Driveshaft Safe Operating RPM Calculator | Spicer Parts
this might help you out and I may just go and check mine to see where the numbers end up for fun.
Back when I had a high HP truck I took my stock driveshaft and had it re-balanced at a local shop because many times they are not perfect from the factory. It took everything I could throw at it.
Unfortunately the longer the driveshaft, the lower the critical speed......
some info:
Balance
Anytime you increase the power output of a stock engine and subsequently increase the speed at which the engine operates, you need to look at driveshaft balance. Most factory driveshafts are balanced between 3,000 and 3,500 rpm. Spinning the driveshaft past that range can have a parasitic effect. Steve Raymond from DynoTech Engineering tells us, “We have had several NASCAR teams tell us our driveshaft saves them 3 to 7 hp at the wheels of their race cars. That’s why balance and design are important and why we manufacture shafts for about 85 to 90 percent of the NASCAR teams.”
DynoTech Engineering uses Balance Engineering driveshaft balancers, which are considered the best in balancing accuracy. DynoTech suggests balancing a performance driveshaft at a minimum of 5,000 rpm and as high as 7,500 rpm. This ensures a properly tuned driveshaft that reduces vibration and efficiently transmits power to the wheels.
Length And Diameter
Other than balance, the length and diameter of the driveshaft directly affect the performance of the unit. Critical speed is the rpm at which the driveshaft becomes unstable and begins to bend into an S shape. The longer and smaller (diameter) a driveshaft is, the lower its critical speed. Critical speed is felt as excessive vibration that could cause the unit to fail. To calculate the critical speed, the length, diameter, wall thickness, and material module of elasticity must be known. Then using the critical speed calculation formula, the numbers are plugged in and you will know the driveshaft’s critical speed.
