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Main
Date: 30 Sep 2007 16:17:18
From: DaveH
Subject: More weight, faster descents??
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I've seen this notion in various forums. It typically takes the form
of "A heavier bike may be more work uphill, but is faster downhill due
to gravity..." Something like that.
The notion -- which I think is incorrect -- really should take the
form of "Greater total mass (rider + bike + stuff) may be more work
uphill, but is faster downhill..."
In any case, reviewing the elementary physics, doesn't mass cancel in
the equations? Same reason a rock and feather both accelerate at g in
a vacuum? Ergo, that heavier bike isn't getting you down the hill any
faster.
Dave
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Date: 01 Oct 2007 04:08:31
From: Donga
Subject: Re: More weight, faster descents??
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On Oct 1, 8:00 pm, Ben C <spams...@spam.eggs > wrote:
> On 2007-10-01, Donga <idomybestworkonab...@hotmail.com> wrote:
> [...]
>
> > The phenomenon has to do with not even trying to go fast in a casual
> > ride, e.g. when we've laboured up a hill and crested it, then start to
> > coast down. Even if I'm sitting up, turned around chatting or gasping,
> > with the aerodynamics of a garden shed, I have to brake to stay with
> > him.
>
> Interesting. I wonder if it's possible that you're riding in a
> straighter line than him. You might subconsciously be better at
> balancing meaning that compared to you he's weaving around all the time.
> Have you tried swapping bikes?
Interesting theory. in case you are right, no way am I letting that
sucker on my bike!
;-)
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Date: 30 Sep 2007 20:24:08
From: Donga
Subject: Re: More weight, faster descents??
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On Oct 1, 11:25 am, A Muzi <a...@yellowjersey.org > wrote:
> >> DaveH wrote:
> >>> I've seen this notion in various forums. It typically takes the form
> >>> of "A heavier bike may be more work uphill, but is faster downhill due
> >>> to gravity..." Something like that.
> >>> The notion -- which I think is incorrect -- really should take the
> >>> form of "Greater total mass (rider + bike + stuff) may be more work
> >>> uphill, but is faster downhill..."
> >>> In any case, reviewing the elementary physics, doesn't mass cancel in
> >>> the equations? Same reason a rock and feather both accelerate at g in
> >>> a vacuum? Ergo, that heavier bike isn't getting you down the hill any
> >>> faster.
> > Mark <remove.mandmlj.t...@remove.comcast.this.net>wrote:
> >> Why don't you go do some cycling in a vacuum and let us know how it
> >> works out. Out here in the real world (no vacuum), heavier bikes (w/
> >> same frontal area) go down hill faster.
> Donga wrote:
> > I've been trying to figure why I accelerate faster downhill (coasting)
> > than my riding buddy. We are about the same weight, size, bike
> > position, tires. Is it my Campag Record hubs versus his Mavic Ksyrium
> > Elites?
>
> Do you tuck in your knees and elbows and drop your chin to the stem? You
> might try descending position tweaks.
> --
> Andrew Muziwww.yellowjersey.org
> Open every day since 1 April, 1971
The phenomenon has to do with not even trying to go fast in a casual
ride, e.g. when we've laboured up a hill and crested it, then start to
coast down. Even if I'm sitting up, turned around chatting or gasping,
with the aerodynamics of a garden shed, I have to brake to stay with
him.
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Date: 01 Oct 2007 05:00:05
From: Ben C
Subject: Re: More weight, faster descents??
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On 2007-10-01, Donga <idomybestworkonabike@hotmail.com > wrote:
[...]
> The phenomenon has to do with not even trying to go fast in a casual
> ride, e.g. when we've laboured up a hill and crested it, then start to
> coast down. Even if I'm sitting up, turned around chatting or gasping,
> with the aerodynamics of a garden shed, I have to brake to stay with
> him.
Interesting. I wonder if it's possible that you're riding in a
straighter line than him. You might subconsciously be better at
balancing meaning that compared to you he's weaving around all the time.
Have you tried swapping bikes?
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Date: 30 Sep 2007 21:02:51
From: Tim McNamara
Subject: Re: More weight, faster descents??
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In article <htvvf3plvq1tq3d38ifquo8qimsc726gsk@4ax.com >,
DaveH <ddhartwick@FUTILITYearthlink.net > wrote:
> I've seen this notion in various forums. It typically takes the form
> of "A heavier bike may be more work uphill, but is faster downhill
> due to gravity..." Something like that.
>
> The notion -- which I think is incorrect -- really should take the
> form of "Greater total mass (rider + bike + stuff) may be more work
> uphill, but is faster downhill..."
>
> In any case, reviewing the elementary physics, doesn't mass cancel in
> the equations? Same reason a rock and feather both accelerate at g
> in a vacuum? Ergo, that heavier bike isn't getting you down the hill
> any faster.
No. The power to weight ratio favors the lighter rider on the way up,
and the mass to frontal area ratio favors the heavier riders on the way
down. You're not descending in a vacuum, so that issue is irrelevant.
Generally the advantage to the lighter rider when climbing is greater
than the advantage to the heavier rider on the descent- it's much easier
for the heavier rider to lose 15 minutes going uphill than to gain 15
minutes on the descent. FWIW, the mass to frontal area ration also tends
to favor larger riders in time trials, because the larger rider
generally has a higher total output.
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Date: 01 Oct 2007 13:40:54
From: Peter Cole
Subject: Re: More weight, faster descents??
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Tim McNamara wrote:
> In article <htvvf3plvq1tq3d38ifquo8qimsc726gsk@4ax.com>,
> DaveH <ddhartwick@FUTILITYearthlink.net> wrote:
>
>> I've seen this notion in various forums. It typically takes the form
>> of "A heavier bike may be more work uphill, but is faster downhill
>> due to gravity..." Something like that.
