I used one of my Mark 3s as a fixed gear commuting bike for almost a year, and while I loved riding fixed, and I loved the simplicity and the low maintenance aspects, I found it did have it's limitations.
Firstly, while I love the simplicity, one gear was just not enough. I had no problem honking up the occasional hill, but my commute at the time was through Dublin's Phoenix Park, which has a lovely 4km stretch with a 1% decline and a prevailing tailwind, that always calls for a higher gear than an all purpose gear.
Secondly, I was worried about my knees. Most fixers will tell you it has no effect on the knees, but I rode while recovering from a knee injury (unrelated to cycling) and I can tell you that even the very subtle pedalling to slow down technique places a considerable strain on the knee.
And most importantly, while I made sure the chainline was perfect, and the chain tension was always ok, I was always worried that it would dump the chain, or catch a shoelace, trouser leg or finger with horrible consequences. Sure, the risk was low, but the consequences were potentially so bad for man and machine, that I found myself unwilling to take even that small risk anymore.
Still wanting a simple, low maintenance setup, I picked up a new old stock Sachs Torpedo Duomatic hub on eBay, built it into a 16inch wheel, and tried it out last night on the same Moulton Mark III.
It's a 2 speed hub, with coaster (back pedal) brake. The "low" gear is direct drive, and the high gear is a 36% overdrive. Gear change is effected by rotating the cranks backwards a few degrees, but not quite as far as required to engage the brake.
It's a really simple setup... there are no gear cables to worry about. And unlike most hub gears, there are no adjustments necessary. It just works. It's got the simplicity of a fixed gear / singlespeed, but with an extra gear for those slight downhills or tailwinds, especially in a mostly flat city like Dublin.
When I rode fixed, I used 56x13, giving around 70 inches on the Moulton. With the Duomatic I'm using 56x16 giving gears of 58 inches and 78 inches. I think that I'll play around with these ratios though.
I'm using a 3/32 inch chain, just because that's the size the chainring and sprockets are. The only problem is that the 56x16 requires the use of a half link. Half links in 3/32 size are hard to find, but I did manage to find some in St John St Cycles.
They're not a very good match for a Sram PC951 chain though. I think they're designed for a chain with taller links. They're also very slightly wider than the PC951 making use of a master link with the half link impossible. The easiest solution is probably to replace the 16t sprocket with a 17t.
Alternatively, I suppose I could just change to a 1/8 chain, with the existing chainring and sprockets... as far as I know the only downside is extra noise.
The test ride was quite nice. I intended doing a 10 minute ride, but it turned into a 30 minute ride.
It takes a little practice to change gears without engaging the brake. The brake is good for a rear brake... obviously useless in an emergency, but perfect for slow easy stops. I used it a lot more than I would normally use a rear brake (i.e. I used it). So it should prolong the life of my front rim, as well as the back rim.
Engaging the brake, also changes the gear, which is great when braking in high gear, as it is quite likely that the low gear will be required next. However, if you brake in low gear, the high gear gets engaged, most likely requiring another backpedal to engage the low gear again before pedalling away. It's only a minor inconvenience, especially when it's actually quite easy to audibly determine what gear is engaged.
How? When the hub is in overdrive, it ticks, whereas direct drive is more silent. The ticking is perceptible as soon as you begin to rotate the cranks and before any effort is engaged, so you know you're in overdrive, even before you try to drive the pedals.
If this hub was to come on the market now, I'm sure the reviews would ridicule it as primitive. But that's it's beauty, it's simple, it works, it's not black. And it requires a little bit of skill and finesse to use it most effectively.
When the fixed gear craze goes out of fashion, as it inevitably will, and cyclists search for something that is simple AND functional... these will be in high demand. Apparently Sturmey Archer are considering remaking the ASC hub, the 3 speed fixed gear hub. Any chance that SRAM would remake the Duomatic??