Driveline Vibration Problem

My 54 Chevy with 350 SB and 700R4 is giving me a fit with driveline vibration at 75-80 mph. The tailshaft and pinion angles are within a couple tenths of a degree as measured with a digital inclinometer. The driveshaft angle is about 1 degree. Less than optimal but that should not be a problem.

I have had the driveshaft checked for balance twice and my shop assures me it's fine. I've checked the tailshaft and pinion and driveshaft. The runout on all three is very minimal and should not be a problem. The engine, tranny and diff are all centered up between the frame rails, also.

The thing is pretty smooth at 65 mph but at 75-80 it goes nuts. Could this be a resonant frequency problem? If so, how do I change the natural frequency of my driveshaft? Is there something else I'm missing?

I'm looking for suggestions. Right now, I can't test it until next weekend but I'm hoping someone helps me come up with the answer before then.

Thanks
Rock

 

What kind of shape are the u joints in? Dry or worn u joints can cause vibration at certain speeds

 

You have covered all the things I could have thought of. How about wheels and tires? Borrow a set you know are good and give it a run.

 

Change the ring and pinion to a lower numerical ratio. This will lower the driveshaft speed at 75-80 and raise the vibration to 90 or more.

 

Here is a good discussion by some circle track guys that talks about drive shaft harmonics. If nothing else it's an interesting read.

http://www.midgetmadness.com/forums/index.php?showtopic=15280

 

This is rare but I've heard of axle shafts being forged rough enough to be slightly off balance.

A brake shoe dragging ?

Your pinion angle should be down not zero because it will rotate up as you load the drivetrain. Also if the car squats down at 80mph the pinion angle will change.

 

I've had it down by a degree. Same result. Trying for equal right now. I don't think this will change anything. Thanks.

 

Thanks. I'll read it tonight.

 

Thanks. I've checked the runout on the wheels and tires in both planes. New tires. It did it with the old ones, too. You cannot feel this in the steering wheel. It's definitely in the driveline.

Also, yes the u-joints are both new.

 

Drive live vibrations are typically happen between 40 & 50, tires and bent axle flanges usually start vibrations at 60-65 and up

 

So....here is a twist.

My old 55 f100 which was a very nice off frame truck would vibrate pretty good with the right window about half way up around 60 or so....vibration went away with it all the way down or all the way up....go figure.

I heard also that some of the recent mustangs would vibrate pretty good at certain speeds....the fix....brace off the floorboards or change their shape.

I am lucky with my vette...just put tires on it about 3 mo ago....radial TA's...coming back from a car show last fri nite, a smart aleck would slow to 60, when I would start to pass, back to 70 he would go...this went on for 3 times.....the last time, I called for back barrels to open some...no vibration at 85...smooth as silk.....and no smart aleck for at least 1/2 mile.

 

broken tranny mount?

 

What happens when its vibrating and you shift to neutral.. Does it go away,get worse/better ,stay the same ?? Trying to rule out convertor imbalance, or engine ,exhaust system "grounding "out on body or frame.. Sometimes tracking down a vibration is a sequence of proving what it Isn't...

A coupla hundred years ago ,when I worked in a dealership, we had a little gizmo that would read the frequency of the vibration.. then you calculated everything that rotated at that frequency or a harmonic thereof... Still , some vibrations was a process of elimination

 

Nope. It originally didn't have any bolts in it to secure the mount to the tranny. Vibration was worse after I put the bolts in. They are still in and I am shimming the tailshaft with washers to try to find a sweet spot.

 

Before I put the bolts in the tranny mount, it would get better when you shifted to neutral. Haven't been able to try it since but will. What do you suppose that points to?

 

A often overlooked part that requires balancing. Drums and rotors. I struggled for 2 years looking for a vibration at speed in a 88 Chevy Monte Carlo. We found a drum was not balanced from the factor. New drum installed and the car road like a magic carpet.

 

Check motor mounts. A friend of mine had an o/t sub that had a similarissue at 75 and up. Turned out to be a bad motormount. Just an ideal.