>>
>> The notion -- which I think is incorrect -- really should take the
>> form of "Greater total mass (rider + bike + stuff) may be more work
>> uphill, but is faster downhill..."
>>
>> In any case, reviewing the elementary physics, doesn't mass cancel in
>> the equations? Same reason a rock and feather both accelerate at g
>> in a vacuum? Ergo, that heavier bike isn't getting you down the hill
>> any faster.
>
> No. The power to weight ratio favors the lighter rider on the way up,
> and the mass to frontal area ratio favors the heavier riders on the way
> down. You're not descending in a vacuum, so that issue is irrelevant.
> Generally the advantage to the lighter rider when climbing is greater
> than the advantage to the heavier rider on the descent- it's much easier
> for the heavier rider to lose 15 minutes going uphill than to gain 15
> minutes on the descent. FWIW, the mass to frontal area ration also tends
> to favor larger riders in time trials, because the larger rider
> generally has a higher total output.
That's about it. I'm very tall and can out coast anybody I've ever
ridden with, I'm almost a 1-man tandem. Despite having a large (68cm)
bike and no aero anything, I'm still noticeably faster in a gravity race.
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Date: 01 Oct 2007 19:27:09
From: Tom \Johnny Sunset\ Sherman
Subject: Re: More weight, faster descents??
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Peter Cole wrote:
> Tim McNamara wrote:
>> In article <htvvf3plvq1tq3d38ifquo8qimsc726gsk@4ax.com>,
>> DaveH <ddhartwick@FUTILITYearthlink.net> wrote:
>>
>>> I've seen this notion in various forums. It typically takes the form
>>> of "A heavier bike may be more work uphill, but is faster downhill
>>> due to gravity..." Something like that.
>>> The notion -- which I think is incorrect -- really should take the
>>> form of "Greater total mass (rider + bike + stuff) may be more work
>>> uphill, but is faster downhill..."
>>> In any case, reviewing the elementary physics, doesn't mass cancel in
>>> the equations? Same reason a rock and feather both accelerate at g
>>> in a vacuum? Ergo, that heavier bike isn't getting you down the hill
>>> any faster.
>>
>> No. The power to weight ratio favors the lighter rider on the way up,
>> and the mass to frontal area ratio favors the heavier riders on the
>> way down. You're not descending in a vacuum, so that issue is
>> irrelevant. Generally the advantage to the lighter rider when climbing
>> is greater than the advantage to the heavier rider on the descent-
>> it's much easier for the heavier rider to lose 15 minutes going uphill
>> than to gain 15 minutes on the descent. FWIW, the mass to frontal area
>> ration also tends to favor larger riders in time trials, because the
>> larger rider generally has a higher total output.
>
> That's about it. I'm very tall and can out coast anybody I've ever
> ridden with, I'm almost a 1-man tandem. Despite having a large (68cm)
> bike and no aero anything, I'm still noticeably faster in a gravity race.
I am 1/2-Chalo in mass, and still out-coast tandems. :) I can pick up a
couple of miles per hour extra speed by sliding to the front of the seat
and un-clipping one foot so both my legs are straight.
--
Tom Sherman - Holstein-Friesland Bovinia
A Real Cyclist [TM] keeps at least one bicycle in the bedroom.
--
Posted via a free Usenet account from http://www.teranews.com
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Date: 30 Sep 2007 17:48:52
From: Donga
Subject: Re: More weight, faster descents??
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On Oct 1, 7:07 am, Mark <remove.mandmlj.t...@remove.comcast.this.net >
wrote:
> DaveH wrote:
> > I've seen this notion in various forums. It typically takes the form
> > of "A heavier bike may be more work uphill, but is faster downhill due
> > to gravity..." Something like that.
>
> > The notion -- which I think is incorrect -- really should take the
> > form of "Greater total mass (rider + bike + stuff) may be more work
> > uphill, but is faster downhill..."
>
> > In any case, reviewing the elementary physics, doesn't mass cancel in
> > the equations? Same reason a rock and feather both accelerate at g in
> > a vacuum? Ergo, that heavier bike isn't getting you down the hill any
> > faster.
> > Dave
>
> Why don't you go do some cycling in a vacuum and let us know how it
> works out. Out here in the real world (no vacuum), heavier bikes (w/
> same frontal area) go down hill faster.
>
> Mark J.
I've been trying to figure why I accelerate faster downhill (coasting)
than my riding buddy. We are about the same weight, size, bike
position, tires. Is it my Campag Record hubs versus his Mavic Ksyrium
Elites?
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Date: 01 Oct 2007 08:38:03
From: Mike Jacoubowsky
Subject: Re: More weight, faster descents??
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> I've been trying to figure why I accelerate faster downhill (coasting)
> than my riding buddy. We are about the same weight, size, bike
> position, tires. Is it my Campag Record hubs versus his Mavic Ksyrium
> Elites?
Most likely not. Mavic isn't known for very aerodynamic wheels, so even if
you've got an air-churning 32-spoke shallow-dish wheel, that's not going to
be much of a difference. More likely you've got to bring you knees not just
"in" but "up", let your gut sag down to your knees (do *not* pedal!!!), and
get your chin down low, as Mr. Muzi suggested. Arms are pretty easy to
figure out, because you can actually feel the wind on the hair of your arms,
so it's not too tough to find the position where you feel that least.
If the grade is, say, 4% or greater, pedaling is probably going to slow you
down in the long run. You can try pedaling for very short periods of time
(to get your speed up if you're below 40mph), but unless you're drafting a
vehicle (or another bike) you're probably doing more harm than good.
--Mike Jacoubowsky
Chain Reaction Bicycles
www.ChainReaction.com
Redwood City & Los Altos, CA USA
"Donga" <idomybestworkonabike@hotmail.com > wrote in message
news:1191199732.136870.309950@w3g2000hsg.googlegroups.com...
