Diagnostics vib w speed

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340inabbody

340inabbody

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Symptom:

At about 40mph I am getting a small vibration that’s almost like a low frequency drone but its a vib. It’s like a sine wave peaking every 1-1.5 second. It’s not real noticeable. It doesn’t seem to increase in frequency with speed BUT I am not 100% sure yet. Right now it seems like a fixed low frequency vib.

What is a good method to start debugging this?

Just spun balanced all 4 tires. No dice.

What should I look at next?
 
Kendog 170 said:
U joints? Flex plate cracked or loose.
Click to expand...
Thanks Ken. I’ll see if I can rule out the u-joints as that’s a quick one. I probably will pull the drive shaft an have it ballanced as well to rule that out.

Process of elimination I suppose going from easy to hard. Flex plate will be difficult. I do recall inspecting it before installing it. It is a weighted unit. Meaning used for an external ballance for the 1973 340 motor which I believe is correct.
 
1WildRT said:
Watch this and consider your comment about a sine wave peeking every 1-1.5 seconds...

Click to expand...

Interesting. I was looking at the FSM last night and looking at this. They use a bubble level gauge. Looks very sensitive actually. I will have to educate myself on what to use to measure this and how to tune for this. If you guys have any suggestions please let me know. Looks like this is a good next step after making sure the ujoints are solid.
 
Is there any way to convert those bubble settings on their level to degrees inclination. I have a way to measure degrees but not this funny level bubble tool that probably only works for this application.

Also do the rear leaf springs affect the pinion/driveshaft angle?
 
Last edited:
I use one of these....
https://www.amazon.com/Digital-Angle-Finder-Magnetic-PT180/dp/B0CQNRSQ4R/?tag=fabo03-20

Yes springs can and do effect pinion angle...
Considering it's a slight vibration a 1-2 degree change will probably cure it...
Most Mopars need the nose to be rotated down slightly....

I've used these on a few cars with good results... Nice thing is you don't need to fully unbolt the axle to install them, just enough to get the thick end under the axle..

https://www.summitracing.com/parts/clv-2-degree?rrec=true
 
1WildRT said:
I use one of these....
https://www.amazon.com/Digital-Angle-Finder-Magnetic-PT180/dp/B0CQNRSQ4R/?tag=fabo03-20

Yes springs can and do effect pinion angle...
Considering it's a slight vibration a 1-2 degree change will probably cure it...
Most Mopars need the nose to be rotated down slightly....

I've used these on a few cars with good results... Nice thing is you don't need to fully unbolt the axle to install them, just enough to get the thick end under the axle..

https://www.summitracing.com/parts/clv-2-degree?rrec=true
Click to expand...
Thank you,
I am trying to understand how the springs can alter the pinion angle. Is this because it’s a unibody?


I will first replace my worn springs and then check the angles and vib.
 
Springs are the connection between the body and the axle... if the front segment of the spring flattens the nose of the pinion rises... there is a direct correlation...
 
Is this a recent build or a well used vehicle that developed a vibration.
If you haven't done anything to change the pinion angle, I'd start by seeing if the driveshaft u--joints gave grease fittings, lube them up.
If the joints don't have grease fittings, take the shaft out. And exersise the joints thru all angles looking for a tight spot, or binding.

I'd start there. Least time, least expense .

Good luck
 
Inertia said:
Is this a recent build or a well used vehicle that developed a vibration.
If you haven't done anything to change the pinion angle, I'd start by seeing if the driveshaft u--joints gave grease fittings, lube them up.
If the joints don't have grease fittings, take the shaft out. And exersise the joints thru all angles looking for a tight spot, or binding.

I'd start there. Least time, least expense .

Good luck
Click to expand...

That rhythmic vibration comes from pinion angle, not bad U joints... But, if the angle is on the edge of being a problem a bad U joint could amplify the issue...
 
Inertia said:
Is this a recent build or a well used vehicle that developed a vibration.
If you haven't done anything to change the pinion angle, I'd start by seeing if the driveshaft u--joints gave grease fittings, lube them up.
If the joints don't have grease fittings, take the shaft out. And exersise the joints thru all angles looking for a tight spot, or binding.

I'd start there. Least time, least expense .

Good luck
Click to expand...

The car (1970 Satellite) is kinda both. I’ve been working on it and haven’t run it much. Just now noticing the vib and its kinda subtle with headers lol.

I installed a 340 in place of the 318.
I just rebuilt the front end however. The rear end has the original suspension making it 55 years old. It lists very little and I have the rear suspension on my list to rebuild. Just haven’t had the money health or weather to get to it yet.

Looks like this is next. Then I can look at the drive shafts joints and angle and the same time. With new springs.
 
Last edited:
Would a bad/or wrong harmonic balancer, or a loose one ?? Or wrong one, a key way failing do this ?
 
