Pulsing vibration
 

I've had a pulsing vibration out of my 68 Gmc since I've gotten it. The vibration starts at 60 peaks at 65 and is gone at 70.

I've had 2 different engines (350&454) and transmissions (turbo 350 and a 700r4), 2 driveshafts both rebuilt prior to install, rear end rebuilt with different carrier, new rims and tires, and redid all brakes. I can't figure out what is causing vibration.

The vibration has stayed constant regardless of configurations the truck has been in.

Any help would be appreciated.

Re: Pulsing vibration
 

It's a harmonic vibration. More than likely it is the operating angle of your Ujoints where the transmission tail shaft meets the drive shaft, or where the drives shaft meets the rear pinion.

Tell us more about your truck.

Long bed or short?
Two piece drive shaft or one
Has it been lowered?
Have you replaced the ujoints, pinion bearing, center carrier bearing (if it's two piece?)

Harmonics (just the science of it) the vibration will start at 64mph. And as you've mentioned it will go away as you slow down or speed up.

It could also be the center carrier bearing has gone bad..

But let's start with the drive shaft and operating angles. Post up some pics if you can.. I'm sure others will chime in. We'll figure it out.

Re: Pulsing vibration
 

It is a swb and lowered 3/4. It has a 2 piece driveshaft. The center support bearing is new and different from old shaft with same vibration. The angles are 4° on trans and 5 on axle. I just ordered a shim for axle to see if that helps. The center support is lower than both trans an axle so the trans shaft points down to bearing then up to axle. Looks like a v. Angle on each shaft is 2.5°

The vibration is also constant regardless of throttle, rpm, or putting in neutral. It's speed dependant.

Re: Pulsing vibration
 

It's know as a beat frequency. Appears when two frequencies, that are close together, go in and out of phase.
You have done a good job of eliminating the motor. Consider jacking it up and running the truck up to 60 mph. It will help eliminate the front-end components.
If you can get someone else to bring the truck up to speed and watch, with a powerful light, from the outside. Possibly your can see something. Do this with jack on the differential and again with it on the frame. See if there is any differences you can feel or spot.
I don't think it's the driveline. Where is the second frequency from if it's coming from the driveline?

Re: Pulsing vibration
 

Lowering 3" front and 4" rear??

If you had a 1 piece DS, you would need to add angled shim (2 maybe 4 degrees) to the rear end to point the nose of pinion down some

But with the 2 piece you made the V angle more so not sure if you can resolve the vibe with the 2 piece DS

Re: Pulsing vibration
 

1 piece drive shaft and 3 degree pinion shim, would be your best bet on a 3/4 Static SWB in my opinion.

Is it a manual?

Re: Pulsing vibration
 

In the first post it's listed "turbo 350 and a 700r4" have been installed.

Re: Pulsing vibration
 

My bad. Sorry.

Re: Pulsing vibration
 

^^^^this

Re: Pulsing vibration
 

The vibration is still there on jack stands but not as bad. I ordered the shim for pinion angle and two new axle shafts. Fingers crossed that fixes it. I've also always been under the impression a 2 piece shift will have less angle issues that a 1 piece.

Re: Pulsing vibration
 

Check out this YouTube video on driveline and driveline induced vibrations.

https://www.youtube.com/watch?v=SH2VIoWOQ0E

Here is a web site on two piece drivelines;
https://www.motortrend.com/how-to/06...iveshaft-tech/
This site figures the first shaft, of a two piece, to be and extension of the transmission. Not always possible but a good start if it's possible on your ride.

Excellent postings from this site that deals with the two piece driveshaft angles:
http://67-72chevytrucks.com/vboard/s...d.php?t=220466

Re: Pulsing vibration
 

Update:
Fixed one of the vibrations with the pinion shim. Still have a slight constant vibration in the same speed but no where near as bad as it was when pulsing.

It'll be another week or two before the new axle shafts show up. Maybe that will take care of the rest. I have one shaft that is slightly pitted causing an axle leak so they need to be replaced either way.

Re: Pulsing vibration
 

if new axle shafts & bearings don't resolve the vibe...try a bigger shim (2 degrees more)

Re: Pulsing vibration
 

I replaced my rear shaft with a slip yoke one which helped after I lowered mine.
Also you said you put in a 700R4. Which when going from a 350 turbo to a 700R4 you have to shorten the front shaft bout 3ins. I didnt see any where you mentioning that you did that. If you didnt you are pushing in against the trans tail shaft. That can and will be an issue.

Re: Pulsing vibration
 

That's where the second shaft came from. I had it rebuilt and cut to length prior to swap. Shop also fixed weak links to help with higher torque of big block.

New axles will be here next Monday so hopefully have some answers soon.

Re: Pulsing vibration
 

As an update axle shafts have been replaced, no change.