> On Oct 1, 7:07 am, Mark <remove.mandmlj.t...@remove.comcast.this.net>
> wrote:
>> DaveH wrote:
>> > I've seen this notion in various forums. It typically takes the form
>> > of "A heavier bike may be more work uphill, but is faster downhill due
>> > to gravity..." Something like that.
>>
>> > The notion -- which I think is incorrect -- really should take the
>> > form of "Greater total mass (rider + bike + stuff) may be more work
>> > uphill, but is faster downhill..."
>>
>> > In any case, reviewing the elementary physics, doesn't mass cancel in
>> > the equations? Same reason a rock and feather both accelerate at g in
>> > a vacuum? Ergo, that heavier bike isn't getting you down the hill any
>> > faster.
>> > Dave
>>
>> Why don't you go do some cycling in a vacuum and let us know how it
>> works out. Out here in the real world (no vacuum), heavier bikes (w/
>> same frontal area) go down hill faster.
>>
>> Mark J.
>
> I've been trying to figure why I accelerate faster downhill (coasting)
> than my riding buddy. We are about the same weight, size, bike
> position, tires. Is it my Campag Record hubs versus his Mavic Ksyrium
> Elites?
>
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Date: 30 Sep 2007 20:25:12
From: A Muzi
Subject: Re: More weight, faster descents??
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>> DaveH wrote:
>>> I've seen this notion in various forums. It typically takes the form
>>> of "A heavier bike may be more work uphill, but is faster downhill due
>>> to gravity..." Something like that.
>>> The notion -- which I think is incorrect -- really should take the
>>> form of "Greater total mass (rider + bike + stuff) may be more work
>>> uphill, but is faster downhill..."
>>> In any case, reviewing the elementary physics, doesn't mass cancel in
>>> the equations? Same reason a rock and feather both accelerate at g in
>>> a vacuum? Ergo, that heavier bike isn't getting you down the hill any
>>> faster.
> Mark <remove.mandmlj.t...@remove.comcast.this.net>wrote:
>> Why don't you go do some cycling in a vacuum and let us know how it
>> works out. Out here in the real world (no vacuum), heavier bikes (w/
>> same frontal area) go down hill faster.
Donga wrote:
> I've been trying to figure why I accelerate faster downhill (coasting)
> than my riding buddy. We are about the same weight, size, bike
> position, tires. Is it my Campag Record hubs versus his Mavic Ksyrium
> Elites?
Do you tuck in your knees and elbows and drop your chin to the stem? You
might try descending position tweaks.
--
Andrew Muzi
www.yellowjersey.org
Open every day since 1 April, 1971
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Date: 30 Sep 2007 16:49:37
From:
Subject: Re: More weight, faster descents??
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carlfo...@comcast.net wrote:
> The little guy rolls down the hill at 27 mph as Chalo pulls away at 40
> mph. (In real life, Chalo would have slightly more wind drag, so
> probably he wouldn't pull away quite that fast, but his weight rises
> much faster than his frontal area, so no one wants to challenge him to
> a coasting race down a long, straight hill.)
And in equally fit riders, muscle mass increases much faster than
frontal area, so they not only have an advantage in higher terminal
velocity, they have a higher power/frontal area ratio.
> That's why climbers usually beat descenders.
But the difference in muscle mass is why climbers rarely win the tdf.
A pure climber does not have the optimal muscle mass to win anything
but the mountaintop finishes. They _usually_ do not have the power to
maintain their lead over a rider with greater muscle-to-frontal area
ratio when the road starts descending. More importantly, they don't
even have the power to maintain their lead when the road is flat. In
other words, climbers (in general) don't beat descenders except when
they are all coasting down the mountain.
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Date: 30 Sep 2007 23:32:58
From: datakoll
Subject: Re: More weight, faster descents??
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WHEN GOOGLE'S ASKED 32 feet per second squared, it gives a group
answer then in the upper left, book recommendations as authorities
http://books.google.com/books?id=ar4AiUkLL3gC&pg=PA199&dq=%2B32+ft+per+second+squared&ei=_zAAR7yvEIia7ALKhKFe&sig=-_GIVVv4kTzrZh78vRQvJ3jsagk#PPA201,M1
http://books.google.com/books?id=LGQSAAAAIAAJ&pg=PA268&dq=%2B32+ft+per+second+squared&ei=_zAAR7yvEIia7ALKhKFe#PPA268,M1
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Date: 30 Sep 2007 23:14:13
From: datakoll
Subject: Re: More weight, faster descents??
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http://www.google.com/search?hl=en&rls=com.microsoft:*:IE-SearchBox&rlz=1I7GGLJ&q=%2B32+ft+per+second+squared&nocalc=1
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Date: 30 Sep 2007 22:17:20
From: sally
Subject: Re: More weight, faster descents??
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DaveH <ddhartwick@FUTILITYearthlink.net > wrote in
news:htvvf3plvq1tq3d38ifquo8qimsc726gsk@4ax.com:
> I've seen this notion in various forums. It typically takes the form
> of "A heavier bike may be more work uphill, but is faster downhill due
> to gravity..." Something like that.
That may be true on a straight road, but real mountain roads are not like
that. On a long descent with lots of sharp turns, the person with the best
bike handling skills will almost always get to the bottom first. In fact,
shorter riders may have an advantage because they have a lower center of
gravity and thus better grip in the corners.
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Date: 30 Sep 2007 18:18:26
From: DaveH
Subject: Re: More weight, faster descents??
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On Sun, 30 Sep 2007 16:17:18 -0400, DaveH
<ddhartwick@FUTILITYearthlink.net > wrote:
>I've seen this notion in various forums. It typically takes the form
>of "A heavier bike may be more work uphill, but is faster downhill due
>to gravity..." Something like that.
>
>The notion -- which I think is incorrect -- really should take the
>form of "Greater total mass (rider + bike + stuff) may be more work
>uphill, but is faster downhill..."
