Speed Limit?

Christopher Brian Colohan said:

But I was thinking, and it seems that the air is thicker (denser) when it is colder. But by how
much? In particular, is this effect significant enough to be noticable?

I am curious... anyone know?

Chris

It is significant for airplanes and parachutes. If it is for you depends on how fast you go. I'd say
the friction in your hubs will be a more significant factor.

--
Perre

You have to be smarter than a robot to reply.

"Christopher Brian Colohan" <[email hidden]> wrote in message
"]news:[email hidden]...

Quoted message said:

On my daily ride to school I go down a fairly long hill. When coasting (i.e., sometimes I don't
pedal just to see how far I can get without any power), I have noticed that on cold days I tend to
have a lower top speed than on warm days... When it is 20 degrees C I tend to go faster on average
than when it is 0 degrees C.

Why is this?

Some ideas I have had include that the warmer clothing I wear on cold days has more wind
resistance, or that the lower temperature decreases my tire pressure somewhat.

But I was thinking, and it seems that the air is thicker (denser) when it is colder. But by how
much? In particular, is this effect significant enough to be noticable?

I am curious... anyone know?

Colder air is denser, but your extra clothing probably has the biggest effect.

Matt O.

Density of dry air (grams/milliliter) = 0.001293/1+0.00367 temperature (Celsius) x H (cmHg)/76
Handbook of Chemistry & Physics 61st ed.

For 76 cmHg, at 10C density of dry air is 0.001247 grams/milliliter and at 20C density of dry air is
0.001205 grams/milliliter From the table.

"Christopher Brian Colohan" <[email hidden]> wrote in message
"]news:[email hidden]...

Quoted message said:

But I was thinking, and it seems that the air is thicker (denser) when it is colder. But by how
much? In particular, is this effect significant enough to be noticable?

I am curious... anyone know?

Air density is why hour records are often set at altitude. Tom

--
Bruni Bicycles "Where art meets science" brunibicycles.com
410.426.3420 Matt O'Toole <[email hidden]> wrote in message
"]news:[email hidden]...

Quoted message said:

"Christopher Brian Colohan" <[email hidden]> wrote in message
"]news:[email hidden]...

Quoted message said:

On my daily ride to school I go down a fairly long hill. When coasting (i.e., sometimes I don't
pedal just to see how far I can get without any power), I have noticed that on cold days I tend
to have a lower top speed than on warm days... When it is 20 degrees C I tend to go faster on
average than when it is 0 degrees C.

Why is this?

Some ideas I have had include that the warmer clothing I wear on cold days has more wind
resistance, or that the lower temperature decreases my tire pressure somewhat.

But I was thinking, and it seems that the air is thicker (denser) when it is colder. But by how
much? In particular, is this effect significant enough to be noticable?

I am curious... anyone know?

Colder air is denser, but your extra clothing probably has the biggest

effect.

Quoted message said:

Matt O.

"Christopher Brian Colohan" <[email hidden]> wrote in message
"]news:[email hidden]...

Quoted message said:

On my daily ride to school I go down a fairly long hill. When coasting (i.e., sometimes I don't
pedal just to see how far I can get without any power), I have noticed that on cold days I tend to
have a lower top speed than on warm days... When it is 20 degrees C I tend to go faster on average
than when it is 0 degrees C.

Why is this?

Some ideas I have had include that the warmer clothing I wear on cold days has more wind
resistance, or that the lower temperature decreases my tire pressure somewhat.

But I was thinking, and it seems that the air is thicker (denser) when it is colder. But by how
much? In particular, is this effect significant enough to be noticable?

I am curious... anyone know?

Temperature has a direct effect on density of air, which has a direct effect on aerodynamic drag.
The aerodynamic drag at 20 C would be only 93% of that at C, a significant and noticeable
difference.

Christopher Brian Colohan <[email hidden]> wrote in message
news:<[email hidden]>...

Quoted message said:

On my daily ride to school I go down a fairly long hill. When coasting (i.e., sometimes I don't
pedal just to see how far I can get without any power), I have noticed that on cold days I tend to
have a lower top speed than on warm days... When it is 20 degrees C I tend to go faster on average
than when it is 0 degrees C.

Why is this?

Some ideas I have had include that the warmer clothing I wear on cold days has more wind
resistance, or that the lower temperature decreases my tire pressure somewhat.

[snip]

Dear Chris,

Whatever it is, you're not crazy. My absurdly detailed spreadsheets show that my daily 15-mile ride
is faster in summer. My times smoothly and steadily drop from late January toward my summer lows and
then climb just as gently up again as Christmas approaches.

Tire pressure change is unlikely be the cause, even though the pressure has been named in other
posts as having a surprising effect on roll-outs. Even if your tires are showing some change, mine
are pumped up by a faithful air compressor to roughly 125 psi.

Possibly the cold increases rolling resistance by simply making the tires less resilient? That is,
our tires may be significantly more unyielding and less springy near freezing.

