More on the Brompton 8-speed conversion….room for improvement?

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.

The Sturmey Archer FCS crank - cheap, ugly and no way to change the chainring

The Sturmey Archer FCS crank – cheap, ugly and no way to change the chainring

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.

There's not room to fit a sprocket with more than 20 teeth, and you can only get that one in a flat version

There’s not room to fit a sprocket with more than 20 teeth, and you can only get that one in a flat version

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.

If it's possible to get a reasonable chainline, the inner ring of this compact double would be a much more attractive and flexible  alternative to the Sturmey crank

If it’s possible to get a reasonable chainline, the inner ring of this compact double would be a much more attractive and flexible alternative to the Sturmey crank

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.

About Bob Jones

Cycling enthusiast living in Wiltshire, England.
This entry was posted in Bikes, Building and fixing, Products and tagged , , . Bookmark the permalink.

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