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Driveshaft Length

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pseudomike

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On the rear drive shaft, is there a magic number where it becomes too long and I should consider a split shaft with the bearing in the middle? I've seen new trucks both ways so kinda curious. I have big 1410 joints and angles are not bad. Is there any disadvantage to running either setup? I'll need a new shaft soon so trying to decide what is the best way out.
 
What else would you need to know? It's a crew cab running 6" of lift. It has a D70 rear with 1410 yoke going to a NP205 fixed with 1410 yoke. I don't have the exact length yet. It has a Cummins w/NV4500/205 so the driveline is much longer than say a 465/Th400/205 combo. I also cut 14" out of the frame behind the cab. In all it's maybe 18-24" longer than a sub or 8ft bed rear shaft. I could do a split shaft but would require making a custom crossmember. So what is the max length you would run a shaft 1 piece. It has about 1000ft/lbs of torque running through it.
 
I have used his shafts before, very pleased.

Anyhow, still haven't answered the question of what is too long and what is not. Keep in mind that by going to a 2 piece unit I also incorporate higher drive line angles, similar to a K5 with 6" of lift. So the trade off isn't cut and dry.

Anyhow, I did some more research on my own and found this:

http://www.pirate4x4.com/tech/billa...Driveshaft engineering data - IJ900-02[1].pdf

which has the actual formula for drive shaft length and critical speed listed. I think I will be plenty fine with a one piece shaft as long as it is 3.5 or 4" tubing.
 
i had a 88-up 2wd std cab long bed. aluminum shaft went bad. got a new steel shaft. guy told me it might vib at or above 60-65 because of length.

sure as heck it did do that. little but not a ton. and i mostly drove in town. so it didnt botherme that much .
 
It's really a matter of RPM. 35" tires with 3.54 gears does not equal many RPM to go fast. Since I have 1410 joints larger tubing or high angles are both an option. I need to price it out and see.

Your shaft should not vibrate like that. I would have the balance checked. Drill a hole in the top and put some oil in it...old hot rodder balancing secret.
 
Well i started really digging into the calculations. DS length is around 79.5". At 4" it would work but marginal because 1/2 critical speed is just about 65mph. I think I will break it in half, just have figure out where. I assume I adjust driveline angles in a similar fashion with a 2 piece shaft?
 
Working on it. I was in the process of extending the frame and now I am in the process of locating a new place to work on it. Everything is sand blasted and ready to rock I just need to get off my keaster and get rolling on it. There is a thread somewhere I started with some pics for teasers.
 
I remember reading somewhere that about 70" in the max for a one peice driveshaft,before it becomes "troublesome" as far as balance and vibration problems..I think it was an artice in 4 wheel & off road ...the shaft in my 1981 G-10 Van with 125" WB is 68+" long,and all it took was one little ding in it to make it rumble and shudder above 60 mph..every try looking for a "good" used shaft at a junkyard??--GOOD LUCK!..ever since they have used forkloaders to lift vehicles,the chances of finding a good shaft is nil to none now!--everyone I found looked like a "U" or macaroni!..
I finally lucked out one day,and found a van someone drove to the junkyard to scrap it,and I took the shaft out while it was still in the parking lot--and almost got speared by the forks when the guy came over to lift it and carry it off!!--I was lucky someone told him I was under it!..
 
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