>
>In any case, reviewing the elementary physics, doesn't mass cancel in
>the equations? Same reason a rock and feather both accelerate at g in
>a vacuum? Ergo, that heavier bike isn't getting you down the hill any
>faster.
>Dave
To all,
Yes, of course, I stupidly forgot that aerodynamic drag induced
forces are very significant wrt to field induced forces parallel to
the incline. {(m x g x [sin of slope])(??--been a while)}
Terminal velocity characteristics certainly apply to real world
descents. If we were riding in a vacuum there would be no advantage.
There would certainly be some adverse physical effects.
Thanks for helping me to oil some rusty mental gears guys--
Dave
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Date: 30 Sep 2007 19:05:02
From: DaveH
Subject: Re: More weight, faster descents??
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>Yes, of course, I stupidly forgot that aerodynamic drag induced
>forces are very significant wrt to field induced forces parallel to
>the incline. {(m x g x [sin of slope])(??--been a while)}
Gack -- Incomplete. An energy analysis provides the clearer view. Work
done on the system versus work done by the system (drag) favors the
heavier cyclist in real world conditions. Got it now, I hope!
Dave
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Date: 30 Sep 2007 16:43:20
From: A Muzi
Subject: Re: More weight, faster descents??
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DaveH wrote:
> I've seen this notion in various forums. It typically takes the form
> of "A heavier bike may be more work uphill, but is faster downhill due
> to gravity..." Something like that.
>
> The notion -- which I think is incorrect -- really should take the
> form of "Greater total mass (rider + bike + stuff) may be more work
> uphill, but is faster downhill..."
>
> In any case, reviewing the elementary physics, doesn't mass cancel in
> the equations? Same reason a rock and feather both accelerate at g in
> a vacuum? Ergo, that heavier bike isn't getting you down the hill any
> faster.
Not to address your actual point, but most of our cycling doesn't occur
in a vacuum.
--
Andrew Muzi
www.yellowjersey.org
Open every day since 1 April, 1971
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Date: 30 Sep 2007 19:38:09
From: DaveH
Subject: Re: More weight, faster descents??
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>DaveH wrote:
>> I've seen this notion in various forums. It typically takes the form
>> of "A heavier bike may be more work uphill, but is faster downhill due
>> to gravity..." Something like that.
>>
>> The notion -- which I think is incorrect -- really should take the
>> form of "Greater total mass (rider + bike + stuff) may be more work
>> uphill, but is faster downhill..."
>>
>> In any case, reviewing the elementary physics, doesn't mass cancel in
>> the equations? Same reason a rock and feather both accelerate at g in
>> a vacuum? Ergo, that heavier bike isn't getting you down the hill any
>> faster.
>
>Not to address your actual point, but most of our cycling doesn't occur
>in a vacuum.
Yes, of course you're right. An egregious error on my part.
I'm so used to battling our chronic westerly winds here that I must
have forgotten about the trivial matter of the presence of atmosphere.
Dave
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Date: 30 Sep 2007 14:51:59
From: Mike Jacoubowsky
Subject: Re: More weight, faster descents??
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>> In any case, reviewing the elementary physics, doesn't mass cancel in
>> the equations? Same reason a rock and feather both accelerate at g in
>> a vacuum? Ergo, that heavier bike isn't getting you down the hill any
>> faster.
>
> Not to address your actual point, but most of our cycling doesn't occur in
> a vacuum.
> --
> Andrew Muzi
After spending time on rbt, it may be an obvious extrapolation to make.
--Mike-- Chain Reaction Bicycles
www.ChainReactionBicycles.com
"A Muzi" <am@yellowjersey.org > wrote in message
news:13g0618a9bg08bf@corp.supernews.com...
> DaveH wrote:
>> I've seen this notion in various forums. It typically takes the form
>> of "A heavier bike may be more work uphill, but is faster downhill due
>> to gravity..." Something like that. The notion -- which I think is
>> incorrect -- really should take the
>> form of "Greater total mass (rider + bike + stuff) may be more work
>> uphill, but is faster downhill..." In any case, reviewing the elementary
>> physics, doesn't mass cancel in
>> the equations? Same reason a rock and feather both accelerate at g in
>> a vacuum? Ergo, that heavier bike isn't getting you down the hill any
>> faster.
>
> Not to address your actual point, but most of our cycling doesn't occur in
> a vacuum.
> --
> Andrew Muzi
> www.yellowjersey.org
> Open every day since 1 April, 1971
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Date: 01 Oct 2007 07:25:40
From: John Henderson
Subject: Re: More weight, faster descents??
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DaveH wrote:
> I've seen this notion in various forums. It typically takes
> the form of "A heavier bike may be more work uphill, but is
> faster downhill due to gravity..." Something like that.
>
> The notion -- which I think is incorrect -- really should take
> the form of "Greater total mass (rider + bike + stuff) may be
> more work uphill, but is faster downhill..."
>
> In any case, reviewing the elementary physics, doesn't mass
> cancel in the equations? Same reason a rock and feather both
> accelerate at g in a vacuum? Ergo, that heavier bike isn't
> getting you down the hill any faster.
Heavy riders certainly go downhill faster, other things being
equal.
That's more to do with basic geometry than with physics per se,
namely the ratio of surface area to volume.
Think of a sphere as an approximation. Double the volume (mass)
and the surface area is much less than doubled. Radius "r"
gets cubed for the volume calculation, but only squared for
surface area. And the ratio of mass to surface area is a main
determinant of terminal velocity.
John
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Date: 30 Sep 2007 15:11:34
From:
Subject: Re: More weight, faster descents??
|
On Sun, 30 Sep 2007 16:17:18 -0400, DaveH
<ddhartwick@FUTILITYearthlink.net > wrote:
>I've seen this notion in various forums. It typically takes the form
>of "A heavier bike may be more work uphill, but is faster downhill due
>to gravity..." Something like that.
