Nimbus - Technical Development 1934 - 1959

HISTORIC DEVELOPMENT

First of all, a short description of the first Nimbus motorcycles, the models A and B, nicknamed the "Stovepipe".

These first Nimbus models were developed and produced between 1919 and 1927 at Fisker & Nielsen Ltd. Copenhagen, Denmark, by electrical engineer Peder Andersen Fisker.

Motorcycles produced from 1919 - 1923 are called 'Model A', and those from 1924 - 1927 'Model B'.

These models are briefly described here, because some elements from the basic design were also applied to the Nimbus-C. The frame, which gave the motorcycle the nickname »Stovepipe«, was constructed around a sloped large-diameter tube, which had the secondary function of a petrol tank.

Models A and B are fitted with a 746 cc four cylinder in-line engine, with individually casted cylinders. And like the Nimbus-C, the engines are placed in the frame between two flat iron frame rails.

All Nimbuses are shaft driven, but because Models A and B have a sprung rear suspension, this drive shaft is more complicated and is in fact a 'live' drive shaft, whereas with the Nimbus- C, the drive shaft was rigid at first but later provided with a flexible/friction intermediate shaft.

While the front fork constructions for Nimbus-A and -B embodied various types of swing (girder fork) suspension, the Nimbus-C is fitted with a telescopic front fork. The method used (bearings, cones, etc) for supporting the front fork at the headstock is identical for all models. In addition to the mentioned head set parts, the ball on the hand-operated gear lever, and the kick starter pedal are identical parts for Nimbus-A, -B and -C. The tool box is virtually unchanged as are the tool bag, and some tools.

It is worth mentioning that Nimbus motorcycles were fitted with Nilfisk vacuum cleaner parts over the years!

BASIC CONSTRUCTION

Nimbus motorcycles from 1934 until 1959 are called 'Model C or 'Type C'. Throughout this book, we have chosen to use the terms 'Make' for the Nimbus brand, 'Model' for Nimbus-A, -B and -C and 'Version' for 'Standard', 'Standard Extra', 'Luksus/Luxus' and 'Sport'. The Nimbus-C is sometimes referred to as »Nimbus II«. The suffix 'C' is a logical consequence of the fact that Nimbus produced from 1919 - 1923 were called 'Model A' and those produced in the following years, from 1924 - 1928, 'Model B'. Nimbus-A and -B were called 'Stovepipe' by the public.

Immediately after launch, because of its distinctive exhaust sound and the shape of its petrol tank, the Nimbus-C acquired the popular nickname 'Bumblebee'.

Based upon information from the archives, 12,715 Nimbus-C motorcycles were produced. In addition, an unknown smaller number were assembled from spare parts after 1959.

The Nimbus-C was never assembled on an assembly line, but rather in small batches of commonly 25 units. This provided the opportunity to implement many changes along the road. Most of these changes are described in this book, but first of all we will deal with the basic construction, viz. those parts that remained basically unchanged during the years of production:

The Nimbus-C is a middle weight motorcycle with a flat-steel frame which is formed around the petrol tank and the engine. The front has been fitted with telescopic forks with front wheel, mudguard and flat steel handlebars, and at the rear, with an unsprung rear wheel with mud- guard. Both wheels are fitted with drum brakes; the front brake is operated by a hand lever, the rear brake by a pedal and a pull rod. The fuel tank has a capacity of 12.1 litre, including 1.5 litre reserve.

The four-stroke engine is a four cylinder in-line engine with overhead valves and cam shaft. The cylinder block is cast in one piece. It has a standard bore of 60 mm, a stroke of 66 mm and a cubic capacity of 746 cc. The pistons are made of aluminum and come in several versions with different dimensions for different compression ratios and power output (this will be discussed later). The engine is air cooled, partly by means of the cooling fins of the cylinder block and head, and partly by the cooling fins of the aluminum crank case, which functions as an oil sump. The aluminum gearbox- and camshaft housing also play a role in the cooling. The overhead camshaft is fitted in a aluminum housing and is driven through the dynamo by means of gear wheels on the Crankshaft. The engine's power is transmitted by a single dry plate clutch, a gear box with three gears and a drive shaft to the pinion and crown wheel of the rear wheel. The lubrication of the Crankshaft, pistons, cam shaft and gear box is done by means of a mechanical oil pump which is driven by the dynamo shaft.