 

I bought a car once that had a vibration at 70mph. I went all through it, balanced the tires numerous times, checked u-joints and the like,etc. Finally noticed that the balance weight was missing off of the left rear brake drum. I put new rear brake drums on it, vibration gone.


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Must have been a 88 Monte Carlo. Check my post above.

 

X2 ^ 1st should be tires/wheels.

 

Ok. We have two guys that have suggested something I had not thought to check - brake drums! I will definitely take a look at that. Thanks!

 

"Before I put the bolts in the tranny mount, it would get better when you shifted to neutral. Haven't been able to try it since but will. What do you suppose that points to?"

This would not totally rule out convertor involvement,since the vibration did not go completely away... however check carefully that something is not "grounding out" under load...i.e. exhaust ,trans mount ,motormounts hitting frame,body etc under load


At the driveshaft yoke to pinion yoke connection , how do the u-joint cups fit the pinion yoke ? is it an exact fit ??
Any play side to side,etc... cups smaller than yoke saddles.. mismatched U/J's to yoke size/type..?

The suggestions about drums being unbalanced is a viable source you might need to check..but generally kicking it into neutral usually has little effect on that

 

I'll check for grounding again. The u joints are perfect plus have been replaced on both ends. . The vibration does not change when I pull it down from overdrive to drive. That sort of rules out the converter. When I checked it in neutral, it seemed to be better but that is subjective plus the tailshaft wasn't bolted down. I'm really suspicious of the brake drums now.

 

at speeds feel the vibration,
a drive line vibration most of the times fills faster than the speed of the tires,
If the vibs feel about the same speed as the tires, then its something that is rotating at the same speed.
If same as tires, put on some jack stands and remove tires and spin it up.
This is how we do it at our shop and have found more than one bad drum or rotor that was out of balance.
Make certain that even with out a wheel you install all the lug nuts to keep the drum from flying off.
If still vibrating, remove drums and try it again., just don't hit the brakes and blow out a wheel cylinder or thow it into park, you just turn off the ignition and let every thing slow to a stop .
Did you check your tail shaft and pinion angles with the weight on the axel? It will change when lifted on a rack

 

I looked at the drums this morning and they had no balancing weights on them. They also are pretty old but still had plenty of life in them. Since I could get a new pair for about $100, I changed them out with some fresh ones that do have balancing weights on them. As stated early in my post, I'm waiting on a brake fitting. When that sucker comes in, I'll test 'er out and let you know the outcome. My gut says this was the right call. It had not occurred to me that this could have been the problem all along. Thanks.

 

Here´s my 2cents:
If your U-joints are fine and your angles are fairly in the ballpark my money is on un-round tires. Are you running bias plys? I´ve seen as much as 1/8th " runout on brandnew ones. The larger the tire, the higher the speed will be were the viration sets in. Maybe you can find an old time tire shop with a tire shaving machine to have them rounded. Balancing won´t cure a out of round tire... I have built several cars over the years, and I only had this problem with bias plys. Shave them round and drive smoothly into the sunset...

 

Nope. Radials and the runout is good. It's almost impossible to find anyone with a tire trying machine any more. Thanks anyway.

 

I fought a vibration problem on an O/T car once, it turned out to be a weight had come off of a rear brake drum.......

 

This is a wild shot but... Did your driveshaft shop build your shaft or just check it? What diameter tube was used? All drive shafts do have a critical speed vibration frequency where they will vibrate. It is all about its natural vibration frequency which by manufacturer is reached above normal driving speed. The electronic devise spoke of in previous post is called an electronic vibration analizer. It measures vibration frequency And is a great diagnostic tool. Call around to some dealerships to see if they have one then be sure someone there knows how to tell you the results.

 

Barnsearcher you could be on to something.Most driveshaft shops can`t spin the shaft fast enough when balancing.Chased a problem like this in a friends car after a gear change.Shop checked the balance twice,Shaft was OK at the max speed they could balance to.A good used driveshaft cured the problem.