glass. Peaks at about 80-90 MPH.
2. Erratic and can smooth out at 120 or so. Yeah, I know - just stay above 120 and be happy.
3. Can be subtle or rather pronounced shaking.
4. High frequency, like a fast flutter.
5. Seen with 3 different sets of wheels and tires.
6. Not felt through the steering wheel - Tie rods and balljoints renewed a couple of years ago. Upper A-arms replaced a few months ago with rebuits.
7. Replaced the torque converter bearings, although the old ones were tight - no change.
9. Examined TT - bearings snug, right location, no noise, left TT alone.
8. Checked all suspension bushings and wheel bearings.
9. Swapped rear axles side-to-side - no change. Axles are rebuilts put on 5 years ago. Seem fine.
10. I don't feel any handling issues related to the vibration.
This vibration seemed to appear soon after I renewed the transmission mounts. The mounts are properly positioned relative to the crossmember per the WSM.
It feels a lot like a rear wheel balance issues I have seen before. I did get new tires about the same time. I went back to the shop to have them check the balance and they found one rear wheel was off 3/4 ounce. I thought that was it, but there was no change and, as I note above, other sets of wheels and tires did nothing to change the vibration.
It does not appear to have gotten worse over time. I've driven probably 20K miles since it appeared.
I'm puzzled and not sure what to do or check next.
1. Starts almost always at 76 MPH. Below 76, smooth as glass. Peaks at about 80-90 MPH.
2. Erratic and can smooth out at 120 or so. Yeah, I know - just stay above 120 and be happy.
3. Can be subtle or rather pronounced shaking.
4. High frequency, like a fast flutter.
5. Seen with 3 different sets of wheels and tires.
6. Not felt through the steering wheel - Tie rods and balljoints renewed a couple of years ago. Upper A-arms replaced a few months ago with rebuits.
7. Replaced the torque converter bearings, although the old ones were tight - no change.
9. Examined TT - bearings snug, right location, no noise, left TT alone.
8. Checked all suspension bushings and wheel bearings.
9. Swapped rear axles side-to-side - no change. Axles are rebuilts put on 5 years ago. Seem fine.
10. I don't feel any handling issues related to the vibration.
This vibration seemed to appear soon after I renewed the transmission mounts. The mounts are properly positioned relative to the crossmember per the WSM.
It feels a lot like a rear wheel balance issues I have seen before. I did get new tires about the same time. I went back to the shop to have them check the balance and they found one rear wheel was off 3/4 ounce. I thought that was it, but there was no change and, as I note above, other sets of wheels and tires did nothing to change the vibration.
It does not appear to have gotten worse over time. I've driven probably 20K miles since it appeared.
I'm puzzled and not sure what to do or check next.
I put a new race torque converter in a different car before.It vibrated alot right after,could put my hand on the dash board and feel it bad even thruout the dashboard.Took the new race converter out put a stock rebuilt in and vibration went away.It ended up being a bad converter even thou new.They sent back another new converter and it was ok.Even new parts sometimes are bad.
04-05-2011, 01:32 PM
#32
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OK, that's at least three votes for road force balance. That would isolate it as well as/better than the dyno idea. I need new tires anyway, so, I'll look for a road force shop.
Jerry - the rear bearings are a good candidate. I did have one go out about 6 years ago and replaced both. A few weeks ago I took everything off the hubs and examined the motion of the bearings. They are firm and snug. I can't find anything wrong with them and I've seen some bad rear wheel bearings.
http://www.youtube.com/watch?v=GtDEvlzDhXU
I don't expect them to be as bad as that. You had some reason to expect them to be bad. Again, I'd like to have some evidence rather than throwing more new parts at this that aren't indicated. I've done these before.
Randy - I got tired or the every 2 year removal drill for CA smog. Also, I wasn't as impressed by it as I had been. After I sold it I discovered why - the manifold for the SC was cracked. Oh, well. Helped the buyer install it and now his car is a rocket.
Brian - it's road speed related. That, again, points strongly to the axles and beyond. I suppose it could be diff and beyond. So, tranny mounts need to be looked at, as suggested and since they seemed temporally related.
