Turning radius and axle loading.
Many of the routes that such 10.6m 3xADD runs on used to be
services plied with Leyland Victory and Dennis Jubliants and not
12m DDs. They have narrow roads and/or turns which require the use
of such short wheelbase buses. An especially good example is NWFB
rt. 15.
However, with the introduction of air-conditioning in 1990s, the
added weight of the additional AC unit exceeds the maximum axle
loadings of 2 axles, thus requiring an additional axle to even out
the weight displaced onto the roads.
Substantial work has been done with the HK E400 and 2x
weg to lower their vehicle weights (partly by using aluminium body
frames). HK TD has also specially given permissions for 2x ADDs to
be operated now despite it still marginally exceeds the axle loads.
These 2x ADDs now weigh no more than a typical 12m SD we use
here, while comfortably accomodating more seated passengers.
Haha oops sorry, totally forgot the 10.3m DM5000s are tri-axle
:P but you're right, with the development of air-con buses and the
lack of requirement for further research has allowed these 3-axle
10.3/10.6 to be on the roads.
However I still think the DMs deserve to stay till end of
lifespan. Coz as you have raised 15 earlier, that service is one
big cashcow for NWFB, many take that service to go to the peak
during weekends and holidays, and so many buses have to be pulled
over, sometimes even exceeding the amount of buses running on 101.
Do you think an SD can cope with ease? At least these DDs can still
comfortably fit in a little more pax.
And also back to safety problems. 15 is a route that goes up and
down the Victoria peak and have sudden and steep inclines. It also
has sharp bends that reached up to 120 degrees. 12m SD? Dream on.
I've ever tried standing on this route, my head was slammed three
times against the wall (was standing near the staircase partition),
and once onto the ceiling. Dangerous isnt it? How would an SD,
which forces more to stand, be able to warrant more safety? Same
goes for 9 and 14. This is why you will NEVER see any other buses
than these DMs on these services.
But it is clear that NWFB will be taking the E400 as
replacement. Not that I'm complaining.