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Discussion Starter · #21 ·
What was the diameter, length (u-joint center to center), tube thickness, and material of the driveshaft that was installed?

For example:
A 3 inch diameter, 52 inch long .083 thick chromoly driveshaft may experience a "half critical speed" vibration at 3600 rpm (see chart below).
View attachment 445208
As I stated above its the same vibration thats been there for 15 years even after changing components in driveline.
 

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Mike - 74 Nova Baltimore, MD
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As I stated above its the same vibration thats been there for 15 years even after changing components in driveline.
My bad. I meant to ask.... What is the diameter, length (u-joint center to center), tube thickness, and material of the new driveshaft that is currently installed?
 

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Discussion Starter · #27 ·
I measured them a while ago and they were correct according to everything I have read. At the present time I do not have these figures.
My original post asked if a bent Axle or worn Axle bearings would cause my vibration only at 3500 in 4th gear
 

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I measured them a while ago and they were correct according to everything I have read. At the present time I do not have these figures.
My original post asked if a bent Axle or worn Axle bearings would cause my vibration only at 3500 in 4th gear
You would think a bent axle would cause problems at all speeds. I would jack the rear off the ground and put the car on jackstands and crank it up and let it idle in gear. If the axle is bent you will see it in the tire movement.


The thing about a vibrations is that it literally can be coming from any moving part in the drive line. Tires are always the first thing to check but it can be coming from anywhere.
 

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The pinion / driveshaft angles are said to be correct.
But no numbers were given, even though they were suggested as a concern.
The pinion /driveshaft angles can cause the problem you have lived with for 15 years.
I chased a driveline issue for years as well. I bought a magnetic digital angle finder and thought I had driveline angles perfect. Even wore out a set of ubolts changing out shims. Decided to do a leg of the power tour and loaded the trunk with some stuff, way more than I ever hauled normally and the rear just sagged too much so I threw on a set of air shocks. The point of this whole story was once I got home and emptied my trunk, I forgot to let the air down on the shocks and to my surprise, just that tiny change in pinion angle eliminated my vibration. I put the QA1's back on and tried to find that sweet spot by changing the shims yet again, but never quite got it perfect.
 

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What was the diameter, length (u-joint center to center), tube thickness, and material of the driveshaft that was installed?

For example:
A 3 inch diameter, 52 inch long .083 thick chromoly driveshaft may experience a "half critical speed" vibration at 3600 rpm (see chart below).
View attachment 445208
Good point!
 

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Discussion Starter · #31 ·
The thing that bothers me,is that its more speed critcal than RPM, I can leave it in 3rd gear and go up and down with the rpm with no issues but in fourth it always comes in at 3500 and out at about 4
 

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The thing that bothers me,is that its more speed critcal than RPM, I can leave it in 3rd gear and go up and down with the rpm with no issues but in fourth it always comes in at 3500 and out at about 4
That would imply it is likely a harmonic created by the drivetrain at that specific RPM and speed as a combination perfect for creating the issue. Do you have a friend (or your own spares) who could loan you shorter tire/wheel combination? See if that moves the RPM issue up and the speed down, then you likely would narrow it down to the rear end as the culprit. Thinking it's either a pinion/yoke or pinion angle issue regardless of what you've measured (this varies based on suspension components, tire and wheel combination as well as use of vehicle [drag/crisuer/etc.] and can be modified +/- 1-2° to seek drivetrain stability regardless of what the books or websites say). Sometimes the car wants what the car wants and you have to go with it regardless of what the books tell you... I would try the tire/wheel combo first then try a 2° shim to remove angle then swap from front to rear of your perch to add some angle and see what happens... Only doing the wheel and tire combo first then putting your old wheel/tire combo back on so you only make one change at a time. It may make it worse, or it may make it better. then, go with replacing parts but I certainly would start by checking that pinion bearing and yoke first... Even new, they are still only as good as the day they were machined and by the runout on the jig standard.... Then move on to the axles and bearings... One thing at a time... Science the F outta that thing!

Just my $0.02
 

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its more speed critcal than RPM
Is your trunk lid securely fastened and compressing the gasket enough? I saw where a door was causing a vibration throughout the cabin of a truck that nobody could pin down. Some old timer came up, grabbed the door and while shut with the window down, rocked the door in and out with about 3/4"-1" slop. He took the seatbelt, folded it up then slammed the door on it while taking it for a test drive. Vibration issue solved! Just another thought...
 