Jerry - the rear bearings are a good candidate. I did have one go out about 6 years ago and replaced both. A few weeks ago I took everything off the hubs and examined the motion of the bearings. They are firm and snug. I can't find anything wrong with them and I've seen some bad rear wheel bearings.
http://www.youtube.com/watch?v=GtDEvlzDhXU
I don't expect them to be as bad as that. You had some reason to expect them to be bad. Again, I'd like to have some evidence rather than throwing more new parts at this that aren't indicated. I've done these before.
Randy - I got tired or the every 2 year removal drill for CA smog. Also, I wasn't as impressed by it as I had been. After I sold it I discovered why - the manifold for the SC was cracked. Oh, well. Helped the buyer install it and now his car is a rocket.
Brian - it's road speed related. That, again, points strongly to the axles and beyond. I suppose it could be diff and beyond. So, tranny mounts need to be looked at, as suggested and since they seemed temporally related.
04-05-2011, 01:34 PM
#33
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What style trans mount?Stock or a solid type?
I put a new race torque converter in a different car before.It vibrated alot right after,could put my hand on the dash board and feel it bad even thruout the dashboard.Took the new race converter out put a stock rebuilt in and vibration went away.It ended up being a bad converter even thou new.They sent back another new converter and it was ok.Even new parts sometimes are bad.
I put a new race torque converter in a different car before.It vibrated alot right after,could put my hand on the dash board and feel it bad even thruout the dashboard.Took the new race converter out put a stock rebuilt in and vibration went away.It ended up being a bad converter even thou new.They sent back another new converter and it was ok.Even new parts sometimes are bad.
04-05-2011, 01:55 PM
#34
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wheel bearings will be a growl and they will make noise and howel. by turning rapidly , side to side, at high speed, the noise will change. that shows wheel bearing
vibration at speed is tires out of round ,or balance. the inner parts dont have enough mass to be out of balance. Heck, i ran the CV joint missing a bolt you and i broke when we did my transmission. smooth as silk, as my tires are balanced well.
3 sets of rims and tires. I think 3 sets of bad balanced rims and tires. want me to stop by and put on my set of rears?
I would take the rears only to a tire shop and have them balanced. it will be pretty cheap. if you are in the area, Ill do it for you for free at Wheel Works. these guys are awesome!
and no, a valve stem wont do it either. remember, at 150mph, the wheels are only turning 1800rpm. at 80mph, less than 1000rpm. thats pretty slow and valve cap cant do much.
vibration at speed is tires out of round ,or balance. the inner parts dont have enough mass to be out of balance. Heck, i ran the CV joint missing a bolt you and i broke when we did my transmission. smooth as silk, as my tires are balanced well.
3 sets of rims and tires. I think 3 sets of bad balanced rims and tires. want me to stop by and put on my set of rears?
I would take the rears only to a tire shop and have them balanced. it will be pretty cheap. if you are in the area, Ill do it for you for free at Wheel Works. these guys are awesome!
and no, a valve stem wont do it either.
remember, at 150mph, the wheels are only turning 1800rpm. at 80mph, less than 1000rpm. thats pretty slow and valve cap cant do much.
04-05-2011, 02:24 PM
#35
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Something I'll just toss out there (admittedly unlikely, but just something else to check)...
Is it possible that this is something that's loose is vibrating due to air flow? I've been surprised before on just how loud wind-driven vibration can be, especially if you happen to hit something's resonant frequency.
The only thing that comes to mind is belly pans.
Chances are, though, it's probably one of the things already mentioned earlier.
Do you know anyone who has an electronic vibration analyzer (EVA)? There's one on eBay right now.
Is it possible that this is something that's loose is vibrating due to air flow? I've been surprised before on just how loud wind-driven vibration can be, especially if you happen to hit something's resonant frequency.
The only thing that comes to mind is belly pans.
Chances are, though, it's probably one of the things already mentioned earlier.
Do you know anyone who has an electronic vibration analyzer (EVA)? There's one on eBay right now.
04-05-2011, 02:50 PM
#36
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Bill,
It sounds as though you have eliminated all the obvious options.
Whereas you are way too 928 street smart to be caught on the TBF hop, is there any possibility your flex plate clamp is out of the correct [zero pre-load] position? I presume you have it well locked whatever method you use and I assume there is no chance you are going to get caught out with the flex plate on the move.
