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Hmm, I was considering the seatpost clamp ats the centre of rotation. Then my seated 70 kg acting straight down through the the saddle clamp, so about 50 mm horizontally from the centre of rotation. Thus moment of 3.5 kgm.
If a bag was 5 kg but its centre of gravity was 300 mm horizontally from the seatpost clamp, that would be an significant additional moment of 1.5 kgm.
Or have I visualised it incorrectly?
chez_jay-
have I visualised it incorrectly?
Your visualisation is close enough, I think it's unlikely that your numbers are right. Unless you're @gaston your CG is likely to be more like 150-200mm behind the fulcrum when you sit up to unwrap a sandwich out of your musette. Even if you could load your seat pack in the way you describe, it's weight would only count double relative to your body weight in terms of the design load of the seat post, not the six times in your illustration.
gbj_tester
Only if the total of your arse and the bag exceeds the maximum rider weight permitted by the frame.