>
>The notion -- which I think is incorrect -- really should take the
>form of "Greater total mass (rider + bike + stuff) may be more work
>uphill, but is faster downhill..."
>
>In any case, reviewing the elementary physics, doesn't mass cancel in
>the equations? Same reason a rock and feather both accelerate at g in
>a vacuum? Ergo, that heavier bike isn't getting you down the hill any
>faster.
>Dave
Dear Dave,
Short answer, heavier objects do indeed coast downhill faster.
Cannonballs and feathers fall at the same rate _only_ when there is no
wind drag.
As the size scales up, wind drag (frontal area) increases with only
the square of one dimension, but mass (gravitational force) increases
with the cube of the same dimension.
The terminal velocity is the speed at which the wind drag matches the
gravitational force. (In a vacuum with no drag, things just keep
accelerating at the same rate.)
Compare two cubes, 5 and 6 feet high:
speed
gravity-mass wind drag force/drag
volume frontal area ratio
6x6x6 = 216 6x6 = 36 216/36 = 6 to 1
5x5x5 = 125 5x5 = 25 125/25 = 5 to 1
The 6-foot high cube is almost twice as heavy, but it isn't even 50%
bigger from the wind's point of view. The larger object has more mass
relative to its wind drag, so it reaches a higher terminal velocity.
So downhill in the same tuck, the heavier rider coasts faster because
he reaches a higher terminal speed. He has much more gravitational
force pulling him down against only slightly increased wind drag.
Here's a calculator that can show you the difference:
http://austinimage.com/bp/velocity/velocity.html
Set the watts at the top of each column to 0 for coasting and the
slope for both columns to -5 (downhill 5% grade).
Then raise one rider's weight from the scrawny default 150 pounds to
Chalo Colina's trim 350 pounds.
The little guy rolls down the hill at 27 mph as Chalo pulls away at 40
mph. (In real life, Chalo would have slightly more wind drag, so
probably he wouldn't pull away quite that fast, but his weight rises
much faster than his frontal area, so no one wants to challenge him to
a coasting race down a long, straight hill.)
The same thing is true of BB's versus cannonballs. Drop them from an
airplane, and the cannonball hits the ground far ahead of the BB
because the mass and gravity-force of the sphere increases with the
cube of its diameter, while its surface-area and wind-drag increases
with only the square of its diameter. As things scale up, mass
outstrips surface area.
But don't get your hopes up. For practical reasons, bicycles usually
lose more than they gain from extra mass.
First, pushing the weight up the hill slows you down more than you can
gain by coasting back down the same hill, just as you lose overall
when you head out and back in with the wind blowing steadily in the
same direction.
The easiest way to illustrate this kind of loss is to imagine a 10
mile stretch of road that you can pedal on a windless day at 20 mph.
With no wind, you pedal out 10 miles in half an hour at 20 mph and
then back in another half an hour, so it takes you an hour to cover 20
miles at 20 mph.
But if the wind blows so hard that you drop to 10 mph heading out, you
can't possibly finish in an hour--you'll spend a whole hour just
fighting your way out to the turnaround point.
Second, bicycles accelerate like snails, but brake like demons. We
slowly speed up to cruising speed, with any extra weight slowing our
acceleration even further, but then we throw most of it away every
time we touch the brakes.
The same thing applies to twisty climbs. We don't have to slow down
for corners as we grind uphill at 10 mph, but we throw away a lot of
our high-speed advantage downhill as we brake for the hairpins.
That's why climbers usually beat descenders.
If you climb 10 mph, but your opponent climbs at 12 mph, then he
reaches the top of a 10-mile pass in 50 minutes, turns around, and
starts descending.
You arrive 10 minutes later (60 minutes), turn, and chase him
downhill.
If he coasts down at 30 mph, which would be fairly easy, he takes 20
minutes to cover the 10 miles to the bottom, so his total time up and
down is 50 up + 20 down = 70 minutes.
You already took 60 minutes to the top, so you have to do 10 miles
downhill in 10 minutes just to tie him for 70 minutes to the finish
line.
Fat chance. You'd have to corner fast enough to do 60 mph where he did
only 30 mph.
Cheers,
Carl Fogel
|
| |
Date: 30 Sep 2007 14:07:15
From: Mark
Subject: Re: More weight, faster descents??
|
DaveH wrote:
> I've seen this notion in various forums. It typically takes the form
> of "A heavier bike may be more work uphill, but is faster downhill due
> to gravity..." Something like that.
>
> The notion -- which I think is incorrect -- really should take the
> form of "Greater total mass (rider + bike + stuff) may be more work
> uphill, but is faster downhill..."
>
> In any case, reviewing the elementary physics, doesn't mass cancel in
> the equations? Same reason a rock and feather both accelerate at g in
> a vacuum? Ergo, that heavier bike isn't getting you down the hill any
> faster.
> Dave
Why don't you go do some cycling in a vacuum and let us know how it
works out. Out here in the real world (no vacuum), heavier bikes (w/
same frontal area) go down hill faster.
Mark J.
|
| |
Date: 30 Sep 2007 16:44:47
From: Paul Myron Hobson
Subject: Re: More weight, faster descents??
|
DaveH wrote:
> I've seen this notion in various forums. It typically takes the form
> of "A heavier bike may be more work uphill, but is faster downhill due
> to gravity..." Something like that.
>
> The notion -- which I think is incorrect -- really should take the
> form of "Greater total mass (rider + bike + stuff) may be more work
> uphill, but is faster downhill..."
>
> In any case, reviewing the elementary physics, doesn't mass cancel in
> the equations? Same reason a rock and feather both accelerate at g in
> a vacuum? Ergo, that heavier bike isn't getting you down the hill any
> faster.
> Dave
I'm 5'10", 130ish lbs. I just got back from the 6 Gap ride in the N.
