Re-gearing the Brompton

Second-hand bikes can be a great way to get a good bike at a more affordable price. The downside is that you  get a bike which has been set-up according to the preferences of its previous owner. When I purchased the Brompton back in February, it had been set-up with obscenely high gearing, a 50-tooth chain-ring with a 13-tooth rear sprocket. With the Sturmey Archer S-RF3 rear hub, this gave gears with 3.7, 4.9 & 6.6 metres development respectively. I put up with this for a long time because although it was much too high, it still worked. Eventually, the stresses to the sprocket and chain from starting from stationary in such a high gear were too much, and the chain would no-longer mesh with the chain-ring.
Not having a 24mm socket to remove the left-hand folding pedal, I had a look at the official Brompton chainsets which would allow me to leave my existing left crank in place without a major mismatch. Needless to say, they were excessively expensive. Instead, I was fortunate enough to find a Stronglight chainset which, online at least, looked similar enough to the Brompton one for me to get away with leaving the original left crank in place.

The unused Stronglight left crank, with the original Brompton one. They could almost have been separated at birth.
The fitted Stronglight chainset with the Brompton original below, again they are very similar looking indeed. I also replaced the rear sprocket and chain, resulting in 42 teeth at the front and 14 teeth at the back. This produces 2.9, 3.8 & 5.1 metres development, respectively. This means the Brompton can now much more easily climb hills and accelerate from stationary without busting my knees. It may not be a very flashy upgrade but it makes a significant difference to how useful the bike is. If your gears are too high (or low) don’t suffer in silence, do something about it. It can make the difference between a bike you merely tolerate riding in certain circumstances to one you actively want to use.

13 thoughts on “Re-gearing the Brompton

  1. If your gears are too high (or low) don't suffer in silence, do something about it. This sounds like a great slogan, something that should go on a sticker!Other great sticker ideas:"Raise your saddle, lube your chain, and put some air in those tires!"

  2. Nice one, Stronglight must be one of the OEMs for Brompton, those cranks look identical. Was there anything stopping you from just increasing the size of the sprocket on the back or was the chainring too worn and needed replacing anyway?

  3. @Smut Pedaller,The rear can only accommodate a 15t sprocket maximum, it was 13t and I've increased it to 14t already. Unfortunately this didn't give the sort of reduction I needed, so I decided to change the front too, especially as the front used to click intermittently and had become bent.

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  5. Wow, great post! I’m in the process of getting a Brompton and despite the -12% and -18% gear reduction on their 3 speeds, I still find the ratios a little high. I’m currently considering getting a 42T chain ring and doing the exact same thing. Can you tell me how many links on the chain do you have to remove for that mod?

    • Thanks for the comment. I’ll be completely honest with you, I didn’t count the number of links I took out, I just broke the chain with the chain tool and pulled the ends together until it looked about right and trimmed off the excess. If the chain is too short, the bike won’t fold/unfold, but beyond that you have a fair amount of leeway with respect to chain length.

  6. Thanks for the input! As I’m a complete newbie when it comes to gears (I’m old ex-BMX rider) and just starting to get back into cycling, I have one last question. With the gear ratio you have on your Brommie, which gear do you use the most in city riding and what speed (approx.) do you start spinning on the 3rd gear? Thanks again for your feedback ๐Ÿ™‚

    • With the gearing as it is, I tend to use the first two gears for getting up to speed as quickly as possible when leaving traffic lights or pulling out of side roads (as is fairly typical of the British cycling experience). If it is flat and not windy I spend most of my time in third gear once I am up to speed (~15-18 mph typical).

      • Thanks again…much appreciated! If you’re ever in Hamburg, Germany…let me know, I’ll treat you to an ice cream ๐Ÿ™‚ (or even better, German Beer!). Cheers!!!

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  8. Hi just wondering, looking to buy a Brompton 50T chainwheel and little bit confused which one right or left hand crank which side is which. thanks

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