Most likely, though, it's just our clothing. My fastest times always occur when I'm wearing bike
shorts and a tucked-in short-sleeve shirt. Under 60 degrees F., my costume changes to long bike
pants. By 50 degrees F., I'm wearing a long-sleeved cycling jersey, looser and longer than my
short-sleeve shirt and wearing heavier gloves. By 40 degrees F., I'm thinking about a plastic
windbreaker. All this slightly flapping clothing, though it seems like nothing to non-bicyclists,
has a dreadful effect in terms of drag and turbulence.

Your coasting removes the other likely culprit, namely that we may not really want to pedal much
faster into a miserable cold wind (which slows my speed down to mph around freezing).

Carl Fogel

Peter Cole said:

Temperature has a direct effect on density of air, which has a direct effect on aerodynamic drag.
The aerodynamic drag at 20 C would be only 93% of that at 0 C, a significant and noticeable
difference.

How did you get that? Doug Huffman pointed out earlier:

"For 76 cmHg, at 10C density of dry air is 0.001247 grams/milliliter and at 20C density of dry air
is 0.001205 grams/milliliter From the table."

Which is only a 0.4% difference of density. Granted, his quotes were for a 10 degree difference, but
still your claim is well beyond what should follow from such a small difference.

--

David L. Johnson

__o | Accept risk. Accept responsibility. Put a lawyer out of _`\(,_ | business. (_)/ (_) |

David L. Johnson said:

Peter Cole said:

Temperature has a direct effect on density of air, which has a direct effect on aerodynamic drag.
The aerodynamic drag at 20 C would be only 93% of that at 0 C, a significant and noticeable
difference.

How did you get that? Doug Huffman pointed out earlier:

"For 76 cmHg, at 10C density of dry air is 0.001247 grams/milliliter and at 20C density of dry air
is 0.001205 grams/milliliter From the table."

Which is only a 0.4% difference of density. Granted, his quotes were for a 10 degree difference,
but still your claim is well beyond what should follow from such a small difference.

Doug's numbers look like about a 3.5% difference to me for a 10 C change in temperature which seems
consistent with the 7% change suggested by Peter C for a 20 C change in temperature. Assuming air
follows ideal-gas properties (which it does quite well under normal atmospheric conditions), the
density will vary inversely with the temperature as measured from absolute zero. So at 20 C (293 K)
the density should be 273/293, or 0.93 as great as at 0 C (273 K).

Christopher Brian Colohan said:

On my daily ride to school I go down a fairly long hill. When coasting (i.e., sometimes I don't
pedal just to see how far I can get without any power), I have noticed that on cold days I tend to
have a lower top speed than on warm days... When it is 20 degrees C I tend to go faster on average
than when it is 0 degrees C.

Why is this?

My vote: Rolling resistance difference due to rubber temperature.

Quoted message said:

Chris

--
Rick "Wild-guessing shot in the dark" Onanian

Rick Onanian <[email hidden]> wrote:
: My vote: Rolling resistance difference due to rubber temperature.

my vote is that he's freezing his ass off and subconsciously doesn't want to go any faster because
it makes him colder.
--
david reuteler [email hidden]

Bruni said:

Air density is why hour records are often set at altitude. Tom

Seems counter-intuitive -- the minor savings of drag vs. the major loss of aerobic breathing...
--
Rick Onanian

"David Reuteler" <[email hidden]> wrote in message
"]news:[email hidden]...

Quoted message said:

Rick Onanian <[email hidden]> wrote:
: My vote: Rolling resistance difference due to rubber temperature.

my vote is that he's freezing his ass off and subconsciously doesn't

want

Quoted message said:

to go any faster because it makes him colder.

Reality is not subject to a majority vote. To know what speed to do pursuit training for a given
power output I calculated the following table in mph.

T(*F) P (in Hg) 29 30 31

50 30 29.5 29 60 30.3 29.8 29.3 70 30.6 30.1 29.6 80 30.8 30.3 29.8 90 31.1 30.6 30.1 100
31.4 30.9 30.4

"David L. Johnson" <[email hidden]> wrote in message
news:<[email hidden]>...

Quoted message said:

Peter Cole said:

Temperature has a direct effect on density of air, which has a direct effect on aerodynamic
drag. The aerodynamic drag at 20 C would be only 93% of that at 0 C, a significant and
noticeable difference.

How did you get that? Doug Huffman pointed out earlier:

"For 76 cmHg, at 10C density of dry air is 0.001247 grams/milliliter and at 20C density of dry air
is 0.001205 grams/milliliter From the table."

Which is only a 0.4% difference of density. Granted, his quotes were for a 10 degree difference,
but still your claim is well beyond what should follow from such a small difference.

