Critical Speed is a term used to describe the speed at which a driveshaft would whip or bend or self-destruct. Exceeding this speed can also damage the transmission or transfer case and differential due to the whipping and vibrations.
CS is determined by calculating the tube length of the propeller shaft or the length between centers of the u-joints, the diameter of the shaft, thickness of the material and material used and the u-joint sizes.
Brent, aluminum shafts are not solid, they are tubes as well. They must use larger diameter tubes and usually thicker wall diameters to equal or exceed a steel driveshaft. In my racing days, aluminum driveshafts were all the rage for a time, until every single one of them twisted off, no match for the horsepower and torque of our motors back then.
https://spicerparts.com/calculators/driveshaft-safe-operating-rpm-calculator
My rear propeller is 53-1/2 inches long, 3-1/2 inches in diameter and has a metal thickness of .083 inches and uses 1350 u-joints. CS = 4,556 RPMs
My front propeller is 28-1/4 inches long, 2-3/4 inches in diameter and has a metal thickness of .083 inches and uses 1344 u-joints. CS = 5,000 RPMs
I have as yet to actually put a tape measure to mine to verify, this information is internet derived.
