Some pictures below of how this works - showing a 16t and 18t 3/32" dual set-up. It is probably not suitable for 1/8" cogs - too much spacing required between the cogs leaving the outside cog a long way from the hub mount. However, with two 3/32" cogs separated by two 2mm spacers it all fits together very well. Note: also not suitable for 15t cogs.
Spaced in this way it places the cogs at chainlines of approx 41mm and 46.5mm so matches up perfectly with a standard road double chainset. Should work well on a mountain-bike too but would perhaps need a bit more fiddling with chainline. Due to the extra spacing 20mm bolts are necessary.
A dual cog set-up like this therfore allows a fixed gear bike to be run with two ratios. A higher ratio for riding to the hills or off-road trails perhaps and then a lower ratio for when you arrive. Simply unbolt the rear wheel, move the the chain over and off you go. Keeping the same tooth difference on the front as on the rear means chain length stays the same (for example 16/18 rear 44/42 front).
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