Category: Planning

OK, so I finally took the plunge and added a manual change small front chainring to the Brompton. Here’s a write up of how what I did and parts used. This will work on a 6-speed 2007 Brompton. I can’t vouch for other models, but with a bit of common sense it won’t be too hard to work out the possibilities for other models. The numbers after the parts are the SJS Cycles part numbers.

Parts:
1 x Stronglight 28t, 86mm bcd chainring (3022)
5 x 2mm chainring spacers (17702S2.0)
5 x 1.2mm chainring spacers (17702S1.2)
5 x double chainring bolts (10652)
1 x 127mm bottom bracket (10859) (this is longer than the original, but needed because the one fitted at the factory isn’t long enough, and the extra chainring will rub against the chain stay unless the whole crank is moved out an extra few mm)

Tools:
Wrench, crank puller, bottom bracket tool, allen keys

Instructions:
Remove the cranks and the bottom bracket. Install the new bottom bracket, bolt on the new chainring, and put the cranks back on. Done, and very simple. Just remember to check the bottom bracket and crank bolts after a few miles of cycling, the crank bolts especially are difficult to get tight from a fresh install, and they can work themselves loose. Now you’ll have a Brompton with a good set of standard gears, changeable with gear levers, and an extra chainring for those really tough climbs, but changeable manually. Once on the smaller chainring, all the other 6 gears work as normal as well.

In response to a comment left on my original gearing post, I think I’ve found the solution to adding lower gears relatively easily. I was notified of the BromptonTalk group on Yahoo!, and searching there finds lots of stuff on dual chainrings.

The standard 50t chainring has holes drilled that will allow a 28t chainring with 86mm BCD. However, the bottom bracket is a bit short, and with the extra chainring, it won’t fold and the chain may rub against the frame. The solution to that is to fit a longer bottom bracket (BB). The guys at BromptonTalk have discussed at length the different BB options. The standard brompton has a 119mm BB, and, as far as I can tell, it needs about another 5mm for the dual chainring. The 125mm one of these seems to be recommended by people on BromptonTalk.

Voila, the gear inches for that set up will go down as low as 22″, with the two highest gears on the small chainring being similar to the two lowest gears on the large chainring. The manual change shouldn’t be too much of a problem to use occasionally.

I know, this is a favourite topic of mine at the moment. I read somewhere (can’t remember where now) that someone put a smaller chain wheel on the front, but with no derailleur. That means it isn’t possible to change the front gears while cycling, but it does give more gears. When I reach a hill and realise I’m going to need a lower set of gears for a while, I can manually put the chain on the smaller chain wheel and off I go. Genius. Wish I could remember who’s website I first read this on so I could give them credit!

Warning – this will be a boring technical post about the number of teeth on chainrings and gear ratios.
Bromptons come with different configurations for gears. There’s a single speed, a two speed with derailleur, a 3 speed with internal hub gears, and a 6 speed with derailleur and internal hub gears. The list gets longer because with the 6 speed, you can choose to have 8% higher or 12 % lower gearing. I got mine with the standard ratio 6 speed, and it’s great, I can get up to 25 mph quite easily (even managed 32 mph once) and have yet to find a hill I can’t get up. But that is around london and when I’ve been at home in Cumbria. Without any luggage.

So I’m thinking with towing a 30kg+ trailer up the Alps, I might need some lower gears, but I don’t want to lose the current top speed I have. One option is to get a Schlumpf Mountain Drive. This acts like an internal hub gear, but for the front chainring. It has two options, a 1:1 ratio, and a 2.5:1 downshift. It would be brilliant, except that it costs £275.

Another option is to use a Rohloff Speed Hub. This would replace all the current rear gearing on the Brompton and give 14 internal hub gears. But they don’t fit in a 16 inch wheel as standard, and the base cost is an astronomical £625. So clearly not an option. Then I thought, why not put a bigger sprocket on the rear?

My standard ratio Brompton has a 50 tooth chainring at the front, with 13 and 15 tooth sprockets allied to 3 speed hub at the rear. I thought I’d see how easy it is to change the sprockets, and discovered that they clip on rather than requiring a heavy chain-whip to remove. The next thing I did was to work out the current gearing. This website lets you put in just about any combination of gears and will tell you the ratios. Here’s a table of my current set-up:

Gear# – Internal – Sprocket – Meters Development
1* – 1 – 13 – 6.7
2 – 1 – 15 – 5.8
3* – 2 – 13 – 4.9
4 – 2 – 15 – 4.3
5* – 3 – 13 – 3.6
6* – 3 – 15 – 3.1

The *’s are the gears I use most often – I change down the hub gears then use the 15 tooth sprocket for big hills. If I was to change the 15 tooth to be a 17 tooth sprocket, I would have:

Gear# – Internal – Sprocket – Meters Development
1* – 1 – 13 – 6.7
2 – 1 – 17 – 5.1
3* – 2 – 13 – 4.9
4 – 2 – 17 – 3.8
5* – 3 – 13 – 3.6
6* – 3 – 17 – 2.8

You can see that the gears I use most often have not changed, except the lowest gear, which is now 10% lower. The cost of this solution? £5.99 from SJS Cycles. Even if it doesn’t work or I don’t like the new gearing, well i’ve wasted £6. And because of the relative ease of changing sprockets on a Brompton, I can always take the original 15 tooth and change it en-route. Or take an additional 19 or even 21 tooth sprocket to change to if the Alps are too demanding.