The engine is started by means of a kick starter, which is fitted in the crank case and operated by a pedal arm at the left side of the engine. The carburettor of F&N's own make is of a horizontal flow type, fitted on the intake manifold; it has a vacuum pipe to the cylinder block.

The flat steel handlebars have one twist grip to operate the carburettor (the throttle twist grip) and one to operate the lights (ignition on/off, main beam / low beam). In addition, there are two hand levers, one for the front brake and one for the clutch. The handlebars have a built-in ignition switch and a push button for the horn. The centre stand and the tool box are placed under the engine. On military machines, the toolbox may be placed behind the pillion seat. Furthermore the frame is fitted with rubber knee supports and foot rests.

The electrical installation is 6 volts and includes a dynamo, a cutter and voltage regulator, and a battery.

It also features a combined ignition and distributor with H.T. leads and spark plugs. Horn, head and taillights are operated from the handlebars, while the brake light, which is fitted in the tail light housing, is operated through a switch activated by the rear brake lever.

CONTEXT

When the Nimbus-C motorcycle was first launched, it was quite naturally compared with other brands. The front forks especially drew a lot of attention.

A comparison between the front fork design principle of the new Nimbus, the British, and American versions respectively is outlined on F&Ns drawings (shown here on page ).

Construction of the front forks for the Nimbus-C can be seen on a 1933 prototype, built by father

(F&Ndirector) Peder.Andersen Fisker and his son civil engineer Anders Fisker.

In a number of details, the prototype Nimbus-C differs from final production: the most obvious visual differences are the front mudguard and the tail light, as well as the engine oil filling orifice being on the right hand side.

Furthermore, the prototype shows the exhaust pipe apparently fitting over the manifold as opposed to the motorcycles produced later in the series, where the exhaust pipe fits into the manifold. In addition, the exhaust manifold of the prototype has no heat shield.

This early machine may, however, be characterized as a 'final version', which was, at the time, far advanced in a considerable number of engineering aspects. There were problems and shortcomings, but these were corrected during the first year, without any cost to the customer.

PATENTS

When the Nimbus-C, later nicknamed »Bumblebee«, was presented to the Danish press on April 20, 1934, A/S Fisker & Nielsen underlined a number of Danish patents related to the construction of this model.

The advantage of having patents is of course protection against imitation and consequently competition in the market, but also to allow third parties to license the patents and pay for them.

Nowadays there are not many Nimbus-C related patents that are applied in other motorcycles, if any.

The patents issued from March - April 1933 are:

Danish patent No. 49451

Motor cycle frame

The flat steel frame is one of the most characteristic features of the Nimbus. It is however not the use of flat steel that is chosen to be patented, but the construction of the frame. The frame embodies the fuel tank and the engine/gearbox and does not just support these. The construction of the frame consequently led to several options that were patented, e.g. the rear mudguard to be hinged and folded back, the position of the centre stand, and the fuel tank fastening. The basic construction of the frame was maintained throughout the years, but the fastening of the headstock varied.

Danish patent No. 49600

Front wheel fork construction for bicycles, especially motorcycles.

The basic principle of a telescopic spring unit is that a tube can move up and down in another tube. The patent includes both the principle of the telescopic spring suspension as well as the attachment of the front wheel to the front forks. The principle of the telescopic suspension was developed at the same time as BMW in Germany, and remained to be used on all later motorcycles. Apart from that, the front fork is one of the Nimbus-C parts that has seen most changes.

Danish patent No. 49189

Handlebars for bicycles, especially motorcycles.

The flat steel handlebars are one of the most characteristic parts of the Nimbus. This patent refers to the construction, and opens...