340inabbody said:
Interesting. I was looking at the FSM last night and looking at this. They use a bubble level gauge. Looks very sensitive actually. I will have to educate myself on what to use to measure this and how to tune for this. If you guys have any suggestions please let me know. Looks like this is a good next step after making sure the ujoints are solid.
Click to expand...
I got this specifically for my pinion angle... worth it. My pinion angle was off by like 3* and the car would shake like a paint mixer over like 35...

P.S. in my experience pinion angle issues get worse the faster you go... so i don't think that's your issue.. especially if you haven't changed the rear suspension angles... BUT.. i could be wrong

 
340inabbody said:
What should I look at next?
Click to expand...
My car has a natural vibration at the same frequency at 55mph. Just enough to notice.
It does it not matter what rear gear is in it, nor matter what tires and no matter the pinion angle, etcetera. That vibration is related to the chassis, and after a few years of looking for it, I just accepted it.

but then, I got me an overdrive, and I don't drive 55 anymore, lol, so I quit caring. Cruizing at 100, she's smooth as glass.......... but the wind-noise is nearly horrendous, lol.

What I did was;
I went for a road test. I ran her up to 65mph, then shut her off. As she came down thru 55mph, there it was.
Next; I Jacked the car up, ran her up into the vibration zone; then started taking things off.
Wheels off, no difference. Drums off, no difference. Driveshaft off, no difference.

So, in my case, it was in the body.
Some years later, I read that it was a common characteristic of our classic A-bodies.
When I got to thinking bout it, I remembered that just about every A-body I ever had, 67 to 75, yes, they all did it.
My 1980 Volare (F-body) did not. Dads 84 New Yorker did not. And my 65 V100 wagon, being a distant memory, I couldn't remember.

I had a 69 Barracuda with a slanty/904 and super big tires on it, that on one 600 mile trip, just kept putting me to sleep at 55. I thought it was those tires. wou-wou-wou-wou down the road we went. 600 miles there and 600 miles home.
When we got back, I installed some nice rally wheels and regular radials on it, and there it still was. It hadn't been the tires....... Daymn those N-50s on adapters looked so good back on there.
I installed the new radials on the front.
Darts and Swingers, same
Dusters and Demons, same
Barracudas, same
wou-wou-wou ............... just noticeable.
 
Kendog 170 said:
Yes, but you can check your bolts easy enough. Did you use Loctite? My friend bought a 66 Dart years ago for $75. Engine had a rap. It was just a loose converter bolt. One ear snapped off the flex plate. We tightened and left it. lol
Click to expand...
Via the inspection opening?
 
AJ/FormS said:
My car has a natural vibration at the same frequency at 55mph. Just enough to notice.
It does it not matter what rear gear is in it, nor matter what tires and no matter the pinion angle, etcetera. That vibration is related to the chassis, and after a few years of looking for it, I just accepted it.

but then, I got me an overdrive, and I don't drive 55 anymore, lol, so I quit caring. Cruizing at 100, she's smooth as glass.......... but the wind-noise is nearly horrendous, lol.

What I did was;
I went for a road test. I ran her up to 65mph, then shut her off. As she came down thru 55mph, there it was.
Next; I Jacked the car up, ran her up into the vibration zone; then started taking things off.
Wheels off, no difference. Drums off, no difference. Driveshaft off, no difference.

So, in my case, it was in the body.
Some years later, I read that it was a common characteristic of our classic A-bodies.
When I got to thinking bout it, I remembered that just about every A-body I ever had, 67 to 75, yes, they all did it.
My 1980 Volare (F-body) did not. Dads 84 New Yorker did not. And my 65 V100 wagon, being a distant memory, I couldn't remember.

I had a 69 Barracuda with a slanty/904 and super big tires on it, that on one 600 mile trip, just kept putting me to sleep at 55. I thought it was those tires. wou-wou-wou-wou down the road we went. 600 miles there and 600 miles home.
When we got back, I installed some nice rally wheels and regular radials on it, and there it still was. It hadn't been the tires....... Daymn those N-50s on adapters looked so good back on there.
I installed the new radials on the front.
Darts and Swingers, same
Dusters and Demons, same
Barracudas, same
wou-wou-wou ............... just noticeable.
Click to expand...
Yikes Ive never seen you admit defeat however if the body has some kind of resonance not much you can do other than trial and error frame stiffeners. Dam Kerndog would kill me for that run on!
 
Welp I don’t know what this means…

IMG_7657.png


IMG_7665.jpeg
 
Last edited:

340inabbody said:
Symptom:

At about 40mph I am getting a small vibration that’s almost like a low frequency drone but its a vib. It’s like a sine wave peaking every 1-1.5 second. It’s not real noticeable. It doesn’t seem to increase in frequency with speed BUT I am not 100% sure yet. Right now it seems like a fixed low frequency vib.

What is a good method to start debugging this?

Just spun balanced all 4 tires. No dice.

What should I look at next?
Click to expand...
I had a driveline vibration in my '68. Checked and rechecked everything. Turns out the tailshaft bushing was too sloppy even though it and the pinion yolk were new. I ended up using a teflon coated bushing from a different manufacturer. Problem solved.

20240824_085609.jpg
 
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