category 'TTX' (renamed as TT Zero) for electric bikes using fuel-cells or batteries. Motorcycle engines can be mounted transversely; with the crankshaft aligned perpendicular to the frame or longitudinally, with the crankshaft parallel to the frame. Motorcycles with transversely mounted engines are more suited to have chain or belt final-drive. Motorcycles with longitudinally mounted engines are more suited for shaft final-drive. Motor scooters often have the engine as part of the rear suspension, so the engine is not fixed rigidly to the main frame. Instead, the combined engine-transmission-swingarm assembly is pivoted to follow the road surface and is part of the "unsprung weight". The chain final-drive of scooters runs in an oil-bath within the engine casings. "Step-through" motorcycles may have a rigidly fixed engine, or may have a scooter-type arrangement. Two-stroke and four-stroke Two-stroke engines have fewer moving parts than four-stroke engines, and produce twice the number of power strokes per revolution. Fuel economy is better in four-strokes due to more complete combustion of the intake charge in four-stroke engines. Nevertheless, two-strokes have been largely replaced on motorcycles in developed nations due to their environmental disadvantages. Cylinder lubrication is necessarily total-loss and this inevitably leads to a smokey exhaust, particularly on wide throttle openings. Two-stroke-engined motorcycles continue to be made in large numbers, but mostly off-road motocross low-power mopeds, small scooters and step-through underbones where they still compete strongly with four-strokes (including the highest-selling motorcycle of all time, the 50 cc Honda Super Cub). The major markets of two-stroke motorcycles are in dirt bikes and developing nations. Cylinder heads (four-stroke) Cylinder head design has a significant effect on the efficiency of combustion, and hence the power output of the engine. The head may be flat, in which case the combustion chamber resides within the cylinder and/or a depression in the piston crown, but usually a "dome" within the cylinder head provides most of the combustion volume. Motorcycles use poppet valves in a range of designs: side valve, overhead valve (OHV) with pushrod operation, overhead cam (SOHC), and double overhead cam (DOHC). A cylinder with desmodromic valves may have three or even four camshafts. An OHC or DOHC cylinder head will have at least two valves per cylinder (1 inlet and 1 exhaust), but multi-valve engines may have three (2 inlet and 1 exhaust), or four (2 inlet and 2 exhaust), or even five (3 inlet and 2 exhaust). Cylinder heads