I don't think either Riv or abcycle have it right. The suitcase analogy doesn't work unless you're carrying your bike. Maybe if it's a suitcase on wheels.
Riv has it wrong too. If I were to go for a 40 minute run today that included at least some small hills, woke tomorrow suddenly 5 lbs lighter and did the same thing, I can tell you I would notice a difference.
And it DOES matter whether the weight is on the bike or on your person. It's easy to demonstrate the difference:
Ride through some moderate potholes on a rigid framed bike keeping your butt on the saddle and your arms as stiff as you can. Now do the same thing with your butt out of the saddle and allowing your arms to flex as you go through the potholes. It makes a big difference. It not only feels better, it's faster. The total weight hasn't changed a bit. What's changed is "suspended" vs. "unsuspended" weight.
Of course most people's commute isn't a continuous string of potholes. But all surfaces have imperfections and your body weight is partially suspended even if you keep your butt on the seat. Your arms aren't steel tubes, they flex quite readily. Your entire upper body doesn't have to move up and down to the same degree as your bike, you're hinged at the waist.
Last edited by tjspiel; 09-23-09 at 08:30 AM.