Gearing: maybe it’s not the most exciting part of a bike, but when you can’t get into the gear you want, you know about it. Straining up hills in too high a gear, or running out of pedal power on the flat with the wind behind you – such moments could be avoided with a more careful choice of gears.
Brompton offer a range of gearing options on their shop-bought bikes, from 1 through to 6-speed models. (Five used to be an option, but four never was!)
Here I’m interested in the 6-speed models which, after all, give you the most gears you can buy on a Brompton. For those new to Bromptons, the six speeds are achieved using a 3-speed hub gear in combination with two sprockets (cogs). By switching the chain between the two cogs, and changing between the three hub gears, six gears are achieved. (i.e. 2 x 3 = 6). Although this is not a new idea in gear design, Brompton have redesigned the 3-speed hub so that its gearing is wide apart, (hence the name ‘Brompton Wide Range’ (BWR)) resulting in an even spread of six gears. The advantage of this system is that it retains the Brompton ideal of being both light and compact. It’s a clever piece of design that Brompton rather undersells on its website.
Now to the detail. Brompton offer three levels at which you can ‘pitch’ your gears – high, standard, and low. Standard gearing is achieved using a 50-toothed (50T) chain-ring (the front cog). If you really hate hills, and want your Brompton geared lower than this, the 44T will lower all your gears by 12%. If you live in a flat place, and have strong legs, you might want the higher version (54T), which raises all your gears by 8%.
The gearing you get for each of these options is represented by the graph above. Along the bottom of the graph is the gear number (from 1st gear through to 6th), and up the side of the graph is a measure of gearing called ‘metres of development.’ I could have used other measures, but I like this one because what it tells you is the number of metres the bike will move forward for one full turn of the pedal. It’s easy to picture!
In the graph above, the low, standard and high levels at which you may ‘pitch’ your six gears are represented by the blue, red and yellow lines respectively. You’ll notice the lines curve gently upwards, demonstrating the even spread of the gears. You’ll also see that in top gear, the bike travels about three times further for each pedal turn than in the lowest gear – in other words the overall range of the gears is around 300% (302 in fact). This is not a bad spread of gears, though obviously can’t compete with a mountain bike or tourer which may have a spread upwards of 500%.
This makes choosing your gears all the more important, since you are working with a compromise. With a spread of 300%, at some point you are going to run out of gears, so you need to decide where you least want this to happen. If you need a low first gear because you have dodgy knees, say, choose the 44T model; if you don’t care about hills, but want speed, go for the 54T model; and if you want to hedge, go for the 50T.
Another way of bringing this choice into the real world is to look at what speeds your Brompton will do in its lowest and highest possible gears. Clearly this will depend upon how fast you pedal (your cadence), and not just what gear you are in. At a comfortable pace you cadence may be around 70rpm (full revolutions of the pedal per minute.) When you are going for it, your cadence may be more like those chaps on the Tour de France, doing 90rpm. So 70rpm and 90rpm are good cadences to compare.
We can now refine the question: what speeds will your Brompton do in its lowest gear at a 70rpm cadence, and in its highest gear at a 90rpm cadence? It turns out that the 44T Brompton will let you crawl up a hill at just 6mph in first gear at 70rpm, while the 54T Brompton will get you up to 28.8mph in 6th gear at 90rpm. Not bad! Obviously, you can’t ride both these bikes at the same time, so if you’re on your 44T and want to go fast, you can only do 23.5mph at 90rpm. Conversely, if you’re on your 54T and hit a steep hill, your 70rpm cadence will mean you will push up the hill at 7.4mph – a brisk jog!
These are Brompton’s off-the-shelf options, but of course there are others. In a previous post I’ve looked at options for installing a Shimano or Sturmey Archer 8-speed hub. In a future post I’ll look at how these can expand the range of gearing options to give you even more choice. If like me you’ve got dodgy knees, it can be good to bring down the ‘pitch’ of the Brompton gears still further, to give you a ‘granny’ gear at the bottom for those big hills. Brompton’s 6-speed offering is (in my opinion) pitched a little too high – I would rather have a decent low gear and run out of gears at the top, than the reverse. If you’re going faster than you can pedal, stop pedalling and enjoy it!