It’s not a bug, it’s a feature
　

In the world of bicycle construction, there are two types of gears - hub and derailleur. The two have their respective strengths and weaknesses and since operating each is quite different, cycle designers never mix hub and derailleur on one bike. The punchline, of course, is that the Brompton is the exception. Brompton’s engineers, constrained by the requirements of an extremely robust folding bike, crossed the streams with a mix of hub and derailleur gears on the six speed Brompton. When Brompton becomes a billion dollar company it will, surely, have a corporate historian on the books who will reveal the heated arguments and shaken spanners over this extremely interesting design decision. Until then, we must simply accept it as a given.

If you’ve never ridden a six speed Brompton, prepare for your cycling mind to be blown.

A two speed Brompton has a two gear derailleur. A three speed Brompton has a Sturmey Archer three speed hub gear. Shifting gears with a derailleur requires you to be pedaling whereas with a hub mechanism you must stop pedalling or have no strain on the chain. So, far so normal. A six speed Brompton has both the three speed hub gear and the two speed derailleur to give (3 x 2) six gears. The hub gears are numbered 1, 2 and 3 and the two derailleur gears are referenced by the minus sign (-) and the plus sign (+). The gear progression (low to high) is,

1-　　1+　　2-　　2+　　3-　　3+

Changing up from 1- to 1+ (or down from 1+ to 1-) is a simple flip of the derailleur gear lever. The same, of course, applies to changes between 2- and 2+, and between 3- and 3+. Going between hub gears is where it gets interesting since a single overall gear step up or down requires two separate operations. Firstly, stop pedalling and change the hub gear and then, while pedalling, change the derailleur. For example, changing from 1+ to 2- has the following steps,

• Stop pedalling momentarily and move the hub gear lever from 1 to 2.
• While pedalling, move the derailleur lever from + to -.

You can swap the order of the above steps according to preference but, even so, cumbersome or what? Well, what, actually. You will quickly get used to the steps involved in a single shift in gears. No, really, it becomes second nature.

Now, consider that a six speed Brompton covers nearly the same gearing range as an 18 or 21 speed road bike. This means the difference between each gear shift is about twice that as on a conventional bike. Changing up one gear on the Brompton is similar to changing gears twice in succession on a conventional bike. If you change the Brompton hub gear up or down without adjusting the derailleur at the same time, for example from 2- to 3-, you have done a double gear shift roughly equivalent to four gear changes on a regular bike. And it takes just a fraction of a second to do so. Crest a hill in a low gear and need to move to a high gear quickly? No problem - go in one step from 1+ to 2+ and, if you have the legs for it, do a second double shift to 3+. That is a monstrous change in gearing. The reverse, of course, is true. Need to stop suddenly while in a high gear? Once stopped double shift down (no pedalling required) to a gear that is comfortable to pull away in.

It’s hard not to think that the mixed the hub and derailleur gearing on a Brompton is a design compromise but it might, just might, have been an inspired first choice. I guess we’ll have to wait for the appointment of The Brompton Historian¹ to find out which it was. The ability to make large gear shifts almost instantly more than makes up for the quirks of a limited gear, dual hub and derailleur system. Actually, the more I think about it, definitely not a bug. It’s a feature.

1. Mr. Butler-Adams, if you are reading this, please consider me a candidate for the position!