Side Valves vs Overhead Valves

I've been doing some reading on the pre-war bikes and getting some sense of models and designs, etc. The book is from BMW Mobile Tradition, Motorcycles from Munich 1923-1969.

It struck me that, up until the war, there were side valve and overhead valve models offered during the same production years. Right off the bat, the R32 was side valve and the R37 was OHV. The R11/R12s were side valve and the R16/R17s were OHV. What was the thinking on having two "competing" versions of valve trains? Seems like that contributed to dual paths for design and manufacturing. What are the pros/cons of side valves versus OHVs?

After WWII, the side valve was dropped (not sure why). OHVs seem to be the way to go in modern motorcycles. I'm just curious as to why side valves were continued for so many years.

Sidevalve motors were considered low revving, high torque motors that were very reliable and good for utility work, touring and towing a sidecar. Although a lot of Americans owned a car in the 30s, it was still very unusual in Europe. Bikes were often basic transportation. Armies loved them, and BMW made something like 25,000 R12s and then another run of the R71 model (BMW's last sidevalve) for the Wehrmacht.

OHV models were considered high revving sport models that needed a lot more fiddling. (The prewar OHV BMWs required oiling the valve gear before putting the valve covers on and then hoping for the best; there's no return lines to the sump and very little of the oil mist in the crankcase makes it out through those narrow pushrod tubes to the head area.)

After the war, the Europeans went back to making basic bikes for transportation. However, BMW had produced the OHV R75M in 1942, so they knew how to apply an OHV motor to utility work while keeping it reliable. Also, BMW could only begin making its twins in 1950, so they were late to the party; and their factory had been bombed so they had limited capabilities. They chose a single prewar model to bring back and get going again, the R51 (an OHV model) which came out as the R51/2. It had to be all things to all people, and those who wanted a more sporting bike actually found ways to put (retuned) 750 motors from the R75M into this bike's frame.

side valves

I have to think that the gas flow through a side valve head is significantly less than on an ohv. I've read that, even in ohv engines, the induction path into an airhead boxer is inefficient, and that you get turbulence in the area just ahead of the intake port. Which is perhaps why some of the racing ones feature intake runners that have been splayed over a number of degrees, in an effort to get a more direct path to the combustion chambers.

It would be interesting to know how much more efficient the gas flow is, into the type 259, with it's extra valves and forward canted cylinders.

Rinty

Sidevalves are incredibly inefficient. The combustion chamber shape is very weird. It's hard to describe it (and my server is down right now so I can't link to a photo), but the cylinder volume is sort of L shaped and when the spark plug sets off the fuel charge, the expanding gases have to make a right turn to push the piston back.

For that matter, in BMW's designs, even though the cylinders are not bilaterally symmetrical, there is only one cylinder casting. The intake on the left side is the exhaust on the right.