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  #1  
Old 10-24-2021, 04:57 PM
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Tracking a floorboard vibration at ~40 mph

In my 1982 w123 300CD.

New tires, good, tight suspension, no steering wheel play.

Tires (15” bundts with 195/65r-15 Cooper CS5 Ultra) we’re roadforce balanced. One is showing 34# of roadforce balance, the rest are all 8-12#. Tires all roll smooth, the one that’s the worst is the drivers front tire, and there is minimal to no shimmy in the steering wheel. Car tracks slightly to the right, but the tiniest bit of adjustment keeps it straight and true.

Tires do not appear to be wearing funny.

At around 40 mph, I feel a rumble and vibration from the floorboard/seat. Minimal to none in the steering wheel. Harder to notice when at faster speeds. Sometimes more notable when decelerating.

Using a vibration meter, at 60MPH, and knowing the tire revolutions/mile, I can see that the third harmonic is as bad as any tire motion. Usually tire motion (14Hz) should be way higher.



This points to the driveline leading up to the diff.

So I took a look underneath this afternoon. Everything looks straight and perfect.

I had done subframe bushings last year:

Squeaky subframe bushing

Diff mount looks good:



As does the driveshaft:





As does the csb as far as I can tell. Can’t really shake the driveshaft and get any motion out of it.







I guess I could get it up while on the kwik lift and cause the wheels to spin, car in drive. Maybe have a helper take it to 40 or so and see. But what else should I be looking for? Is the best bet to just remove the driveshaft and check things from there, with it out and able to be moved independently?

Thanks!

__________________
Current Diesels:
1981 240D (73K)
1982 300CD (169k)
1985 190D (169k)
1991 350SD (113k)
1991 350SD (206k)
1991 300D (228k)
1993 300SD (291k)
1993 300D 2.5T (338k)
1996 Dodge Ram CTD (442k)
1996 Dodge Ram CTD (265k)

Past Diesels:
1983 300D (228K)
1985 300D (233K)
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  #2  
Old 10-24-2021, 05:22 PM
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I see both sides of the center mount bushing, and the rear driveshaft guibo/flex disc…
But I don’t see a pic of the front guibo…
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  #3  
Old 10-24-2021, 07:54 PM
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Try shimming the center support bearing away from the tunnel and see if it changes anything. I had a W124 start a vibration at 38 MPH felt in the drivers seat going away on the highway. A shim between the nose of the diff and the floorboard changed the pinion angle and the vibration completely stopped. W123 diff is set-up/suspended differently.

Good luck!!!
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  #4  
Old 10-24-2021, 08:08 PM
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Quote:
Originally Posted by vstech View Post
I see both sides of thecenter mount bushing, and the rear driveshaft guibo/flex disc…
But I don’t see a pic of the front guibo…
All are very boring. Original guibos in fine shape.







And the other one…



__________________
Current Diesels:
1981 240D (73K)
1982 300CD (169k)
1985 190D (169k)
1991 350SD (113k)
1991 350SD (206k)
1991 300D (228k)
1993 300SD (291k)
1993 300D 2.5T (338k)
1996 Dodge Ram CTD (442k)
1996 Dodge Ram CTD (265k)

Past Diesels:
1983 300D (228K)
1985 300D (233K)
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  #5  
Old 10-24-2021, 10:27 PM
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Quote:
Originally Posted by Sugar Bear View Post
Try shimming the center support bearing away from the tunnel and see if it changes anything. I had a W124 start a vibration at 38 MPH felt in the drivers seat going away on the highway. A shim between the nose of the diff and the floorboard changed the pinion angle and the vibration completely stopped. W123 diff is set-up/suspended differently.

Good luck!!!
Interesting. So, since my center bearing looks fine (it could be binding but nothing says it is), I suspect IF it is driveline related, it would be the u-joint.

