Did you know that thanks to Denby Marshall there is no need for expensive dyno’s to work out the horse power of your bike? Using this formula my 650 Bonnie makes 28.9 HP which explains why I have trouble chasing Hinkleys up the Kaimai Ranges. I’ve had the Bonnie for many years and it’s on it’s third rebore (60″ over) which gives it a staggering additional 1.25 HP!
The Denby Marshall formula is :- Bore x Bore x Stroke x No Cylinders x rpm / 200,000,000 = HP where bore and stroke are in mm.
i.e. for the standard Bonnie 71mm x 71mm x 82mm x 2 x 7000 / 200,000,000 = 28.9 HP
This is extremely useful information until I did the same calculation for my Hinkley Triumph Sprint which calcuates as a miserable 61.5 HP. Geeez it’s not the power of the motor that slows the Sprint going up the Kaimais – it’s only limited by the size of my balls!
Perhaps Mr Denby Marshall failed to account for Mr Hinkley’s ingenuity when he developed this formula. Denby also invented revolutionary new valve gear for steam engines in the early 1900’s and is better known for authoring books on railway history and, most interestingly, a history of the post office.
Try this for your bike by entering the data from your Specification in the table below.