Originally Posted by
Roody
My physics is very rusty, but I think that the tires--especially the rims of the tires--is the worst possible place to add weight to a bike. Somebody told me that 1 pound on the rims is like 10 pounds on the frames. If that's true, using the two hard inner tubes would be like adding 40 pounds to the frame or in your backpack or panniers. Too much for me!
I hope that somebody with better understanding of the physics can explain and correct all this!
It's a torque (which is effectively another word for leverage) thing.
Torque is nothing more than force applied at a particular distance to rotate something. 10 pound-feet of torque is 10 pounds of force exerted 1 foot from the center of rotation (the hub of the wheel), or 1 pound of force exerted 10 feet away, or 100 pounds of force exerted 1/10th of a foot away. More distance means greater torque for the same force. If you remember playing on a playground teeter-totter, the farther away from the pivot (hub) you were, and the closer to the pivot someone else was, the easier it was for you to move them. Torque is leverage.
Objects with mass, meaning wheels in this case, have rotational inertia. That's how prone they are to stay not-rotating if they aren't rotating, and how prone they are to continue rotating if they already are rotating. Much like a given force creates more torque the farther away from the hub it is, the farther away from the hub the mass of a wheel is the more rotational inertia the wheel has. The more the mass of a wheel/tube/tire assembly is distributed towards the rim, the harder it will be to get started but the longer it will keep spinning on it's own (all other things being equal). Whether the tendency to keep spinning on it's own is noticeable during riding is something I can't readily answer, and would easily be confounded by things like tire pressure and stiffness, rolling resistance, etc.
You can observe all this pretty easily if you have a free-spinning bike wheel handy. Just use your finger on a spoke out near the rim to spin the wheel, stop the wheel, then try again up close to the hub. It will be easier to start the wheel spinning near the rim than near the hub.
That's if I remember my physics correctly. I definitely don't remember enough physics to get very specific on the effects of the weight of the frame, non-wheel components, and rider.