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I. My initial conversion
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, carbondrivesystems.com). 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 (shapeways.com). 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!
II. Update #1 - custom rear cog - 3D-printed stainless steel for Sturmey-Archer five-speed hub
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.
III. Update #2 - custom rear cog - 3D-printed plastic for hyperglide splines single-speed hub
A bit more than a year ago I got a new Brompton (the main frame broke -- metal fatigue. The bike was about 15 years old). I thought I would move the five-speed hub over, but I decided I liked the single-speed (a couple of pounds lighter, for one thing!). But I still wanted belt drive, of course.
The Brompton single-/two-speed hub has hyperglide splines (see the photo). I wondered if a plastic cog – with matching splines – might work. The edges of the splines are not large, but there are nine of them, so maybe the stress would be over a large enough area that the plastic could handle it. The part is
The splines held up well -- I did have to make the side flanges thicker -- the first one I made (with thinner flanges) broke after a month or so. The current version is here. I've been using it for about eight months now (between five and ten miles most days, including a great deal of uphill stress!). It's working well.
I also made a new, smaller and thinner idler pulley, which is here.
The two photos show the current setup. The cog is hard to see behind the rear-triangle frame parts -- the 12-tooth idler is in front (to the right). Both parts show a bit of accumulated grime. Note that they were printed in "black," but it's only a surface dye, which quickly wore off!
IV. Update #3 - custom rear cog - stainless steel plus plastic
While the plastic cog has performed well for a year-and-a-half now, a closer look showed that it was quite worn. Photo IV.A shows the new stainless steel cog in comparison to the worn plastic cog -- the plastic cog originally had the same tooth profile as the new one!
The belt also had a tendency to "skip" under heavy load (steep hills). I could alleviate it a bit by increasing the belt tension (moving the rear axle by small fractions of a millimeter!), but it would resume skipping after a few days -- it's hard to imagine the cog wearing that fast, but I don't have another explanation.
I had tried making a hyperglide-splined stainless steel cog. Because the dimensional accuracy of the stainless steel process is not that good, I made the cog splines slightly undersized, and filed them out carefully to fit the hub. Nevertheless, it started rattling on the hub within a day or two of riding, and efforts to add various shims were likewise effective for only a short while.
Photo IV.a. New stainless steel, old plastic cogs.
Photo IV.b. Cog partly installed on hub
Photo IV.c. Cog installed on hub
V. Thomas Inderka's conversion
Thomas Inderka of Cologne, Germany, has completed a conversion of his Brompton to belt drive.
Rather than filing the dropouts, Thomas uses an eccentric bottom bracket to tension the belt.
All in all, a very elegant bit of work!
VI. Update, June 2015
Thomas provided the following update:
The new 14-tooth 9-spline cog would have made my recent changes a lot easier!
First I must say that I rode a lot with my initial setup and it worked great except that it skipped a tooth when I pushed it very hard all of a sudden. The thing was that my filing on the 15-tooth sprocket was not accurate enough so that after a couple of months it started to rattle a little. So I ordered a new one at Shapeways. I had my instrument maker doing the filing work but somehow he accidentally cracked the part very slightly. This slight crack caused the whole sprocket to break into two pieces after riding it for a couple of weeks.
So, after being a bit frustrated I decided to try a new setup with a larger gear ratio. I replaced the original Brompton BSR hub (Brompton Standard Range three-speed) with a Brompton BWR (Brompton Wide Range three-speed), got a 70-tooth chainring, a 120-tooth belt and a 20-tooth rear sprocket (which fits exactly on the nine spline BWR hub). This setup is actually tighter than original setup, so that I had to file the dropouts by 2 mm, in spite of the eccentric. But it works!!!
It would have probably been easier with your new 14-tooth cog, but I didn't know. Anyway, the new setup feels good.