Alex Moulton -> Gravel tourer

[a.twiddler]

Something to bear in mind, too is that careful chain length adjustment (even though the minimum length is ultimately dictated by big/big plus 2 links) might let you get away with a bigger low gear cog (hence derailleur arm length) than you might expect. The derailleur will point down or back more on the smaller cogs/higher gears where you might be moving faster on better surfaces, with arguably less likelihood of contacting the ground, taking up more chain slack, whereas on the bigger cogs/ lower gears where you might be going more slowly on poorer surfaces the derailleur will be pointing forward and less likely to make contact with the ground. Also, in higher gears the derailleur will be further out from the centre line while in lower gears will be closer to the wheel and partly shielded by it unless you drop into vertical ruts. There could be a lot of trial and error involved while working things out.

As long as there is some clearance between the ground and the derailleur when fully extended the balance of probabilities is that in dry conditions there probably won't be a problem. Most of the time, the derailleur arm is likely to be away from that position. In mud, or rain with puddles there will be maintenance needs, but anyone with a Brompton will be familiar with the muck that accumulates around the transmission. It's something that goes with the territory.

Small wheels and derailleurs can sometimes have problems with grass if long enough, but I've had to stop and unclog a 700C bike when grass has caused the gears to bind, so it's not just a small wheel thing.

 

[dimrub]

Do you need the 42T on the rear cassette? What about an 11-34 instead and something like the 105 R7000 rear derailleur which is a shorter medium cage.

Yep, as discussed above, I don't strictly need the 42T cog (except quite rarely), and 11-34 cassette is the exact solution I'm going for.

 

[404 Not Found Anywhere]

At the risk of stating the obvious: what about a double chain set? You have a braze on mount on the rear triangle. No need to change handlebar controls, you could control it with a bar end lever on the handlebar. Even though my AM has 17” wheels I have a good spread of gears with a 2 x 9 setup. And I have found it surprisingly handy on rough forest tracks, just so long as things don’t get muddy.