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An air cooled transverse vertical twin is a horrible design. First off, it's much wider than it needs to be. The longer radius that all that metal has to traverse has an adverse effect on handling.
Second, the bore size is limited by how far apart the rider can comfortably spread his knees. This is the reason Norton had to cant the cylinders forward to get enough bore to make 850 CC's.
Third, there are cooling fins on only 3/4 of the cylinder because the other cylinder is in the way of the remaining fourth. This results in uneven heating/cooling that distorts the cylinders, increasing wear and causing seriously complex design problems.


The rear cylinder didn't always run hot on a traditional V twin. Before the politicians fancied themselves to be engineers, the 90 degree timing offset caused the rear cylinder to get less than a fair share of air/fuel mixture. At idle, it would only have enough mixture to fire maybe every second or third power stroke, resulting in the traditional raggedy idle. At small throttle openings, both cylinders fired every power stroke, but the rear still had a smaller air fuel charge. By the time the throttle was opened enough and the engine turned fast enough to offset the timing differences, the bike was going fast enough to supply plenty of cooling air front and back.

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