Progress

It has been about a year since my last posting. Life sometimes gets in the way of, well, life.

Progress has been made. For the frame, I decided not to try to stiffen the prewar frame as was done with the original R51RS. I did not want to modify a rare frame. I was able to locate a 1952 R51/3 basket case. The forks that came with the basket case was a 1954 set of forks. The forks required rubber gaiters. I decided to use rubber gaiters for the front and back, a homage to the pre-war compressor race bikes.

I had some small repairs made to the frame and I had it powder-coated and front forks installed.

The fork tubes were made by Frank's Maintenance and Engineering, Inc. It took almost a year of calls before a production run was made and I got myself a few of the forks for different projects.

 
 

I had some simple aluminum "spools" machined up to allow me to attach the rear rubber gaiters. Those gaiters are Triumph part number 97-1645.

They fit very well and tight! One of the spools had to be heated up a little to get to fit.

I was able to get both front and rear suspension parts all complete

Next step is to get the hubs brake surfaces machined and trued and new brake pads arced. I plan to make the 3 hour trip to Vintage Brakes in the next few weeks and get both the race brakes and the '54 R51/3 brakes refurbished.

Also the motor is nearing completion. I sent out my flywheel to get lighten up and I hope to have a complete motor in the next couple of months. I can't wait.

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.

The R51RS motor

The engine for the R51RS is heavily based on the R51 motor from 1939. BMW produced basically the same engine from 1936 starting with the R5 to 1940 ending with the R51. In 1950, BMW's first twin cylinder motor was a copy of the R51, denoted as the R51/2. In 1951, the R51/3 was introduced with a completely revised motor. There is an interesting connection between the RS engine and the R51/3 engine which I will get to shortly.

What made the production R5/R51 engine interesting were the twin cams; one cam for the right cylinder and one for the left. The idea was to keep the pushrods short. This reduce the flexing of the pushrods at high RPM. The chain driven R5/R51 produced 24 hp at 5500 RPM.

Cutaway of a R51/2 engine

Photo by Bench Mark Works

For the R51RS engine, the BMW engineers changed how the cam was driven. They replaced the chain driven cams with gear driven cams. I assume the rational was to get more precise and consistent timing between the cams and the crank.

R51RS gear driven cams

The intermediate, floating gear in the middle of the cluster allows the cams to rotate in the same direction as the chain driven cams. Also the alternator on top of the motor is replaced with a magneto.

Note that BMW went to a gear driven cam on the R51/3 in 1951, although with only one cam. This was not the first time a production engine from BMW was gear driven, but all twins from 1951 to 1969 were gear driven. In 1970, with the introduction of the /5 series, BMW went back to chain driven cam.

Ok, so my project. Last year I saw an ad for a R51/2 engine block that was modified to accept a prewar Bosch Magneto, a Bosch Magneto, and a R51RS timing cover.

 Here are some photos that compare the modified R51/2 block with a R51 block. The R51 block is the more corroded block. The main difference is that block is flattened on top to accept the Bosch magneto. Also for this block, the vent holes used on the R51/2 generator has been welded over.

Here is a comparison of timing cover for the R51RS (left) and a R51 (right). The R51RS cover is made of a magnesium alloy, known as Elektron. This alloy was used by many race machines in the prewar era (and maybe after too).

Note that the production timing cover has a location for the points that is driven by the left cam (left as seen when sitting on the bike).

The points for the R51RS is part of the magneto that sits on top of the engine case. 

More on the engine in the next post.

History of the BMW R51RS

In the mid 1930's, the best German privateer's ride of choice for the German 500 cc championships was the Norton. You can image the pride of the Germans was on the line, especially with BMW. The BMW Compressor dominated their class in the late 30s with multiple wins and championships throughout Europe. BMW challenged the Senior Isle of Man TT race in 1938 but Karl Gall was injured during practice and Georg Meier retired at the start line. Jock West carried the BMW banner with a 5th place finish.


In 1939 Georg "Schorsch" Meier was the first foreign competitor to win the Senior TT race on the factory prepped BMW Compressor. West came in 2nd for a BMW one, two. Karl Gall crashed during practice in 1939 and died from complications a few days before Meier's win.


BMW had a winning presence with the factory bikes, but the best German privateers had to look elsewhere. BMW had produced limited run Super Sport models with the R5SS and the R51SS in 1938. For 1939, the race division at BMW was tasked to create a competitive 500 cc machine, on par or better than the Norton, for the best private German racers.


A factory works replica was not practical as the setup and maintenance of such a machine was out of the realm of a privateer's team expertise and budget. The decision was to create a race machine based on the R51 production motorcycle. The twin cam R51 was an excellent, high performance machine and had plenty of opportunities for improvement. All of 17 examples of the R51RS were produced and only a few are known to exist today. While the race machines were available to the general public, in principle, only selected individuals were allow to purchase them.


One restored example can be found in the BMW factory museum.

Photo by stkone

Photo by pilot_micha

Photo by stkone

Photo by pilot_micha

One of the few original models came up for auction in January of 2011. It was sold to Emil Recke, a New York importer, in 1939 and was raced successfully in the US. It sold for $130,000 in 2011. 


This blog is about my adventure to recreate my own 1939 R51RS and campaign it in vintage races. I will talk about some of the history of this machine and differences between the production R51 and the RS.