Fork movement - primarily fore-aft. BB movement - along the axis of the BB axle. Magnitude? I'm guessing pretty small, maybe 0.1 inch or less?
OK, you weren't very specific and I hope I don't put words on your mouth. You're saying the fork moves fore-aft by 2.5mm? Is that forwards by 1.5mm and backwards by 1.5mm? Yes/No?
Anyway, the fork is making a pendulum movement fore-aft, that can indeed be seen. However, a fore-aft arc of 2.5mm translates to a vertical movement (that's what we are after, the suspension effect if you want) of less than a 10th of a millimeter. You can feel that over and above the 5mm flex in your tyres and 5mm flex in your saddle? Remember, a sheet of paper (80gsm) is a tenth of a millimeter. Go ride over one and see if you can feel it. Don't forget your earplugs, cause the tyres may give secret away.
BB movement - along the axis of the BB axle.
Granted, that flex is indeed visible, but how does it contribute to compliance (suspension effect)?
Short answer, it doesn't
I'm sure that when riding over rough tarmac at speed and looking down at the fork (in the singular, just to please you)
Thanks for pleasing me. I have a thing about multiple forks on bicycles.
The long and the short of it all is that you cannot feel the difference between a steel and aluminium bike. This was widely perpetuated in the bike magazines around the time that Cannondale popularised the first aluminium bikes. The word "harsh" was used by an American magazine editor with zero engineering cred, yet he made the word go viral.
It is nonsense. The rear triangle is for all intends and purposes, completely rigid. That's where all the butt jarring comes from, the rear. It has no compliance.
The differences are acoustic and it is easy to assign properties to sounds. I have to resist the urge to do so myself.
I've taken this thing to the n-th degree over many decades, to the point of copying a Cannondale CAAD 4's frame in steel. I used exactly the same tube lenghs, angles, weld techniques etc and made a steel Cannondale. I still have it today. I cannot tell you what bike I'm riding other than to look down and see. Obviously I saw before I get on, but still. There is NO DIFFERENCE. The Cannondale is noisier because the gear cables ping against the fat downtube.