Side Valves vs Overhead Valves

[schrader7032]

I posted these questions on the MOA Forum and Darryl provided an interesting reponse. With his permission (at least I hope he's OK with it!!) I thought I cross-post here so others can read what he wrote...and to save him from having to retype! If there are other thoughts, I'd certainly like to hear them...I'm getting more interested in the history of the pre-war machines.

Kurt in S.A.

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From MOA Forum by Darryl:

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.

[Bruce Frey]

Uh-oh.......looks like Kurt is nibbling on the prewar hook!

Sidevalves are also less expensive to manufacture. Because of poor (compared to OHV) combustion chamber configuration and volumetric efficiency, sidevalves 'run out of breath' quickly, limiting their RPM. Their cams were ground with this in mind.

Sidevalves are by nature less powerful than OHV in both torque and HP, but they deliver it at a lower RPM which is why people consider them "torque-ier".

I am a big sidevalve fan having an R6, R12 and an R71 (project).

An interesting side note is that all BMW singles were OHV.

I think sidevalve production for some European Marques like Universal continued int the 60's.

One of the items on my wish list (quite far down, though) is a big British sidevalve thumper like the Norton Big Four.

Best regards,

Bruce

[Peter D. Nettesheim]

I read a bit on this thread and wanted to shed additional light on the topic.
The statement is completely accurate that the side valve engines were the choice for the services (post office, police, military, gov't agencies...).
If you look at the engine specifications you will see that these engines had long strokes which were excellent for operations in the above services that were served better by engines that produced hi torque and torque rises (rate of torque development). The problem is that an engine normally needs high engine rpm to produce higher hp. The increased piston speed that this high rpm engine experiences is detrimental to the engines life, so a shorter stroke is designed enabling the engine to turn faster without a faster piston speed. Understand?
None of this has anything to do with the OHV vs. side valve configuration. The ohv configuration enables the engine to have a more efficient compression ratio and more importantly better air flow. Just look at two similar displacement engines of the period. The OHV engines had a 50% increase in hp that was due mostly to the efficiency in air flow as well as the higher rpm permitted with the shorter stroke.

Lubrication was not a problem for these OHV engines as this was handled very simply. Unlike what many of you are familiar with when we talk of upper end lubrication on the post war BMW twins, these earlier engines had no such lubrication system. The answer to this difficult lubrication system is not answered by just adding some oil and putting the valve cover on. Nor is there any oil that travels down the push tubes as there are drains right after the lifters to prevent this. The answer is that in these early OHV engines you have three seperate oil sumps. The first is in the main engine case operated by a high pressure oil pump lubricating the crank, cam, lifters, gear train... The roller bearing rocker arms (yes, roller bearings, like a modern race engine) are lubricated by their own separate oil sump in each head. The valve cover is installed, 250ml of oil is then inserted, and the valves have the proper splash necessary to lubricate the 8 (yes eight) bearings in the heads.
Now here is the final answer as to why they produced both types of engines. If you look at the complexity of the OHV design, you can instantly see that this must have been very expensive to produce. Some customers could afford this and others could not. Thats why there were very few built (36000 of the R12 vs. 454 of the R17, same bike one with side valve, one with OHV). There is a tremendous performance dirfference that is clear if you ever get a chance to ride both.
For clairification purposes, I am writing of the R32,42,52,62,47,57,63,12,17,11,16 series of bikes.
Please let me know if this answers your questions. Regards,
Peter D. Nettesheim

[schrader7032]

Thanks, guys...interesting to understand a little more what BMW might have been thinking as well as the markets they were trying to serve. This book I'm reading does address the market and cost of machines (BMWs were always very highly priced...still are!!) but didn't provide the engineering reason why they chose the valve configuration for the given market...at least not so I could discern it.

...and yes, a prewar might be somewhat interesting...

Kurt in S.A.

[Darryl.Richman]

Peter, I'm confused a bit about how the valve gear is lubricated in a prewar OHV BMW. I've never owned one nor even personally seen one openned up.