Georgia mountains. I was smoking people going uphill, but then they
smoked my ass going down and I couldn't catch up no matter how hard I
tried (save for a few with a high pucker factor).
Mass does cancel out in elementary physics, but if you add fluid
mechanics into the picture (drag force), low weight loses going down.
\\paul
|
| | |
Date: 01 Oct 2007 00:50:35
From: Mike Jacoubowsky
Subject: Re: More weight, faster descents??
|
> I'm 5'10", 130ish lbs. I just got back from the 6 Gap ride in the N.
> Georgia mountains. I was smoking people going uphill, but then they
> smoked my ass going down and I couldn't catch up no matter how hard I
> tried (save for a few with a high pucker factor).
There's a *lot* more to descending than rider weight. Otherwise, the TdF
guys would float downhill like a feather, but if you watch the coverage,
you'll find plenty of lightweight guys who can descend plenty fast.
My theory is that most of the bigger guys descend faster entirely by
accident. I was watching the coverage of the TdF a few years ago, and
noticed as they were flying down the hill just how fat some of the guys
were. Yet I knew they were anything but (fat). I even looked up some of
their weights, just to make sure. But they did look like they had big guts
hanging out.
And that, as it turns out, is the secret. You want to not only get low, but
bring your chest down and your knees up. Close up the airspace and your
aerodynamics improve dramatically. And the guys who really are big? Their
guts hang down whether they want them to or not. By accident they're often
more aerodynamic (despite thinking that their increased size would render
them less so).
--Mike-- Chain Reaction Bicycles
www.ChainReactionBicycles.com
"Paul Myron Hobson" <phobson@gatech.edu > wrote in message
news:fdp1rv$fma$1@news-int2.gatech.edu...
> DaveH wrote:
>> I've seen this notion in various forums. It typically takes the form
>> of "A heavier bike may be more work uphill, but is faster downhill due
>> to gravity..." Something like that. The notion -- which I think is
>> incorrect -- really should take the
>> form of "Greater total mass (rider + bike + stuff) may be more work
>> uphill, but is faster downhill..." In any case, reviewing the elementary
>> physics, doesn't mass cancel in
>> the equations? Same reason a rock and feather both accelerate at g in
>> a vacuum? Ergo, that heavier bike isn't getting you down the hill any
>> faster. Dave
>
> I'm 5'10", 130ish lbs. I just got back from the 6 Gap ride in the N.
> Georgia mountains. I was smoking people going uphill, but then they
> smoked my ass going down and I couldn't catch up no matter how hard I
> tried (save for a few with a high pucker factor).
>
> Mass does cancel out in elementary physics, but if you add fluid mechanics
> into the picture (drag force), low weight loses going down.
>
> \\paul
|
| | | |
Date: 30 Sep 2007 22:14:52
From: DaveH
Subject: Re: More weight, faster descents??
|
On Mon, 01 Oct 2007 00:50:35 GMT, "Mike Jacoubowsky"
<mikej1@ix.netcom.com > wrote:
>> I'm 5'10", 130ish lbs. I just got back from the 6 Gap ride in the N.
>> Georgia mountains. I was smoking people going uphill, but then they
>> smoked my ass going down and I couldn't catch up no matter how hard I
>> tried (save for a few with a high pucker factor).
>
>There's a *lot* more to descending than rider weight. Otherwise, the TdF
>guys would float downhill like a feather, but if you watch the coverage,
>you'll find plenty of lightweight guys who can descend plenty fast.
>
>My theory is that most of the bigger guys descend faster entirely by
>accident. I was watching the coverage of the TdF a few years ago, and
>noticed as they were flying down the hill just how fat some of the guys
>were. Yet I knew they were anything but (fat). I even looked up some of
>their weights, just to make sure. But they did look like they had big guts
>hanging out.
>
>And that, as it turns out, is the secret. You want to not only get low, but
>bring your chest down and your knees up. Close up the airspace and your
>aerodynamics improve dramatically. And the guys who really are big? Their
>guts hang down whether they want them to or not. By accident they're often
>more aerodynamic (despite thinking that their increased size would render
>them less so).
>
>--Mike-- Chain Reaction Bicycles
Is that gut fat, or a lot a abdominal muscle mass?
|
| | | | |
Date: 01 Oct 2007 03:16:05
From: Mike Jacoubowsky
Subject: Re: More weight, faster descents??
|
> Is that gut fat, or a lot a abdominal muscle mass
On me? Gut fat! But for the TdF riders, what they're doing is intentionally
letting their stomach sag down, countering the natural tendancy to kinda
suck it in. A cyclist isn't going to intentionally build abdominal muscle
mass, as it's not much good for anything having to do with cycling.
--Mike-- Chain Reaction Bicycles
www.ChainReactionBicycles.com
"DaveH" <ddhartwick@FUTILITYearthlink.net > wrote in message
news:pql0g3tgm83e964naru9ec4lop82nl2vk5@4ax.com...
> On Mon, 01 Oct 2007 00:50:35 GMT, "Mike Jacoubowsky"
> <mikej1@ix.netcom.com> wrote:
>
>>> I'm 5'10", 130ish lbs. I just got back from the 6 Gap ride in the N.
>>> Georgia mountains. I was smoking people going uphill, but then they
>>> smoked my ass going down and I couldn't catch up no matter how hard I
>>> tried (save for a few with a high pucker factor).
>>
>>There's a *lot* more to descending than rider weight. Otherwise, the TdF
>>guys would float downhill like a feather, but if you watch the coverage,
>>you'll find plenty of lightweight guys who can descend plenty fast.
>>
>>My theory is that most of the bigger guys descend faster entirely by
>>accident. I was watching the coverage of the TdF a few years ago, and
>>noticed as they were flying down the hill just how fat some of the guys
>>were. Yet I knew they were anything but (fat). I even looked up some of
>>their weights, just to make sure. But they did look like they had big guts
>>hanging out.
