3p141592654;1713663 said:
Why 1 pieces DS are not an engineering win.
Nc = K sqrt(E*I/m) / L^2
Nc = critical speed (rev/s )
K = constant typ between 1 and 4
m = shaft Mass
L = Length of shaft
E = Young's Modulus
I = Second Moment of Area
The problem is obvious, the non-linear relationship to shaft length is a real killer.
If you fix Nc, which is what you want to do since we want to avoid the Cressida problem, then since Nc is proportional to one over the length squared, if you make the shaft longer you need to dramatically increase the strength of the shaft to keep Nc constant. To make matters worse, the restoring force is a square root of the shaft material properties.
Example, doubling the length (i.e. going from 2-piece to 1-piece) means you need to make the shaft 16 times stronger to compensate, but unless you plan to switch to exotic materials, then your only solution is a bigger diameter and thicker wall thickness and that adds a lot more weight and results in a shaft that is heavier and hence has more inertia than the sum of the two shafts in a 2-piece. You never want to be on the wrong end of a nonlinear equation!
The loss argument is specious. The losses in a driveshaft are essentially negligible compared to the rest of the drivetrain. I mean its obvious right, if they weren't these things would be glowing red hot and be enclosed in cooling oil. Well, they don't and the carrier bearing in a 2 piece has negligible friction as well.
zombie;1713665 said:
you can't go bringing real math into this, this is supposed to be a mythbusters style way of saying why one is better than the other 
Mathematics are fine on paper, and hypothesis are fine in a classroom, but where the rubber meets the road, where first place gets paid and everyone elso goes home; at the dyno, or at the dragstrip, or in many, many other forms of motorsports throuoghout history, and currently, the 1pc. wins.
If drag and friction were not a consideration, why don't we have toothpick-thin 3, 4, or 12 piece driveshafts supported by multiple bearings?
Do you consider aluminum alloys and the carbon fiber on some folks hoods to be "exotic".
If budget wasn't a consideration, we would all have torque-tubes or be on some Ferrarri Forum.
But when considering all the variables in the equation (including budget), there are tradeoffs that have to be made.
Braking is a nonlinear equation. Brakes are part of a safety system. Driveshafts are part of a propulsion system.
When the real-world Budget variable is introduced into the equation, I use the money saved on my propulsion system to advance my safety systems so that I am not on the wrong side of a nonlinear equation. The hundred dollars I saved on the 1pcr went to replacing the 20yr old rubber brake lines with new SS braided lines.
If I lose on the nonlinear propulsion side, I simply lose propulsion and have to walk home.
If I lose on the nonlinear braking (safety) side, I might never walk again. So do you know which nonlinear equation I have the most respect for?
Where are your priorities?
The money I save by not overbuilding or overspending on a 2pc build or rebuild I can put towards better braking, suspension, and steering systems, including wheels and tires. These, and many other components in the safety side of the nonlinear equations common to automobiles are far more important to me than losing a part of the propulsion system.
FWIW, the transmission tunnel in a unibody car carries a major portion of the chassis load, and is built like a brick sh*thouse. The chances of a lightweight driveshaft of any type of construction penetrating it are between nil to none.
