NavList:

For me, the key thing to realise was that while pushing the pedal backwards, your feet push the ground in the opposite direction. It's as if there was a rubber band stretched between the bottom pedal and the bottom of the tyre, trying to pull them together. In fact, your body is the rubber band here, and the Earth is a connecting component. So we can rephrase the question equivalently: if we attached that rubber band to the pedal and the wheel, what will the mechanism do, from the point of view (reference frame) of the bike frame?

A picture gives a quick answer: the torque on the (rear) wheel from the chain is Frm/M. The torque on the wheel from the tyre is FR. Since m/M < 1 in most cases and r/R < 1 always, Frm/M will generally be smaller than FR and the wheel will start turning backwards. The bigger wheel wins, quite simply. So unless your bike has an exotic gear ratio, it will start rolling backwards, too.

Fortunately, it looks like David Pike's experiment corroborates this armchair analysis. :)

Now I just need to find an excuse why this is on-topic on NavList...

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