daka

Let me pose this in another way - Two identical bikes are rolling down two identical grades. One grade has perfectly smooth pavement, the other is patched asphalt, cobblestone, chipseal, your choice, it doesn't matter as long as the imperfections aren't enough to flat the tire or bend the rim. Which bike will get to the bottom first? In another words, which bike is not getting a force vector opposite to the direction of travel, in the direction that would push the fork back?

daka

You know, if we were able to get together over a cup of coffee and do a sketch or two, we'd reach 100% agreement in just a few minutes. Here's another way to try and visualize my point - When a bike encounters a bump (any bump, it scales), the wheel has to roll up and over the obstruction. So picture in profile the front wheel against the 2 x 4 you mentioned earlier - Note that the point of contact has moved forward on the tire relative to where it was when the surface was smooth. So now, the spokes in tension are not the ones that are vertically aligned but those that are at, say, 11 o'clock or thereabouts. This translates to the direction of our force vector; looking at the orthogonal components, we have the vertical like we had before and we have a new horizontal component in the opposite direction of travel. The magnitudes of these go back to our discussion on mass and kinetic energy but we can ignore that for now, you should see there is a force vector component acting to pull the hub axles back and slow the bike, hence my ramblings in the post above this.

As you have pointed out, the reality of the situation is that (thanks to the head tube angle) we were already pushing the axle/fork forward due to the vertical, gravitationally aligned force so hitting a small bump may just reduce that momentarily since the magnitude is not great. But as the bumps get bigger and the relative magnitude of the horizontal component gets larger we could shift into bending the fork back, as happens when we strike something as large as a curb.