Rear driveline vibration

[72cs20]

I've been fighting an annoying driveline vibration for a few weeks now and have not nailed the cause yet.

1966 K20 with a sm465 w/married np205 and a 14 bolt rear axle.

Everything is smooth up to 50 mph, then vibration starts and goes to about 57 mph and then disappears by 60 mph. Happens under acceleration, deceleration and coasting.

I put the rear axle on jack stands and did a run. Vibration present 50-60 mph.

Pinion angle right within spec for a cv type driveshaft.

Driveshaft is new and all u-joints move freely but had it re-balanced. No change.

Wheels and tires removed. No change.

Axle shafts removed. No change.

Disconnected driveshaft from t-case. Vibration gone.

Rear diff is solid, no noises or looseness that I can tell.

However, there is movement at the t-case output yoke. If I grab the driveshaft and move it side to side or up and down, there is noticeable movement at the yoke and output shaft. I did replace the output shaft bearing, but no change. Is this movement normal for an np205?

Could sloppy t-case internals induce a vibration when a driveline load is connected but go away when disconnected?

Any thoughts? Thanks.

[57taskforce]

Based on the fact that it’s obviously harmonic (happens in a certain speed range) I’d be inclined to say maybe the pinion angle needs some fine tuning. I know you said it’s in spec and I believe you, but since you’ve replaced the rear end it’s always the first thing that is suspect to me followed by u joints. You might try some pinion angle shims to see if that helps, they are cheap enough that if it doesn’t work it’s not the end of the world. That’s where I’d start if it was me.

[mongocanfly]

Are the yokes phased correctly on the driveshaft?
A buddy of mine has been fighting a vibration in a station wagon he built, hes spent thousands of $$ trying to fix it..we all kept telling him we thought it was the trans, but he just kept throwing money at it, come to find out it was the trans, the main shaft was bent..chasing this stuff down can be very aggravating..
I scrolled thru your build but couldnt see the whole shaft in the installed position..nice truck btw..

[Zoomad75]

What are the actual drive line angles? If you have a cv on the front of the shaft your pinion should match the shaft angle. If both the shaft angle and pinion angle match at rest it may be the issue.

I know I just contradicted myself but hang with me. Under acceleration the pinion will try to “climb” up the ring gear. So when setting the pinion angle you might need to set the pinion angle one to two degrees below the shaft angle to allow for that natural action to happen and put the pinion angle where it needs to be dynamically.

What can happen if your pinion angle matches at rest is that under acceleration the pinion climbs higher than the shaft angle and now the u-joints are not cancelling each other out correctly.

It’s a balance for sure. But knowing where you are at now should help get to the adjustment that is needed.

The slop at the 205 output is not helping but depending on how long this vibration has been occurring the loose output may have been caused but the vibration from the incorrect drive line angles. I’ve seen it many times where improper drive line angles caused damage to transmission/t-case outputs as well as pinion bearings on the rear axle.
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[kwmech]

Any kind of a lift on this truck? Post up a couple of pics of truck stance and the offending driveshaft and angle. Ever pull the driveshaft apart at the slip yoke?

[72cs20]

Thank you all for replying!

My driveshaft angle is 10 degrees down. I initially set the pinion angle 10 degrees down with a two degree shim under the springs to match the driveshaft angle. I set the pinion like this because this truck does not see much, if at all, pinion rotation under load.

So then I changed the pinion angle to 8 degrees down to allow for a two degree pinion angle rotation under load, should that happen. No change in the vibration.

A long story short - I've adjusted the pinion angle to multiple points in relation to the driveshaft angle with no change in the vibration characteristics.

The yokes are phased correctly.

The only thing I've done so far that makes any change to the vibration was changing out the t-case output shaft bearing. When I noticed the play in the output shaft I swapped the bearing with one out of another 205 I have. This made the vibration worse. I then ordered a new bearing and installed it, getting me back to square one.

I would say this truck is maybe two inches higher than stock. I will try and get some good photos taken and post them up.

Thanks!

[mongocanfly]

Your pinion angle should up...not down
It should be on a imaginary parallel line with the output of the trans
I always try to shoot for 3deg down on my trans, and 3deg up on my pinion

[Richard]

Originally Posted by mongocanfly

Your pinion angle should up...not down
It should be on a imaginary parallel line with the output of the trans
I always try to shoot for 3deg down on my trans, and 3deg up on my pinion

Pretty sure from his explanation the pinion is rotated up inline with the drive shaft. As I tried to explain in another post, the angle is down though. If you draw a line through the t-case, follow the driveshaft and finally through the diff the line angles down the entire way.

[72cs20]

Here are a few pics. Visually the driveshaft is phased properly but when measured with a bubble level, it is not exactly true. The yokes are a full bubble off. I don't know if this would have an impact or cause a vibration. Not sure what the tolerances are for phasing.

[Richard]

Everything looks pretty good to me. Pretty easy to see if clocking the slip yoke one spline will get the yokes straight on, then test it.

