70 mph on a bike in the TDF?

It occurs to me that one of the things missing in the
discussion of length required to achieve terminal velocity
on a descent is the starting speed and initial acceleration
from pedalling. The meaningful math sims use zero power
input, assuming a tuck the whole way. I suspect the
fast way is to sprint up to a good clip, then get into
a tuck to fight the air. The faster you go, the
faster you go faster, until you hit the drag wall. If
you go over the top at 35 mph, it isn't taking long to get
to 40 or 45.

-dB






--
Butterflies tell me to say:
"The statements and opinions expressed here are my own and do not necessarily
represent those of Oracle Corporation."

On 13 Sep 04 23:04:51 GMT, (David Brower)
wrote:

It occurs to me that one of the things missing in the
discussion of length required to achieve terminal velocity
on a descent is the starting speed and initial acceleration
from pedalling. The meaningful math sims use zero power
input, assuming a tuck the whole way. I suspect the
fast way is to sprint up to a good clip, then get into
a tuck to fight the air. The faster you go, the
faster you go faster, until you hit the drag wall. If
you go over the top at 35 mph, it isn't taking long to get
to 40 or 45.

-dB

Dear David,

This may be the calculator that you want:

http://www.analyticcycling.com/DiffE...lope_Page.html

You can put in any speed you like for the starting speed,
regrettably in meters per second, and then get a graph as
well as the raw details.

To coast, just set power to 0 watts.

Carl Fogel

Trevor wrote:
Zog The Undeniable wrote in message 4139dbdb.0@entanet...
Like most of your other posts, Trevor, I call bollix. Speed limits
apply only to *motor* vehicles in the UK (apart from a few special
locations covered by local bye-laws, notably some Royal Parks).
Speed limits apply to a pedalling cyclist but not someone freewheeling.

This is a total fiction; you will be completely unable to produce any
reference to it. You are confusing speed limits with the offence described
by Zog of "pedalling furiously".

is not a cycling specific law, it is a speed limit law. Cyclists are
excluded if freewheeling. Try and find it. Where does it say a bicycle is
excluded from speed limits. All vehicles are subject to speed limits.

The most obvious place is the Highway Code rule 103, which describes the
national speed limit (and is a rule of the type that describes the law).
You will notice that the classes of vehicle do not include any
non-motorised vehicles.

The RTRA is regrettably not available online but if you actually care about
the truth, which seems unlikely, find a copy and refer to it.
--
David Damerell Distortion Field!

On 2004-09-06 20:09:10 -0600, said:

n you name anyone who exceeds 50 mph on a level course of
any kind with a roughly normal bicycle?

You probably could do it in Boulder during a Chinook wind. I
experienced some of them back in the 70s and rode a bike in one of them
(no speedometer at that time). There is a good road down from the NOAA
with a broad curve that gets tailwinds at that time that would
certainly allow 70mph.
http://www.cdc.noaa.gov/Boulder/wind.html

Larry Coon Wrote:
Tom Nakashima wrote:

Weisse, I believe you that you hit 63 mph, because 4 cyclist showe
me their
cyclometers as their top speed was over 60 mph in this year's Deat
Ride.
I hit a little over 50 mph, but I don't weigh that much

What does your weight have to do with it?

and never come close
to 60 mph.
-tom


Larry Coon
University of California

undefined

Gravity makes you faster :rolleyes

--
kathybeast

On Thu, 23 Sep 2004 00:42:33 +1000, kathybeast
wrote:


Larry Coon Wrote:
Tom Nakashima wrote:

Weisse, I believe you that you hit 63 mph, because 4 cyclist showed
me their
cyclometers as their top speed was over 60 mph in this year's Death
Ride.
I hit a little over 50 mph, but I don't weigh that much

What does your weight have to do with it?

and never come close
to 60 mph.
-tom


Larry Coon
University of California

undefined

Gravity makes you faster

Dear Kathy,

I can't tell whether you're rolling your eyes because
everyone should know that heavier riders coast downhill
faster, or because you mistakenly think that this is a myth.

As the thread illustrates, a surprising range of posters
from casual riders to bicycle shop owners are understandably
puzzled by this odd situation.

After all, we usually think of heavier things as going
slower, and everyone who's had a physics class has heard
that all objects fall at the same rate in a vacuum.

But heavier riders do coast noticeably faster downhill.
Anyone with a heavy friend or a few moments to increase
weight on this calculator can see the speed increase:

http://www.analyticcycling.com/ForcesSpeed_Page.html

For the link above, set the slope to -0.10 for a 10%
downhill grade, the watts to 0 for coasting, run the model,
and the predicted speed is 21.46 meters per second. Bump the
weight up from 75 kg to 100 kg, and the speed rises to 24.79
meters per second. That's an increase from about 48 mph to
55.5 mph.

The heavier rider coasts downhill faster because he's the
same aerodynamic shape, just a little larger. Wind drag
increases roughly with cross-section, a two-dimensional
surface, but the force of gravity increases roughly with
volume in three dimensions. Given a cube of height H, wind
drag will be H x H, while mass will be H x H x H.