When I had this problem the resulting vibration was very peaky at 3050 rpm [about 85 mph] but in a very narrow band. The vibe was not felt in the steering wheel. Interestingly I only noticed this in top gear and it would run out of it at higher rpm's- wheel balance issues do not generally run out- they just gets worse the faster you go.
Noting that you had a high power output with the s/c fitted, is there any chance it has knocked something out of true in the drive line?
Just a shot to nothing given the ground you have covered.
Best wishes
It sounds as though you have eliminated all the obvious options.
Whereas you are way too 928 street smart to be caught on the TBF hop, is there any possibility your flex plate clamp is out of the correct [zero pre-load] position? I presume you have it well locked whatever method you use and I assume there is no chance you are going to get caught out with the flex plate on the move.
When I had this problem the resulting vibration was very peaky at 3050 rpm [about 85 mph] but in a very narrow band. The vibe was not felt in the steering wheel. Interestingly I only noticed this in top gear and it would run out of it at higher rpm's- wheel balance issues do not generally run out- they just gets worse the faster you go.
Noting that you had a high power output with the s/c fitted, is there any chance it has knocked something out of true in the drive line?
Just a shot to nothing given the ground you have covered.
Best wishes
04-05-2011, 02:56 PM
#37
Drifting
Since it's not gear related, I think what your describing as a "high frequency", "fast flutter", is actually a low frequency vibration. Low frequency would likely be a wheel, axle, or tire noise.
With an audio recording of the noise, you could use a tone generator on your PC to determine the exact frequency being generated.
Have you checked the wheel alignment? Could be tire noise/vibration from improper rear toe. Also, what about shock absorbers? Combination of bad alignment and poor shocks could result in increased amplitude of the noise.
With an audio recording of the noise, you could use a tone generator on your PC to determine the exact frequency being generated.
Have you checked the wheel alignment? Could be tire noise/vibration from improper rear toe. Also, what about shock absorbers? Combination of bad alignment and poor shocks could result in increased amplitude of the noise.
04-05-2011, 03:08 PM
#38
Burning Brakes
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Since it's not gear related, I think what your describing as a "high frequency", "fast flutter", is actually a low frequency vibration. Low frequency would likely be a wheel, axle, or tire noise.
With an audio recording of the noise, you could use a tone generator on your PC to determine the exact frequency being generated.
Have you checked the wheel alignment? Could be tire noise/vibration from improper rear toe. Also, what about shock absorbers? Combination of bad alignment and poor shocks could result in increased amplitude of the noise.
With an audio recording of the noise, you could use a tone generator on your PC to determine the exact frequency being generated.
Have you checked the wheel alignment? Could be tire noise/vibration from improper rear toe. Also, what about shock absorbers? Combination of bad alignment and poor shocks could result in increased amplitude of the noise.
04-05-2011, 04:56 PM
#39
Race Car
I'll throw this out there. I had vibration issues in both my 928's for almost 10 years. Brand new wheels and tires would help, but the vibration would come back in a short time. Always happened around ~70mph. About 6 months ago, when I bought a new set of tires (same brand and size as last time) I had a 4-wheel alignment done at a much more up-scale place and actually watched them do it. The vibration is gone and has not returned. I guess my cars were not aligned properly and this would affect the way the tires wore as I drove, and pretty quickly caused a vibration issue.
Dan
'91 928GT S/C
475hp/460lb.ft
Dan
'91 928GT S/C
475hp/460lb.ft
04-05-2011, 04:57 PM
#40
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Something I'll just toss out there (admittedly unlikely, but just something else to check)...
Is it possible that this is something that's loose is vibrating due to air flow? I've been surprised before on just how loud wind-driven vibration can be, especially if you happen to hit something's resonant frequency.
The only thing that comes to mind is belly pans.
Is it possible that this is something that's loose is vibrating due to air flow? I've been surprised before on just how loud wind-driven vibration can be, especially if you happen to hit something's resonant frequency.
The only thing that comes to mind is belly pans.
This is one thing that I havad been beating around as well. There are times when I am bombing along and there is no vibration and then it appears. And the flutter frequency seems too fast for wheel imbalance. The cowling inside the nose is all busted up and could be vibrating in the wind, but I can't quite accept that it is shaking the whole car. My bellypans are in good shape (plastic front one was repaired) and are totally secured. Nothing loose.
