Gearing for a Brompton

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.


5 thoughts on “Gearing for a Brompton

  1. interesting. I plan Geneva to Nice on a standard 6 speed Brompton via ‘the Grand Traverse’ at the end of July and have endured similar deductions to arrive at the same question. Schlumpf and rholoff, tho interesting are out of the question financially. Large front cogs always feel more efficient to me (any basis to this???). i suspect any more than 18T on the rear, combined with the 50T front chain ring, would be overkill and have you pedalling loads but going nowhere. One problem I see from your calculations is that already with 17T on the rear, the 2nd and 3rd, as well as the 4th and 5th are nearly the same. So with the 18T i guess you would loose all the ‘inbetweens’. Maybe with a 21T you’d regain them, 1 gear up!!Do you have SPD pedals? they improve the efficiency massively, although they wont fold.

  2. the spline pattern on the 2 speed freehub body is the same as shimano so you can use shimano 2 speed as a 12/16 with a 54chainring..thinking fo converting to a single speed… poss 54/13also moving from marathon tyres to stelvio as I have just done lowers your gear inches slightly

  3. Hi Jon,
    I am planning to cycling Taroko Gorge( Taiwan with brompton M6R.
    The bike is currently on standard gearing, which I intend to change the 15 tooth sprocket to 17 or bigger.
    Will appreciate if you can share:
    – did you implement the change ?
    – if so, is it possible to have 13, 17 or even 13, 18. Any problems with shifting?
    – need to change to a longer chain?

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