Max weight of child, for rear bike seats - why is it always (seemingly) 22kg? I had a rear child's bike seat on my old bike (which was stolen, boo hoo).  It was a Yepp Maxi and was the kind that sits on a rack, rather than being mounted to the seat post.  

The maximum weight of child rating for both versions (rack mounted and seat-post mounted) is 22kg.  And, this seems to be the maximum child weight for all rear seats.  I don't understand how something mounted on a rack can have the same maximum weight rating as something attached on a diagonal bar to the seat post (which I'd be more worried about breaking) - surely it depends on how strong the rack is?  If the rack has a maximum weight of, say, 50kg, then I'd feel very confident about putting a bike seat on it with a 22kg child in it.

The fact that it seems to be 22kg across the board, which is slightly under fifty pounds, makes me wonder if this limit is set not due to any properties of the seat itself but rather due to some regulation.   I'm much less worried about breaking a regulation than I am about cycling along with my son and the seat falling off, or something.  

I understand why we have regulations but often they are very "dumb" in the sense that they don't take other factors into account:  for example, maybe some tests were done with an average bike and rider and found that any greater weight at the back made the bike too prone to tipping backwards.  This wouldn't take into account the size of the bike (XL in my case) or the weight of the rider (100kg in my case), which might mean that the bike can hold a heavier child at the back without tipping.  This is pure speculation on my part, anyway.

Does anyone know why this limit seems to be so consistent across different makes and designs of child's rear bike seat?  thanks
While Criggie's answer explains the reasoning behind the limit, the reason why the official limit is almost always exactly 22kg is simple: that's the standard.

The 22kg limit comes from the European standard EN 14344:2004: 


  This document specifies requirements for child seats for cycles, which
  are intended to be mounted on pedal cycles and electrically power
  assisted bicycles, in order to transport children with a weight from 9
  kg up to 22 kg (approximately 9 months up to 5 years) and who are
  capable of sitting unaided. NOTE Some European countries have special
  legislation for child seats for cycles. Compliance with this document
  may not meet this legislation.


So that's why it's always 22kg, and not 23kg or "approximately 22kg".