04-05-2011, 05:01 PM
#41
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Bill,
It sounds as though you have eliminated all the obvious options.
Whereas you are way too 928 street smart to be caught on the TBF hop, is there any possibility your flex plate clamp is out of the correct [zero pre-load] position? I presume you have it well locked whatever method you use and I assume there is no chance you are going to get caught out with the flex plate on the move.
When I had this problem the resulting vibration was very peaky at 3050 rpm [about 85 mph] but in a very narrow band. The vibe was not felt in the steering wheel. Interestingly I only noticed this in top gear and it would run out of it at higher rpm's- wheel balance issues do not generally run out- they just gets worse the faster you go.
Noting that you had a high power output with the s/c fitted, is there any chance it has knocked something out of true in the drive line?
Just a shot to nothing given the ground you have covered.
Best wishes
It sounds as though you have eliminated all the obvious options.
Whereas you are way too 928 street smart to be caught on the TBF hop, is there any possibility your flex plate clamp is out of the correct [zero pre-load] position? I presume you have it well locked whatever method you use and I assume there is no chance you are going to get caught out with the flex plate on the move.
When I had this problem the resulting vibration was very peaky at 3050 rpm [about 85 mph] but in a very narrow band. The vibe was not felt in the steering wheel. Interestingly I only noticed this in top gear and it would run out of it at higher rpm's- wheel balance issues do not generally run out- they just gets worse the faster you go.
Noting that you had a high power output with the s/c fitted, is there any chance it has knocked something out of true in the drive line?
Just a shot to nothing given the ground you have covered.
Best wishes
04-05-2011, 05:03 PM
#42
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I did have higher speed vibration before plus a leaking rack and leaking oil pan.Took it to the Porsche dealer they put new tires on,they also tightened and repacked the wheel bearings and did a good 4 wheel alignment and the car was smoother riding than all the years I owned it.I have to say it was one dealer that I did go to that still did some good work on the 928.So between the rack and tires which were needed,tighteneing and packing the wheel bearing the car ran really smooth.
04-05-2011, 05:06 PM
#43
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vibration at speed is tires out of round ,or balance. the inner parts dont have enough mass to be out of balance. Heck, i ran the CV joint missing a bolt you and i broke when we did my transmission. smooth as silk, as my tires are balanced well.
3 sets of rims and tires. I think 3 sets of bad balanced rims and tires. want me to stop by and put on my set of rears?
I would take the rears only to a tire shop and have them balanced. it will be pretty cheap. if you are in the area, Ill do it for you for free at Wheel Works. these guys are awesome!
and no, a valve stem wont do it either. remember, at 150mph, the wheels are only turning 1800rpm. at 80mph, less than 1000rpm. thats pretty slow and valve cap cant do much.
I would take the rears only to a tire shop and have them balanced. it will be pretty cheap. if you are in the area, Ill do it for you for free at Wheel Works. these guys are awesome!
and no, a valve stem wont do it either.
remember, at 150mph, the wheels are only turning 1800rpm. at 80mph, less than 1000rpm. thats pretty slow and valve cap cant do much.
04-05-2011, 05:09 PM
#44
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I did have higher speed vibration before plus a leaking rack and leaking oil pan.Took it to the Porsche dealer they put new tires on,they also tightened and repacked the wheel bearings and did a good 4 wheel alignment and the car was smoother riding than all the years I owned it.I have to say it was one dealer that I did go to that still did some good work on the 928.So between the rack and tires which were needed,tighteneing and packing the wheel bearing the car ran really smooth.
04-05-2011, 05:12 PM
#45
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I'll throw this out there. I had vibration issues in both my 928's for almost 10 years. Brand new wheels and tires would help, but the vibration would come back in a short time. Always happened around ~70mph. About 6 months ago, when I bought a new set of tires (same brand and size as last time) I had a 4-wheel alignment done at a much more up-scale place and actually watched them do it. The vibration is gone and has not returned. I guess my cars were not aligned properly and this would affect the way the tires wore as I drove, and pretty quickly caused a vibration issue.
Dan
'91 928GT S/C 475hp/460lb.ft
Dan
'91 928GT S/C
475hp/460lb.ft