It's not just the rear suspension that is affected by excess weight. The entire seating area is mounted on the frame, cantilevered by the enormouse cast alloy bracket with the footrests, by two massive bolts. The seat frame supports the entire weight of rider and pillion, two paniers and the top box.
The seat is bolted to the engine frame at the front of the seat, and the rear of it is supported by two bolts mounted to the footrest bracket by two bolts. The top box and panniers are also fastened to the seat frame.
The seat frame is tubular. The shape formed by the engine frame, tubular seat frame, massive footest bracket and the rear support for the seat frame (also tubular) is not a triangle, so the sahpe isn't a rigidd structure. But the footrest bracket is pretty rigid, so that is probably close enough. But only two parts of it are tubular - the top seat frame and the short supports to the rear of the massive footrest bracket. There is a temptation to think that bracket will always be there. But it has a heck of a lot of weight bouncing up and down on a very long lever, held in place by the casting surrounding two very large bolts. That casting is all there is. Everything has a breaking point.
And of course, all of this weight and more under heavy braking is taken by two more tubes sticking out at the front of the bike.
As for the seat tubes and the welding of those tubes. There's nothing supporting the middle section of the seat frame. Tubes are quite strong, vertical to the load. Less so if the load is on them horizontally (but still stronger than solid). But tubes are like the proverbial camel's back. They support the weight until they don't - at which point they buckle and are pretty much useless for supporting anything.
Trouble is - we don't have a clue what the actual breaking point is. But there will be a margin for safety, for the odd sudden bottoming out, for manufacturing variations.
All we know is what we need it to be to be safe. My 2009 model came with stickers all over the place saying clearly what the maximum load is for each pocket, pannier, luggage, total. The Handbook is similarly very clear with warnings printed in bold letters. None of this was there in that detail in the manual that came with my 2006 model.
Perhaps there was (ahem) some incident in the past that made Honda think that it was essential that they make it clear where their liability stops.
The total weight of the accessories and luggage added to the rider and passenger's weight should not exceed 196kg (432lb). (2009 UK handbook and 2003/4 Workshop Manual)
The luggage weight must not exceed 31kg under any circumstances. (2009 UK Handbook)
I've only ever guessed the weight of my riding gear - I must weigh myself with it all on one day. Goodness knows how much heavier it will be when wet.
There is an interesting article written in RIDE magazine last month, by a lawyer who specialises in motorcyclist representations. Someone had written in - his new bike's suspension had failed and he didn't want to foot the bill as the manufacturers said that his claim wasn't covered by the warranty. The reason being he had loaded the bike beyond the manufacturers gross vehicle weight. (Way beyond in fact). Which in the UK is a criminal offence. The top of the shock absorber mount had split.
We used to get pretty close to the maximum ourselves, but I always kept an eye on it and reduced the weight of the camping gear. Posted the dirty washing home to make room for other stuff !
