New rear driveshaft or rebuild stock one?

[77 K20]

So I've owned my truck now for something like 11 years. The whole time I've owned it I've had a vibration at around 65 mph. The sound it makes when it is vibrating "surges" in and out. This was the case when the truck was stock, and also after I lifted it 3" (rear has blocks).
Now with the somewhat new HT383 engine I've found that if I romp on it from a stop I get a horrible vibration in it until I back off. I'm guessing this is from the rear axle tilting the pinion up from the torque?

I've spent the last week or two reading up on driveshafts (not something I've had to mess with before) and I'm thinking I should get a double cardan shaft. Seems this should cure my issues?

I went to a driveline shop that is local and tried talking to them. Even though the truck was sitting in the parking lot they won't go out and look at it. They just said "take the driveshaft off and bring it in, then we'll talk about options and prices". Looking around online and in my Fourwheeler magazines I've always seen lots of Tom Wood's Custom driveshaft ads. Went to their website and seem like they build good stuff. I like the idea of a rubber boot over the slip yoke to keep dirt/crap out. I tried measuring everything for their quote request form. I probably won't get a response back for a week. I think I got the info right:
u joint out of NP203: 1310
u joint near 14 bolt FF: 1350
3.5" tube diameter
60" driveshaft length (yoke to yoke)

Anyone used Tom Woods? They claim 24 hour build time and 2 days shipping where I live.

Originally I thought about shimming the rear axle more or less, but that wouldn't fix the acceleration issue. Also from time to time I load it down with thousands of pounds of weight then the back sits several inches lower. This would throw off the angles again.

Am I on the right track with a double cardan shaft?

 

[PrairieDrifter]

When I had my differential go out I had a surging or slow pulsating vibration type sound but never really felt any vibration.

Have you tried replacing u joints? You could be getting too much axle wrap with those blocks if you have a beefier motor

 

[77 K20]

I checked my u joints today. They feel nice and solid with no slop in them. The rear differential was rebuilt a few years ago. And just before that I had new tires and wheels installed and that didn't make any change. Probably 10 years ago I put in a rebuilt transfer case and that didn't change anything either.

I've thought about getting rid of the blocks and going with a lift spring but I like the weight capacity and the ride my stock springs have. I then thought about the shackle flip style lift, but I do tow trailers and have heard there can be some stability issues with it, and then it would raise the pinion even more that what I have now. Might make the vibration even worse.

Just found the whole thing a bit odd because I didn't have vibration issues with previous squares (they were all 1/2 tons) and I had 4" blocks on one of them anyway.

 

[87scotty]

What about shackle flip that way you can keep stock springs and get them blocks out I have also heard good things about the double cardans

 

[Keith Seymore]

A vibration that "surges" or "pulses" is called a "Boom/Beat".

It occurs when there are two vibrations whose resonant frequency is close together but not exactly on top of each other. The pulsing is a result of the amplitudes adding together and subtracting as they go in and out of phase.

Band nerds have experienced this as they tune their instruments (so I'm told ); eliminating the pulsing ensures you are perfectly in tune.

So - bottom line - there are two things going on and if you kill one of them you get rid of the boom/beat.

I'd probably just have a new shaft made, since you've (apparently) always had the problem and since they are not that expensive.

Also - you should know what your u joint working angles are or you are just shooting in the dark. They need to be around 3 to 7 degrees or you will have problems like you have described. That is something that can be fixed for "free".

K

 

[77 K20]

Is it normal for the 3/4 ton driveshaft to have different series u joints on both sides of it? I'm wondering why have a big heavy duty series one near the differential and a much lighter version near the transfer case.
Just trying to figure out if this was stock or did a previous owner change something?

I'll need to buy one of those protractors so I can measure driveline angles. I know the transfer case comes out straight and the pinion is pointed up to the transfer case, but don't know the exact angle.

 

[77 K20]

So I got a quote back from Tom Woods drivelines. $469 for a new driveshaft with a 1350 double cardan joint at the transfer case, and a 1350 u joint near the differential.
The yoke out of the NP203 would need to be replaced with a flange. Additional $85. New seal would be needed also for $15. Then $10 for packaging and shipping.

So I'm sure this would be a lot stronger, but then if I ever stressed the system what would break? Transfer case would be my bet...

 

[77 K20]

Found some info on u joint strength:

1310: 1,600 ft/lbs
1350: 2,200 ft/lbs

 

[87scotty]

I suppose but its not just making it stronger it allows you to run more angle and less vibration

 

[77 K20]

They say the 1350 CV joint can flex between 20 and 30 degrees before binding. Having the CV joint there will cancel out the vibrations- as long as my pinion angle is pointed right at the transfer case output.

 

[Keith Seymore]

Having the CV joint there will cancel out the vibrations- as long as my pinion angle is pointed right at the transfer case output.

And as long as your problem is being caused by the U joints.

Driveline imbalance is a possibility, too, which we haven't discussed yet and which a new shaft would likely address.

K

 

[PrairieDrifter]

I was going to mention driveline balance, but he had stated they haven't really been touched since he has owned it so I figured it was good

Basically if your u joints were ever replaced, who ever did it may not have installed the yoke on the driveshaft in its original position, which would cause a vibration

 

[77 K20]

So I got the new Tom Wood's driveshaft and installed it a few days ago. The vibration from accelerating from a dead stop is 95% gone, and the vibration while cruising at 65 is almost all the way gone also.
It is much much better, but not completely gone. Not sure what it could be at this point- but it is livable. The new driveshaft is much stronger and was well built.

 

[PrairieDrifter]

Getting your tires balanced would be the next thing

 

[Wildjoe330]

So I got a quote back from Tom Woods drivelines. $469 for a new driveshaft with a 1350 double cardan joint at the transfer case, and a 1350 u joint near the differential.
The yoke out of the NP203 would need to be replaced with a flange. Additional $85. New seal would be needed also for $15. Then $10 for packaging and shipping.

So I'm sure this would be a lot stronger, but then if I ever stressed the system what would break? Transfer case would be my bet...

I bet you have a 203, I am going to the 205 soon I bet my
203 (past full time worn out Chain) vibration goes away. It's going to cost
less than the driveline except for adding over drive in a 700r (core) trans.
that came with my 205 and adapter. I will probably trade in my 700r
for a rebuilt one now that will bring it up $500.

I don't think the 203 chain is getting the lubrication that it
should be getting. every 200 miles run it 10 in full time
to oil it.