>>
>>And that, as it turns out, is the secret. You want to not only get low,
>>but
>>bring your chest down and your knees up. Close up the airspace and your
>>aerodynamics improve dramatically. And the guys who really are big? Their
>>guts hang down whether they want them to or not. By accident they're often
>>more aerodynamic (despite thinking that their increased size would render
>>them less so).
>>
>>--Mike-- Chain Reaction Bicycles
> Is that gut fat, or a lot a abdominal muscle mass?
|
| | | | | |
Date: 01 Oct 2007 04:57:33
From: Ben C
Subject: Re: More weight, faster descents??
|
On 2007-10-01, Mike Jacoubowsky <mikej1@ix.netcom.com > wrote:
>> Is that gut fat, or a lot a abdominal muscle mass
>
> On me? Gut fat! But for the TdF riders, what they're doing is intentionally
> letting their stomach sag down, countering the natural tendancy to kinda
> suck it in.
>
> A cyclist isn't going to intentionally build abdominal muscle mass, as
> it's not much good for anything having to do with cycling.
I heard Indurain's paunch was caused by his elephantine lungs displacing
all his inessential organs downwards.
|
| | |
Date: 30 Sep 2007 17:40:55
From: DaveH
Subject: Re: More weight, faster descents??
|
On Sun, 30 Sep 2007 16:44:47 -0400, Paul Myron Hobson
<phobson@gatech.edu > wrote:
>DaveH wrote:
>> I've seen this notion in various forums. It typically takes the form
>> of "A heavier bike may be more work uphill, but is faster downhill due
>> to gravity..." Something like that.
>>
>> The notion -- which I think is incorrect -- really should take the
>> form of "Greater total mass (rider + bike + stuff) may be more work
>> uphill, but is faster downhill..."
>>
>> In any case, reviewing the elementary physics, doesn't mass cancel in
>> the equations? Same reason a rock and feather both accelerate at g in
>> a vacuum? Ergo, that heavier bike isn't getting you down the hill any
>> faster.
>> Dave
>
>I'm 5'10", 130ish lbs. I just got back from the 6 Gap ride in the N.
>Georgia mountains. I was smoking people going uphill, but then they
>smoked my ass going down and I couldn't catch up no matter how hard I
>tried (save for a few with a high pucker factor).
>
>Mass does cancel out in elementary physics, but if you add fluid
>mechanics into the picture (drag force), low weight loses going down.
>
>\\paul
Paul, this assumes that the heavier rider/bike combo (henceforth
referred to as moving mass) will always present an aerodynamic profile
with more drag. No?
Dave
|
| | | |
Date: 30 Sep 2007 22:41:01
From: Paul Myron Hobson
Subject: Re: More weight, faster descents??
|
DaveH wrote:
> On Sun, 30 Sep 2007 16:44:47 -0400, Paul Myron Hobson
> <phobson@gatech.edu> wrote:
>
>> DaveH wrote:
>>> I've seen this notion in various forums. It typically takes the form
>>> of "A heavier bike may be more work uphill, but is faster downhill due
>>> to gravity..." Something like that.
>>>
>>> The notion -- which I think is incorrect -- really should take the
>>> form of "Greater total mass (rider + bike + stuff) may be more work
>>> uphill, but is faster downhill..."
>>>
>>> In any case, reviewing the elementary physics, doesn't mass cancel in
>>> the equations? Same reason a rock and feather both accelerate at g in
>>> a vacuum? Ergo, that heavier bike isn't getting you down the hill any
>>> faster.
>>> Dave
>> I'm 5'10", 130ish lbs. I just got back from the 6 Gap ride in the N.
>> Georgia mountains. I was smoking people going uphill, but then they
>> smoked my ass going down and I couldn't catch up no matter how hard I
>> tried (save for a few with a high pucker factor).
>>
>> Mass does cancel out in elementary physics, but if you add fluid
>> mechanics into the picture (drag force), low weight loses going down.
>>
>> \\paul
> Paul, this assumes that the heavier rider/bike combo (henceforth
> referred to as moving mass) will always present an aerodynamic profile
> with more drag. No?
> Dave
If I understand you correctly: Yes, but:
-Linear dimensions (size) scale, well, linearly.
-Frontal area scales with the square of the linear dimensions.
-Weight or Volume (inertia) scales with the cube of the linear dimensions.
So a slightly bigger (L) guy has an more than slightly bigger frontal
area (L^2) but then even bigger weight (L^3), which is the force
counteracting the drag force (L^2). I'm undoubtedly doing a horrible
job of explaining this. I must admit, I'm pretty tired.
\\paul
|
| |
Date: 30 Sep 2007 13:48:34
From:
Subject: Re: More weight, faster descents??
|
On Sep 30, 4:28 pm, Lou Holtman <lholremovet...@planet.nl > wrote:
> DaveH wrote:
> > I've seen this notion in various forums. It typically takes the form
> > of "A heavier bike may be more work uphill, but is faster downhill due
> > to gravity..." Something like that.
>
> > The notion -- which I think is incorrect -- really should take the
> > form of "Greater total mass (rider + bike + stuff) may be more work
> > uphill, but is faster downhill..."
>
> > In any case, reviewing the elementary physics, doesn't mass cancel in
> > the equations? Same reason a rock and feather both accelerate at g in
> > a vacuum? Ergo, that heavier bike isn't getting you down the hill any
> > faster.
> > Dave
>
> Yes it is. More weight, equal frontal area (= air resistance).