How is oil in the valve cover splashed up to the bearings? I don't see any provision for that in the parts books I have (R12/17 and R52/62/57/63).

Also, Craig Vechorick wrote an article not too long ago about rigging a sort of drip tray between the bearings and under the oil cap, that would slowly drip oil (or, I think in his case, STP) onto the top two bearings (and presumably then on to the bottom bearings), and would also allow him to see how much was left in the tray by removing the cap.

One more question, if I may... in the hypothetical case that I should ever be lucky enough to have one of these, could lubrication for these bearings be provided by using sealed bearings?

[Peter D. Nettesheim]

D,
Here's what we have:
When the valve cover is filled to the proper level with oil the valve springs are almost half submerged in oil. They are not fully submerged as doing so would then cause oil to be at a level equal to or higher than the valve guide (made this mistake). You know what that would mean, you would have oil constantly trying to get through the guide past the stem and would be blowing a lot of blue smoke (been there, done that). Now think what happens when those valve springs start compressing and de-compressing at 25 times a second. You have oil everywhere, top, bottom, sides, everywhere. This is why there is no additional anything needed to get proper lubrication to all eight bearings. If you take the small cap off the very top of the valve cover you will even get oil out of there at any speed above idle and its the highest point so how could the bearings which are lower not have oil?. Sealed bearings are not available as these bearings are specific to this design. The ones on my recent R16 restoration were machine made by a friend. The only thing stock are the rollers themselves. We made the races. The bearings need to be disassembled to insert the rockers as the rockers are one piece. The bores where the races go were bored to size. Now maybe you know why there are so few running properly. And most of them have bushings made of bronze instead of the roller bearings. There are no sealed bearings and if there were you could not use them as they would have to be taken apart to even install them.
let me know if you have any other questions.
Peter Nettesheim

[EuroIron]

one of the most highly developed........ in stark contrast to the BMW, and most other flatties..... is the HD WR and KR flatheads......

over 52 horsepower from 45 inches of 7:1 CR farm implement engine

some are actually developed to actually return excellent fuel economy which is not a forte of your typical flathead engine

a typical WLA military flatty returned between 30 and 35 mpg...... hard for an engine to be fuel efficient with just 5:1 CR

my street 45 will cruise 50-60 mph nearly 20 miles to the Waffle House on just a few drops over 0.2 gallon

it gets much thirstier when cruising over 70mph and will trot along at 80+....... all day long....... returns better than 60 mpg at those speeds

The Hudson Automobile flathead engines were also highly developed but much of that stopped when people quit racing them....... and even more so with the passing of Jack CLifford

whereas the HD flatties are still being tinkered with, and even raced extensively

people are now actually flow benching them, smoke testing (with clear heads), and the smarter ones are actually flow benching them while the crank and pistons are in place and actually measuring flow with the crank spinning due to very intelligent reasons that still elude many flathead tuners

the one charming thing about flatheads is that they are so very easy to work on

I fixed one on the side of the road for a guy in Daytona, he had a fresh engine and forgot to put assembly lube on the valve stems and of course they seized quickly. Un-keyed the springs with a couple screw drivers, stoned the galling off the stems with a couple pieces of choice hand picked gravel, and honed the new guides with a road side borrowed gun cleaned kit having it's patch impregnated with gravel dust and then somebody produced some BonAmi.......

it ran just fine afterwards and went for several years before having to come down again

made for some interesting pix

I'd love to build a really fast BMW flatty

[EuroIron]

here is a WL in the progress of becoming a highly developed flatty with some real force fed muscle

those tulips were replaced in favor of some heavier tulips without the depressions

top fuel nimonic 80A stuff and cheaper than stock vintage BMW valves

also Total Seal Gapless rings....... again cheaper than OEM BMW vintage or modern ring sets

and 2% leakdown and I urge all of you doing topends on vintage low compression engines to give them a try..... biggest favor you will ever do for your antique



4 thou squish, really tight

[VBMWMO]

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.

Kurt in S.A.