Re: How to make my bike faster!

carlfogel@xxxxxxxxxxx wrote:
On Fri, 28 Apr 2006 15:33:44 -0700, "Phil Holman"
<piholmanc@yourservice> wrote:

"Donald Gillies" <gillies@xxxxxxxxx> wrote in message news:e2tjf1$bnl$1@xxxxxxxxxxxxxxxxxxxx
"Big Al" <ajmwareham@xxxxxxx> writes:

People, what is the single factor in making my road bike faster? I have
a 2001 Specialized allez with an aluminium frame, Ive considered
getting carbon forks, stem, seatpost etc to ake it lighter in the hope
it goes faster and up hills easier. I dont want to fork out thousands
of pounds for a new bike because I love the bike ive got. Do you think
new wheels and tyres would do the trick?
Nothing will give you more speed than a faring. You could easily go
from 20 mph on a club ride to 25 mph. Wind resistance is the dominant
form of resistance above 10 mph, and wind resistance grows according
to your velocity CUBED.
squared

PH

Dear Phil,

As I understand it, wind drag does rise with the cube of
velocity.

When velocity doubles, to use the usual easy-to-follow
example, each air molecule strikes at twice the original
speed.

So that gives each molecule four times as much kinetic
energy, according to the equation ke = 1/2 mass x
velocity^2.

But since things are going twice as fast, twice as many air
molecules strike per second, so we have twice as many air
molecules hitting us, each with four times as much kinetic
energy, so the overall effect is 4 x 2, or a 2^3 increase in
drag for 2x velocity.

Am I missing a distinction between wind drag versus wind
resistance? Or just woefully misinformed?

Airily,

Carl Fogel

From <http://en.wikipedia.org/wiki/Drag_%28physics%29>
(Note: in physics, air is considered a "fluid"):

"In physics, the magnitude of the force experienced by an object moving through a fluid is given by

F_d = 1/2 rho C_d A v^2

where

F_d is the force of drag,
rho is the density of the fluid (about 1.29kg / m3 for air),
C_d is the drag coefficient (a dimensionless constant, e.g. 0.25 to 0.45 for a car).
A is the reference area, and
v is the velocity of the object relative to the fluid"

"Of particular importance is the v^2 dependence on velocity, meaning that fluid drag increases with the square of velocity. When velocity is doubled, for example, not only does the fluid strike with twice the velocity, but twice the mass of fluid strikes per second. Therefore the change of momentum per second is multiplied by four. Force is equivalent to the change of momentum divided by time."

"Another interesting relation, though it is not part of the equation, is that the power needed to push an object through a fluid increases as the cube of the velocity. A car cruising on a highway at 50 mph (80 km/h) may require only 10 horsepower (7 kW) to overcome air drag, but that same car at 100 mph (160 km/h) requires 80 hp (60 kW). With a doubling of speed the drag (force) quadruples per the formula. Since power is the rate of doing work, exerting four times the force at twice the speed requires eight times the power."
.

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