2nd Gen Non-Engine/Transmission 2 piece to 1 piece rear driveline

[JGheen]

Hey guys,



Just wanted to get your opinion on going from the stock 2 piece rear driveline in my '98 3500 4x4 to a 1 piece 77" long driveline. I want to get rid of the carrier bearing and 2 piece shaft as it is just a hassle for me and I think a 1 piece is much more practical for my application and use of the truck.

Another couple questions I have are, the driveline place can make this driveline in regular steel or even aluminum. Do you guys think I would take a chance in twisting or breaking that aluminum driveline with the mighty Cummins? What about the steel shaft?

Also, if I went with the steel shaft, do you think the weight of the shaft would end up putting any unnecessary stress on the rear output shaft of the transfer case splines or bearings? I doubt there would be any issues with the pinion bearings on the axle but I was curious about the transfer case side.

So, what do you guys think?



Thanks, Joe

 

[RustyJC]

One reason for the stock 2-piece shaft is that as the shaft gets longer, the torsional and lateral critical (harmonic) vibration frequencies get lower. You could well tune a significant critical vibration frequency right into the truck's operating range. This could produce driveline vibration problems or even driveline failures. Without performing a torsional and lateral vibration analysis, there's no way of determining whether or not this would be a problem.



Rusty

 

[drrhoades]

First why do you want to change to a one piece driveshaft? Just curious.



I would go with a steel driveshaft to handle the torque. Next like Rusty said you may open a whole can of worms as far as vibrations go. Remeber they put a 2 piece driveshaft in for a reason. If you have to do it go ahead and spend the money and see what happens, but that will be alot of stress on the transfer case. Remember there is a dampner at the front of the driveshaft for a reason.



Good luck



Doug

 

[JGheen]

First why do you want to change to a one piece driveshaft? Just curious.



Remember there is a dampner at the front of the driveshaft for a reason.



Good luck



Doug

The reason I want to go to a 1 piece is to get rid of the wear prone carrier as well as driveline angles.



What dampner at the front of the driveshaft? There isn't one on my rig. Do you have a pic of this dampner?



Joe

 

[drrhoades]

Sorry I dont have a pic. Mine is a 2500 though. I understand the driveline angles that would go along with your lift. Have you thought about lowering the centerbearing to help with the angles. Remember when you do this your angles have to cancel otherwise you will have vibrations.

 

[JGheen]

Sorry I dont have a pic. Mine is a 2500 though. I understand the driveline angles that would go along with your lift. Have you thought about lowering the centerbearing to help with the angles. Remember when you do this your angles have to cancel otherwise you will have vibrations.

Yeah, I was just out there again lookin' at the driveline and there is no dampner at the front of it as you described.

My driveline angles are not too bad the one off the transfer case has not changed as I have not lowered the center bearing. But the ones after the center bearing and at the axle have, but I have them at the same angles. The one at the center bearing is the same as the one at the axle, so that wouldn't really cause a vibration as it is no where near an angle it would bind up at. I believe the angle is around 3-5*.

I think I found out why I had a very mild vibration. The vibration was only at my top highway speed of about 70 and it was very mild, but after I inspected the driveline again last night, I found that the grease fitting on the axle u-joint was missing! The dealer had installed all new u-joints about a little less than a year ago and I didn't realize they were the greaseable kind until last night. The dealer service manager had told me the non-greaseable ones were installed at the time, so I never bothered to get under the truck and look.

But after I discovered the fitting missing, I went and got another, installed it and pumped the joint full of grease. A good amount of black grease came out as I pumped new stuff in, but the grease smelled a little burnt, so I hope everything is Ok with that u-joint. As I drove to work this morning I mysteriously didn't feel the vibration again, so i think I may have fixed it. Who knows, if the driveline flys out on a road trip one day then I know it was not fine.

 

[SS]

Joe,



I think I may be tempted to try a 1-piece driveshaft in your situation also. The cost of a quality shop to make one may not be worth it though (please don't make it out of well pipe! ) Personally, I would go with steel, but there are some aluminum shafts out there that would probably be fine. I would also tend towards the stock diameter and thickness (fairly large diameter and relatively thin material) to keep the weight down with the ability to handle torque. On short Jeep driveshafts I have used 1/4" thick DOM, but would not in this case. The transfer case would not concern me since 1-piece shafts are available from the factory on slightly shorter wheelbases. I'm not sure that the rear axle pinion angle is correct for a 1-piece shaft, so that would be something to check and adjust as necessary. Without a damper you could end up with some unwanted driveline moans at different speeds - won't know until you try it. And lastly, if you do this, please try to keep your speeds near the speed limit. Good luck with your plans!

