Matching the Future with the Past: 1928/1974 BMW R52 Similaria

Motorcycle builder and rider Bob Vail turns a 1974 BMW R75/6 into a 1928 R52 “Similaria.”

article image
by Mark Most

BMW enthusiasts who appreciate the purity and performance of a bone-stock BMW might want to skip the next few pages in this issue of Motorcycle Classics; a 1974 R75/6 lost its pedigree in the making of the accompanying feature bike, what its builder describes as a 1928 BMW R52 Similaria.

But readers who forge onward with this narrative are in for a treat because you’re about to get a glimpse of how one man converted a mid-1970s Slash 6 BMW into what visually replicates a 1928 BMW R52, an early model that shared a similar silhouette with BMW’s first-ever model, the iconic 1923 R32. Those early R models set the stage for BMW’s prewar signature slab-side gas tank enveloped by two upper frame tubes that swooped from steering head to rear axle. It was a classic formula defining the simplistic yet graceful lines of practically every rigid-frame BMW from that era.

What about Bob?

The man responsible for the R52 Similaria is Bob Vail, who unabashedly describes himself as someone who enjoys “building and riding motorcycles equally.” Bob hails from Cleveland, Ohio, where, as he’ll tell you, “the cold snowy winters and beautiful summers are completely conducive to that equality.”

Consequently, Bob spends a good portion of those cold snowy winters turning metal on his 1940s-era South Bend lathe, plus shaping, filing, whittling and welding pieces of metal into bikes of his own creation — Similarias, as he calls them. Among his stable of retro-style bikes you’ll find a 1914 Harley-Davidson Similaria based on a 1997 H-D XL883 platform, and two Yamahas — a 1977 XS650 that he terms a Yamaton and a 1981 XV920R he morphed into a Yamacent. The remainder of his bike fleet consists of original bikes including a Ducati, a Laverda, a Moto Guzzi, a BMW, a Harley-Davidson and a Triumph, plus two non-Similaria retro-theme customs. And all his bikes are runners; as the man says, he likes to build and ride motorcycles.

Now, about that name “Similaria.” Bob explains it as “a made-up word used to describe a motorcycle with the similar look of an old original machine, but made more practical and functional for riding … lots of riding!” Each Similaria is based on distinct features that tie it to the targeted motorcycle’s design. Those features include isolated parts such as engine, frame, gas tank, front end, wheels and tires, fenders, headlight, and so on of the original model.

And with that, let’s focus on the R52 Similaria. With two of his riding buddies, Ray Shaw and Doug Horner cheering him on, Bob decided that his 1974 R75/6 donor bike’s boxer engine was suitable for the retro look. He figured that he could trim its bulbous valve covers that conceal overhead valves to more closely resemble those of the original R52’s slab-side flatheads. Mimicking those pre-war cylinder heads required some creative engineering, though, so Bob turned to a group of young and eager engineering students at The Gow School near Buffalo, New York, for help. First they created CAD drawings of each head, feeding their data into a 3D printer to set the pattern for a large five-axis Haas CNC milling machine. Huge chunks of billet aluminum were then tirelessly whittled into their 1928 shapes. Bob then turned down the heads’ cooling fins on his trusty milling machine to more precisely replicate the R52’s classic features.

Bob also removed the R75/6 engine’s massive front and timing case covers, plus the two CV carbs were trashed. As Bob put it, “The timing case cover and front engine cover would have to be cut off,” to make way for a flat cover that he fabricated. Bob continues: “Yes, I see you purists in the back row cringing at the thought that I would actually cut these pieces off.” Matching 32mm Mikuni carbs, their round-slide bodies favoring the R52’s more rudimentary mixers of old, were attached to either side of the otherwise stock engine, and to complete the engine’s breathing circuit Bob fashioned an exhaust system using tubing with J bends from automotive supplier Summit Racing’s inventory.

Naturally, creating the 1928-like mufflers required some additional cutting and welding by our man. Although Bob located a suitable shorty muffler, he “could not stand either the look of the 1-3/4-inch-interior-diameter outlet pipe or the thought that the baffle could not be adequately packed around [with sound deadening material].” So he cut four shorty 2-1/2-inch-diameter mufflers in half, using only the halves with 1-1/2-inch-inner-diameter ends for the project. He next created inner sleeves to join the anointed muffler halves together before fabricating baffles from 1-1/4-inch-diameter stainless steel tubing that he wrapped with 5/8-inch-thick packing.

The engine itself created another minor obstacle — BMW originally positioned the R75/6’s boxer motor at a 6-degree downward slant (front to rear) in its frame. Most certainly a no-no for this project because those 6 degrees would disrupt the classic flowing lines that the R52 offered, and so the engine needed to be elevated 6 degrees at its rear.

