Steel drive shafts flex more than aluminum shafts. Over time, when a steel shaft's balancers become even slightly off balance, they can "wobble" when they spin at high RPM, especially when combined with a gear. Aluminum on the other hand is a very rigid metal, and flexes very little. The aluminum drive shafts on the market are manufactured with as little offset as possible to eliminate the need for balancers, as compared to the mass manufactured and often sloppy stock shafts. The attention taken when making the aluminum shafts, combined with their ability to flex less and absorb more vibration than a steel shaft, is what eliminates flex.
I used to race BMX when i was younger, and aluminum frames were all the rage, but not to save weight as you might first think. The frames had to be made thick in order to be strong enough to withstand jumps and all the torque that theyre put through. A stronger racer can actually cause so much torque through the back of the frame that they can break them in half with a good launch and traction (sounds a lot like drag racing huh?) Anyways, due to the thickness that they have to be made with in order to withstand all that flex, they end up not being any lighter than their steel conterparts, and often times were even heavier. In return, the frames flexed little if any at all, which eliminates parasitic loss of drivetrain power, similar to the loss that's created in a car when transferring the power from the motor through the drivetrain and rear axles. A car loses power to the ground through the cheap stamped steel control arms. Replacing them with a strong aluminum version will have the same effect, which is why you gain so much traction with aftermarket control arms (example would be something like Steeda's aluminum LCAs).
Contrary to popular belief, aluminum is not as weak and fragile as it's made out to be. Many of the aluminum items we face durring normal life, like pop cans, are made of extremely cheap and often thin aluminum, which as we all know bends and crushes very easily. There are many types of aluminum which can be thought of as "blends" because of the varying density and strength that each one has (not sure about how this is done, but its similar to cheap low tensil steel vs. chrome moly steel or stainless steel). Some aluminum can even withstand pounding it with a hammer and hardly show a dint. Many car manufacturers are making their frame structures out of aluminum due to the stiffness of the metal, which creates a higher performance car, such as the Acura NSX which has aluminum body panels.
OK...lesson is over, class dismissed.
