gravitation and the ministry
the max weight limit of 18,000 kg for a 2 axle bus is to do with physics and not the ministry of transport.
In practice, the issue of the 'G' force [gravitation] comes into play when you have an extremely heavy bus such as this BYD.
on a normal single deck bus you can carry more passengers, but in practice, this is limited by the space available for standing passengers.
The ministry can fix a lower passenger capacity than the one dictated by weight limits, but not vice versa if you see what I mean.
[you see, 2 axle d/d buses in London also have a max weight of 18,000kg, and they weigh more than s/d buses, so 'everyone' in London is nowadays aware of weight issue!]
Chinese buses are heavy in comparison to European ones, simply because the lightweight materials and new construction techniques employed in Europe are most costly than the existing stuff and technology. If you want cheap buses, China would supply you with cheap buses, but they are heavier buses.
I am amazed that Dan is so confident that the top heavy BYD would preform well in service, because the BYD service history elsewhere, and in Shenzhen in particular, is fairly patchy. all you have to do is google the web, and search for comments on Chinese websites!
one of the reasons why the bus offers less internal space is that some of the battery 'grids' run alongside the bus 'wall', so it takes some space, thus narrowing the available width of the bus.
Regards
Yoram