I might be wrong (what else is new?), but I thought that the new Bonneville's frame used the engine as a stressed member (ie: it's part of the frame). That's why the lower frame rails are removable, and you could, in effect, remove them completely from the bike. I know my '98 Adventurer, with a steel spine frame, had no lower frame rails at all. The lower frame rails were only added to give "the look" of a loop frame, as the older bikes had.
If you are correct then I supposed the whole thing is a moot point because Triumph elects to have redundancy with the engine shouldering the burden of frame loading and yet there be token frame members purely for aesthetic reasons. I sure wouldn't want to be the guinea pig on removing the rails but if somebody has taken them off and ridden successfully it would be good to know as a weight savings opportunity for those that believe that function should drive form.
In my experience, having a sectioned frame with removable frame rails across brands is more about ease of assembling the bike and installation/removal of the engine when the engine isn't weight bearing. Frame rails connected with fasteners as in the case of the bonneville is more expensive than welding the entire frame as a sectioned unit. By contrast, a unitized or welded frame would compromise packaging and space efficiency if the engine wasn’t weigh bearing without being able to remove rails for access. The motor in other words fits more efficiently inside a sectioned frame from a weight and packaging standpoint versus sitting on the frame which would have to be taller without having the ability to disassemble the frame.
Below show two Triumph frames. Notice the contrasting frame designs. The Street Triple clearly has the engine as a load bearing member. It is part of the frame. The frame doesn't close on itself which is critical to strength if the engine is isolated and not load bearing. Also note the access for the engine to the frame. In the case of the Street Triple, nothing has to be removed to install the engine into the frame because it is part of it. Contrast that with the old school construction of the bonneville.
Other noteworthy traits as a technical sidebar include steel having smaller frame sections because it has 3x's the yield strength of aluminum. Much of the weight advantage of aluminum in fact is diminished by the fact that a much larger volume has to be used to offset its strength disadvantage. Also, Aluminum is inherently a much more flexible material than steel and yet the net stiffness of most aluminum frames is higher than that of steel because of the larger sections of aluminum necessary to make it strong enough. The net result is Aluminum frames are lighter than steel frames but not as much of a difference as many would believe by the difference in respective material densities. Not using the engine as a load bearing member is a bigger factor than material selection. Also steel tends to ride a bit smoother because of inherent damping qualities.
Bottom line in this discussion is a bonneville even with comparable components will never be as light as a Street Triple due to the native frame differences. Does this matter to most that buy a bike like the bonneville?
It doesn't to me but if I have a chance to casually take weight out of the bike as others have, I try and do it.
