I think you'll find that the only way to render sega circolare a cardano into English with no ambiguity whatsoever would be to say tractor-driven circular saw with universal-jointed driveshaft.
Hello there, Blackman,
Your motorbike is shaft-driven as opposed to being chain-driven (or belt-driven). I imagine you've got a BMW; although Moto Guzzi sometimes prefers this method of transferring power from the engine to the wheel; Triumph and the top four Japanese manufacturers have also produced shaft-driven bikes.
Your bike has got a driveshaft; this driveshaft (sometimes - but very rarely - called a "cardan shaft" in English) has got two universal joints, one at each of its ends. Hence the "problem": in Italian you refer to one of the two extremities of the mechanical part, i.e., il cardano, whilst in English we call that whole part a driveshaft, with no direct reference to cardano.
I hope this helps.