Re: WIND
- From: thirty-six <thirty-six@xxxxxxxxxx>
- Date: Sun, 22 Nov 2009 14:27:31 -0800 (PST)
On 22 Nov, 21:43, <pastorgreg...@xxxxxxxxxxxxxx> wrote:
"Jobst Brandt" <jbra...@xxxxxxxxx> wrote in message
news:4b095fb2$0$1583$742ec2ed@xxxxxxxxxxxxxxxxx
RIDING IN THE WIND
Wind is the bicyclists greatest and invisible adversary, however,
being invisible doesn't mean it should remain a mystery.
By Jobst Brandt
Why small changes in wind direction seem to make large differences in
speed is not readily apparent and is why these effects are worth
analyzing. Of course wind seems to be just fine when coming from
behind, although that is not entirely true either but the opposite is
true when riding into a headwind, especially if it is gusty. Winds
often attack from directions other than straight ahead or behind but
side winds are not quite what they seem to be. Part of this
perception arises because wind speed and direction are difficult to
gauge by a moving observer.
It may seem that only headwinds slow us down, that side winds are only
a nuisance, and that tailwinds always help. This is not entirely
true. For instance, a direct side wind, at 90 degrees to the
direction of travel, always slows speed. Even a side wind slightly
from behind, one that is obviously from behind when standing still,
retards progress. These effects are what make this a subject
interesting and worth investigating.
When moving, a bicyclist experiences relative wind consisting of
riding through still air plus "real" wind (Fig 1). Wind drag
increases with the square of its velocity with respect to the rider
and this is what makes it so burdensome. That even a small increase
in speed without wind requires significantly more effort is a result
of this squared relationship between speed and drag.
Because aerodynamic drag increases as speed squared, doubling speed
quadruples drag, and being encountered at increased speed this drag
requires more power to overcome than the same drag would at lower
speed. For this reason power increases as the cube of rider speed.
For example, riding in still air on level ground at 25 miles per hour
requires almost twice the power than riding at 20 mph, even though
this is only 25 percent faster. Or more directly, doubling speed
requires eight times more power, regardless of initial speed. In
contrast, climbing a steep hill, typically at a speed where air drag
is insignificant, increasing speed by 25 percent requires 25 percent
more power.
Two other measurable power losses that occur, tire rolling resistance
and mechanical friction (chain and bearing friction) increase roughly
proportional to speed. As long as tires are reasonably inflated and
bearings lubricated, these losses are insignificant in comparison to
air drag even at high speeds.
Thus bicycling speed on level ground is governed almost solely by air
drag. Because drag increases rapidly with speed while power to
overcome it even faster, wind direction is difficult to assess. This
makes bicycling appear to always have headwinds. Unlike with cars,
where the gas pedal is depressed until a desired speed is attained,
the bicyclist is limited by power, so that his speed is limited by
prevailing winds. Bicycling speed levels off at the point where wind
drag power equals rider power. With this in mind, it's easy to see
how tailwinds can feel like headwinds once underway.
How rider and wind speed combine to generate drag is the question at
hand. Given the variables involved in riding in wind, determining
possible riding speed for winds from the side requires some geometry.
Considering two simple situations using the same rider power, one at
25 mph in still air on level ground and a second with a headwind of 25
mph. With the headwind, speed will slow to 11.6 mph because at this
speed the relative wind of 36.6 mph (25 + 11.6 mph) squared times the
speed of 11.6 is the same as 25 mph cubed (relative wind speed squared
times rider speed).
In contrast, a 25 mph tailwind would not increase speed to 50 mph as
might be expected, but rather to 46.7 mph, because at the higher
speed, air drag demands more power (work = force x distance, power =
work x rate). With constant power (that of 25 mph cubed), only 21.7
mph of additional wind (25 + 21.7 = 46.7) can be overcome, because
46.7 squared times 21.7 equals 25 cubed. Therefore, the greater the
speed the less drag can be overcome.
Applying a 25mph wind to a 50 mile out-and-back time trial for a rider
who would take two hours with no wind, the rider would take nearly two
hours and ten minutes battling the headwind at a speed of 11.6 mph,
while the tailwind portion would take little more than half hour for a
total elapsed time of 2:41.
If performance is not an issue, the power of relative headwinds
suggests not to fight them and be miserable, but rather to relax and
exert effort when the route changes direction and the winds are more
favorable.
Deciphering the effect of side winds is more complicated because a
direct side wind doesn't actually come from the side of a bicyclist in
motion. Its effect comes from the relative wind which, as was shown,
is a combination of the side wind and rider speed while the power
required is the in-line component of its drag times rider speed.
The relative wind is neither from straight ahead nor from the side,
but from an angle somewhere in between, and its velocity can be
greater than either of its components (Fig 1). The direction of the
relative wind can be found just as the sailor does with a thread tied
to the mast, however, this can be assessed analytically. By vector
addition of rider speed and wind speed, the magnitude and direction of
the relative wind can be found (Fig 1).
It might appear that side winds create more or less drag than direct
headwinds or tailwinds as a result of a different rider and bicycle
profile that determines a drag coefficient. Practically the
aerodynamic profile is the same from any direction Because all parts
of the rider, including arms, legs, torso and head, as well as all
major bicycle parts, are round. Using a round model called a bluff
body to compute drag power from any wind directions was verified in
the wind tunnel (Fig 2), and has been used for these calculations.
Fig 3. shows power required to overcome wind of various speeds from
all directions at at constant rider speed. The curves show power
required to ride as one would drive a car; that is, at the same speed
regardless of wind. This graph, as the others, is normalized to the
rider's speed so that the results are applicable to any rider at any
chosen speed. Thus, wind speeds are expressed as a percentage and
must be multiplied by rider speed. For example, at 20 mph rider
speed, the 80-percent wind curve means a wind blowing at 80 percent of
rider speed and shows what additional power must be expended at 20 mph
with a 16 mph wind (20 mph x 80 percent = 16 mph), blowing from
various directions.
-----------------------------------
Jobst Brandt
I was thinking of the greatest cycling victory ever, and having wind
resistance against the rider came the inovatation of new cycling strategize,
componants and riding position to win the race.http://www.youtube.com/watch?v=AyvwtOQYQ-E
I believe Jobst has posted this article before, I remember reading this or
similar post- good info.
I don't like side winds, my bike is sensitive to side winds for some reason.
As for headwinds, an intelligent wheel sucker knows how ride in such a way
as to avoid headwinds.
Why do you think we moan when someone isn't wearing mudguards despite
there being no rain nor forcast of it? Because it permits closer
ridiing (as long as any wire ends are clipped short) and getting
behind someone with Salmon guards is ideal.
.
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