Shoot me down if my schoolboy physics is wrong, but here is the calculation

If we assume I went from 20mph to 0mph in 1 second we have

Kintetic energy = 1/2 * weight * (M/S)2

so KE = 1/2 * 80kg * (9 m/s)2 = 6.885 KJ

Covert this to watts to give an idea of power transfer through the stay

P(W) = 1000 × 6.885kJ / 1s = 6885W

That is assuming all the energy was transferred through the stay which it will not, but even if we said 3000W is a stay designed to take such wattage.

Interesting

the advantage is not stepped up by the gear ratio. the maximum tension I can put into my chain is my weight times the crankarm length divided by the effective radius of tha chainring.

In a normal use that is true, but there is additional mechanical advantage during the process of chaining gears. Whilst the chain is between sprockets, the cassette is effectively acting as a cam, gradually growing the required length of the chain by 1″ as the wheel rotates through half a turn. In my example above, that 1″ of chain “growth” comes from 25″ of pedal movement, hence the 25:1 MA.

the maximum tension I can put into my chain is my weight times the crankarm length divided by the effective radius of tha chainring.

Well, technically that’s the maximum difference you can put in between the upper and lower chains. Normally the lower chain is under negligible tension (it’s only the mech spring) but in this situation the chain gets winched onto a larger sprocket and both upper and lower chains are under very high tension.

There’s a critical sprocket difference which I can’t be bothered calculating – if the sprocket difference is small, then the chain will just be a bit tight. If the sprocket difference is large, then the chain won’t climb up much or at all, and it’ll stop dead*. But in the middle it’ll climb up with your full weight behind it, and be under a very high tension indeed.

*of course stopping dead isn’t ideal either, but the wheel will still freewheel so unless something else breaks it’ll probably not be catastrophic.

Yeah, what pdw said about cams 😉

bencooper and pdw, thanks for your patience. I felt there was something wrong with what I was saying – “maximum difference” made it clear.

okay, I’m with those guys. now how do I add a smiley using my phone?…

B-)