>
> Lou
> --
> Posted by news://news.nb.nu (http://www.nb.nu)
The only problem is that work done to overcome air resistance goes up
by the square of velocity while the work done/input to the system by
the mass going up and down the hill is linear with velocity. For any
significant grade, more mass will never speed you up on the descent as
much as it slowed you down on the climb.
|
| | |
Date: 30 Sep 2007 23:01:31
From: Lou Holtman
Subject: Re: More weight, faster descents??
|
unforgiven99@juno.com wrote:
> On Sep 30, 4:28 pm, Lou Holtman <lholremovet...@planet.nl> wrote:
>> DaveH wrote:
>>> I've seen this notion in various forums. It typically takes the form
>>> of "A heavier bike may be more work uphill, but is faster downhill due
>>> to gravity..." Something like that.
>>> The notion -- which I think is incorrect -- really should take the
>>> form of "Greater total mass (rider + bike + stuff) may be more work
>>> uphill, but is faster downhill..."
>>> In any case, reviewing the elementary physics, doesn't mass cancel in
>>> the equations? Same reason a rock and feather both accelerate at g in
>>> a vacuum? Ergo, that heavier bike isn't getting you down the hill any
>>> faster.
>>> Dave
>> Yes it is. More weight, equal frontal area (= air resistance).
>>
>> Lou
>> --
>> Posted by news://news.nb.nu (http://www.nb.nu)
>
> The only problem is that work done to overcome air resistance goes up
> by the square of velocity while the work done/input to the system by
> the mass going up and down the hill is linear with velocity. For any
> significant grade, more mass will never speed you up on the descent as
> much as it slowed you down on the climb.
>
True.
Lou
--
Posted by news://news.nb.nu (http://www.nb.nu)
|
| |
Date: 30 Sep 2007 22:28:47
From: Lou Holtman
Subject: Re: More weight, faster descents??
|
DaveH wrote:
> I've seen this notion in various forums. It typically takes the form
> of "A heavier bike may be more work uphill, but is faster downhill due
> to gravity..." Something like that.
>
> The notion -- which I think is incorrect -- really should take the
> form of "Greater total mass (rider + bike + stuff) may be more work
> uphill, but is faster downhill..."
>
> In any case, reviewing the elementary physics, doesn't mass cancel in
> the equations? Same reason a rock and feather both accelerate at g in
> a vacuum? Ergo, that heavier bike isn't getting you down the hill any
> faster.
> Dave
Yes it is. More weight, equal frontal area (= air resistance).
Lou
--
Posted by news://news.nb.nu (http://www.nb.nu)
|
| | |
Date: 30 Sep 2007 17:36:33
From: DaveH
Subject: Re: More weight, faster descents??
|
On Sun, 30 Sep 2007 22:28:47 +0200, Lou Holtman
<lholremovethis@planet.nl > wrote:
>DaveH wrote:
>> I've seen this notion in various forums. It typically takes the form
>> of "A heavier bike may be more work uphill, but is faster downhill due
>> to gravity..." Something like that.
>>
>> The notion -- which I think is incorrect -- really should take the
>> form of "Greater total mass (rider + bike + stuff) may be more work
>> uphill, but is faster downhill..."
>>
>> In any case, reviewing the elementary physics, doesn't mass cancel in
>> the equations? Same reason a rock and feather both accelerate at g in
>> a vacuum? Ergo, that heavier bike isn't getting you down the hill any
>> faster.
>> Dave
>
>
>Yes it is. More weight, equal frontal area (= air resistance).
>
>Lou
Assume same air resistance or drag for both scenarios -- identical
frontal area and configuration.
|
| | | |
Date: 30 Sep 2007 23:46:18
From: Lou Holtman
Subject: Re: More weight, faster descents??
|
DaveH wrote:
> On Sun, 30 Sep 2007 22:28:47 +0200, Lou Holtman
> <lholremovethis@planet.nl> wrote:
>
>> DaveH wrote:
>>> I've seen this notion in various forums. It typically takes the form
>>> of "A heavier bike may be more work uphill, but is faster downhill due
>>> to gravity..." Something like that.
>>>
>>> The notion -- which I think is incorrect -- really should take the
>>> form of "Greater total mass (rider + bike + stuff) may be more work
>>> uphill, but is faster downhill..."
>>>
>>> In any case, reviewing the elementary physics, doesn't mass cancel in
>>> the equations? Same reason a rock and feather both accelerate at g in
>>> a vacuum? Ergo, that heavier bike isn't getting you down the hill any
>>> faster.
>>> Dave
>>
>> Yes it is. More weight, equal frontal area (= air resistance).
>>
>> Lou
>
> Assume same air resistance or drag for both scenarios -- identical
> frontal area and configuration.
Air resistance is a function of speed. A heavier bike accelerates to a
greater speed, because the pulling force is larger
(Fgravity.sin(grade-angle)).
Lou
--
Posted by news://news.nb.nu (http://www.nb.nu)
|
| | | | |
Date: 30 Sep 2007 19:30:22
From: DaveH
Subject: Re: More weight, faster descents??
|
>> Assume same air resistance or drag for both scenarios -- identical
>> frontal area and configuration.
>
>
>Air resistance is a function of speed. A heavier bike accelerates to a
>greater speed, because the pulling force is larger
>(Fgravity.sin(grade-angle)).
>
>Lou
Right Lou -- It is clear now. I used to tutor this stuff in college.
Believe that? Embarrasing.
Dave
|
| | | | | |
Date: 30 Sep 2007 22:47:39
From: Paul Myron Hobson
Subject: Re: More weight, faster descents??
|
DaveH wrote:
>>> Assume same air resistance or drag for both scenarios -- identical
>>> frontal area and configuration.
>>
>> Air resistance is a function of speed. A heavier bike accelerates to a
>> greater speed, because the pulling force is larger
>> (Fgravity.sin(grade-angle)).
>>
>> Lou
>
> Right Lou -- It is clear now. I used to tutor this stuff in college.
> Believe that? Embarrasing.
> Dave
I was a hydraulics TA for three semesters :p
Crowning achievement: showing my student how to calculate P_cK (critical
keg pressure - the pressure at which the beer will come out foamy).
\\paul
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|