“I never realized this [6-degree drop] because the angled top of the R75/6 engine is hidden under the gas tank, the oil pan’s bottom is wedge-shaped making its bottom horizontal, and the engine badges are mounted level, at a 6-degree angle to the engine.” Those crafty Germans, concealing their Teutonic truth like that!

Repositioning the engine should be a rather straightforward fix, you say. Well, yes, it should be but Bob also was going to change the frame’s rake and trail to 30 degrees and 3.5 inches respectively, to better mimic the 1928 BMW’s stance as well as accommodate a Kiwi Indian leaf-spring front fork matched to a cantilevered Works Performance rear twin-shock assembly. “If change was made on one item,” allowed Bob, “then something else was bound to change.” What to do?

The hip bone’s connected to the thigh bone …

His solution was to divvy the frame’s makeover into four sections — front, top, back and lower sections. The front section, he determined, established location of the steering head relative to the rest of the frame. “My calculations told me the front downtubes had to be lengthened with spacers to accommodate the height of the leaf spring front end and the steering head had to be tilted back to achieve the desired rake and trail.”

Regarding the frame’s top section, Bob said that he “cut out the one large tube on the donor bike frame, replacing it with two parallel tubes. These tubes would run on top of the future gas tank to achieve one of those distinctive [R52] features.” Bob continued, focusing on the back (or rear) section: “I also wanted the distinctive feature of the rear loop on the frame. The real R52 had it on one side only because the rear drive was on the other side, but I chose to have this feature on both sides.”

Sounding more like a math professor, not a typical motorcycle enthusiast, Bob explained how he engineered the rear suspension: “Referring back to geometry class, physics and vector analysis, I’m sure you remember that side O is half the length of side H.” Uh-huh. “This means two things: First, if you want 3 inches of vertical axle movement, the shock only moves approximately 1-1/2 inches. Secondly, a 300-pound force vertically on the axle transmits a 600-pound load on the shock.” Of course. “This and the fact I now needed a 15-inch length shock rather than the 13.5-inch stock shock, meant I had to find someone to build a custom set of shocks. Enter Works Performance. They built these with the needed travel and stiffness, and added a very reasonable engineering fee.” Problem solved. Next!

Next turned out to be sizing and welding the two replacement backbone tubes to the steering head and rear portion of the frame. “I needed to build some sort of a fixture to hold the frame pieces in alignment while I cut the top section out and while the front section was cut off awaiting modification.” The solution was to use his trusty wooden work table to hold a Unistrut base fixture with turnbuckles and wire to secure the frame during surgery, and a digital level/angle gauge to monitor vital signs to assure that the entire, and somewhat chopped up, frame structure remained in alignment during welding. He then placed a tension wire as close to level as he could, to serve as the centerline of the frame. All measurements were taken from that line.

“The stock frame was then mounted to the fixture.” After a brief sacrificial ceremony, Bob made the necessary cuts, until all that remained in the fixture was a series of floating frame sections. He then cut and welded in the two filler pieces to the downtubes, followed by the two top tubes that boasted the necessary bends to further replicate the R52’s persona. Added Bob about this step: “This was by far the most complex ‘fitting’ work of the whole project.” Essentially he had to prevent the frame’s dangling rear section from moving while he welded the top tubes at both ends, keeping them parallel in the process. “And at the front of the top tubes there is a complex compound connection into the round steering neck,” Bob pointed out, which made completing the weld especially critical, and difficult. “If any one of these four joining surfaces were cut or filed 1/32 inch too much, the top tubes would be out of alignment with each other. Fortunately the process produced no profanity … that I can remember,” Bob said, adding, “and have I mentioned what a bad memory I have?”

That concluded much of the necessary engineering work to convert the R75/6’s frame and engine for the tribute R52. The easy (?) part was next, that of replicating the 1928 bike’s sheet metal and body work. No problem, and at this point we see just how creative and clever Bob had to be.

It’s all about style

First order of business was to fashion a prototype gas tank out of Styrofoam to visualize how the hand-formed aluminum tank would fit. The prototype completed, Bob’s questions began: “How will it be supported? How can it be removed? Oh, and look, the tank can only be removed from one side because the handshift will be on the other.” Yeah, he also had to engineer a handshifter. And to counter engine vibration (“Aluminum fatigues and cracks over time when exposed to vibration,” Bob reminds us), he ultimately positioned the tank on two parallel inverted beams running front to back, using 1/8-inch soft silicone rubber at four points (two on each beam) to isolate vibration. Gas tank fabrication was an adventure in itself.

