I’ve looked at Brompton gearing in a previous post – this graph adds in the gear ratios of two popular 8-speed hubs, the Sturmey-Archer x-rf8(w) (SA), and the Shimano Alfine 8. The top three lines of the graph are the three Brompton 6-speed options. I like my gears low, which is reflected in the choices below.
There are four 8-speed options above. The first two to notice are the brown and yellow lines (the top two longest). The brown is the Alfine 8 using a 54T chainwheel, and a 16T rear sprocket. The yellow is the SA x-rf8(w) using a 33T chainwheel, and a 20T sprocket. Both of these start in gear 1 in a similar gear-ratio to one another and to the low Brompton 6-speed gearing. Both end in gear 8 just slightly higher than gear 6 of the Brompton set-up. In other words, these two 8-speed options reproduce the same spread of gears as the low Brompton 6-speed gearing, but obviously they make this climb over 8 gears instead of six, so there are more choices when pedalling.
The second two provide a lower-geared option, which is more suitable for touring (managing fiendish hills under load). Here the red line represents the SA x-rf8(w), with the sprocket beefed up to a 23T. The blue line is the Alfine 8, still with a 16T sprocket, but with the chainwheel dropped to 44T. These hubs start and end in about the same place, although remember the SA has a couple of advantages: it has a gear range of 325% compared to the Alfine’s 307%, and its first gear is direct drive – delicious for getting up nasty hills with ease. I’ve written a fuller comparison of the two hubs elsewhere.
You can play around with these gear ratios to find the range and pitch that suits you best using the excellent Brompton Gear Calculator – only, remember that a) not all sprockets will fit the Brompton rear frame; b) your sprocket size will determine the kind of modification you will need to make to the chain tensioner so that its jockey wheels fit; c) you may not achieve a perfect chain line with the SA x-rf8(w) (the best I can manage is about 5mm error) d) any modification needs to work with the fold! Happy planning!