Even more gearing options! If Brompton doesn't offer what we're looking for, what can we do?
We're Pam and Gilbert of 2Bikes4Adventure. In a recent video, we reviewed the various gearing
configurations offered by Brompton. Despite the dozen or so options presently available,
some of us may still not find the gearing setup we need from long distance touring or fast cycling.
In this video, we'll review some third-party solutions to improve your Brompton drivetrain.
We'll use the C-Line Explorer 6-speed with a 44T chainring as a baseline
but most upgrades are applicable to other configurations as well.
Brompton's 6-speed cassette consists of two sprockets, 13T and 16T. One of the cheapest
modifications is to change one or both sprockets to a different size. For example, a 17T
sprocket would lower the first gear by 6%. This update doesn't require much technical skill
and is very inexpensive, around $5. If you can fix a flat on a rear tire, you'll be able to
change sprockets. Chris by Bike and Brilliant Bikes YouTube videos can walk you through the procedure.
When selecting new sprockets, three points to be aware of: Brompton uses two different
sprocket fitting standards, the 3-speed A and C-Line Utility models use the 3-spline
and the 1,2,4 and 6-speed models use the Shimano 9-spline.
You'll need to select the correct fitting when ordering. The combination of sprockets will affect
the spacing between gears. Some combinations are better suited for climbing or speed.
And finally, larger sprockets 18T or 19T may interfere with the rear
frame structure, meaning you'd have to file some metal from the rear triangle.
Limiting yourself to a maximum of 17T will reduce the risk of mechanical interference.
It's also possible to add a third sprocket to convert a 6-speed Brompton to a 9-speed.
While not particularly expensive, about $100 in parts, it requires some
DIY effort Berlinonaut has an interesting article on the subject, there's a link in our description.
Another simple solution is to replace the chainring. Besides the 44T, 50T and 54T
provided by Brompton, third-party providers offer a variety of options.
Since 2013, Brompton has been using a standard 130mm bolt circle diameter removable chain
ring known as 130BCD. A 40T chainring will cost about $150 to achieve a 26.5 gear-inches or
2.1 meters of development first gear. This online video from Brilliant Bikes explains how to replace
a chainring. For a wider gear range, the most cost effective solution is to install a dual chainring.
Various combinations are available, such as 33T/ 50T and 33T/54T. Most users do not install a front
derailleur, instead opting to manually change the chain line depending on riding conditions.
With some practice, you can learn to use your right heel to move the chain from the big to small chainring.
Brompton Family Time channel has a good video on the installation of a dual chain ring.
Finally, if cost is not an issue, you can simulate a dual chainring by installing a Schlumpf
speed drive. This push button control dual gear system fits in the Brompton bottom
brackets and provides a 1.65 ratio increase for total gear range of almost 500%.
The trade-offs are the expense, about $700 and the 700 gram weight increase. We've covered a
few of the third-party options available to improve your Brompton gearing configuration,
from the replacement of a $5 sprocket to the installation of a $700
planetary gear bottom bracket, there's likely one that will fit your need and budget.