Hand-crafted in Ireland, Don Cronin’s latest creation looks like the sort of bike that Captain America would ride, not a Royal Danish postman.
Words and photography by Phillip Tooth
When it comes to choosing an engine for his Medaza motorcycles, Don Cronin has form. Forget about a big Harley mill from The Motor Company, or the latest big-bore Triumph twins and triples. He’s used a 500 single with a ‘bacon slicer’ external flywheel that once powered a Guzzi Nuovo Falcone, a V-twin liberated from a Morini Camel enduro, and even a utilitarian two-stroke lifted from an MZ 300ETZ. So, when he wanted an inline-four for his latest project, he was guaranteed to come up with something different. And in a world of high-revving, huge horsepower fours, there’s nothing quite like a Nimbus.
Vacuum cleaner manufacturer Nilfisk branched out into making motorcycles way back in 1919, but the Nimbus we are interested in was launched in 1934. The unit construction 750cc engine featured an overhead camshaft. Just like the original MG sports car, the vertical camshaft drive to the bevel gears doubled as the armature spindle for the dynamo, which was mounted in front of the cylinder block. The lower half of the crankcase was cast in aluminium and carried a couple of litres of oil, but the upper half of the crankcase and the finned cylinder block were a single piece of cast iron. The detachable one-piece cylinder head, with its hemispherical combustion chambers, was also cast iron and incorporated the inlet manifold. An aluminium camshaft housing was bolted to the head, with the rockers supported in ball and socket bearings operating on the vintage-style exposed valves.
The one-piece, drop-forged crankshaft runs in two large-diameter ball-bearing journals, with the flywheel incorporating a large single plate clutch fixed at the rear. A three-speed gearbox was bolted to the clutch housing, with drive shaft to the rear wheel.
The cradle frame was made from lengths of 40mm x 8mm flat steel, riveted to the steering head and the unsprung rear end. It might not have been sophisticated, but it was cheap and easy to make, and practical. Up front was the first modern telescopic fork, patented in 1933. That was two years before BMW introduced its tele fork, but the Germans pioneered oil damping. Hold on to your hair, Nimbus lovers. The standard 1934 model managed with 18bhp, but a sports version introduced for 1937 had the compression ratio upped to 5.7:1 and delivered a thrilling 22bhp. When the four was revved towards the 4500rpm limit, the straight-through exhaust with its little fishtail really buzzed, which is why Danish enthusiasts nicknamed the Nimbus the ‘Humlebien’, or Bumblebee.
But the Nimbus was never meant to be a sports motorcycle. It was a workhorse, and the biggest buyers of Denmark’s homemade motorcycle were the Army and the Royal Danish Post Office, which finally pensioned off its fleet of sidecar outfits in 1976. Numerous upgrades were made to the engine and cycle parts before production officially ended in 1959, by which time more than 12,000 had been built, but factory policy meant that earlier models could easily be updated to the latest specification.
If Don wanted a 1265cc Indian or a 1301cc Henderson Four as his donor, he would have had to pay north of 100,000 Euro. “And those are big engines,” he says. “A late Henderson measures 360mm across the front of the crankcase. The Nimbus is tiny, only 200mm wide.”
You can pick up a fully restored Nimbus motorcycle for about 10,000 Euro, while a project bike costs less than half that. And that’s what Don shipped from Denmark to his home in Ireland.
Confirmed as a 1946 model, there was rust under the frame paint and a Japanese fork was a non-Nimbus upgrade, but at least the engine turned over. “Denmark’s Nimbus Touring Club is brilliant. They are so proud of their national motorcycle, and everyone is so supportive,” he says. “They have all the spare parts that you can possibly need, including new crankshafts, cast iron cylinder heads, and the combined cylinder block/crankcase top – all beautiful quality and they don’t cost silly money.”
But Don had no intention of building a stock motor: “The engine was designed for long-term reliability, not performance. The ports of the integral inlet manifold are square-section with right-angle bends, which is never great for gas flow. It is impossible to clean them up. And besides, the single Nimbus carburettor has a tiny 22mm choke. You could barely pass a man’s wedding ring through it!” His solution? Design a new cylinder head with detachable inlet manifolds that carry twin carbs.
Contacts count when you are designing and building a special. Don was at a party in the Californian home of one of the members of Eagles of Death Metal when he met Mark Atkinson. “He is Speed of Cheese Racing,” laughs Don. “What a name! But after talking for only a few minutes, I realised that he’s the real deal. Mark broke world speed records at Bonneville on his RD400 and runs his own machining and design service in Salt Lake City.”
Don used Solidworks to create the drawings that were zipped over to Mark for machining from a billet of 6082 grade alloy.
New carburettors, valve guides, valves and springs were supplied by Jesper Jensen, of Nimbus-Shop.com. Don did think about making the cylinder block/crankcase top in alloy but soon changed his mind. “It would have been insanely expensive. There’s only a couple of millimetres between the bores, so there’s not enough room to fit cast iron liners unless I used smaller pistons. And I didn’t want a 600cc Nimbus!”
