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1949 Nimbus Model C

The Danish Model C inline 4-cylinder makes speed easy to obtain but double-declutching is best for a transmission lacking synchronizer rings.

front left
by Nick Cedar

“I saw my first Nimbus in 1974 in Copenhagen,” remembers Allen Siekman. “I was chasing family heritage. The rider had stopped at a traffic light. He was wearing a Barbour jacket and half helmet and goggles, and looked right out of the Thirties. I was mesmerized by the ends of the rocker arms waving at me on the sidewalk.”

In the month Siekman was in Denmark, he was able to watch several more Nimbuses traverse the streets. Many years later, Siekman finally has his own Nimbus. He has found it is as enjoyable to ride as it is to look at.

Engine: 746cc air-cooled 4-stroke SOHC inline four, 60mm x 66mm bore and stroke, 5.7:1 compression ratio, 22hp @ 4,500rpm
Top speed: 75mph (est.)
Carburetion: Single barrel carburetor, made in house by Nimbus
Transmission: 3-speed, left foot shift, shaft final drive
Electrics: 6v, battery and coil ignition, 6v generator
Frame/wheelbase: Spring steel cradle frame/1,435mm (57 inches)
Suspension: Telescopic forks front, rigid rear
Brakes: 7in (180mm) drums front and rear
Tires: 3.50 x 19in front, 3.50 x 19in rear
Weight (dry): 375lb (170kg)
Seat height: 28in (711mm)
Fuel capacity/MPG: 3.3gal (12.5ltr)/45mpg (est.)

Dutch made

Nimbus, possibly the only motorcycle ever produced in Denmark, hasn’t been built since 1960, but continued popularity has kept many on the road. The Danish Nimbus Club estimates that 4,000 Nimbus motorcycles are presently registered in Denmark, and 2,000 in the rest of the world. Since only 1,300 were built between 1919 and 1928, and 12,000 were built between 1934 and 1960, that is an excellent survival rate. Many people, especially in Denmark, continue to ride them on a regular basis. Nimbus motorcycles never had a U.S. importer, and most of the approximately 275 in the U.S. came to this side of the pond via an enthusiast who met one of the engaging 4-cylinder machines in Europe. Some people in the Antique Motorcycle Club of America remember a club member who brought a container load of Nimbuses (Nimbii?) to a meet and sold them for $495 each.

Some motorcycles are awe-inspiring, The Nimbus is friendly. The Danes nicknamed it “Humlebien” (Bumblebee), after the buzzing sound it makes going down the road. “It has a vintage, comfortable riding position. I do enough laying down flat on the tank when I race,” says Siekman, who, when not chilling out and enjoying the scenery on his Nimbus, campaigns a Honda CL160 with a 12,000rpm redline.

left drive shaft

Image by Nick Cedar

The shaft drive runs on the left side of the bike to the rear wheel. The rear footpegs mount directly to the frame.

The friendliness of the Nimbus does not mean that it is mechanically primitive. All Nimbus motorcycles built after 1934 are powered by an inline 4-cylinder engine with an overhead camshaft. Power is transmitted to the rear wheel via a driveshaft and bevel gears. Craftsmanship and good machine tooling are evident in the finished product: the 4-cylinder block, including the upper half of the crankcase, is cast in one piece. The well designed and reliable carburetor has an accelerator pump. In one of the few nods to vintage design, Nimbus valves do not have covers, which is why you can see them bouncing up and down at stops.

In the beginning

Fisker and Nielsen of Copenhagen started life in 1906 as a manufacturer of electric motors. In 1910, the company graduated to consumer products with a well-made and popular vacuum cleaner. Management got the idea of building a motorcycle after World War I.

At the time, there was a lot of experimentation in the motorcycle industry, with factories trying different engine configurations. Inline 4-cylinder engines were common, due to quieter and smoother operation than the single cylinder machines that made up the bulk of the bikes on the largely unimproved roads of the time. Although inline 4-cylinder motorcycles are often thought to be uniquely American designs, the first Four was built by FN in Belgium, and Pierce (manufacturer of the first American Four) got the idea from a FN he saw while touring Europe. Fisker and Nielsen may also have gotten the idea of a Four from FN.

valve-springs

Image by Nick Cedar

The exposed valve springs are visible on the left side of the head.

