Friction vs speed

"Dieter Britz" <[email hidden]> wrote in message
"]news:[email hidden]...

Quoted message said:

Some years ago, I read that the force required to push the
bike forward goes up as the cube of the speed. Is this
correct, and if so, why? It can't be wind resistance, as
that gives a force (for a sphere {:] ) that goes up
proportionally with the speed. Can this be friction of the
bearings and tyres?

--
Dieter Britz, Kemisk Institut, Aarhus Universitet,
Danmark.

It is indeed aerodynamic drag that exponentially increases
with speed. Check the drag formula for automobiles out. I
believe it is squared or cubed w.r.t. speed.

Cheers,

Scott..

"Dieter Britz" <[email hidden]> wrote in message
"]news:[email hidden]...

Quoted message said:

Some years ago, I read that the force required to push the
bike forward goes up as the cube of the speed. Is this
correct, and if so, why? It can't be wind resistance, as
that gives a force (for a sphere {:] ) that goes up
proportionally with the speed. Can this be friction of the
bearings and tyres?

--
Dieter Britz, Kemisk Institut, Aarhus Universitet,
Danmark.

Fluid drag force is proportional to the square of velocity:

F=p/2*Cd*A*V^2

p= density of fluid Cd=drag coefficient of object
A=projected area in direction of motion V=velocity

Dieter Britz <[email hidden]> wrote in message news:<[email hidden]>...

Quoted message said:

Some years ago, I read that the force required to push the
bike forward goes up as the cube of the speed. Is this
correct, and if so, why? It can't be wind resistance, as
that gives a force (for a sphere {:] ) that goes up
proportionally with the speed. Can this be friction of the
bearings and tyres?

The force required is proportional to the square of
velocity. The power required is proportional to the cube of
velocity (power = force X velocity).

So the faster you cycle on level ground the more power is
required to overcome air resistance.

The next most important force required (assuming level
ground) is rolling resistance - this is related to the hub
bearings, tyres and road surface (mainly the latter two; in
other words normally adjusted hub bearing require a trivial
amount of power to overcome their resistance).

The next force required is to overcome the drivetrain - this
is the chain and derailleur.

As an example consider Chis Boardman's UCI legal world
record. On a smooth indoor velodrome using the highest
qaulity tubs available on single speed bike (without
derailleur) over 92% of his power output is required to
overcome air resistance when travelling @ 49.442 km/h.

*** "Chris Boardman (GBR) Athlete's Hour Record,
Manchester, 2000"
***

Input Parameters [Metric Format]
--------------------------------
Cyclist Velocity [km/h] ................ 49.442

Power Total [W] ........................ 401.2 Power Air
Resistance [W] ............... 372.8 92.91% Power Rolling
Resistance [W] ........... 20.4 5.09% Power Drive Train [W]
.................. 8.0 2.00%

In article <[email hidden]>,
[email hidden] says...

Quoted message said:

Some years ago, I read that the force required to push the
bike forward goes up as the cube of the speed. Is this
correct, and if so, why? It can't be wind resistance, as
that gives a force (for a sphere {:] ) that goes up
proportionally with the speed. Can this be friction of the
bearings and tyres?

For fluid (air/water) resistance, the _force_ required goes
up with the square of the speed, and the _power_
requirements therefore go up with the cube of the speed
(because power is proportional to force times speed).

--
Remove the ns_ from if replying by e-mail (but keep posts in
the newsgroups if possible).

Dieter Britz said:

Some years ago, I read that the force required to push the
bike forward goes up as the cube of the speed. Is this
correct, and if so, why? It can't be wind resistance, as
that gives a force (for a sphere {:] ) that goes up
proportionally with the speed. Can this be friction of the
bearings and tyres?

No, it's wrong ;-) The force required to overcome
aerodynamic drag varies with the SQUARE of the speed and the
power required varies with the cube.

Friction is roughly proportional to speed and is negligible
compared to aerodynamic drag at race speeds.