Long-term readers may recall that the bankruptcy of hub gear manufacturer Sturmey Archer in the summer of 2000 left Brompton short of a suitable 5-speed hub. Other bike manufacturers migrated to the similar SRAM 5-speed, but the Brompton frame is unusually narrow, and it was not until May 2002 that the company came up with its own solution – the 6-speed (more correctly 2×3-speed) SRAM/Brompton derailleur/hub gear.

The new system took advantage of the Brompton chain tensioner. On a basic 3- speed hub-geared variant, the tensioner only moves when the bike is folded, but by fitting tensioner wheels designed to ‘float’ from side to side, it was possible to fit two sprockets side by side, doubling the number of gears. It all sounds a bit Heath Robinson, but engineered with Brompton’s usual attention to detail, the 6-speed soon established itself as a neat and efficient conversion.

The company decided to fit 13- and 15-tooth sprockets.The very similar sizes helped to give a slick gear change and resulted in evenly-spaced ratios.The bad news was a rather disappointing overall gear range of 215%, which was not much more than the 187% of the basic 3-speed. At 86″, top gear was the same, but first was reduced from 46″ to 40″. Just to recap, lower gears (measured in terms of the effective wheel size), give better low-speed hill-climbing, and high gears allow you to ride faster on the flat.

Fortunately, when the chain tensioner assembly was re-engineered for the 6-speed, the opportunity was taken to leave space for bigger sprockets, enabling the company to introduce the new 2-speed variant. Home tinkerers soon discovered that a sprocket as big as 18-tooth could be squeezed in, and paired with a 12-tooth sprocket, this stretched the gear range to 282%; better than most 7-speed hub gears.

…this stretched the gear range to 282%; better than most 7-speeds…

We tried a Steve Parry 12/18-tooth conversion in August 2002 and used it successfully for a couple of years until the 12-tooth sprocket shattered. As we had been warned, this was the weakness – the 12-tooth is so slim that it eventually cracks. A 13-tooth sprocket is stronger, but it reduces the gear range, and gives two pairs of ratios that are almost identical, resulting in a rather disappointing 4-speed.

Now, thanks to Highpath Engineering, the wide-ratio Brompton is back, and the new conversion seems to work very well.The basis is a combined 12/18-tooth sprocket assembly, the two components being tig-welded together, then re-hardened to give a reasonable working life.

Complicated Things

The first decision for those contemplating a gear upgrade is to calculate the sort of gears they want. Using Brompton’s 50- tooth chainring and the new Highpath sprocket set (£39.50), the standard gears of 40″, 46″, 55″, 63″, 75″ and 86″ become 33″, 46″, 50″, 62″, 68″ and 93″ – in other words, the high gears are higher and the low gears lower. Note that the fairly even spaces of the original have been lost, but as with the Sturmey Archer 8-speed (see A to B 40), we now have broad gaps at the extremities and narrower spaces between the middle gears, which can be useful.

If you’re looking for lower gears, you’ll need to fit a smaller 44-tooth chainring as well as the sprockets.This produces ratios of 29″, 40″, 44″, 55″, 60″ and 82″.Top is now lower than standard, but first is almost as low as a Speed Drive conversion! A new 44-tooth chainring costs £23.40, but if you’re planning to order a new bike, the option adds only £9.

For those without a 6-speed bike, a post-April 2001 3-speed (with the SRAM hub) can be upgraded to 6-speed spec for £78.40, but this would be a lot more expensive on an older Sturmey-geared machine.

A word of warning about chains, chainrings and widgets: for years Bromptons came with 1/8″ chain and 1/8″ chainrings (some are a little narrower), but since the arrival of the 6-speed the situation has become more complicated. Retro-fit 6-speed kits and a few early production bikes were supplied with 1/8″ chain, but all other 6-speeds will have narrower 3/32″ chain.The standard 50-tooth, and smaller 44-tooth, chainrings have been produced in both chain sizes, although Brompton expects to standardise on the narrower 3/32″ soon.The wide-ratio kit will work with an 1/8″ chain, but we’d strongly recommend 3/32″. A 1/8″ chain will run on either chainring, but a 3/32″ chain will only fit a 3/32″! Before doing anything, it’s a good idea to try a new piece of 3/32″ chain on your chainring and see what happens – if it fits, great, if not, you’ll need a narrow version.

