Side Valves vs Overhead Valves

[20774]

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.

[DarrylRi]

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.

[rinty]

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

[DarrylRi]

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.

[EXR911]

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.

Early on (1905 - 1920) the ohv/ohc layout was problematic due to the poor valve steels available - stem fracture on a sv was no problem, on an ohv design it was catastrophic. Also many cylinders were cast with the barrel and head integral, eliminating a hard-to seal cylinder head joint. Side valves were installed/serviced through valve caps in a "blind head" design. OHV engines had to have the valves inserted up through the cylinder bore - a more complex operation.

While WW1 aeroengines proved the higher efficiency of the ohv layout, (and promoted better valve steels) there was still a substantial commercial place for the cheaper-to-make, lower compression sv engine which could use lower quality fuel with its lower output requirements. Sir Harry Ricardo's invention of the "squish" turbulent head sidevalve design gave a new lease on life for the sidevalve design in commercial vehicles where fuel economy on low octane fuel and easy maintenance were requirements. The "Ricardo head" (used by many American companies without royalty payments) gave the performance potential of the old sv design a considerable boost. A good sv 1,000cc v-twin with Ricardo heads could easily manage 90-95 mph, the ohv version of the same motor in the same bike (and geared taller) would reach 105-110 mph. In the 1930s few roads could accomodate that extra 15 mph at the top end.

In many ohv/ohc motorcycle designs there were open valve rockers/springs well into the 1930s and even post WW2 ( eg.Norton) due to the valve spring steel problems and the difficulty of good valve gear lubrication and sealing. This created a problem in dusty/dirty operating conditions which did not affect sidevalve engines which had generally gone to enclosed "valve chests" or sealed covers by the early 1930s. Probably one of the main reasons that American bikes stayed with sidevalves so long.

Of course many utility engines (B&S, Tecumseh, etc.) have stayed with the sv design until quite recently when air pollution considerations (and probably competition from foreign ohv designs) have pushed the ohv design to the fore of the utility market.

For strange but high performance sv head design, the HD dirt track engines of the 1960s are worth a look by anyone interested in such technology.

PT9766