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. The first decision for those contemplating a gear upgrade is to calculate the sort of gears they want. 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. 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. As we had been warned, this was the weakness – the 12-tooth is so slim that it eventually cracks. 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. …this stretched the gear range to 282% better than most 7-speeds… 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. 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.įortunately, 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. At 86″, top gear was the same, but first was reduced from 46″ to 40″. 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. 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. 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. The new system took advantage of the Brompton chain tensioner. 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. 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.

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