Originally Posted by swingset
All frames flex, whether or not you feel it is neither here nor there. A well designed frame flexes in accordance with the suspension, minimally, and feels planted; a poorly designed frame flexes and opposes the suspension, and that is an ill-handling bike or one whose suspension will be harder to manage or to feel consistent. This is true for Aluminum and Steel frames.
It's all hypothetical anyway, but there's got to be a reason aluminum is used in Japanese offroad bikes.
Originally Posted by rxcrider
All things being equal, thickness of material, the steel typically used in motorcycle frames flexes more than aluminum. So, all things being equal Al flex is more rigid or more specifically the flex is more controlled...and I think that's the benefit for the manufacturer, among other things. There's a good article in MX Action I read discussing why and how the Japanese use aluminum and continue to improve it.
In many instances, flex is used to tune the chassis and even incorporated into both steel and aluminum designs (this is especially true in road-racing bikes). But it complicates offroad where the lighter, skinnier and taller frames on long suspensions see torsion and force from many different directions while being expected to track and feel "true". Al works very well in that regard, especially when you don't have to stick to one thickness. It's easier when you're hyrdoforming aluminum to make thick/thin areas than steel, where you can more easily "tune" the frame for desirable characteristics than you can with steel. The Japanese feel it's a better platform on which to develop chassis'.
Most people who ride the same bike (like opposing steel/al framed WR450's) with different chassis have a preference one way or the other.
There's no "right" or "better", really, there's different and if I'm going to make a dream bike that will never happen, I'll never build, nor will anyone else....I'll take an aluminum frame. Anyone who disagrees with my choice can have steel, or magnesium, or make it out of unicorn bones, and that's ok too.