I mentioned in my last post about the Kinetics 8-speed conversion kit for the Brompton that the new crankset looks rather cheap and nasty. It is. I’ve discovered that it’s a Sturmey Archer FCS30 model with 33 teeth (why it’s not called the FCS33 is a mystery), which retails for about £16. So we now have a bike that’s cost the best part of £1000, with a somewhat poorly matched crank that’s ugly.
Worse still, I found that the chainring fouled intermittently on the rear triangle, preventing the bike folding properly. I started looking around for alternatives. The first thing to note is that the Brompton bottom bracket has a 119mm ISO square taper axle. By far the majority of today’s cranksets are JIS taper, so the choice is more limited. The differences are explained most clearly here by Sheldon Brown. According to Sheldon, “if you install a J.I.S. crank on an ISO spindle, it will wind up about 4.5 mm farther in than it would on a J.I.S spindle of the same length.” More about chainline in a moment.
It seems relatively uncommon to find a single-speed chainring with less than 39 teeth, and the most commonly available versions start at 42 teeth or bigger. I need to stay close to 33 teeth to achieve the gear range of the original, which seems ideal for the rather hilly countryside around here. The hub has a 20 tooth sprocket on the back, so according to the online gear calculator, it produces a range from 27.3 to 88.7 gear inches.
Chainline is the next consideration. According to the Sturmey Archer spec, the 8-speed hub with flat 20-tooth sprocket has a chainline of 39.7mm. I measured it and came up with about 38mm, so that checks out. At the FSC30 chainring, I found the chainline to be around 40mm – not perfect but not too bad either. If I’m going to cure the folding problem, the front chainline will need to be bigger at the front, about 41mm. Again, it won’t be perfect, but 3mm difference between front and back doesn’t seem to likely to be much of a problem.
There is no easy way to correct the chainline at the back. You can buy 25-tooth dished sprocket to fit the 8-speed hub. Reversing it would add about 1.5mm. However, the chain tensioner is so close to the sprocket that it’s not possible to use one with more than 20 teeth.
The solution at the front may well be to fit a JIS taper compact double crankset and remove the outer chainring – changing the fixing bolts and using washers to adjust the exact position of the inner ring to get the desired 41mm chainline. This will give me a chainring of 34 teeth, rather than 33, so I’ll need one more link in the chain to take it to 101 links. Fitting a new chain at this point seems like a good idea anyway. The gearing will move up very slightly to a range of 28.1 to 91.4 gear inches.
It may even make sense to change the bottom bracket for a JIS taper version at some point – Shimano sells them in 118mm and 122.5mm axle widths, so it’s likely I can achieve a fit one way or the other. The FSA Vero compact crankset looks infinitely better than the Sturmey one, so it’s on order and I’ll know if it’s going to work later this week.
Incidentally, the 6-speed parts taken from my Brompton fetched over £100 on eBay, after deducting the costs and PayPal commission, so the final cost of the upgrade will be £200 plus whatever I end up spending on the crankset and/or bottom bracket.
Pedalitis 
Hi,
Thanks for your explanation of your thinking here. I have recently done an 8-speed Brompton conversion, and I used a Shimano Nexus 33T single crankset. The only problem here was that the left crank was too wide to allow the folding pedal to fold over the top, so I re-used the Brompton L crank. The chainwheel misses the rear frame on folding by a good 4mm.
At the hub I installed a 23T sprocket on the Sturmey Archer x-rf8 hub. I was keen to imitate the gearing on my ‘big bike’ (26″ wheels) which uses a Shimano Alfine 8-speed hub (39T front, 20T rear.) Because the overall range of the Alfine 8-speed hub is slightly narrower than the SA 8-speed hub (306% against 325%), it means you effectively have an extra gear. Depending on how you choose your number of teeth front and back, you can match the gear range of the Alfine, and then choose to place your ‘extra’ gear either at the high end or the low end. The online Brompton Gear Calculator is fantastic.
I wanted a low ‘granny’ gear, so I chose a 33T at the front of the Brompton, and a 23T sprocket. Steep hills are now a glorious ride on the Brompton, if you don’t mind looking like ferret on speed. Downhill is not so good if you want to be able to accelerate beyond about 23mph – I’m happy to coast! As you suggest, I dished the sprocket inwards – the only option to avoid it fouling the frame. The chain line is a good 10mm out (maybe more) but this is no worse that a chain puts up with on a cassette derailleur set-up.
I didn’t buy this as a kit, so the biggest challenge was to adapt the chain tensioner. The wheels on the tensioner have to be extended away from the sprocket, AND laterally towards the wheel, to match the new chain line. In my first attempt, I only extended the wheel in the fixed arm of the tensioner. I shortened the chain so that the wheel of the sprung arm rested under tension just beneath the wheel of the fixed arm. This enabled me to ride the bike, but folding was a disaster. The chain scrunched around the rear sprocket (instead of being held clear), and worse, the front forks collided on folding with the nut holding the wheel of the fixed arm – pressing on it, and stripping paint from the fork. My second attempt was successful – I shortened the extension on the fixed arm, and extended the sprung arm such that its wheel could swing round in front of the wheel of the fixed arm. This enabled the bike to fold without damage, and the chain to be held clear of itself by the sprung arm.
Creating the extension takes some thought. There are several places where using bolts to attach brackets to the arms will foul the gear change mechanism around the hub, or hamper the fold, or hamper the sprung arm. (Too complex to explain here…) Suffice to say, it is do-able with a lot of thought, some ingenuity, trial and error, and lots of rummaging in boxes filled with random bits of metal.
I’m happy to say, it all works now. Perhaps this would be a more common project if we could persuade Sturmey to make an extended version of the Brompton chain tensioner, although I doubt they would approve of the skewed chain line it demands.
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Hi Duncan
thanks for your comment – glad you managed to get everything working.
Bob
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