Originally Posted by
Kontact
You'll find that dodgy logic alive and well in all the lightest aluminum, steel, titanium and carbon road bikes. A 2.5 pound S3 steel road frame only gets that light because the tubing is oversized and very thin walled. If you made the walls that thin and allowed the tubing to flex, the walls would collapse from the load. Make the tubes oversize and the structure becomes stiff enough to not have to deal with with localized bending.
This is the reason that late '80s Cannondales with massive down tubes were lighter than Vitus frames with all skinny tubes.
And it isn't unchallenging to make something stiff and light - that's been the game since the '80s. But it is impossible to make something as light as the lightest frames with only moderate stiffness - they would flex past their plastic deformation limit and fail. The original Columbus EL tubing does not make for long lived frames, but S3 does.
I was talking about bike frames in general vs much heavier motorbike frames. If you only have 1kg of a particular material to design a frame, then there is a finite limit to how stiff you can possibly make it. If you have 2kg or more of the same material you then have scope to make it considerably stiffer.
I would imagine all current road race bikes are aiming for maximal torsional and lateral stiffness given the relatively small amount of material they have to play with ie 1kg or less of carbon. Motorbike frame designers are less limited in this respect and AFAIK they aim for a target torsional stiffness that is below what they could achieve if they wanted to i.e. they are not necessarily designing for maximal torsional stiffness.