[mongocanfly]

Looks like its 1 spline off to me

[kwmech]

Originally Posted by mongocanfly

Looks like its 1 spline off to me

Yep.............see post number 5......AND..... there is a big fat weight on the slip looking at the pic. Could be out 180 also

[mongocanfly]

Didnt even see the weight, but wow, i do now..that's a biggun...kws right you could be 180 + - out

[72cs20]

Gentlemen, thank you very much for your inputs! The problem is in the driveshaft and relates to your posts about clocking or phasing adjustment.

Looking from the axle to the t-case I clocked the slip yoke counter clockwise one spline from original position. Did a test run on the jack stands. This slightly worsened the vibration and started it at a lower speed. 47 mph vs 50 mph.

So then I clocked the slip yoke one spline clockwise from original position. Did a run. Vibration much reduced.

Clocked it one more spline clockwise(two splines from original position) and vibration all but gone. Test drive on the highway good.

Visually the shaft is completely out of whack, but at least the truck is drivable.

[mongocanfly]

Did you try flipping it 180deg?

[72cs20]

Originally Posted by mongocanfly

Did you try flipping it 180deg?

Not yet. I'll have more time tomorrow to experiment with it.

[duallyjams]

Good info on setting pinion angle https://4xshaft.com/blogs/general-te...veshaft-angles

[kwmech]

It's either 1 spline off or 180 degrees and 1 spline. When the shaft was built and balanced it was not welded together 2 teeth off to make it close. U-joints were in phase with each other. If it vibrates in either place you'll need to take to a shop and have it re-balanced or something is loose in the CV. Look at the slip where welded to the tube and closely at the sliding part. Most driveline shops will have a marker like a line, couple of dots, arrow etc. marking both pieces where it should be for instances like this. PLUS the pinion (in the 3rd pic) is too high. 1st pic looks correct. It needs to come down about 4 degrees or more (2'' ish). Under load, the pinion is going to climb higher than that. Pinion needs to point at the bottom of the t-case yoke so when it climbs it points directly at the center. Make sure the rear u-bolts are the correct size and torqued down to specs

[mongocanfly]

I was just gonna say look for a witness mark from the shaft builder, all mine have one
But kw beat me to it

[72cs20]

Originally Posted by kwmech

It's either 1 spline off or 180 degrees and 1 spline. When the shaft was built and balanced it was not welded together 2 teeth off to make it close. U-joints were in phase with each other. If it vibrates in either place you'll need to take to a shop and have it re-balanced or something is loose in the CV. Look at the slip where welded to the tube and closely at the sliding part. Most driveline shops will have a marker like a line, couple of dots, arrow etc. marking both pieces where it should be for instances like this. PLUS the pinion (in the 3rd pic) is too high. 1st pic looks correct. It needs to come down about 4 degrees or more (2'' ish). Under load, the pinion is going to climb higher than that. Pinion needs to point at the bottom of the t-case yoke so when it climbs it points directly at the center. Make sure the rear u-bolts are the correct size and torqued down to specs

There are no alignment marks on this driveshaft.

I did a lot of experimenting today and found that three splines over from the original position is the sweet spot, no vibration. Turning it 180 degrees results in vibration.

The pinion angle is good. The shaft is 10 degrees down and the pinion is 8 degrees down. This should allow for a 2 degree upward movement of the pinion, should there be any.

All the suspension components are properly sized and torqued to spec.

You are right. The bottom line is that I will be taking it back to the shop that built it so they can make it right.

I'll post back the results. Thanks everyone!

[Richard]

Good to see you are making progress. I kind of do not understand why setting pinion for higher load would be necessary or even important, unless binding would occur. Steady cruise is what? Maybe 70%-90% of the time. Why set pinion up for load that is 30%-10% of the time. Just throwing some numbers in there but I am sure you get the idea. IMO keeping within 1-2 degrees max will keep phasing close and is all that is needed.

[Zoomad75]

The action of the pinion climbing the ring gear is dynamic not static. If it’s at part throttle cruise and the pinion and shaft angle were set to the same angle static (for a cv shaft) there is still the possibility that slight load still has the pinion rising slightly higher than the shaft angle. There is a small window between being ok and not ok. 1-2 degrees is the standard. If you go an extra degree beyond is when the vibration comes in.


Back when I was dealing with class A Rv chassis I fought driveline issues on a regular basis. RV companies would stretch or shorten our chassis and use standards for class 8 over the road trucks for shaft angles over our engineering standards. If they were sometimes bad enough the vibration could be felt up and down the floor if the RV and typically wipe out the front pinion on a Dana S135 giant rear axle assembly. Usually it took a minor adjustment of 1-3 degrees to get it back in line.

Look at it this way, there is always some throttle input to maintain speed while driving. That’s changing pinion angle ever so slightly. 1-2 degrees is not a lot but it still keeps it within the hood range that it doesn’t vibrate while coasting either.
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[kwmech]

Any more on this vibration?

[72cs20]

Originally Posted by kwmech

Any more on this vibration?

Not yet. Just haven't had time to get down to the driveshaft shop. Shootin for this week.

[LONGHAIR]

I don't recall ever seeing a counterweight on the actual slip-yoke itself?
Maybe I'm just not remembering, it has been years since I did this regularly.
The double-cardan joint on the rear shaft, especially with such little driveline angle, seems odd too?