The major difference between European Fours and U.S. Fours was horsepower. Americans bought Fours for touring on long, straight roads, and wanted to get wherever they were going in a hurry. Fours were a mainstay of U.S. police forces that needed to run down speeders and rumrunners. In contrast, until the 1950s, Danish police were happy with a motorcycle that could only do 75mph in short bursts. Before World War II, the Danish speed limit was 50 miles an hour.

P. A. Fisker, the designer of the new motorcycle, used a large diameter tube as the frame backbone and gas tank, and spring steel for the rest of the frame, with the tank welded in place. The front end used trailing link suspension, with springs connecting the frame rear to a swingarm. The valves on the four freestanding cylinders were pushrod operated. This bike was popular through the 1920s, but motorcycle production competed with other items produced by the factory. When orders for vacuum cleaners overwhelmed the production space, Fisker and Nielsen stopped building bikes.

Moving forward

In 1934, Fisker and Nielsen completed work on a new factory building, and started building motorcycles again. The newly designed Model C had many ingenious details. The generator shaft also ran the overhead camshaft. All bearings were pressure lubricated. The bike was the first production motorcycle with telescopic forks — one year before BMW. Unlike Models A and B, there was no rear suspension, but the seat was insulated from bumpy roads by either springs or (as on Siekman’s Model C) thick rubber bands. He says they work surprisingly well. The wet sump at the bottom of the crankcase was cast from aluminum. The pistons have two scraper rings, instead of the one scraper used on most bikes of the time. The 3-speed transmission works through a car-type single plate clutch. Although the first bikes built in 1934 were hand-shift, by 1939, the factory had changed over to foot-shift. The drum brakes were enlarged from 150mm to 180mm during this period as well, and the telescopic forks received oil damping in 1939.

speedometer

The Model C’s frame was all spring steel, except for the tubular telescopic forks. Even the handlebars were made of pressed steel, in unit with the instruments.

On August 17, 1939, just before Germany invaded Poland, The Motor Cycle, a British magazine, published a test of a Nimbus. As usual with press reports of the time, more effort was made to please a potential advertiser than to provide an honest evaluation of a motorcycle. Kickstarting was “effortless,” slow speed running was, “like a car engine … I was soon zooming along, reveling in the smoothness of the 4-cylinder unit.” Speed was never a Nimbus strong point, but The Motor Cycle claims, “the machine would accelerate quickly up to 60 m.p.h. in top.”

Learning to shift

One of the few annoying features of the Model C Nimbus is the way it shifts. The transmission was designed without synchronizer rings, so it is prone to gear clashing and grinding. Allen Siekman says that shifting up on the 3-speed box is usually no problem, “but when I shift down, I have issues.”

The Motor Cycle tried to soft pedal the double clutching needed to successfully shift. “Gear changing required care and, as with nearly all gear boxes that run at engine speed, the best results were obtained by double-declutching. It was not that there was any difficulty in engaging the gears, because the positive stop foot change was efficient.” Siekman also points out that the engine has a very wide power band, and a rider doesn’t have to shift nearly as often as on other bikes. “When slowing to a halt I just wait to downshift till the bike is stopped.”

Nimbus-engine

Image by Nick Cedar

The 746cc inline four makes 22 horsepower at 4,500rpm, fed by a single barrel carb.

Production of the Model C continued on a very limited basis during the Nazi occupation, since most raw materials were diverted to the war effort. After Denmark was liberated, Fisker and Nielsen considered upgrading the bike, but decided that meeting pent up demand was more important than designing an up-to-date model. Postwar Model Cs came in two versions: the regular, low-compression version and the sport model, boasting 5.7:1 compression and 22 horsepower. The engine in Siekman’s bike uses the same pistons as the Sport model, even though the first owner of his bike was the Danish Army. After a period of military service, bikes were declared surplus and sold to the general public.

End of the road

By the late 1950s, the postwar motorcycle boom was over, and the Danish post office and military (government purchasing accounted for at least 20 percent of Nimbus production) were phasing out motorcycles. Private riders often preferred the faster British machines. Coincidentally, the market for vacuum cleaners, Fisker and Nielsen’s other major product, was booming. The factory converted the space used for motorcycle production into vacuum cleaner assembly, and the last Nimbus motorcycles were shipped in either late 1959 or early 1960, depending on who you talk to. The Danish Post Office continued to use some Nimbuses until at least 1972, proving how reliable these bikes can be. In 2014, an entrepreneur tried to get the Nimbus brand rolling again, but failed to locate enough investment to make the project a reality.

