The right time for some historic speedometers
PREVIEW: Classic Car, Motorcycle & Automobilia Sale, Dee Atkinson & Harrison, Sledmere House, Driffield, East Yorkshire, 3 November
Three examples of the revolutionary Bonniksen isochronous speedometer are included in the Dee Atkinson & Harrison sale at Sledmere House, East
Yorkshire on Saturday, 3 November – two 50mph and one 100mph – with estimates from £500-800.
Danish horologist Bahne Bonniksen introduced the isochronous speedometer in 1912 that was a great improvement on what had gone before as it featured a clock-type movement. Bonniksen had moved to the UK as a young man to learn his trade and his business was based at Coventry.
Previous speedometer mechanisms relied mainly on ‘governor’ type systems where spinning bob-weights would be flung further outwards as the road speed increased. These devices tended to be inaccurate as they were affected by vibration, friction and wear in the moving parts. Bonniksen reasoned that speed is simply distance divided by time. His mechanism measures time using a watch-type movement, with a balance wheel and escapement similar to that in a clock or watch. Distance is measured by counting the number of revolutions of the speedo cable, which is proportional to road speed. These factors are measurable to a high degree of accuracy. Particularly clever is the way the movement samples time and distance over fixed two-and-a-half or five-second increments and feeds the resulting speed information to the indicator arms.
Also included in this sale is a Halda Speedmaster Pilot Rally Timer, c1960s, with dash mounting bracket and cable, as used in a 1966 Saab 99 V4, estimate £150-200.
The sale also includes a large number of Triumph spares including useful items for models including Renown, Mayfair and GT6/Vitesse, plus some TR7 parts and bits.