Dear David and Peter,

Here's an interesting page that lets you fill in the temperature and calculates air density for you.
I can't vouch for the accuracy of its formulas, but it seems worth a look.

analyticcycling.comForcesAirDensity Page.htmlOpen ↗

For 30.10 barometric, sea-level, and 20 degrees C. :
1.21071 kg/m3

For 30.10 barometric, sea-level, and 0 degrees C. :
2.29936 kg/m3

This does seem to suggest that density rises from roughly
3.21 to 1.30 with a 20 degree C. drop, roughly a 7% increase. I'm startled, but it's a pleasure that
ignorance often affords
me. (My first impulse was to point out that target-shooters carry range-finders, not thermometers,
but I may have to check that now. Time to go confess that my earlier post on this is looking
even less accurate than usual.)

Maybe that cold air really does get dense enough?

Carl Fogel

[email hidden] (Carl Fogel) wrote in message
news:<[email hidden]>...

[snip]

Dear Chris,

Oops! Other posters raised the question of air density, which led me to check the easy-to-use
temperature-altitude-barometric calculator on this page:

analyticcycling.comForcesAirDensity Page.htmlOpen ↗

It looks as if there might be a 7% increase in air density in the drop from 20 degrees C.
to freezing.

This sounds like another promising excuse not to go riding when it's cold and a good lesson for me
not to shoot from the hip.

Carl Fogel

Peter said:

Quoted message said:

"For 76 cmHg, at 10C density of dry air is 0.001247 grams/milliliter and at 20C density of dry
air is 0.001205 grams/milliliter From the table."

Which is only a 0.4% difference of density. Granted, his quotes were for a 10 degree difference,
but still your claim is well beyond what should follow from such a small difference.

Doug's numbers look like about a 3.5% difference to me

Oops. You're right. Long day.

--

David L. Johnson

__o | And what if you track down these men and kill them, what if you _`\(,_ | killed all of us?
From every corner of Europe, hundreds, (_)/ (_) | thousands would rise up to take our places.
Even Nazis can't kill that fast. -- Paul Henreid (Casablanca).

Christopher Brian Colohan said:

On my daily ride to school I go down a fairly long hill. When coasting (i.e., sometimes I don't
pedal just to see how far I can get without any power), I have noticed that on cold days I tend to
have a lower top speed than on warm days... When it is 20 degrees C I tend to go faster on average
than when it is 0 degrees C.

Why is this?

Some ideas I have had include that the warmer clothing I wear on cold days has more wind
resistance, or that the lower temperature decreases my tire pressure somewhat.

But I was thinking, and it seems that the air is thicker (denser) when it is colder. But by how
much? In particular, is this effect significant enough to be noticable?

I am curious... anyone know?

Chris

From my High School physics, I recall the Ideal Gas Law

P(ressure)V(olume)=n(moles)R(Universal Gas Constant)T(emp in deg.K)

so assuming the pressure remains constant (for the sake of the argument), the difference in temp 0C
-> 20C = 273K -> 293K or an increase of about 7%, which means the volume the gas occupies must also
increase by the same ratio, which then means the gas density is lower by about 7%.

These are really rough calculations, but it should give you the idea. If I could remember any of the
fluid dynamics I was taught in university, I could probably work out what impact the drop in air
density means for drag.

pjb

Rick Onanian <[email hidden]> wrote in message
news:<[email hidden]>...

Quoted message said:

Bruni said:

Air density is why hour records are often set at altitude. Tom

Seems counter-intuitive -- the minor savings of drag vs. the major loss of aerobic breathing...

Dear Rick,

The savings in drag may not be so minor.

Home runs in the Colorado Rockies home park are a scandal. At over 5,000 feet, the ball goes
noticeably further when hit by the same batters. And before major-league ball came to Denver,
Atlanta was the highest park in the nation and notorious for the same thin-air long-ball hitting.

As for respiration, people who come to higher altitudes suffer at first, but usually show impressive
improvement within six months or so, as shown by various blood tests. Meanwhile, they still enjoy
the reduced drag.

I started out dismissing the denser-air-on-cold-days-increases-drag explanation, but I'm feeling
more and more like Benedict Arnold (who lived out his days quite happily in England, as I recall).

Carl Fogel

Carl Fogel <[email hidden]> wrote:
: And before major-league ball came to Denver, Atlanta was the highest park in the nation and
: notorious for the same thin-air long-ball hitting.

umm, atlanta's altitude is 1,057 feet. you are correct about it being formerly the highest park:

atlanta.braves.mlb.comatl ballpark history.jspOpen ↗

but .. that's like 100 feet higher than my old apartment in minneapolis which was a few miles
south of the metrodome. if there were a lot of home runs hit in atlanta i doubt it was because
of thin air.
--
david reuteler [email hidden]

Carl Fogel said:

Rick Onanian <[email hidden]>

Quoted message said:

Seems counter-intuitive -- the minor savings of drag vs. the major loss of aerobic breathing...

Home runs in the Colorado Rockies home park are a scandal. At over 5,000 feet, the ball goes
noticeably further when hit by the same batters.

But batsmen are not performing a steady aerobic task like cycling; they are rather producing a
single burst of energy.
--
David Damerell <[email hidden]> Distortion Field!