Warning – this will be a boring technical post about the number of teeth on chainrings and gear ratios.
Bromptons come with different configurations for gears. There’s a single speed, a two speed with derailleur, a 3 speed with internal hub gears, and a 6 speed with derailleur and internal hub gears. The list gets longer because with the 6 speed, you can choose to have 8% higher or 12 % lower gearing. I got mine with the standard ratio 6 speed, and it’s great, I can get up to 25 mph quite easily (even managed 32 mph once) and have yet to find a hill I can’t get up. But that is around london and when I’ve been at home in Cumbria. Without any luggage.
So I’m thinking with towing a 30kg+ trailer up the Alps, I might need some lower gears, but I don’t want to lose the current top speed I have. One option is to get a Schlumpf Mountain Drive. This acts like an internal hub gear, but for the front chainring. It has two options, a 1:1 ratio, and a 2.5:1 downshift. It would be brilliant, except that it costs £275.
Another option is to use a Rohloff Speed Hub. This would replace all the current rear gearing on the Brompton and give 14 internal hub gears. But they don’t fit in a 16 inch wheel as standard, and the base cost is an astronomical £625. So clearly not an option. Then I thought, why not put a bigger sprocket on the rear?
My standard ratio Brompton has a 50 tooth chainring at the front, with 13 and 15 tooth sprockets allied to 3 speed hub at the rear. I thought I’d see how easy it is to change the sprockets, and discovered that they clip on rather than requiring a heavy chain-whip to remove. The next thing I did was to work out the current gearing. This website lets you put in just about any combination of gears and will tell you the ratios. Here’s a table of my current set-up:
Gear# – Internal – Sprocket – Meters Development
1* – 1 – 13 – 6.7
2 – 1 – 15 – 5.8
3* – 2 – 13 – 4.9
4 – 2 – 15 – 4.3
5* – 3 – 13 – 3.6
6* – 3 – 15 – 3.1
The *’s are the gears I use most often – I change down the hub gears then use the 15 tooth sprocket for big hills. If I was to change the 15 tooth to be a 17 tooth sprocket, I would have:
Gear# – Internal – Sprocket – Meters Development
1* – 1 – 13 – 6.7
2 – 1 – 17 – 5.1
3* – 2 – 13 – 4.9
4 – 2 – 17 – 3.8
5* – 3 – 13 – 3.6
6* – 3 – 17 – 2.8
You can see that the gears I use most often have not changed, except the lowest gear, which is now 10% lower. The cost of this solution? £5.99 from SJS Cycles. Even if it doesn’t work or I don’t like the new gearing, well i’ve wasted £6. And because of the relative ease of changing sprockets on a Brompton, I can always take the original 15 tooth and change it en-route. Or take an additional 19 or even 21 tooth sprocket to change to if the Alps are too demanding.