 

[supersonic]

I wouldn't do it. When I changed my 3500 c&c to 4 wheel drive, I replaced the two piece driveshaft with a one piece. It's not the cost that bothered me, and its not the strength. If you build it right strength won't be a problem (ie: our 1,300 hp pulling truck uses a 76" one piece shaft and weve never had any trouble). But on my dodge the harmonic vibrations will drive you nuts, especially all through 4th gear. A one piece shaft that long is also very difficult to balance correctly, and can cause pinion shaft bearing and seal problems down the road. Don't believe me? I can show you a pile of junk parts and a bill for 1,800. 00 worth of rearend fixes in the last year, all caused by harmonic vibrations and out of balance conditions. Besides the sound will drive you out of your seat, sounds like the world is coming to an end.

 

[JGheen]

Joe,



The transfer case would not concern me since 1-piece shafts are available from the factory on slightly shorter wheelbases. I'm not sure that the rear axle pinion angle is correct for a 1-piece shaft, so that would be something to check and adjust as necessary. Without a damper you could end up with some unwanted driveline moans at different speeds - won't know until you try it. And lastly, if you do this, please try to keep your speeds near the speed limit. Good luck with your plans!

Thanks for that advice and opinion! I hear ya on the transfer case side of things. I have decided if I do go to the 1 piece, then steel would be the ideal way to go. I can adjust my rear pinion angle as necessary with my adjustable traction bars, so that is no issue.

Do you or anyone have a pic or a diagram of what this DL dampner would look like. I can see the possible need for it with a 1 piece shaft.

And don't worry, I like to only go as fast as my tires will let me and with 37" tires that tops out at about 70 MPH. After that I take a chance in loosing control and who knows what else. I accelerate fast but when it comes to top speed I kinda steer clear.



Keep the advice and opinions coming, guys, I need all I can get for this idea of what to do with my rear driveline.



Thanks again, Joe

 

[JGheen]

Don't believe me? .

Of course I believe you, that's why I'm asking.

I definately see what you are getting at. But I wonder if one could install a vibration damper on the shaft at about 4" or so from the pinion u-joint and absorb some of those nasty harmonic vibrations?



I take it you went back to the 2 piece shaft, but how many times did you go through pinion parts? How long was that 1 piece driveline and why was it difficult to balance? I talked to a couple of driveline places and they said they have sold them longer than that and can balance them almost perfectly(nothin's perfect), so do the vibrations through the engine/transmission/transfer case cause the end of it to just wanna fish tail right off the pinion yoke?

 

[RustyJC]

Torsional vibration has nothing to do with balance and everything to do with the natural resonant frequencies of the driveshaft. These natural resonant (critical) frequencies become lower as the driveshaft becomes longer. Therefore, a long 1 piece driveshaft may well exhibit torsional vibration problems that weren't present in the 2 piece design because the engineers had tuned the criticals in the 2 piece design above the operating range of the stock driveshafts.



Lateral criticals can excite lateral vibration with minimal levels of imbalance - operation at or near the critical frequency means that the energy transmitted through the driveshaft is used like an amplifier to drive the vibration levels higher and higher until failure occurs.



Rusty

 

[supersonic]

Ya, what he said. Still a one piece shaft, got it balanced good now but the noises are unbearable. Think it's somewhere around 63" long. I dont have the time right now to chang it back, as I will have to make a custom crosmember and bearing plate to make it work. My truck is a standard cab and the factory mount is too close to the transfer case for it to work. As to my comment which you quoted, I wasn't trying to be a smart arse, just getting my point across. Alot of people don't respect what i say because of my age, I guess, but i've been there and done that a few times, don't consider myself an expert by any means and i cant say many of them fancy words like RustyJC but I understand what he's sayin and concur.

 

[JGheen]

Alot of people don't respect what i say because of my age, I guess, but i've been there and done that a few times, don't consider myself an expert by any means and i cant say many of them fancy words like RustyJC but I understand what he's sayin and concur.

It's all good there supersonic, not too many people on here respect what I say either. Sometimes it may sound like I don't know what I am saying due to my wording, I guess. But I hear ya and definately understand what Rusty is saying.

After getting under the truck last night and thinking about the current driveline geometry, I think going to a 1 piece would be stressful on parts as well as cause vibration overkill. I am no trying to think about maybe going to a CV joint right after the carrier to compensate for that angle there and run the transfer case and axle pinion angles straight as the CV would cancel those angles out. But then again, I have heard some say that a CV in the rear will eventually go out and/or cause a nasty vibration as well.

But in the meantime I am going to the auto parts store to get a magnetic angle finder and see what I can do this weekend to get those carrier and pinion angles a little closer and see how it goes from there.

This whole rear driveshaft thing gets worse the higher I lift the truck. I think next time I lift a truck I will consider doing it to a 2500 single cab long bed or something to that extent. Oh the joys of lifting trucks Oo.