Despite the end of production, the friendliness and reliability of the Nimbus kept it on the road. Owner’s clubs kept the Nimbus torch lit and provided a venue for buying and selling. There are some Nimbus clubs in the U.S., and the Chicago club and the U.S. Nimbus Club are particularly active.

Allen’s Nimbus

Allen Siekman came home from Denmark with pleasant memories, including the Nimbus motorcycles, and went back to work. He got interested in classic Japanese motorcycles, and started a collection. He learned to race. Eventually, the desire for a Nimbus percolated to the top of the list, and Siekman started the search for one of his own.

Nimbus-side

He decided that the best bet was to look for one in Denmark, and, as a starting point, tried to join the Danish Nimbus Owner’s Club so he could look through the For Sale ads. Despite hours of trying, it proved impossible to join the Owner’s Club without a Danish address. Siekman then contacted one of the Danish Nimbus parts distributors, and struck pay dirt. The great folks at Nimbus-Shop in Denmark not only found him a nice bike, but also crated it and helped with arranging shipping.

Before he brought in the Nimbus, Siekman had imported several classic motorcycles from Japan. In the process, he located a skilled customs broker who makes sure all the paperwork is in order and eases the import process after the ship docks. Once he got the call that the crate was available for pickup, Siekman went to get the bike with his 99 year old father-in-law, who helped with uncrating and had a margarita to celebrate. “He had a ball.” Of course, once the bike was out of the crate, Allen just had to try starting it. “It started in three kicks.”

During the first month Siekman had the bike, the engine would run rough after it warmed up. Siekman ordered a carburetor rebuild kit (most drivetrain parts are easily available) and rebuilt and tuned the carburetor, which fixed the problem. Nimbus carburetors were made in house and are easy to work on and reliable. Since then, the Nimbus has run well, with a few minor issues typical of classic bikes that haven’t run in a while. The coil fried during a Danish Days rally in Solvang, California. Once the new part, ordered from Denmark, arrived and was installed, the Nimbus started immediately. “Now it’s seeping a little oil — I need to replace an O ring on the camshaft housing.” The tank, frame and wheels needed no restoration other than a good cleanup.

front back Nimbus

The Nimbus weighs just over 400 pounds with oil and gas, has no rear suspension, and the telescopic front forks are not up to modern standards. One would expect that the bike handles like a wheelbarrow, but both the The Motor Cycle and Allen Siekman attest that the Nimbus actually handles very well. Siekman explains, “It is smooth on smooth pavement and tracks very well. It seems much lighter than its actual weight and is very stable in corners. Steering feels quick and nimble.”

“The rigid tail does make you understand the benefit of having rear suspension. On rough roads the rear tends to hop and move about. It’s what bikes with no rear suspension do but once you get used to it, the ride feels safe — if not comfortable when a sudden pot hole or bump is encountered. The steering is quick and the bike is easy to flip side to side in the twisties and it tracks well on big sweepers. The only thing it is not good at is choppy, bumpy roads and it does not like freeways. The bike has a top speed of around 65mph and is happier cruising at 50-55.”

Like most older motorcycles, starting the Nimbus requires a routine. As Siekman explains, “Key off, on cold mornings choke to full. Since the carburetor has an accelerator pump, two twists of the throttle delivers a bit of gas to the intake, kind of like a tickler on a British bike with an Amal. Kick the engine through twice. Key on and kick. It usually starts on the first kick. Once it starts, turn the choke lever to the half choke setting and let it warm up. Once it is warm it will usually start with one throttle twist and half or no choke.”

Allen-Siekman

Image by Nick Cedar

Allen Siekman and his Nimbus, which he imported from Denmark.

The friendly nature of the Nimbus extends to easy maintenance. There is no chain to lube or adjust. Allen continues, “I change the oil [nondetergent 30W] every 1,000 miles but that may be earlier than needed. The oil looks pretty clean when I drain it. I do check valve adjustment regularly. With the rocker arm and valve exposed to the open air I would expect wear and frequent adjustment. To my surprise the valve clearances are always spot on.”

It is a truism that one never sees a motorcycle outside a psychiatrist’s office. Siekman says that a Nimbus is good therapy. “It’s incredibly smooth and not very powerful. It doesn’t go very fast, so if you are not in a hurry, it is very relaxing. Riding a Nimbus just feels good.”

“You cannot ride this bike and not smile.” MC

Updated on Oct 6, 2021  |  Originally Published on Sep 28, 2021

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