By Mike Smithson
My ideal bike would be a Brompton equipped with the new SRAM Dual Drive 27 gear system.
This would combine the still unsurpassed portability of the former, which means that you can take it almost anywhere, with the latter’s extraordinary range of gears that allows you to pedal almost anywhere. Alas for a whole series of reasons this looks like staying a dream for some years yet.
SRAM Dual Drive 27 is the latest version of what used to be the Sachs 3 x 7 system that has been hugely popular with many of the leading manufacturers of small-wheeled bikes including the Pashley Moulton, Dahon, Bike Friday and the Birdy. By using a three-speed hub and a 7,8 or, now, 9-speed derailleur Dual Drive allows a range of gears and spacing that is very difficult to achieve for small wheels with a more conventional triple chainring system.
A huge benefit is that, unlike a triple chainset, all 21/24/27 available gears are useable and they are all easy to get at. With a 46T chainring on a 20 inch wheel bike the Dual Drive 27 gives you a range of 19-113 inches with almost no replication of gear ratios. There are also an extraordinary number of ratios available in the 50-70 inch range that mean that you can keep on making minor adjustments to maintain your cadence.
But it is not just the mathematics that make this impressive – the shifting system has evolved to the new single-sided shifter that has been introduced for DD27. This is so sweet and easy that it allows you to get the most out of what is available.
An argument against hybrid gearing systems is that they combine the inefficiencies of the derailleur with those of the hub. That clearly is correct but the latest version of the hub gear, introduced with the DD24 system, was shown in tests carried out for the Human Power journal in 2001 to be as efficient as the best detailers. Even if there is an efficiency loss this is more than offset by the gains of always have suitable gear ratios available.
Removing the rear wheel, a problem with all hub-geared bikes, has been made easier with SRAM’s new “click box”. It takes about ten seconds to attach or detach the hub gear cables and there is no need to make adjustments once the wheel has been re-fitted. The only down-side, as I recently discovered trying to fix a roadside puncture, is that you have to remember how the click-box works.
When I first experienced this gearing a year ago with my new Moulton APB Dual Drive 24 I was completely overwhelmed. Cycling was totally transformed because I felt I could go almost anywhere. For six months the other bikes in my shed were ignored.. At Christmas I decided to give a present to myself by upgrading my Bike Friday to Dual Drive 27 and this has been wonderful.
Now if only I could fit it to the Brompton……….!!
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Last updated: 7 May 2003
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