# Allowy Wheels Weight and Energy

1. Jun 18, 2004

### noobie_physics

Hello,

Was just wondering if anyone has the equations and examples of how light an 18" alloy wheel would have to weigh so that the same amount of energy could turn it at the same rate as a 16" wheel.

Cheers.

2. Jun 18, 2004

### Gza

To calculate rotational kinetic energy use $$\frac{1}{2} I \omega^2$$

I stands for the rotational inertia of a solid wheel around its axis, while omega is its angular velocity. I'll leave it up to you to make the necessary calculations, since its almost 3 am here, and i'm about to pass out on my keyboard. If you need further help, i'd be glad to help you tomorrow.

3. Jun 18, 2004

### Chi Meson

The "I" in the above formula stands for "moment of inertia" which is a combination of mass and distance from the center. For bicycle wheels, you usually simplify the problem by assuming most of the mass is at the rim, This makes I = m(r^2).

I'm assuming your problem gives some information regarding either the masses of the wheels. If not, then you have to assume that the alloy wheel is essentially aluminum, and the other one is steel. Use the densities to determine a ratio of M_alloy / M_steel and go from there.

4. Jun 18, 2004

### ray b

tyre weight difference counts too
but a much bigger factor will be any size difference
both hight that effects over all gear ratio
and width that effects rolling resestance
allso wider tyre have more air drag
so in the real world the tyres count more than the wheels

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