If in doubt, renew anything that moves, because the new sprockets ask a lot more of the changer mechanism. It’s good practice to use Brompton chain, because the nominally 3/32″ chain pin length can vary, and not all work happily in the Brompton derailleur. If your eyes are glazing, simply trot down to a hub gear expert, such as Bicycle Workshop of West London (tel: 020 7229 4850) who will solve all the technical bits.

Fitting

Changing the sprockets is easy, but if working on an older bike, you’ll need to check that the system is working properly first. Remove the rear wheel and chain, and flick the gear changer back and forth whilst observing the ‘chain pusher plate’ at the rear end. If the operation seems sticky or unreliable, release the cable assembly and try moving the pusher plate by hand – any tightness or roughness will have to be sorted before you go any further. On early ball-bearing units, in particular, the bearing will almost certainly need to be thoroughly cleaned or replaced.

With the spring clip popped off the hub, and the old sprockets removed, the new sprocket assembly will slide into place after refitting the original dust shield and the new shims. At this point things get a bit more difficult, because it’s necessary to remove a few millimetres from one of the rear frame tubes to clear the teeth of the larger sprocket.We did this by gently squeezing the tube and grinding away the last millimetre or so.

…Fitting the sprockets gives the Brompton… a gear range similar to a typical hybrid…

With the dodgy bit out of the way, all should now be plain sailing.With the rear wheel in place, feed in a new 98-link (100-link for a 50-tooth chainring) x 3/32″ chain and connect it up. As the new 18-tooth sprocket fits slightly outboard compared to the standard 15-tooth one, the chain pusher plate will need adjusting. Disconnect the cable assembly from the pusher plate, and move the plate by hand as far as it will go, checking that the plate either gently touches, or just clears the chain at each extreme. If it needs adjustment, turn the relevant stop screw until you’re satisfied that the pusher plate is just going far enough to change gear smoothly.

Finally, reconnect the cable assembly, turn the bike upside down (or get someone to lift the rear frame) and try changing up and down through the gears.The 2-speed changer gives twice as much cable movement as is required to change gear, so it should work well enough, even when slightly out of adjustment. If one or other of the gears is failing to engage properly, remove the trigger cover and move the outer cable to another of the four location slots until both gears engage smoothly and cleanly.

In Use

The most noticeable thing is the very different characteristics of the two sprockets: the 18-tooth feeling silky-smooth and the smaller 12-tooth relatively ‘coggy’.The change quality is a little lumpier than standard too, but it should remain crisp and fast – obviously new components will cope better than well worn examples.

In normal riding, we tend to stay in the top range, using the top two gears. As a steep hill approaches, the change from middle gear/top range to top gear/low range is one of the closer ones, but it sets the bike up for more serious climbing. Over the top, and it’s back into the high range and up to top gear. Being slightly lower than standard, top does tend to run out of steam on the descents, but for most people this will be a small price to pay for the much lower gears.

A really steep hill will defeat the bike, even with a 29-inch bottom gear, primarily because the front wheel begins to lift as you pedal – in any event, walking may now be quicker. In all other circumstances, this is more or less a go anywhere Brompton.

Conclusion

Fitting the 12/18-tooth sprocket block takes a few hours, but it’s a cheap, light and effective solution, giving the Brompton a gear range similar to that on a typical hybrid. Efficiency is better than a 7- or 8-speed hub, and although the double changers may seem confusing, the system is no more complicated to use than a 3×7 derailleur.

How long will it last? As our prototype has done only a few hundred miles, it’s hard to judge.The 18-tooth sprocket should last forever, but the 12-tooth will have a relatively short life, and of course, when it eventually fails, you’ll have to throw the whole assembly away.We would expect to see 2,000 miles at the very least, and 3,000 miles or more with care. Given the relatively low purchase cost, that sounds quite acceptable against the alternatives, and would equate to many years of leisure use.

Brompton 12/18-tooth block £39.50 plus postage of £3 (UK), £7 (Europe) or £15 (worldwide) . Manufacturer Highpath Engineering tel/fax 01570 470035 mailadmin@highpath.net

A to B 47 – April 2005