Shake/Vibration when Towing

[Hipster]

FWIW, driveshaft was balanced 2 days ago.

I saw that

 

[Hipster]

With front end stuff you can look all you want. Until you get under there with a prybar and a hammer tapping on stuff you might not find a problem. Worn components can settle into the worn position and stick. As an example, I put 3 Moog idler arms on my truck in a 3 moth period. The third one has been on there 12 years. New can be problematic.

 

[Caman96]

Low miles since State inspection last fall. As pointed out earlier in thread, owner/tech who inspected it was so impressed with front end, he called over to other 2 working there and said “this is how a front is supposed to be”, then when paying for inspection he offered to buy truck. I also just went under there with pry bar, everything is solid. When not towing there is zero issues. Zero play in wheel and tracks perfectly straight..

 

[Caman96]

Have you checked pinion angle loaded and unloaded? May need a slight shim with your new springs.

Bare with me on this @Orpedcrow because I might as well know nothing about this. Very limited. So these are the angles I’ve come up with.
For the record, I used my iPhone 12 to get angles. I’ve also worked in multiple high end wood working shops and installing high end millwork for 44 years. I know how to assess angles and believe it or not at least the iPhone angle finder’s I’ve checked are dead on. The numbers are at the minimum relative to each other. Still this is what I’ve got as it’s sitting in driveway.
Transfer Case - neg. 11°
Driveshaft- neg. 7°
Pinion - neg. 5°

Transfer case to driveshaft is 4° difference.

Pinion to driveshaft is 3° difference.

Transfer case to pinion is 6° difference.

Reading up on it a bit, I’m getting some conflicting information so I’ll let you speak or any other.

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Disregard the black horizontal 0° line.

 

[Caman96]

I’m seeing pinion and output should be within 1/2°, other articles say within 1°-3°. Either way, my numbers don’t look good!

 

[Hipster]

I'll just say most of my knowledge is as it pertains to drag cars which is not remotely close. In those situations you want the pinion nose down from the trans output shaft about 3 degrees to account for loading the suspension on the line. If you have six degrees difference + any truck squat when loaded might be close to nine degrees and too much and cause u-joint binding or induce vibrations under load and that's where the vibes are coming from? The worn u-joint that got replaced during balancing the indicator.

 

[Orpedcrow]

I’d say that be your issue. You’re looking for a difference between the output (t case) and input (pinion) to be ideally within <1/2*. 1-3 is pretty typical and usually won’t see an affect.

I’d bet you’ll see an even greater delta with the truck loaded (causes the pinion to roll upwards)

Not questioning anyone’s abilities to pay attention when assembling something, but it’s possible (I do it every daggum time) that the pinion shims were installed backwards or not even at all.

 

[Orpedcrow]

The actual angles are pretty irrelevant, the difference between them is the concern. Crawl under and see if there’s a wedge shaped thingy between the spring pack and axle mount

 

[Caman96]

The actual angles are pretty irrelevant, the difference between them is the concern. Crawl under and see if there’s a wedge shaped thingy between the spring pack and axle mount

I don’t believe there is. I understand the angles are irrelevant, but you have to have some baseline.

Ok, I spent about 1/2 on the phone with driveshaft balancer. We went through all the measurements (driveshaft length, difference in output to pinion height difference…). He said everything was easily within spec. He also said it being a long bed with longer driveshaft, it’s not as critical, that those numbers affect short driveshafts way more. Then he asked diameter of driveshaft, it’s 3 1/2”. Again, he said that would be fine. He then asked if driveshaft was smaller diameter at ends, which it does step down to 2 1/2” at each u-joint, had conversations with his person who does calculations and said that is the cause of my issue. A driveshaft that didn’t reduce and was 3 1/2” would resolve.
So, now I’m really confused, isn’t a K1500 with 3.42 rated to tow 6,000 lbs.? That’s what my manual says, and I’m only towing 2,200 lbs. max.
I wish someone on this site could confirm their Z71 RCLB w/tow package had the same driveshaft as I do. What’s everyone’s thoughts.

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[Orpedcrow]

A driveshaft that didn’t reduce and was 3 1/2” would resolve.

I’m far from a physics genius, but the only thing I see that affecting is the resonant frequency of the driveshaft. 2 u-joints, in the proper phase alignment and in the same rotational plane, will cancel out their inherent resonance. I’m probably wrong but it makes sense to me that the bigger driveshaft would just mask the misalignment by changing the frequency of the u joints.