Originally Posted by
SpongeDad
In the sense of center of mass (COM), if you're COM is moving up a steep hill at 10mph and you stand, you're probably moving 90% of the bike weight (your body) forward faster than 10mph, meaning the bike itself has to either slow down or go backwards (just for a moment). Now, I suppose you could apply sufficient force just as you started standing up that you keep things moving forward.
Even if all the wheel does is slow down, someone close on your wheel could bump it.
It depends on your frame of reference. As the rider who's standing, it appears that you are moving forward when you stand, because your frame of reference is anchored at the bike.
If you're another rider, your frame of reference is your own bike. When the guy in front stands up, his weight moves forward relative to his bike, but his COM continues at the same speed. Since the bike is lighter and the COM didn't move relatively, the bike moves closer to yours.
The effect would be much smaller if we just rode much heavier bikes
