I wonder why no one has come up with this before, myself. The amount of drop out length does not need to be equal; hub goes into drop out with chain not necessarily on the cog, then the pin on reaction arm engages into the slot. Then hub can be moved forward enough to slip chain onto the cog and then, back deeper into the drop out for the chain tension. Of course this idea could also be used with rear facing track style drop outs , as well.