This thing, an Ariel Square Four.
Not, it’s not all square. Just the engine.
Back in 1928 British engineer Edward Turner decided to design a four cylinder bike engine and rather than doing what you’d expect and coming up with a conventional inline four he got the notion of effectively sticking two twins together to give a more compact design. He hawked this idea around until he got a buyer, the Ariel company from Birmingham, who made what they logically termed the Square Four in a variety of models from 1931 to 1959. They then gave up on four strokes and turned to small and cheap two strokes.
Turner used two contra rotating cranks which were joined by their central, geared flywheels and shared a common block and head. The biggest issue was one that should have been obvious at the time - the engine was aircooled and so the two rear cylinders were cut off from most of their cooling air by two hot cylinders in front of them. Ariel addressed this by providing a gap through the middle of the block and (for reasons unclear) switching from OHC to OHV but they never really eliminated overheating and the engine always had to be restricted in power output and rpm. The final iteration of the Square Four, the Mk2 of 1953 to 1959, had 40 bhp from 997cc which wasn’t spectacular even 60+ years ago. Still did 100 mph though. Stopping from that speed with those small drum brakes was another matter.
What somewhat surprises me is what a Square Four sounds like. I expected something like, well, two twins but no. It sounds more like a modern four. Like so:
Want to buy one? You’ll be paying something like £14,000 or €15,000 for a bike in this condition.