As standard the B'twin Vitamin came with a 36 tooth front chainring. the Nexus hub I got off ebay had a 20 tooth sprocket fitted. Using Sheldon Brown's Internal Hub Gear Calculator I can see that this gave the bike a gain ratio range of 1.8 - 5.6 (or 24.4 -74.9 in gear inches) which felt too low for my legs, but good enough to put up with for the time being.
I had planned to fit a smaller rear sprocket to the Nexus hub inorder to raise the gearing. But there is a problem with the shimano gear shifting design that isn't apparent in other mechanisms such as the Sturmey Archer style shifter toggle. The Shimano system rotates around the axle and takes up quite a bit of room, this means that there is a minimum sprocket size that can be accomodated on the hub before you start getting issues with the chain fouling the shifter mechanism. The range of gears I wanted needed a 14 tooth sprocket, but the minimum size the hub can safely accomodate is 16 teeth.
So since that was a dead end I bought a new longer 1/8" chain and fitted an old 44 tooth front chain ring I had left over from when I raised the gearing on the Brompton.
Shimano recommend that you keep the gearing as close to a 2:1 ratio as possible. This is to do with the amount of torque being transferred through the hub and I won't pretend to understand it. But basically 44 divided by 20 equals 2:2 which is closer than some of the acceptable ratio examples that Shimano give in the Service Instructions so it will do just fine for me.
On square tapered cotterless cranks you'll find a bolt under a cap on the chainring like this:
Usually a normal wrench socket will be too thick to fit in here and undo this bolt, so you'll need a tool like this one that has very thin clearance allowing you to undo the bolt.
Once that's out you'll need yet another special tool called a crank puller. This screws into the crank and chain ring and allows you to use either an allen key or spanner to pull the crank away from the sqaure tapered axle.
With the old chainring removed you can fit the new one, making sure its on straight and also opposing the other crank arm at 180 degrees. Then use the bolt you removed earlier to tighten it onto the axle.
This bigger chainring means the chain has to be extended. I choose to simply add a few more links to the existing chain rather than fit an entire new chain. I test fitted the chain to see roughly how many new links were needed.
Then spliced in the new links. Mounting the chain splitter in a vice can make things alot easier.
And finally the finished upgrade.
I had planned to flog the 36T chainring on ebay but in clumsily removing the pedals I managed to break the plastic guard. This is why the blog is subtitled 'making, breaking and riding bikes in manchester' because if something is flimsy enough to be broken easily, I will inevitably break it at some point!.
The crank arms don't match anymore, but that doesn't matter, at least I've made use of something that was just sitting in a draw.
This set up now gives me a range of 2.2 - 6.8 (or 29.9 - 91.5 gear inches) which is just about right for setting off in 3rd and running up through the gears and cruising in around 6th or 7th. That leaves 1st and 2nd for going up steep hills and 8th for coming down them!.
Coincidentally 20T - 44T is also the right size for a Hebie Chainglider to fit....hmmmm.....