if you got the 2pc steel on your HD like mine, I have about 14 passes on the dyno to 6k rpm, no sweat. Of course that's not a guarantee either

 

i wouldent even worry about a 2piece. 1 piece stock d/s suck for

 

As the others are mentioning, usually you'd be a 2 piece, if you are indeed a 72" shaft, you just ca't go wrong having a loop. No matter how nice your DS is. Especially if that f-er is steel, imagine how much damage will happen to the under side if it let go spinning at 1:1 at 6K rpm, or even worse...4th gear at .75:1 (8Krpm). I am sure there are plenty of pics floating around of broken trans, fuel tanks, and such.

The 2 piece will suck up alot of any issues (along with some HP, unless you are like grippy and are making so much it won't be missed )

1slowhoe, that critical speed you spec is nice, but again, enter your specs into the calculator and make sure the length says 72". Watch how fast the critical speed drops. Pass 60" and it's another ball game.

 

I think his post above said ~54" right. So you had a "short" AL go? I had only seen (and experienced) the 72" AL's making a nice sin wave.

 

Absolutley. A 72" driveshaft seems insane but I think that's what the Avalanche has as well. At that length I'd seriously consider a carbon fiber shaft. Big $$$

 

It was my 54" (give or take) steel shaft that gave out. The aluminum better not give out. I paid $600 for that damn thing.

 

My shaft is one piece (4x4 truck). It is 66" from u-joint to u-joint and about 61 1/2" from weld to weld. 3 1/2" dia. For the critical speed calc, does it consider the yoke length or tube length only?

 

yoke center to yoke center.

I think that calculator only does takes aluminum into consideration. One of the driveshaft companies and tell them your contemplating a new shaft but would like to know how to figure out your current driveshaft's critical speed. If they can help you, GREAT, you have a good answer and you'll know they know what they are doing.

 

I'll call around tomorrow. I would feel much better to have the loops on before I dyno. I may wait or dyno in 2nd if I have a chance. How much difference is there between 2nd and third for comparison? If I dyno in 2nd now, then mod some more and dyno in third am I comparing apples to oranges? My critical speed is 5303 rpm, I can get the HP and TQ to meet then guess at the rest.

 

The calc shows that shaft was good for 7900 rpm. I bet the balance has a lot more to due with failure than RPM.