Having given up hope of getting my Brompton T5 gears fixed, I decided to upgrade to a new six speed version. As I have kept the Schlumpf Mountain Drive from my old Brompton, I have no need of any lower gears. In fact, the transition between Schlumpf top and Brompton bottom is perfect, but I find the Brompton derailleur gears so close together that the difference between them is hardly noticeable. Is it possible to fit a smaller high gear cog, so that I can coast at over 20 mph at a reasonable pedal cadence?
Anthony Lamb, London
Apologies to those who do not own a Brompton, but there is a great deal of interest in this subject and developments are continuing apace. In short, yes, it is possible to fit a smaller sprocket to the Brompton 6- speed, and bigger sprockets with up to 18 teeth can be fitted too.
I am indebted to Steve Parry, engineer of the SP Brompton, for researching this matter. After some experimentation, Steve is now producing a ‘wide ratio’ kit for the 6- speed Brompton consisting of Shimano 12- and 18-tooth sprockets adapted to fit the SRAM hub.The conversion takes less than an hour, but be warned – it does involve cutting, grinding or gently crushing the very rear of one of the frame tubes (invisible, and strength should not be affected) to make room for the 18-tooth sprocket.
The result works surprisingly well.The shift quality is almost on a par with the standard 13/15-tooth combination, but the bike now has a total gear range of 282%.That’s more than a Nexus 7-speed hub, equal to the SRAM 7-speed and almost as wide as the SRAM 3-speed plus Speed Drive (formerly the Mountain Drive Type 2). Leave the standard 50-tooth chainring in place and this extra range will give you a 93-inch top gear which is certainly enough for a 20mph cruise if you have the strength! A better alternative for most people would be to fit the optional 44-tooth chainring, giving a top gear of 82 inches and bottom of 29, as in the chart overleaf.
Before hundreds of readers leap for their cheque books, I should outline a few complications. As the Brompton tensioner is not designed for such a dramatic gear differential, it barely provides sufficient movement to keep the chain tensioned and allow folding in either gear, so a slightly worn chain might cause problems.
Another difficulty, as a glance at the chart will reveal, is a considerable overlap between the gears (although none are identical, as occurs all too frequently with derailleur systems).To avoid disappointment, it’s best to think of this as a four speed system – effectively a three speed with an extra-low bottom gear, or three low ratios plus ‘overdrive’.This leads to another complication: with four ratios where seven would be more usual, the gaps between the gears are quite wide.The final criticism arrived by fax from Holland, courtesy of Simon Korn, Brompton’s inestimable Dutch distributor and another engineer with a long association with the little bikes.
Simon has played with 12-tooth sprockets for years, and claims that sprocket life can be very short – less than a few hundred kilometres in some cases.This seems to be because the derailleur sprockets are designed to be driven by six small pegs, and three must be ground off to fit the SRAM hub. Apparently the pegs ride out of their grooves, bursting the sprocket. All the same, sprocket life depends largely on the rider’s weight and strength, so one fellow’s 200 kilometres, might well be another’s 2,000. An answer might be to braze the 12- and 18-tooth sprockets together, producing a very rugged little block, but that sort of thing can get a bit complicated…
Criticisms aside, the SP conversion does some remarkable things, adding nothing to the weight of a standard 6-speed, and giving a similar range to much costlier, heavier and less efficient options.
My personal preference is for the nippier standard ‘close-ratio’ option, but if I had to choose one bicycle for all occasions, I would probably choose this wide ratio 6-speed Brompton! Whether the sprockets can be made to last will no doubt be revealed. If they don’t, a 13/18-tooth combination would provide a less radical solution. Simon Korn markets all manner of gear options, including Anthony’s set-up: a standard 6-speed plus The 18/12 tooth conversion – space is tight, Mountain Drive, giving 12 evenly spaced gears but it works and a greater range than the Rohloff hub!
Finally, several readers have grumbled that last month’s item lacked fitting instructions and other technical information. Unfortunately, lack of space in a magazine of this kind makes such detail impossible. Both Brompton and the Mountain Drive company can provide written or verbal advice if required (although both may curse me for saying this). But don’t expect Brompton to endorse odd conversions like the above!
