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there's a big difference in that area between an Arkose and a couple of other Al frames I've ridden
How much in actual units please? To be honest, I don't believe the difference could be detected in a proper blind user test (which admittedly is almost impossible to do).
Edit: I own a notoriously light and flexible Ti road frame, and when ridden in a turbo trainer the BB visibly moves at least 10 mm side to side. But I'm not at all sure I can genuinely detect that through displacement of my feet, and given it's clamped at the dropouts the tyre flex is removed from the problem.
chez_jay-
I rode them, I didn't do a load deflection test. But steering and bar leverage against rider weight can all put a twist load into a frame and they vary a fair bit in stiffness there. It won't be all the frame, the whole system flexes bars particularly but from a whippy steel bike to a stiff Al frame and somewhere in between I'd say a perceptive rider can feel it. I'd agree that the finer differences would be hard to call. And I'd not be able to feel varying levels of vertical flex in a frame. I think when most people talk about what they think is vertical flex they're feeling twisting flex (or imaging the whole thing via better fit/comfort).
jameso
In the vertical plane, not a lot. Esp in the rear triangle where it's often said to be. But Steel bikes are more likely to have lateral twist kind of flex and that accounts for a lot more comfort and ride feel (good and bad, depends how much etc) than most give it credit for or realise. Al frames can also do this too. The Arkose was always designed and ride-checked not to be too stiff against those lateral twist forces. Eg there's a big difference in that area between an Arkose and a couple of other Al frames I've ridden - some are silly-stiff imho.