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Discussion Starter · #34 ·
That would imply it is likely a harmonic created by the drivetrain at that specific RPM and speed as a combination perfect for creating the issue. Do you have a friend (or your own spares) who could loan you shorter tire/wheel combination? See if that moves the RPM issue up and the speed down, then you likely would narrow it down to the rear end as the culprit. Thinking it's either a pinion/yoke or pinion angle issue regardless of what you've measured (this varies based on suspension components, tire and wheel combination as well as use of vehicle [drag/crisuer/etc.] and can be modified +/- 1-2° to seek drivetrain stability regardless of what the books or websites say). Sometimes the car wants what the car wants and you have to go with it regardless of what the books tell you... I would try the tire/wheel combo first then try a 2° shim to remove angle then swap from front to rear of your perch to add some angle and see what happens... Only doing the wheel and tire combo first then putting your old wheel/tire combo back on so you only make one change at a time. It may make it worse, or it may make it better. then, go with replacing parts but I certainly would start by checking that pinion bearing and yoke first... Even new, they are still only as good as the day they were machined and by the runout on the jig standard.... Then move on to the axles and bearings... One thing at a time... Science the F outta that thing!

Just my $0.02
All good perspective. Car original came with 14" and thats what was on it when it vibrated coming from wisconsin. After a few changes to driveline,Yokes,Shaft,Tranny,I still had the vibe so I went to 15". Still there. I have bought spring perch shims to try the angle thing but before doing that I was thinking maybe the axles were worn at the bearing surfaces
I'm now thinking I will look at the input pinion bearing at the same time I look at the axles
 

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1969 Nova . . 2dr . . Chino Valley,Az USA
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ronsnova - - all really good questions & answers , and one of those were : try too change (basically) ONE Item at a time : so you might narrow this cause down closer.
Keep looking and do-not rule out the small issues . I do agree that you can not "see" those rear axle bearings (going down th' road) . . and, even changing out the complete rear axle
could help narrow down your cause (but, who wants to go that far / or even have an extra rear laying around) .
I also thought : jacking up the rear - and putting it in gear - too look at your 'rear wheels , spinning' - was a great idea. (just be careful) .

To me , it is kind of weird , that it has gone on this long (and, only in 4th gear) (seems like a "vibration" - is usually not that limited ; too just one thing / gear / speed).
But, stranger things have & do happen - - keep asking those questions and keep us posted - - some really great help on this site .

jim
 

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Discussion Starter · #38 ·
ronsnova - - all really good questions & answers , and one of those were : try too change (basically) ONE Item at a time : so you might narrow this cause down closer.
Keep looking and do-not rule out the small issues . I do agree that you can not "see" those rear axle bearings (going down th' road) . . and, even changing out the complete rear axle
could help narrow down your cause (but, who wants to go that far / or even have an extra rear laying around) .
I also thought : jacking up the rear - and putting it in gear - too look at your 'rear wheels , spinning' - was a great idea. (just be careful) .

To me , it is kind of weird , that it has gone on this long (and, only in 4th gear) (seems like a "vibration" - is usually not that limited ; too just one thing / gear / speed).
But, stranger things have & do happen - - keep asking those questions and keep us posted - - some really great help on this site .

jim
Over the last 15 years that's exactly what I have done . Not necessarily in order from front to rear but have changed everything front the engine to my clutch,driveshaft and transmission plus wheels and tires. Not all at once,but one at a time. I'm now at the rear diff and drive axles as the last things I haven't changed. Wintertime project
 

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Mike - 74 Nova Baltimore, MD
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The thing that bothers me,is that its more speed critical than RPM, I can leave it in 3rd gear and go up and down with the rpm with no issues but in fourth it always comes in at 3500 and out at about 4
While leaving your car in 3rd gear only, can you drive at the same speed that the vibration occurs when you are in 4th gear?
  • If so, what happened?
 

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"I still have the vibration in fourth gear at 3500 RPM. Does not vibrate in 3rd gear at 3500 RPM."
Based on this statement, your vibration is related to speed, not engine rpm. I would first check the rear axle to make sure it is tracking straight and is square under the car. Maybe car was wrecked or damaged by previous owner. Maybe a 4 wheel alignment would check this, idk.
Next thing I would check is the pinion angle,again. You're convinced it's correct but provide no data as to what your car is currently set at. I would check this again. This would certainly cause this. Double check it.
Then I would pull the axles and have them checked to make sure one isn't bent, have a machine shop check them. Replace bearings regardless.
 
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