The Bonniksen Boy
Restoration, Repair and Supply of Bonniksen Speedometers and Ancilliaries
Technical and Historical Detail
A Brief History of the Development of the Bonniksen Isochronous Speedometer
The isochronous speedometer was first patented in 1912 by it's inventor Bahne Bonniksen. Bonniksen (1835-1935) was a Danish horologist who came to England as a young man to learn his trade. He was a watchmaker of considerable talent and is most famous for having invented the 'Karussel' mechanism in 1892 which revolutionised watch movements. When the isochronous speedometer became available in 1912, it likewise revolutionised the accuracy of the speedometer due the clock-type movement at it's heart.
Before 1912 speedometer mechanisms relied mostly on 'governer' type systems where spinning bob-weights would be flung further outwards as the road speed increased. These devices were rather inaccurate, being affected by vibration, friction and wear in the moving parts. Bonniksen's new speedo used a totally fresh approach. He reasoned that speed is simply distance divided by time. His mechanism measures time using a watch-type movement - it has a balance wheel and escapement familiar to anyone who has taken apart a watch or small clock. And measures distance by 'counting' the number of revolutions of the speedo cable, which is proportional to road speed. Both of these factors are measureable to a high degree of accuracy. The clever bit of the movement is the way it 'samples' time and distance over fixed 2.5 or 5 second increments - and feeds the resulting speed information to the indicator hands.
How to read your Speedo - the Bonniksen speedo has two 'speed' hands, not a single one like any other speedo you may come across. Click here to learn how to read what were poetically described as 'those whirling hands'!
How to date your Speedo - numbers can be found stamped on the case and movement of most Bonniksen speedos, but they are not -to my knowledge anyway - an indicator of date of manufacture. The following guide may help you to estimate the date of your speedo, or what you should be looking for when seeking a speedo for a particular machine.
The speedo was first patented in 1912 and had a dial reading up to 50mph. It had a heavy milled bezel and thick bevel edged glass. Two subsidiary 'trip' dials were available as an option. A speedo identical to this one is shown in 'The Motorcycle' on 26th September 1912.
A speedo identical to this one is shown in 'The Motorcycle' on 11th December 1913.
Some time between 1916 and 1919 Bonniksen sold the rights to his speedometer to Rotherhams of Coventry - a successful watchmaking company. This example is identical to one featured in 'Motorcycling's' Olympia Report of 16th November 1919. Note that the dial now reads 'Made by Rotherhams Coventry'. Before 1916 the dial simply reads 'B Bonniksen Coventry'.
In 1922 Bonniksen brought out another patent showing an enhancement to his 1912 design. The facility to measure elapsed time on a journey as well as distance was provided by the left hand subsidiary dial, both time and trip dials now having two hands each. The hands are sadly missing on the example, but if you click here you will learn how to read the improved design.
About this time the (no doubt cheaper to produce) spun bezel replaced the heavy milled one and plain glass replaced the thick bevel edged type. Advertisements in the motorcycling press showed both types in 1922. This example shows the new bezel design.
1925 saw the introduction of the 100mph dial, shown here with the optional, cheaper 'non-trip' movement. The black face was also an option from the early days, the majority of customers opting for the nickel plated variety.
A time and trip 100mph speedo with black face. The white hands may have been fluorescent, the speedo then being useful for night riding.
A 'trip only' 100mph speedo with nickel plated brass dial. Some dials were made in aluminium and these oxidise over the years leaving very dilapidated-looking faces.
Production of Bonniksen speedometers probably ceased in the early 1930's. By this time the 'chronometric' movements made by Jaeger/Smiths were equally accurate but considerably cheaper to produce.