Roger,
The bike rack you referenced was a factory option and as they say: 'if
it's good enough for them it's good enough for me'. But I think it's
good enough because the monocoque construction has enough reserve load
carrying capacity, not just because it's bolted to the frame.
In architecture the 'beam' (frame in this case) is sized to handle both
'dead' and 'live' or dynamic loads, with a margin of safety over that.
I think that is the case here also. If the frame were at it's max
design load and then the bike rack was bolted to it, the frame would
deflect beyond acceptable limits either causing frame separation or
'rear-end sag' we so often hear about. One example would be Bob &
Bertie's trailer before stiffeners. The rear bath sat on the flooring
which in turn loaded the frame, which caused it to 'over-deflect'. The
ultimate load carrying ability before separation occurs probably varies
with size of frame members, cantilever distance from the rear-most axle
and how much excess capacity was designed into each trailer model.
So... I don't totally agree with your logic, but it's probably just fine
nonetheless. The factory must have thought so also.
Rob Davis
'64 GlobeTrotter
WBCCI #1824, VAC