I have bought a BoBike junior bike seat for the rear of my bike which is a Cargo bike but the seat instruction says it can’t be mounted on carbon frames, baffles me as to why?

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    That's a pretty wide-ranging blanket statement, but I understand where they're coming from. Some particularly beefy carbon fiber seatstays might be strong enough for a child seat, but overbuilt carbon frames are the exception rather than the norm as the target customer wants light and just strong enough for a rider, not sturdy with cargo capacity. I can imagine some lightweight aluminium frames not being able to withstand child seats too. – Gabriel C. Feb 11 at 14:43
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    Just clarifying - Your cargo bike is a carbon fibre frame ? I'd not heard of such a thing, but yes they do exist. – Criggie Feb 12 at 8:44
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    @Criggie I'd assumed that the asker's bike isn't carbon but they're asking just out of curiosity, after seeing the restriction in the instructions. – David Richerby Feb 12 at 15:45

Carbon tubes tend to be very strong at withstanding forces in the direction they are designed for, but weak at resisting other forces (such as clamping forces compressing the tube), hence why bolts for carbon bars, seat posts etc are usually only torqued to 4-5Nm.

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    and that seat (I've got one on my Al hybrid) clamps onto the seat tube and seatstays. The latter clamps in particular need to be quite tight – Chris H Feb 11 at 15:23

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