Originally Posted by
MrPolak
I really want to let this go, but I have to point just a few things, and then I'm done... really!
--Lighter bikes change direction more easily when acted upon by an outside force. This is true, but it's a little different when rolling down hill. Now we're talking about potential energy and the object with greater potential energy will accelerate faster unless friction overcomes the difference.
No. If you are just rolling downhill, then gravity is the only force causing the bike to move. As we know, the acceleration is independent of mass, a 10 lb ball will fall at the same rate as a 50 lb ball. The heavier bike will have greater potential energy. As the bike moves, the speed of the bike is not greater in the heavier bike because of the mass, but the kinetic energy is greater as potential energy is converted into kinetic energy.
Originally Posted by MrPolak
--My wheels are 700c and lighter by 300-400 grams than his, so my wheels have smaller rotational inertia.
Well, I was giving a general statement, different masses will change the rotational inertia. I don't know the weight of your wheels, so let's say your's are 1500 grams. This means the 26" wheel would be 1800 grams according to you. I=mr^2
Your 700c wheels would have the inertia of: 0.145 kgm^2
The 26" wheels would have the inertia of: 0.141 kgm^2
This is of course using some rounding. As you can see, the 26" wheels, though heavier still have a lower inertia. This should help him get moving faster and you should be able to keep the speed easier.
Originally Posted by MrPolak
--Tire compound - yes, this is a good point! Is it enough to make up for 40lb advantage?
What advantage? The mass does not matter to the acceleration of gravity. But tire compound is a huge factor.
Originally Posted by MrPolak
-- Tire size -- we were about equal in that department. His are 25c with a round profile and mine are 28c but run narrow.
--My hubs are road hubs which are more efficient. My wheels are larger and roll over road imperfections better. Blue Ridge Parkway, where we repeated this amazing feat time and time again, is anything but smooth, but the views are spectacular!
You say that your hubs are more efficient. I assume you mean that your hubs have less friction. Yes, this will matter.
You say that you accelerate faster down a hill than your friend. This is most likely because your tires have less rolling resistance, your hubs have less friction and the aerodynamics or some combination of these. You say that aerodynamics doesn't matter that they are the same, but I highly doubt that two different people on two different bikes have the same amount of drag.
In conclusion, Weight does not matter on how fast you go down hill with only gravity as the force, before terminal velocity is reached. With all things equal, 26" wheels have a lower moment of inertai, allowing easier change of velocity, 700c wheels will resist a change of velocity more, tire composition plays a major rule in rolling resistance and aerodynamics play a major part in overall speed.