Re: wheel inertia testing

Ron Ruff said:

Didn't check out your testing, but rotational inertia is a really easy
thing to calculate if you know the weight of the parts... and it is
also a pretty trivial component of performance.

Weight of the 'parts'? Please elaborate.

bicycle_disciple said:

Ron Ruff said:

Didn't check out your testing, but rotational inertia is a really easy
thing to calculate if you know the weight of the parts... and it is
also a pretty trivial component of performance.

Weight of the 'parts'? Please elaborate.

Dear BD,

Ron probably means that what you can take the weight of the important
parts and plug them into the equations that Ben explained earlier.

That is, if you know the weight of the tire, tube, rim strip, and
nipples, you can figure out what that mass is doing as it spins.

You can treat their combined mass as an idealized circle out at the
rim and not be significantly off.

The mass of the spokes, hub, and gear cluster aren't too important,
since they turn so close to the center of the circle.

When you spin the very outside of a 2120 mm tire up from 0 to 20 mph,
it ends up doing about 250 rpm.

A 16-tooth rear sprocket is about one-tenth that circumference, so at
the same 250 rpm when you're doing 20 mph, it's only accelerated from
0 to about 2 mph. The hub is a considerably smaller circle than such a
sprocket, while the spokes extend past it, so it's a rough average for
the missing parts.

As Ron says, the difference in actual effect between two reasonable
700c wheels, stock and stupid light, is trivial, though fun to ponder.

It only matters during acceleration, the mass is tiny compared to the
total bike and rider, and you end up needing enormous calculations to
predict a 0.7 second difference in a 4 km time trial, which is less
than the effect of what you ate for breakfast.

Cheers,

Carl Fogel