Does your driveshaft have a u-joint? I’d suspect that in your case, maybe the slight change in angle caused the bearings to sit and roll slightly different. It seems like my best option is to pull the driveshaft (I’m assuming shimming would just be some washers on the bolts?), versus dropping the thing. But isn’t it prudent to check all the components for binding?
__________________
Current Diesels:
1981 240D (73K)
1982 300CD (169k)
1985 190D (169k)
1991 350SD (113k)
1991 350SD (206k)
1991 300D (228k)
1993 300SD (291k)
1993 300D 2.5T (338k)
1996 Dodge Ram CTD (442k)
1996 Dodge Ram CTD (265k)

Past Diesels:
1983 300D (228K)
1985 300D (233K)
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  #6  
Old 10-27-2021, 05:28 PM
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The fundamental looks to be below 10hz with 2nd and 3rd order harmonics occurring around 14 and 42 hz respectively. Provided that these measurements are accurate they are too low to be either tire or driveshaft related. I believe you are dealing with an aggregated suspension harmonic issue due to excessive play. Id check all joints for tightness and double check alignment.
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  #7  
Old 10-27-2021, 05:33 PM
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If you need a visual youtube heavy duty ford death wobble.
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  #8  
Old 10-27-2021, 06:16 PM
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The 3:1 ratio of frequencies of peak amplitudes (42:14) suggests that the driving force is in the driveline and an axle shaft is being driven, or contrariwise.
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  #9  
Old 10-27-2021, 07:48 PM
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Quote:
Originally Posted by JHZR2 View Post
Interesting. So, since my center bearing looks fine (it could be binding but nothing says it is), I suspect IF it is driveline related, it would be the u-joint.

Does your driveshaft have a u-joint? I’d suspect that in your case, maybe the slight change in angle caused the bearings to sit and roll slightly different. It seems like my best option is to pull the driveshaft (I’m assuming shimming would just be some washers on the bolts?), versus dropping the thing. But isn’t it prudent to check all the components for binding?
Yes it has an almost identical driveshaft set-up. I had the driveshaft rebuilt and balanced, it had zero effect. All of the vibration ceased when the nose of the pinion was shimmed downward.

Yes a washer as a shim between the body and the center support bearing hanger would work as a test.

Pull everything to check it? You could, but I'm not sure if it is worth it.

Good luck!!!
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  #10  
Old 10-28-2021, 01:06 AM
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Quote:
Originally Posted by R.Diesel View Post
The fundamental looks to be below 10hz with 2nd and 3rd order harmonics occurring around 14 and 42 hz respectively. Provided that these measurements are accurate they are too low to be either tire or driveshaft related. I believe you are dealing with an aggregated suspension harmonic issue due to excessive play. Id check all joints for tightness and double check alignment.
The tires are 195/65-15, and do 829 revolutions per mile. This at 60MPH, it’s one mile per minute, or 829 revolutions / 60s… thus 13.8Hz. 14Hz x 3 = 42.

So I’m not seeing how 14 and 42 become the 2nd and 3rd harmonic, but that’s just me and my cursory math.

The 34lb roadforce tire, though on the front axle and not causing steeering issues or wobble in the steering wheel, could be in play, as could anything else I guess…

It just seemed obvious to me that if a tire vibrated once per revolution, and a driveshaft spun three times per tire revolution, that the math works out…. And was a pointer.

Everything feels tight which is also what confuses me. No funny play or movement anywhere….
__________________
Current Diesels:
1981 240D (73K)
1982 300CD (169k)
1985 190D (169k)
1991 350SD (113k)
1991 350SD (206k)
1991 300D (228k)
1993 300SD (291k)
1993 300D 2.5T (338k)
1996 Dodge Ram CTD (442k)
1996 Dodge Ram CTD (265k)

Past Diesels:
1983 300D (228K)
1985 300D (233K)
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  #11  
Old 10-28-2021, 01:11 AM
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Quote:
Originally Posted by Frank Reiner View Post
The 3:1 ratio of frequencies of peak amplitudes (42:14) suggests that the driving force is in the driveline and an axle shaft is being driven, or contrariwise.
That was my thought, but I don’t get your comment about axle shaft. The axle shaft spins at the same rate as the tire. The driveshaft spins at roughly 3x.

What do you mean by “being driven” and “contrariwise”?

There is some slop in the differential, and I did get new axle washers.

