Turf Zone – Fishmongers
Good news! After some considerable effort to get my credit card to actually authorise the purchase, I’ve placed an order for a new Brompton C Line Urban 2-speed, direct from Brompton as they seem to be as hard to find as cyclists that actually ring their bell when passing from behind. A pet hate of mine. The scarcity might be down to the colour choice I was after, that of Bumblebee Yellow, also one of the most popular colour choices. And at this point I must mention that this Brompton is henceforth named Mr Bumble. With the Brompton factory closed over the Christmas and New Year break, I hope to get delivery from the 6th January onwards.
The plan with this bike, which comes with two gears, is to convert it to single speed. Yes, I know that seems daft to most people but I’ve converted a few bikes to single speed over the years and love the neatness and simplicity that riding single speed offers. Yes, you may find yourself struggling on the hills, even walking, or spinning out on the flats, but the simplicity of not changing gear and just riding along can be very meditative. It’s simple, neat and saves some weight, though not very much, is quieter than gears and you get a fantastic workout.
The conversion to single speed on this model is fairly straight forward if you are used to working with bicycles and only requires a small number of additional components and some basic bike tools. Basically, all that is required is to change the sprocket and the chainring to suit your personal requirements. A new chain is recommended if the existing chain is old/worn. In this case, the bike is new, so not required. The chain may require shortening or a longer chain may be needed, depending on the final sizes of the sprocket and chaining.
The required components, in this instance, are:
- Brompton 12T sprocket set including spacer.
- Brompton chain tensioner for single speed.
- Brompton 44T chainring and guard assembly.
The first task was to decide what final gear ratio I was after. The gear ratio is how many times the rear wheel turns with one rotation of the cranks. It can be calculated and described by various different methods but Brompton use the term “gear inches” so I’ll stick with that for ease. Note that the size of the tyre also affects the gear ratio, though with the narrow tyre range available for the Brompton, that is not significant.
I did some testing on Mr Orange, my other Brompton, a 6-speed C Line Explore, and drew up a chart of the gear inches for all the combination of gears available from the 3-speed hub gear and the 2-speed sprockets. This was to give me something to try and help determine what combination of sprocket and chainring would be best for single speed and my own abilities.
The C Line Explore has a 50T chainring and the sprockets are 13T and 16T. T stands for the number of teeth. With the hub gear in the middle setting, which equates to 100%, this gave me two gear inch options, 69.2″ on the 12T sprocket and 51.6″ in the 16T sprocket. I also added to the chart all the sprocket and chainring options readily available to purchase from Brompton. Note, I’ve not looked at any after-market products.
I then spent three hours cycling around Edinburgh to test things out and found that the 69.2″, that’s 50T/16T, the lowest gear, was generally fine but tended to have me spinning out in the flats and downhill, though that was not too much of a problem. The other gear, 51.6″, running 50T/12T was generally way too high a gear for me, especially on the hills. So, what I was looking for with the single speed was something in between 51.6″ and 69.2″.
This gives an average about 60.0″, so that’s where I’m starting, ordering a 44T chainrings and a 12T sprocket which will start me off with a 60.9″ gear. I know that the C Line Urban already comes fitted with a 12T sprocket but I need the spacer from the 12T single speed sprocket set. I may need to order the 13T/16T sprocket set and a 50T chainring if I want to play around with the nearest others ratios, i.e. 56.0″ and 63.9″. There is one unknown, and that is if various combinations of sprockets, 12T, 13T or 16T, and spacers, will actually work together. No doubt time with tell. Fingers crossed all the parts are compatible.
The final component in the conversion to single speed is the chain, specifically the chain length. Each combination of sprocket and chainring requires a different length of chain. The fewer the number of teeth on either, the shorter the chain needs to be. Similarly, the greater the number of teeth, the longer the chain needs to be. Thankfully, that information is available online and I’ve drawn up another table of the various options available for my single speed conversion.
With my chosen starting option of 44T/12T, I require a chain with 96 links. As the C Line Urban, sorry Mr Bumble, comes with a 102-link chain, I’ll need to remove a few links from the chain to allow it to work correctly. However, there is a downside to that, namely once a chain is shortened it cannot be lengthened again, so cannot be used should I increase, for example, the rear sprocket to a 16T. Back with more soon.
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