Re: Pulsing vibration
 

While your pinion angle to motor centreline isn’t a problem, have you thought of the relationship between the pinion and centreline on the other plain.
Viewed from above is your pinion and motor centrelines parallel. If the motor sits crooked you can get strange vibrations.
A lot of people think the centre of the trans output shaft is supposed to be dead centre between the frame rails. It’s supposed to be offset to the passenger side about 3/4 to 1 inch.
Hopefully you understand what I’m saying.

Re: Pulsing vibration
 

A speed related vibration could be a tossed wheel weight or if theres drums back there they often have weights welded to them and they might fall off.

Re: Pulsing vibration
 

I do. I'll have to check. Motor mounts and trans mount are in original locations so it should be right. Only thing that has changed is the bolt locations front to rear on frame (has small and big block bolt holes from factory). Left to right has no adjustments.

Re: Pulsing vibration
 

Really digging in the dirt for the cause. I have included some more information on the driveline offset below. However, my intuition says the cause is more basic than this complicated driveline issue.

The vast majority of domestic OEM high-performance applications, be they GM, Ford, or Chrysler, feature rear-ends with the pinion offset to the passenger side of the vehicle to match the position of the engine and transmission. Each manufacturer has its reasons for doing this, from fitment of the steering column to providing the driver more room in the cabin. The tried-and-true Ford 9-inch is offset 15/16-inch, Ford’s 8.8-inch housing is offset 9/16-inch from the factory, and GM 10- and 12-bolts rears are offset 1/2-inch, for example.

https://www.dragzine.com/tech-storie...s-engineering/


I read where you jacked it up and noticed it wasn't as bad. Are the tires out of round? The modern balancers do what is called a road force calculation. Based on balance and out of roundness. Has this been done? If the tires are out of round you can see it with them turning while the truck is jacked up. Lastly consider running the motor, in gear, without the drive line installed. Might loose a little transmission oil but it would prove the source, driveline, differential or tires.

I am interested in if the vibration happens at the same speed in different gears. Say 60 in OD and 60mph in drive.

Another check it to travel at the vibration speed and with your left foot step on the brake while maintaining speed with the throttle. Only use enough brake pressure to activate the brake light, this will unlock the converter. However, it has no application if you haven't connected the converter wiring.

A properly phased driveshaft has the in-board yokes of the shaft in line with each other. I didn't mention this as every profession driveline shop knows this and would not make this mistake.

Re: Pulsing vibration
 

I'm following as I also am fighting a vibration. Sounds very similar. I eliminated tires, wheels and brake drums by running on jack stands with all these removed. Just don't touch the brakes. Also removed the rear portion of the driveshaft and it was super smooth, eliminating engine, trans and front portion of driveshaft. Shimming helped some but not gone.

Re: Pulsing vibration
 

Get rid of the two piece drive shaft. You're removing 2 of the 4 operating angles just swapping over to 1 piece.

Did the truck have the vibration before you lowered it? I'm betting it didn't. So by lowering it you have effectively changed the operating angle from the pinion to the carrier bearing only. The angle from the tail shaft to the carrier bearing shouldn't have changed. If you convert to 1 piece, get rid of the carrier bearing and mount then add a 3 degree shim you'll be very happy with it.

Talk to your local Rock Crawler fab shop about it. Ask them to make you a drive shaft for it, Spicer (servicable) UJoints, balance it for daily driving and you're good.

Every SWB I build is 1 piece.

What size shim did you put in it?

Re: Pulsing vibration
 

The vibration is speed dependant. Regardless of 3/4/neutral. Applying brakes doesn't change anything until out of speed range.

Tires have just been balanced on a normal balancer but at two different shops, no change.

Vibration is not in steering wheel more so shakes whole truck.

The biggest thing I don't get is at 55 it's very smooth then 60-70 is bad, 75 back to glass smooth.

Re: Pulsing vibration
 

I'm worried about dropping a bunch of money on a one piece after dropping as much as I have into the 2 piece that's on it now just to have the same vibration after.

I put a 2.5 degree shim in it and it fixed the pulsing part of vibration.

Vibration is very isolated to speed only from 60-70 everywhere else it drives very well for an old truck.

The truck wasn't running before I lowered it. Did a full restore on it myself. So I'm unsure how it drove before. Every part of truck is new which is why it's so frustrating to have a bad vibration at the average speed limit where I live.

Re: Pulsing vibration
 

I'm worried about dropping a bunch of money on a one piece after dropping as much as I have into the 2 piece that's on it now just to have the same vibration after. "Quote from above"

Point well taken. Borrow one. If you were closer I would let you try mine for an afternoon drive. We have a machine shop in town that will build you a nice shaft for around 350 bucks ready to slap in and go. Every truck that I have cut down gets the 3/5 static drop, new shaft and I go thru the brakes. Lucky so far, no vibrations. Also, no pinion angle shims. This leads me to believe that your problem may not be in the shaft. I would re-visit my rear end unit for premature bearing wear. Does this vibration have any whining or sound that accompanies it? One more thing, I would have a passenger ride in the bed, laying prone to see if this vibration transfers to the bed and if it is worse towards the rear or front. It will be simple when you do find it.