A propshaft is a shaft that transfers power from your engine, via the gearbox, to the driven wheels, so generally speaking in the case of BMWs that means the rear wheels. The propshaft connects to the rear diff while a pair of driveshafts or halfshafts (one for each wheel) transfer the power to the wheels themselves.
A propshaft can be a single-piece item or, more commonly, a multi-piece shaft connected by one or more joints that allow the shaft to be angled beneath the car. These universal joints allow the shaft to be split and angled whilst still rotating. At each end of the shaft a CV joint (constant velocity) allows the propshaft to transmit power through an angle at a constant rotational speed. In cars with serious power upgrades people often fit larger CV joints to cope with the increased torque.
Most propshafts are made of steel as it is tough and relatively cheap. However, it is also heavy and that’s not what you want on a performance car. A lighter propshaft will reduced rotational mass and improve throttle response and acceleration and there are two main options: an aluminium propshaft will offer a decent reduction in weight over a steel item whilst also being affordable; or, if you really want to push the boat out, you could go for a carbon fibre propshaft – as fitted to the new F8x M3 and M4. Naturally, this is a much more expensive option but it is also significantly lighter, physically smaller in terms of diameter, and much stronger.
Carbon fibre has numerous advantages over aluminium and allowed BMW to construct a singlepiece propshaft with no centre bearing compared with the two-piece item on the BMW E9x M3. The carbon shaft weighs 40% less and delivers a significant reduction in rotating mass, improving drivetrain dynamics and response as the engine expends less energy rotating the heavy propshaft, allowing more energy to be transferred to the rear wheels, and that is obviously a very good thing.