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As previously promised, here's a pile of stuff I wrote recently when I was wracking my brain over trying to find the simplest possible ways to add a lower gear ratio to the Franken-Peugeot, before I walked the thing all the way through one of the trails that I was hoping to be able to use said lower gear on and realised it just wouldn't be viable for other reasons. A little glimpse into a diseased mind which is far too liable to spend weeks stewing on this kind of thing. There is one correction I should make to the below: I've since double checked and the Franken-Peugeot's rear spacing is 125mm, not 120mm, which means that 6- and 7-speed freewheels (still in production by Shimano and easy to source) are viable options on top of the somewhat more obscure 5-speed freewheels. Anyway, enjoy.
This is the obvious, traditional, "why would you try anything else first?" solution. And as much as I love the simplicity of my current single speed setup, I'm not entirely opposed to it. But it comes with a whole heap of practical difficulties. The rear dropout spacing is 120mm, so I'd need to either cold-set the frame to accept modern 130mm freehubs, or redish my current wheel and add a 5-speed freewheel, which would either need to be a NOS part or one of the cheap new ones that Sunrace makes (and who knows how much longer those will be made for?). Then there's the fact that the frame's previous owner, presumably in a bid for that "clean fixie look" did an excellent job of removing every useful bit of the frame for gearing: there's no derailer hanger, no cable stops, no downtube bosses, nothing. All of these things can be replaced with clamp-on equivalents, but the unusual dimensions of French tubes mean I'd need to either hunt down French-sized parts (most likely by Simplex, whose parts have a poor reputation for reliability due to their use of plastic) or shim everything. None of this is insurmountable, but at some point one has to ask whether it makes more sense to keep an eye out for a good deal on another frame, one with its original braze ons, or standard sized tubing. Even, oh wonder of wonders, standard English threading for the bottom bracket! In short, this would be enough of an uphill battle that it's well worth considering other options. Also, after happily single speeding for years, it seems a shame to go, for want of just a single extra gear for occasional use, all the way to the opposite extreme of a fully geared bike.
All of these options involve mucking around with the back wheel without adding a derailer. A flip-flop hub is a pretty standard approach, but I'm not sure it works well here. Unless I wanted to carry a second chain around with me, I would not be able to have much difference at all between the size of the two freewheels without the chain being too loose on the bigger gear. I guess I can't know until I try, but I don't think one or two teeth more is quite what I need here. Bigger problems are that, with fenders fitted, removing my rear wheel requires letting down the tire, which adds to the hassle of changing gears. Plus, the forest is the last place you want to be wrangling with your rear wheel, because at least at this time of the year it's liable to be quite muddy.
Another option would be to have a second dedicated wheel for this kind of riding. I got this idea from the sadly defunct Pondero blog's post:
wherein the author muses on paring down his bike collection to a single bike (a Rivendel Quickbeam, which is a single speed - ish, more on that soon) and a "quiver" of wheels. Basically, three wheels with different sized cogs or freewheels and different sized tyres, including, at one extreme, a camping / off-road wheel with "some kind of knobbly 40mm CX tire" and a lower gear than the other wheels. This basically requires gear changing to happen at home, so at least you don't have to carry chains with you and the tyre can be clean when you wrangle it. But I don't think this works too well for me, since I ride the Franken-Peugeot to work during the week, so I'd be swapping wheels all the time, which could get old fast.
Then there are much weirder options, like the kind of thing sometimes called "dingle-speeds" (which I think is specifically Surly's name for a concept that they didn't quite invent). I'm talking about an approach where you have two freewheels (either on opposite sides of the hub, or with an expensive dedicated two-speed freewheel like:
White Industry's ENO DOS wheels)
and either two chainrings such that exactly the same chain fits nicely with both pairs of front/rear cogs, or cleverly shaped dropouts (which the Quickbeam has) so that you can adjust chain tension by moving the wheel forward or backward without messing up the alignment of the brake pads on the rim, in which case you can have a single chainring if you want. So this arrangement permits either two or four gears.
At the far end of the weirdness scale in this section are:
Sheldon Brown's 1-, 2- and 3-speed mountain bikes
which he built somewhat along the lines above but using old Suntour 5-speed freewheels with some of the cogs removed and replaced with spacers to basically build a cheaper and more flexible version of the ENO DOS freewheel (which may not have been around at the time Sheldon was doing this). Sheldon used a flip flop hub, with a fixed cog on one side and one of these double freewheels on the other, in conjunction with a double chainring to achieve three speeds, but I don't see why you couldn't use a triple freewheel and a triple chainring to get three speeds without the need to flip the wheel over. This approach requires building a truly unique rear wheel, but it has its merits: you can change gear on the side of the road without removing a wheel, and you only ever need the one chain.
The ultimate weirdness: keep the single-speed freewheel, but also add a rear derailer. Don't put a cable on the derailer, just adjust the limit screws to keep the chain fixed right in line with the freewheel (basically, use it as a chain tensioner - I've read many people say that this works much better than dedicated chain tensioners). Run a double or even triple chainring, and change gears manually by moving the chain between them. This is definitely the lunatic fringe of drive trains, but I have found a few scattered forum reports of people getting this working. The general consensus seems to be that this arrangement makes no sense and that once you have a rear derailer in there you might as well go with a conventional setup. But, at least in my specific case, this plan does make a crazy kind of sense. Compared to going "full derailer", I wouldn't need to worry about finding or bodging up French tube compatible clamp on cable stops or shifters, I could use easily found, in-production singlespeed freewheels instead of NOS or bargain basement 5-speed freewheels, and I could run a totally undished rear wheel for maximum strength. Compared to flip-flops, dingle-speeds and anything like that, I never have to flip the rear wheel or move it around to adjust tension, so there's no problem with brake pad alignment, no need to touch a muddy tyre and no reason to stop shunning quick releases as silly theft-enablers. Okay, instead of a muddy tyre I need to touch a greasy chain, but that can probably be done with a stick. As weird as it is, this seems to be the way to add just one or two extra gears to my bike that require the fewest parts, and zero exotic parts. It would be a shame to lose the efficiency of a derailerless drive train, but there's no such thing as a free lunch.