Comparative Anatomy - Frames

I haven't been able to get a lot of information on the frame used on the R51RS. Clearly it was based on the R51 frame. From photos, the frame an additional brace in front of the rear fender, under the seat. Unlike the brace that was used in R51/3 frames, the brace was centered. The R51/3 brace was off center in order to accommodate the battery. I have some pictures of a couple of frames I own; 1938/1939 frame and a 1954 frame.

First the R51 frame:

 Simple gusset around the steering head

The frame had no bracing in the "interior" volume. Looks to be very flexible, not in a good way.

There is a spot for the battery right in front of the rear fender, on the right side

The R51/3 frame:

 Brace on the across the down tubes, right below the steering head (a little hard to see) and a second one that is easy to see

 Brace from front to the seat post, just as with the post-55 /2 frames

 Brace from top to bottom. Note that the battery box on the '54 frame not only had a "floor" but also a "back". Prior to '54, the battery only had a floor.

Don't mind the bandages. 

I will add that I weighed the two frames, both powder coated. The '54 frame is 5 pounds heavier that the '39 frame. The price of stiffness.

Timing Gears

I want to share some photos from JH who is also recreating a R51RS. From the photos sent to me from JH, he appears to be much further along than I am.

JH took some photos of the timing gears from a generous owner of R51RS who was willing to show what is behind the cover. I really have to thank JH. I spent many months searching the internet for photos of the the R51RS timing gears. The picture I have on an earlier post is the set that Josef Heft sells. I hope to get my set from Josef in the next month or so.

 Copyright JH

 Copyright JH

In the blog post Weiss und Blau, it was noted that in the 1950s, one could have bought the timing gears from Ernst Hoske (known in the /2 world for his tanks). I'm always surprised by the number of R51RS replicas that are out in the world, given that only 17 were only produced and only about five original are know to exist today.

The RS engine, part 2

The R51RS in 1939 followed the R51SS of 1938 and R5SS previous to that. The R51SS was also based on the production R51 with more modest tweaks than the RS. The SS used the standard chain driven cams that the production bike used. It also used the production ignition system. Some of the 1938 SS modifications included high crown pistons which increased the compression ratio from the stock 5.6:1 up to 8:1. Larger carburetors, 25.4 instead of the 22 mm. The alternator robbed less power from the engine by using a 14 tooth gear instead of the stock 12 tooth gear. About 50 of the SS versions, both R5 and R51, were built.

Beppe Stelzer, lead of the BMW race department, oversaw the R51RS changes with the replacement of the  chain drive with a intermediate helical spur gear. The battery was eliminated with the used of a racing magneto, Bosch's W2R48. The used of the magneto required the top of the engine case, where the alternator typically is housed, to be machined flat.

The production round iron cylinder was replaced by an aluminum "porcupine" casting, similar to that used by the BMW R66. Of course, an original porcupine cylinder is impossible to find. Fortunately one can get reproduction cylinders from SMA Racing in Germany. This is where I got my cylinders. Note that SMA Racing has some very cool RS54 engine and transmission casings as well. Maybe for my next project. I'll start saving my pennies.



SMA Racing also produces R51 heads and valve covers that can be used for a R51RS project. I fortunately was able to find some original heads and valve covers. The R51RS used magnesium valve covers which can be gotten from SMA Racing. I may go down that path as well, but I have a ways to go before I have to  make that decision.

Note that all these parts (and a lot more!) can be gotten through Josef Heft. A quick google search will show the massive extent of parts he has to offer. For me, I only embarked on this project because Josef had a very comprehensive catalog of parts.

Let's finish up on the engine. The R51RS used the standard 300 degree cam opening but with a 6.5mm lift. This translated to a valve lift of 10 mm instead of the stock 5.8 mm. As with all race engines, polishing and balancing was employed. A lightened flywheel was used and an air-scoop was used to keep the clutch cool. The air-scoop was located where the timing hole would be used in later models, right behind the left carburetor. You can see the polished air-scoop in the pictures from the BMW museum.

The RS produced 12 more than the production engine. At 36 hp, that is a 50% increase over stock!

I should note that most of the technical information I have on the R51RS came from a 2002 article in Motorrad Classic as well as internet searches.