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.

First TLS brake came out in 1968 and this was the only year it came with the operating cable parallel to the road. The 1969/70 versions the cable was parallel to the forks as in the photo. The 1971-73 brake is also known as the ‘comical’ brake as it’s performance is laughable – at least until you hit the side of a bus!

The 500’s from 68 onward until they were discontinued in 71 used a 7″ TLS brake similar to the 68 – 70 8″ brake.

Here’s a good one to try – on second thoughts if you like scraping your footpegs on your OIF Trumpy read no further or it will only worry you. Shaddy’s new to these, he’s been riding Hinkleys for years. Round in his shed he showed me this on the 72 Bonnie he’s doing up. With the bike on the centre stand he grasped the front wheel firmly between his knees and twisted the handlebars from side to side. Watch the forks stanchions (upper tubes) bend and twist as you are doing this. Shaddy was quite alarmed by this and asked if it was normal. As an old Meridan owner from way back I was able to reassure him the he was in possession of a finely tuned example of British engineering at its peak and that they have a reputation for fine handling. While this is true I’ve felt a bit uneasy hammering round a bumpy corner ever since. When I got home I tried this on my 07 Sprint – stiff as the Pope in a nunnery, good show Mr Bloor!