Why a 2 piece driveshaft?

[Coley]

Wow, interesting thread.
In addition to the very important 'critical speed' issue is the ability to work around this speed....of which they are indeed ways.
For instance, if you went with a (less desireable) solid steel shaft you would very quickly find you couldn't rotate it as fast as you could a larger diameter hollow shaft due to its concentrated (central) mass wanting to initially bend at both rest conditions and at higher speeds.....and definitely quicker to approach this condition than the hollow one.
That is why you find most (all?) driveshafts, that are subject to high speeds, made of a hollow (larger diameter) tube material that is inherently stiffer over its length than a solid steel smaller diameter shaft over the same length.
Thus a great result of this is the ability for a large diameter tube to successfully deliver torque with a minimal amount of axial twist relative to its length....nice to have.
A lot of folks would really be surprised to see just how thin the material on a high hp driveshaft, or any typical driveshaft is....lol. Its actually, physically a fairly light part relative to its length and this is achieved by increasing the diameter which keeps it stiff while allowing excellent torque transmission characteristics.
If you want to have some fun....make a driveshaft out of solid 1-1/4"ø steel and wind up the vehicle on a hoist to say....4500 rpm, and watch the driveshaft....(from behind a screen or shield)...lol.
Interesting discussion....lets keep it going!
All Good
Coley