“Building the tank was a lot of fun. Well, mostly,” Bob recalls. After making his own wooden buck, he cut and shaped sections of 3003 aluminum (.090 inch thick) into what eventually became a piece that closely resembled the gas tank of an original 1928 R52. Bob’s simple explanation describes just how intricate and elaborate the build time was: “After a bung for the fuel petcock and gas cap neck were welded in place, hours of filing and sanding ensued to hide the welded seams.”

He also fashioned fenders, using blanks from Wild Card’s Long Boy Front Fenders. Mounting them to the Kiwi leaf-spring front end required additional engineering such that the fender attaches to bronze bushings on each side of the axle, allowing the fender to travel vertically with the front wheel/tire. “The rear fender had similar issues of traveling with the rear wheel, so it’s attached to the swingarm in several places.”

While we’re examining the front end, check out the headlight. It was fashioned from a pair of cut-and-welded 8-inch sauce pans that Bob sourced from the local Target department store’s housewares section. Bob cites the pans’ vertical sides and rounded bottoms as the key ingredients needed to replicate the R52’s front headlight.

Just as amazing are those stylish footboards that resemble miniature toboggans. After making templates from corrugated cardboard, wood and copper tubing, Bob cut a 6-foot section of 1/4-inch-thick aluminum angle (with 6-inch legs) into a pair of 14-inch pieces before fashioning them into mirror images of each other. Careful thought and measuring went into the footboards’ final placement on the bike, allowing for the angle of the rider’s feet, clearance for the carburetors and foot controls, and so on.

Indeed, on and on the planning and engineering went, each part requiring its own special fabrication or modification. For instance, the handlebar was bent to 1928 shape, the handshifter also incorporated a “cheater foot shift lever to allow for occasional need to shift quickly,” and Bob created a multi-functional gauge (digital speedometer, analog tachometer, plus 15 other functions) that he based on a Motogadget instrument harvested at a motorcycle trade show.

Ode to electrodes and diodes

I would be cutting this narrative short if I didn’t mention the electrical system that Bob strung throughout the bike. Bob describes a motorcycle’s electrical system as being “like the nerve system in the human body,” so he takes this part of the build rather seriously. Turns out that Bob is somewhat of an electrical wizard who maps out his own wiring looms for each bike he makes. So he started with a Euro Motoelectrics unit that puts out 450 watts of juice from a high-output alternator. Remember, Cleveland is home to cold snowy winters, so Bob enjoys his electrically heated vest and gloves.

Mounting the Odyssey AGM battery to the bike created its own problems, not to mention that the battery’s modern plastic case just looked … modern; this was to be a bike based on nearly a 100-year-old design, so he needed to rethink this part of the build. While making coffee one morning Bob noticed that the creamer’s cap in front of him was the perfect size and shape for the R52 Similaria’s battery fill caps. His brain began to percolate: “My wife makes art objects out of polymer clay, a soft malleable plastic-like material that hardens when it’s heated in a toaster oven. It’s available in a plethora of colors; I made a silicone mold of the cap, filled it with red polymer clay, baked it, and voila, my first of three faux battery caps.” Too much caffeine, Bob! And did I mention that Bob produces his own complete and color-coded electrical schematics for all his bikes?

Finally, the Similaria was ready for its maiden lap around the block. One shakedown run led to another, and another, until it was deemed suitable and ready for paint. As usual, Bob delved into the home stretch with verve and enthusiasm. “Parts were coming off the motorcycle like clothes on prom night,” he exclaimed, and before long those naked parts returned fully dressed, clothed in shiny paint and rich nickel plating.

The entire project took more than 2-1/2 years to complete. This essay brushes on but a few of the many obstacles and hurdles that Bob had to overcome that required 277 sketches and drawings that led to the fabrication of 257 pieces for the finished bike. No doubt an entire book could be written to showcase how this one bike came into being.

But wait, there’s more! Bob Vail actually produced a book, of sorts, about the making of his 1928 R52 Similaria, and he launched a blog ( that serializes the bike’s construction process, including photos. Bob also is taking the knowledge that he acquired from Similaria and his other bike projects to share at seminars sponsored by Skidmark Garage, a do-it-yourself shop hosted by Brian Schaffran in Cleveland, Ohio. With Bob’s blog up and running you can bet that he’ll be elbow deep into future, and interesting, Similaria projects — when he’s not riding one of his many bikes. MC

Motorcycle Classics Magazine
Motorcycle Classics Magazine
Motorcycle Classics Magazine Featuring the most brilliant, unusual and popular motorcycles ever made!