“Could you tell us the options for putting very low (sub 30-inch) gears in a folding bike? Which folding bikes will accept a Rohloff hub gear? It’s very hilly around here.”
Margaret Lunnon, Coulsdon, Surrey
Alot depends on whether you are prepared to put up with a general lowering of the gears, or are hoping to keep the high gears as they are.The simplest and easiest solution, common to most small-wheeled bikes, is to fit a smaller chain ring.These are cheap and available in a variety of sizes. An even cheaper answer with hub geared bikes, is to fit a larger sprocket. Most small machines come with 13- or 14-tooth sprockets and most will accept a much larger sprocket, giving an instant and cheap gear reduction. Bear in mind, though, that these solutions lower all the gears and have no effect on the gear ‘range’ – the difference between first and top gear.
Schlumpf Mountain Drive - the assembly fits into the bottom bracket
There are many options to increase the gear range, some more practical than others. On a derailleur bike, larger sprockets and long-arm derailleurs tend to be impractical with small wheels, but the other extreme of fitting a small top-gear sprocket and smaller chainring is fine, if a little complex.The Shimano Capreo system is specifically designed for small wheel bikes and gives a gear range of almost 290% from a very neat little unit. Assuming a just practical 80-inch top gear, the Capreo would give ratios down to 28-inch, just inside your requirement.This system is available on the Birdy White and the new Mezzo D9, and can be retro-fitted to most derailleur machines at a price.
An easier solution open to just about every folder is the Schlumpf Mountain Drive.This is a little hub-style gear set that fits inside the chainring. In direct drive, it has no effect, but click your heel on the little control button and the bike changes down to a low ‘hill-climbing’ set of gears.The Mountain Drive gives a reduction of 2.5:1 and the similar Speed Drive a more reasonable 1.65:1. Assuming a top gear of 80 inches, and a 3-speed hub, the Speed would give a bottom gear of 27 inches and the Mountain an almost unusably low 18-inch gear.With a wider range hub, the effect is even more pronounced. The nice thing about this system is that there are no cables to worry about, but it’s heavy, quite expensive and somewhat inefficient in the more extreme ratios.
Brompton Wide Ratio Conversion - 12/18t sprockets squeezed onto a standard 6- speed hub
My personal favourite is a wide-range conversion of the Brompton 6-speed.The standard 6-speed has a range of only 215%, so try as you might, you won’t get anywhere near your 30-inch target and keep a decent top gear. But fit 12- and 18-tooth sprockets to the Brompton hub and the range increases to 282%.This is wide enough to give a range from 80 down to 28 inches, with good efficiency. Unfortunately, there are technical issues involved, and no one produces a kit at present, although I understand this omission is being rectified. More in a later issue.
Particularly fashionable at the moment are the 8-speed hubs from Sturmey Archer and Shimano.The Sturmey is designed around very large sprockets and is thus too cumbersome for most small bikes.The Shimano is much more practical, and its gear range of 307%, would give a bottom gear of 26 inches with our 80-inch top gear. Several small- wheelers are available with this hub, including the Airframe, and conversions are available for most small-wheelers. Even the Brompton can be adapted, although the extra weight and folded width would be a disincentive for regular commuters.
Rohloff 14-speed - the widest range of any hub gear
Finally, we come to the Rohloff 14-speed hub. This offers an enormous gear range of 526%, but it’s a big, brutal device, and more or less doubles the price of a typical high-end folding bike.The range is as much as most people could ever use, from a wobbly walking pace, to a flat-out down hill top gear. Using our example of a bicycle with an 80-inch top gear, the Rohloff would reach Margaret’s 30-inch criteria in gear 4, and go as low as 15 inches in bottom gear! The hub is fitted as standard on the £1,880 Birdy Grey, the £2,000 Brompton- based SP and can be shoe-horned into most other bikes if your budget extends to that sort of thing.
Elsewhere: Puncture-resistant tubes A to B 35 Brompton Wide-Ratio Conversion A to B 31 Brompton Mountain Drive Conversion A to B 21 PBW/Rohloff Speedhub A to B 31