So a rider of height 1 has a drag of 1 x 1 and a mass of 1 x
1 x 1 and a drag-to-mass ratio of 1 to 1. As he rolls
downhill, he accelerates until the increasing wind drag
matches the constant force of gravity.

Scale the rider up 10% to a height of 1.1. Now his drag is
1.1squared, and his mass is 1.1 cubed, so his drag-to-mass
ratio is not 1 to 1, but 1.21 to 1.33, or 1 to 1.1. At the
previous terminal speed, his drag will no longer match the
force of gravity, so he keeps accelerating until the
increasing drag counters his larger gravitational force.

Wind drag simply doesn't increase as fast as mass when
objects stay the same shape. This is why a bowling-ball
sized bearing will roll faster down a slope than than a
marble-sized bearing. It's a slightly different application
of the wind drag principle that explains why a a penny falls
faster than a large feather in air, but at the same rate in
a vacuum.

Carl Fogel

Well, this 70 mph thing is not something I have ever done, (broke 50 o
a big hill in Wisconsin) but I know a guy from Boulder who has
Colorado state Highway Patrol ticket framed in his house on which h
was clocked going over 70 mph in a 50 mph zone. And yes of course i
was going down a mountain. Bob Fourney is his name. So you can't ge
much more official than radar

--
Chris Fish

Here's another data point for descending speed, taken from
yesterday's Everest Challenge race in Bishop, CA.

Notes: I'm 5'11" and don't weigh much (133 lbs this morning--lost
some weight during the race), My jersey pockets were stuffed with a
jacket, glove liners and shells, and several gel packs, making my
frontal cross section a little bigger than normal. I was in a tuck
similar to this one:

http://tinyurl.com/42ppk

except that my elbows were in, my knees were touching the down tube,
and I was off the front and beneath the saddle. In other words, I
was in a slightly tighter tuck than that guy. The road was Glacier
Lodge Road, a very nice road surface with extended straight sections
of 8%:

http://tinyurl.com/5gkle

There was little wind in the early morning hour. My highest recorded
speed was 53.2 mph. This calculator:

http://www.kreuzotter.de/english/espeed.htm

predicts 48.7 mph, using the "triathlon bicycle" option.

Another interesting tidbit: I easily descended faster than my buddy
on this descent. On the next descent, he dumped off his jacket from
his jersey pocket and was able to stay in front of me down the next
road (Death Valley Road, 5%, 40.6 max mph). This demonstrated how
sensitive descending speed is to changes in frontal cross section.
--
terry morse Palo Alto, CA http://bike.terrymorse.com/

On Mon, 27 Sep 2004 16:15:41 -0700, Terry Morse
wrote:

Here's another data point for descending speed, taken from
yesterday's Everest Challenge race in Bishop, CA.

Notes: I'm 5'11" and don't weigh much (133 lbs this morning--lost
some weight during the race), My jersey pockets were stuffed with a
jacket, glove liners and shells, and several gel packs, making my
frontal cross section a little bigger than normal. I was in a tuck
similar to this one:

http://tinyurl.com/42ppk

except that my elbows were in, my knees were touching the down tube,
and I was off the front and beneath the saddle. In other words, I
was in a slightly tighter tuck than that guy. The road was Glacier
Lodge Road, a very nice road surface with extended straight sections
of 8%:

http://tinyurl.com/5gkle

There was little wind in the early morning hour. My highest recorded
speed was 53.2 mph. This calculator:

http://www.kreuzotter.de/english/espeed.htm

predicts 48.7 mph, using the "triathlon bicycle" option.

Another interesting tidbit: I easily descended faster than my buddy
on this descent. On the next descent, he dumped off his jacket from
his jersey pocket and was able to stay in front of me down the next
road (Death Valley Road, 5%, 40.6 max mph). This demonstrated how
sensitive descending speed is to changes in frontal cross section.

Dear Terry,

To be fair to the calculator, its predictions are for
someone pedalling a triathalon bike, a considerably less
aerodynamic posiiton than someone coasting downhill.

It's convenient to be able to set the pedal power and rpm to
zero, but I doubt that our evil purposes were anticipated by
the nice fellows who put the various calculators together.

In any case, it's likely that the road grade varied as much
as the predicted versus noticed speeds. That is, your
speedometer's highest speed was about 10% above the
prediction on an 8% grade (average of distance versus drop
or highway posting or whatever) that probably varies from
7.5% to 8.5% over any significant stretches.

And while I'm sure that it seemed windless in the early
morning, I also suspect that riders have little ability to
sense a 5 mph wind in any direction when they're tucked in
and zooming down exciting descents.

Your 53 mph for at least an instant on a roughly 8% grade on
a calm early morning in your best 133-lb tuck strikes me as
perfectly plausible. It's claims from other posters of over
70 mph while pedalling 200 rpm down short descents of 10% or
less that raise my eyebrows.

Stuff a few more tools in your bag for weight and you might
hit 55.

Cheers,

Carl Fogel