I had this thread sometime back with some video and pics…

W123 Axles: What do I have and what is this play?
__________________
Current Diesels:
1981 240D (73K)
1982 300CD (169k)
1985 190D (169k)
1991 350SD (113k)
1991 350SD (206k)
1991 300D (228k)
1993 300SD (291k)
1993 300D 2.5T (338k)
1996 Dodge Ram CTD (442k)
1996 Dodge Ram CTD (265k)

Past Diesels:
1983 300D (228K)
1985 300D (233K)
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  #12  
Old 10-28-2021, 10:57 AM
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Location: Modesto CA
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Quote:
Originally Posted by JHZR2 View Post
That was my thought, but I don’t get your comment about axle shaft. The axle shaft spins at the same rate as the tire. The driveshaft spins at roughly 3x.
What do you mean by “being driven” and “contrariwise”?
There is some slop in the differential, and I did get new axle washers.
I had this thread sometime back with some video and pics…

W123 Axles: What do I have and what is this play?
A fault, that is the source of the vibration, could be in either the driveline (u-joint, center bearing, centering bushings, etc.), or in an axle shaft (bent shaft, bad CV-joint, loose splines). The other element, be it the axle shaft or the driveline, then may be vibrating in sympathy, at the ratio of the final drive - 3.07:1. The element which is at fault is the driver of the vibration, and the other element is being driven. The use of the word "contrariwise" was to indicate that each of the elements can be either driver or driven.
[Just as a curiosity, a 3.07 final drive has tooth counts of 15 & 46.]

The frequencies that have been observed are way below the first order (natural) resonant frequency of any of the driveline components, let alone the frequencies of 2nd or 3rd order harmonics.
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  #13  
Old 10-28-2021, 12:43 PM
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Quote:
Originally Posted by Frank Reiner View Post
A fault, that is the source of the vibration, could be in either the driveline (u-joint, center bearing, centering bushings, etc.), or in an axle shaft (bent shaft, bad CV-joint, loose splines). The other element, be it the axle shaft or the driveline, then may be vibrating in sympathy, at the ratio of the final drive - 3.07:1. The element which is at fault is the driver of the vibration, and the other element is being driven. The use of the word "contrariwise" was to indicate that each of the elements can be either driver or driven.
[Just as a curiosity, a 3.07 final drive has tooth counts of 15 & 46.]

The frequencies that have been observed are way below the first order (natural) resonant frequency of any of the driveline components, let alone the frequencies of 2nd or 3rd order harmonics.
Sorry, not understanding how the sympathetic vibration could be so high a magnitude compared to the primary if it was something past the differential. It’s typical to pick up that ~14Hz vibration on any vehicle, not just this one. I had used this vibration sensing app on other vehicles when tracing wheel-reared vibrations.

I get the driver or driven aspect now, thanks for that. It just seems like the relative magnitudes would be the indicator.

It could be that there is play in the diff, a beating someplace, the driveshaft, the one iffy tire… whatever. Question now is how to isolate and move stuff. I have a lift that puts the car 20”+ up in the air at the bottom of the tires. It’s a nice height for diy without a pro lift, the downside is to get the height it is roll-on, so I need to jack the rear on the lift somehow.

Thank you for your explanations!
__________________
Current Diesels:
1981 240D (73K)
1982 300CD (169k)
1985 190D (169k)
1991 350SD (113k)
1991 350SD (206k)
1991 300D (228k)
1993 300SD (291k)
1993 300D 2.5T (338k)
1996 Dodge Ram CTD (442k)
1996 Dodge Ram CTD (265k)

Past Diesels:
1983 300D (228K)
1985 300D (233K)
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  #14  
Old 10-28-2021, 02:01 PM
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I’m interested how this gets solved: I’ve got the same/similar problems with new-looking guibos and CSB. I expected to find a misalignment of the arrows on the driveshaft halves but they look correct.

I did isolate my vibration to driveline by putting the car in neutral at about 40mph; the vibration remained and slowed with speed irrespective of engine idle rpm.


Sent from my iPhone using Tapatalk
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  #15  
Old 10-28-2021, 08:11 PM
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2nd, and 3rd harmonic because I see a spike in the single digits which could be the fundamental or spurious.

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