NIMBUS SWEET PEA
How a sedate Danish postman’s bike was transformed into what a Nimbus Brooklands racer could have been like – complete with supercharger
Brooklands-inspired re-imagination of the Danish postman’s favourite
Iam a long-time admirer of the four-cylinder Nimbus that was built by Fisker & Nielsen in Denmark. So successful was the innovative 1934 design that it remained in production, with very few changes, until 1959. I fancied owning one, but prices have risen in recent years and not many come up for sale in the UK. It seemed a distant dream. This all changed when a rusty Nimbus frame appeared on ebay and my successful of bid of £100 meant I’d made a small, uncertain step towards owning a Nimbus.
Over the next couple of months various other parts started appearing in my workshop, as though the frame was acting as a giant magnet. A petrol tank from Ireland and a wheel from Cornwall joined my growing collection of bits. After this initial progress things slowed down, leaving me wondering if I would ever have a rideable motorcycle. Months passed and still the Nimbus bits pile obstinately refused to grow. The frame seemed to have lost its mojo. I’d sit there in my workshop looking at the bits – and the bits sat there looking back at me.
Just when it seemed that my dream had entered a one-way street leading to a cul-de-sac, an idea of Zeppelin-like dimensions drifted into my mind. What if Fisker & Nielsen had built an official 1934 ‘works’ machine to race at Brooklands to promote their newly-launched Type C motorcycle? What would this racing machine have looked like ? What might its technical specification have been? In that moment I decided I would build ‘Sweet Pea’, the forgotten racing Nimbus. The project now had an exciting new momentum. The frame regained its magnetism and the pile of bits started to grow again. I already had a pre-war Brooklands silencer that would have been compulsory on the old race circuit just south of London. Before long, I was able to acquire a complete engine, gearbox and high-ratio rear wheel. As Fisker & Nielsen also manufactured vacuum cleaners, it’s very likely that they would have used this expertise to supercharge their race bike engine. An AMR 500 unit turned up at a good price, so that aspect was taken care of. The bike was gradually starting to take shape and resemble the vision I had in my mind.
If this bike had raced on the notoriously uneven Brooklands track, the surface would have been too punishing for the revolutionary, but somewhat frail, telescopic forks on the first production Type C machines. I’m sure they would have been replaced by a robust pair of girder forks. So I delivered my frame and wheels to Jake and Will Robbins’ Vintage Engineering Company in Hastings, where they built and expertly fitted a magnificent pair of period-style girders.
After returning home from Hastings with the rolling frame – including a 1970s 19in front wheel with a Triumph
conical hub and twin-leading-shoe brake and inverted Harley-davidson ‘Beach Bars’ – I could now start to bolt on the accumulated parts. Firstly, I needed to decide how to mount the AMR 500 and find the best routes for the pipework. In planning this layout, I had the benefit of being able to study how Lars Nielsen had plumbed a supercharger on his stupendous Nimbus, ‘Odin’s Fury’. Lars used a different model, the AMR 300, but the basic principles are the same and he generously published his design on the web. Thank you, Lars.
Staying in the fuel delivery department, I chose an SU MC2 carburettor of the type fitted to the Ariel Square Four and Triumph’s 6T Thunderbird. The MC2 is a constantvelocity carb and therefore compatible with supercharging. At the other end of the system is a Shorrock C75/C142 pressure relief valve that, hopefully, avoids you getting your nuts blown off in the event of an engine backfire igniting fuel vapour present in the delivery pipe. The Shorrock valve is mounted in a stainless steel housing that was machined from an industrial 100mm end cap. A small tube connects this housing to a genuine World War II Spitfire boost gauge that still works perfectly after 80 years.
It took me four years to transform the rusty frame into a fully-assembled motorcycle, and the first public outing was to Lorne Cheetham’s fabulous Kickback Custom Show in Cheltenham. I was surprised at the interest shown in the shaft-driven, three-speed Nimbus and thrilled when it was awarded a trophy, sponsored by renowned custom builders Lamb Engineering of Salisbury. Truth is, I was still fiddling with the Nimbus on the night before the show and hadn’t yet had the opportunity to ride it. With the Kickback show over with, I now needed to focus on getting the bike roadworthy.
I knew the supercharger would cause the engine to overheat when under load and that this potential problem needed to be addressed. As well as keeping the boost pressure fairly low, and running the engine on castor oil, I have installed an Westach EGT gauge alongside the MOS Spitfire boost gauge. If the EGT gauge indicates a rise in the exhaust gas temperature, such as when climbing a hill, I can turn a two-way tap and switch from petrol to cooler-burning methanol held in a separate tank. When the engine temperature has returned to normal, the tap can be turned back to petrol. This arrangement has been working very well and suits me better than using 100% methanol, fitting a water/methanol injection system or running on a methanol/petrol mix. Like the early Nimbus models, my bike has a hand gearchange/foot clutch set-up – and mastering this has been a bit of a challenge. I do have the advantage of living in a rural location and on summer evenings there’s hardly any other traffic, which is just as well. Overall, the bike behaves itself and is a lot faster than I expected. Yes, I’ve had a few scary moments – but there’s an excellent ambulance service locally, so I haven’t been too bothered. I hope to be out on the bike as often as possible and look forward to local bike meets and some charity motorcycle runs.
The fictitious racing motorcycle I’ve created now has some ‘racing history’ after I inserted an image of ‘Sweet Pea’ into a genuine 1930s photograph of the Brooklands paddock (the tank colour, by the way, is Halfords Garden Green, applied by Ady Trevorrow, with the ‘Sweet Pea’ signwriting by Joeby Slight). The owner/rider is the equally fictitious Major Barrington-thrubwell. The Major’s Facebook debut on April 1 was so convincing that numerous Nimbus experts fell for it – hook, line and sinker. We all wanted to believe, myself included, that Fisker & Nielsen’s rather docile motorcycle, favoured by the Danish Post Office and Army, had actually been lapping pre-war Brooklands ridden by the fearless Major at over 100mph!
I could not have completed this project without considerable help from Danish Nimbus enthusiasts. Apart from Lars Nielsen, who I’ve already mentioned, I must thank Kim Scholer, Gregers & Vibe Tuxen and a Danish resident in the UK, Bo Nielsen, provided many parts from his own workshop as well as expert advice. Thank you, Bo. Most of all, I would like to thank Chris Smith of CCS Restorations in Chilcompton, whose skills, ideas and good humour were sunshine on the darkest days.
With the bike finished, another thought has drifted into my mind. What if Fisker & Nielsen had built a land speed record contender in 1938? What would it have looked like? I already have a nice old sprung saddle and front wheel...
‘THE PRESSURE RELIEF VALVE HOPEFULLY AVOIDS YOU GETTING YOUR NUTS BLOWN OFF IN THE EVENT OF AN ENGINE BACKFIRE’