Custom motorcycle built with a 7-cylinder radial engine from the Australian company Rotec Engineering. Builder is JRL Cycles of Black Hawk, South Dakota, who also a distiributes the Rotec radial engines in the USofA.
I sort of understand radial engines in aircraft, but in a motorcycle?
Apart from an exercise in producing something very different, why bother?
Air cooled, large displacement and high horsepower? What’s not to like? 
How long a list do you want?
Let’s start with rotating mass.
Torque effect from the rotating mass.
Weight. …
I think it was to do with weight distribution in aircrafts.
An inline engine is now more efficient than a radial.
However in the early years, engines were just not that powerful, so you didn’t want a huge, long, heavy lump of an engine with 8 cylinders in front of the wings ,so why not fit a flatter slightly lighter radial 8 instead.
By the 1930’s combustion became more efficient so you startto see an increase V block engines in aircrafts. V8’s V12’s etc and a few in lines too.
Let’s accept radial engines being the best option available for aircraft, say, 100 or even 60 years ago. (Yes, I do know that some radial engines are still in production.)
Still seems the worst option for a 21st century motorcycle.
Here’s a pretty cool animation of how the radial engine works.
The site has a load of these for quite a few engines…
Nope that’s a rotary engine. 
My apologies for the wrong link.
It is still a cool site though 
An air cooled radial is nothing but a BMW boxer with more cylinders.
I think they got them a bit more efficient than the push rod BMW boxers though.
Not a very difficult job as my 800cc develops just about 68 b.h.p. on a good day.
Not really, that engine is 2800cc & produces a claimed 110 bhp so that’s only 39 bhp per litre or 31 for 800cc.
Almost all radials are pushrod. All the Sherman Tanks prior to the A3 were powered by a 21 cylinder air cooled radial producing 300-400 bhp from 16,000 cc.