Background:
What we call a universal joint is named a Cardan joint. What many of you do not know is that when the Cardan joint is transmitting power at an angle, the output is not a constant angular velocity even though the input is.

That is why Cardan joints are used in pairs.

The input to the first joint is at a constant angular velocity (rpm) while the output is not. It changes from faster to slower to faster to slower with every revolution of the shaft. The second Cardan joint takes this varying angular velocity and turns it back into the original constant velocity that was at the input shaft.

Even though the driveshaft itself looks like it is spinning at a constant angular velocity it is not.

Another thing to consider with driveshafts is their harmonic frequency. When the rpm approaches the harmonic frequency or an even multiple of it, the shaft becomes unstable. So one either wants to travel through the first or second or whatever harmonic at a rapid speed so the shaft doesn't disassemble itself, or make the shaft harmonic frequency be above the fastest the driveshaft will ever spin.

One of the problems with adding overdrives is that the driveshaft is spinning much faster than the designer of the driveshaft had intended. Say the car came with 3.23s, but you change to 4.11s and add an overdrive. The driveshaft will be spinning 27% faster at any speed, so at 75mph the driveshaft will be spinning as if the original car was traveling 95mph. This will put the driveshaft speed much closer to a harmonic frequency.

To counter this, the driveshaft must be made stiffer in some way. The easiest way to make it stiffer is by adding diameter.

End of lecture.

R.