Don is a sculptor whose work is on display in art galleries, museums, and public spaces around the world. In a review of one exhibition, The Irish Times newspaper reported that “Don Cronin’s sculptures exude an air of technical elegance… of moving at speed through space.” If only they knew about his passion for motorcycles and getting grease under his fingernails.
Don has his own foundry, where he casts huge bronze and aluminium pieces, so using the ancient lost wax process to cast a pair of detachable inlet manifolds, each with ports in a V formation, was easy for this master craftsman. The same process was used to make the ribbed exhaust manifold, but the carburettor shields were sand cast. “The Americans call those bird catchers,” says Don. “Only a bumblebee can get sucked into one of these tiny carbs…”
Instead of shells, the Nimbus has vintage-style big-end bearings made by pouring molten white (babbit) metal into the eye of the connecting rods and machining to size. Continuing the vintage theme, instead of force-fed oil, the bearings are lubricated by splash, so Don was expecting them to be knackered. But when the crankshaft was stripped, they were well within specified tolerances – and that tells you something about the legendary long-term reliability of these engines. The crankshaft was carefully balanced, new ball bearings fitted to the camshaft housing, and the oil pump rebuilt.
Danish riders might think that the riveted steel strip frame is an iconic part of the Nimbus, but it had to go – Don crafted one from tubular steel. “In the old days, frames were made by brazing steel tubes into malleable cast iron lugs,” he explains. “I wanted that vintage look but couldn’t afford to make the castings, so I fabricated my ‘lugs’ by TIG-welding sections of steel before cutting them in half along the length and welding them over the tubes.” You could have fooled me…
Nimbus might have pioneered telescopic forks, but Don went vintage again with girders and asked the lads at Cork CNC to machine the legs to his design. Strong but light, excess metal has been machined away from the inside faces. The contrast between the polished and satin alloy is often seen in his sculptures, and his artist’s eye has repeated the horizontal cooling vents in the front brake drum as fins on the fork legs. While the triple clamps are machined from billet, to avoid distortion, both top and bottom fork links were machined in one piece from stainless steel. Don bent his own handlebars and fabricated the inverted levers and open twist grip for that vintage look. “There’s no point in making it look like a Superbike if there’s less than 30bhp to play with,” he adds.
Styling cues came from the streamlined Milwaukee Road ‘A’ Class, the largest and most powerful steam locomotives to thunder across the United States in the 1930s. Check out the hand-formed alloy headlamp cowl with its stainless-steel visor and you’ll see what we mean, but don’t forget to peek at the Fournales air shock hidden behind it.
And while you’re paying attention, take a closer look at that front stopper. Recognise it? That’s an enclosed, ventilated twin disc brake with inverted twin-pot calipers from a 1982 Honda CBX550F. Don’s daily ride is a modified (surprise!) Mk.3 Le Mans. “Moto Guzzi big twins have linked brakes, so I realised that the foot pedal master cylinder is just what I needed to operate the Honda set-up,” he says. The rear brake started life as a Nimbus unit, but you wouldn’t know it. Now, the steel drum has three alloy ribs shrunk on, while the new brake plate, fabricated from alloy plate, is a work of art in itself. Check out the brake lever. Is that a propeller blade?
Don ordered alloy wheel blanks from Californian company MSI. These have the rim and bead pre-formed, with a ¾-width solid slab in the middle. Don was limited by the 19in tyre size, matched to the new solo gearing in the final drive crown and pinion, but he had a free hand in the design file that he sent to Mark, at Speed of Cheese. There’s a hint of Honda Comstar in the way the brakes appear to be bolted to the wheel rims, and, of course, more of that sublime satin and shine contrast.
Just about everything on this motorcycle has been designed or made by Don. He continued to work with Cork CNC, sending more files for machining components like the silencer box with its internal baffles, toolbox, fuel cap and neat little tail-light. But he hand-formed the 7L (1.8 US gallon) fuel tank, mudguards, and frame side skirts. Although much of the build was done during Covid-19 lockdowns, it wasn’t until the spring of 2023 that he was ready to ride… and we were there to see the action.
Don wasn’t interested in checking the power on a rolling road dyno before our test ride. “The twin carb cylinder head has probably made bugger all difference to the performance,” he laughs. “For me, it’s all about the challenge – can I make one?”
Starting is effortless – a gentle swing on the kick-start pedal and the four immediately purrs into life, the external valves rustling in the breeze. But after Don snicks into first and opens the taps, the revs rise rapidly to a frenzied buzz as he charges up the Healy Pass, moving smoothly through second and into top and sweeping through curves with effortless grace. This looks like the sort of bike that Captain America would ride, not a Royal Danish postman.
“Sounds more like a wasp than a bumblebee!” says Don, with a satisfied grin. Medaza Wasp? Sounds good to me!
Thanks to:
Special thanks to old friends Chris Harte for wiring the bike, Mick O’Shea for invaluable help and advice, and Jim C for generous time and effort on the engine build. Check out other creations at medaza.com
