Belt-drive Brompton

Original post: September 2012

This shows my Brompton folding bicycle, which I converted from chain to belt drive. I'm very happy with it. It's very quiet compared to a chain, and no more oil, grease, and dirt!

The images show the setup. The frame had to be cut to allow the belt to be fitted. Those little metal plates and small bolts hold the frame together (the frame is cut right in the center of the dropout).

The "CenterTrack" belt is from Gates, as is the "chainring" and "sprocket" (roughly $80 each, The chainring has a standard bolt-circle diameter, which required the Brompton cranks to be replaced. (It turns out BMX bikes are the main use for single-ring cranks, which accounts for the color choices.) The sprocket I got is for a Shimano Alfine hub, but it's roughly compatible with the Sturmey Archer five-speed hub on my bike – the Shimano has nine splines, the Sturmey Archer only three. An hour or so of filing...

The idler sprocket, which has the primary purpose of taking up the slack in the belt when the bike is folded, is plastic - I made it with 3D printing ( While it took quite some time to learn how to specify the part (with the Openscad computer-aided-design program), actually having it "printed" cost less than $12!

The standard chain-drive Brompton has a second idler that helps "fold" the chain. I replaced the second idler with a little "hook" (you need something – otherwise there's too much slack belt in the folded position). I tried a second small idler, but the belt is stiffer than a chain, and didn't want to run around the idlers in any case. As a result, I'm not able to reposition the pedals once the bike is folded. I've learned to position the pedals correctly before folding the bike.

I worry a bit about the tight bend the belt makes around the hook in the folded position – you're not supposed to kink the belts – but so far so good.

As long as one has an internal-geared hub, I don't see any point to a chain – the belt seems to have all the advantage!

Update: October 2014

The bike and belt have been performing well these past two years. I'm still completely happy with the change from steel and oil!

I decided I wanted a higher gear ratio, so created a new 15-tooth rear cog to substitute for the 20-tooth cog from Gates. It is 3D printed with Shapeways's stainless steel process (15 tooth GT2-11 Sturmey Archer pulley). Gates recommended against the smaller-diameter cog, saying that it was less than the recommended minimum radius, and that the belt cords (Kevlar fibers) might be damaged. Since this change, however, (almost two years ago) I've had no problems. I may have been lucky, however, since I am a light rider (138 lbs). Obviously a 200-lb rider would be putting a lot more muscle/force into the pedals!

With the smaller cog I also got a shorter belt (113 teeth instead of the 115 I started with). This is actually a better fit. Note that the belt tension is NOT provided by the smaller white idler sprocket. Rather, the positioning of the hub in relation to the crank is what counts. I had filed out the drop-outs a bit to provide for some adjustment -- without the filing the 20-tooth cog and 115-tooth belt were actually too tight (in fact, the Brompton couldn't be completely unfolded!). But I think that filing would have been completely unnecessary had I started with the 15-tooth cog / 113-tooth belt combination.