On Mon, 28 Mar 2005 05:14:33 -0800, Ryan Cousineau
<[email hidden]> wrote:
[snip]
Quoted message said:First, if I may, an assertion: nobody here really cares about efficiency
at 10 mph (16 km/h). The data has some uses, but if you're going that
slowly on level ground, you're already outside the parameters of a
cyclist who should bother caring about performance.
[snip]
Dear Ryan,
Since the topic was the physicis of how much more power is
required to double speeds, an original range of 10 to 20 mph
actually is what we care about--that's what we can double
to 20 to 40 mph.
And the calculators all predict roughly the same thing,
which is that the wind drag is about 50% to 75% (not the 80%
that you later mentioned) of the total at 10 to 20 mph,
rising to 88% at 40 mph.
If the 75% versus 80% wind drag seems like a quibble at
first, remember that it's actually 74% versus 80%. This
means that the rolling resistance and transmission loss are
26% of the total rather than 20%--which is a 30% increase
from their point of view.
Because the wind drag increases wildly when the speed
double, but the other losses don't, this leads to the point
that it doesn't take 8 times as much power to double the
ordinary range of bicycle speeds. It takes about 4.5 to 6.5
times as much power because the non-aerodynamic losses are
in fact quite significant.
Again, this was not a question of what can be easily
improved. The calculator fuss was a question of
understanding what the total drag is, what's significant,
and what changes with speed, all in terms of the convenient
doubling-of-speed example. When almost 50% to 26% of the
rider's power is used to overcome non-aerodynamic drag at 10
to 20 mph, that drag is significant.
Carl Fogel
