Descending steep hills on a tandem

David Damerell has a letter in the recent UK Tandem Club Journal on
this subject. He makes 2 claims that I think are very controversial.

The first is this: it is widely accepted that there is a "worst speed"
to descend, at which the rims are most strongly heated (go faster, and
air resistance dissipates the potential energy; go slower, and the rate
of conversion of gravitational potential energy to heat drops). DD
claims that this worst speed is "around walking pace, maybe lower". I'm
sure I have seen calculations presented on tandem@hobbes (where I have
also posted this message), and have done calcs myself, indicating that
the worst speed is likely to be in the region of 10-20mph. Of course,
it depends on gradient and load, but walking pace seems implausible.
Were that to be the case, tyres would never be under threat, because
the energy dissipation would be too low.

The second claim is the old chestnut about going fast on the straights
and braking hard on the corners. In principle this means a greater
dissipation of energy to air resistance (for the same average
descending speed), due to the nonlinearity of this effect. However, I
cannot see how this can be "much better" than a steady speed as DD
claims - any time I have tried what I believe to be reasonable
calculations, I find the effect to be close to negligible. It seems to
only help if the riders can get close to terminal velocity on the
straights, which IME is far from the case for most steep descents in
the UK. Can anyone show different?

Ultimately, I have no disagreement with his conclusion that an Arai
drum is a sensible choice for most teams (at least in the UK). But I
would prefer such an argument to be based on an accurate understanding
of the physics involved.

James

James Annan wrote:

The second claim is the old chestnut about going fast on the straights
and braking hard on the corners. In principle this means a greater
dissipation of energy to air resistance (for the same average
descending speed), due to the nonlinearity of this effect. However, I
cannot see how this can be "much better" than a steady speed as DD
claims - any time I have tried what I believe to be reasonable
calculations, I find the effect to be close to negligible. It seems to
only help if the riders can get close to terminal velocity on the
straights, which IME is far from the case for most steep descents in
the UK. Can anyone show different?


I have always viewed this as because the thermal disippation of the rims
is super-linear with temperature. Therefore to disippate the same
energy, it is better to do so in a short burst which creates higher
temperatures that disippate the energy quickly than in long slow periods
which gives more opportunity for the heat to conduct through the rims.
I was taught similar braking techniques on hills for cars from the days
where brakes were not as good as today and tended to fade with overuse.

However the technique wa not braking hard on the corners but braking
hard approaching the corners but stop braking before you started to
turn. Braking and turning is not generally a good idea.

I would think that the calculations would be near impossible to do
without sophisticated thermal codes because you have a complex mix of
conduction, forced convection and radiation in a complex geometrical
shape. I have certainly found from experience that if I do continuous
braking, the brake pads start to make all sort of nasty rasping noises
as they heat up which does not happen with short bursts of hard braking.
The drum brakes on our tandems have taken care of that though now for
long downhills.


--
Tony

"I did make a mistake once - I thought I'd made a mistake but I hadn't"
Anon

Tony Raven wrote:
James Annan wrote:

The second claim is the old chestnut about going fast on the straights
and braking hard on the corners. In principle this means a greater
dissipation of energy to air resistance (for the same average
descending speed), due to the nonlinearity of this effect. However, I
cannot see how this can be "much better" than a steady speed as DD
claims - any time I have tried what I believe to be reasonable
calculations, I find the effect to be close to negligible. It seems to
only help if the riders can get close to terminal velocity on the
straights, which IME is far from the case for most steep descents in
the UK. Can anyone show different?


I have always viewed this as because the thermal disippation of the rims
is super-linear with temperature. Therefore to disippate the same
energy, it is better to do so in a short burst which creates higher
temperatures that disippate the energy quickly than in long slow periods
which gives more opportunity for the heat to conduct through the rims.

Rims are very thin and conduct very well: it seems unlikely to me that
heating them to a _higher_ temperature, even if intermittently, can be
much less likely to cause a tyre/tube problem than maintaining a lower
but steadier temperature. At least, it would only work for some set of
circumstances, and could surely increase the likelihood of failure in
others. But I'd certainly be open to any evidence that supports this
theory - has anyone ever seen any experimental results?

I was taught similar braking techniques on hills for cars from the days
where brakes were not as good as today and tended to fade with overuse.

I have seen what seems to me to be a plausible explanation for this
advice, which may help explain the origin of the myth (if indeed it is
a myth). Light application of a drum brake means that only a small part
of the pad is in contact with the shoe, with the result that it can
overheat and glaze even for light braking. In contrast, intermittent
firm braking spreads the load more widely. Of course this explanation
does not apply to bicycles with rim brakes, but that wouldn't
necessarily stop the advice from spreading.

James

"James Annan" wrote:

Tony Raven wrote:
James Annan wrote:

The second claim is the old chestnut about going fast on the straights
and braking hard on the corners. In principle this means a greater
dissipation of energy to air resistance (for the same average
descending speed), due to the nonlinearity of this effect. However, I
cannot see how this can be "much better" than a steady speed as DD
claims - any time I have tried what I believe to be reasonable
calculations, I find the effect to be close to negligible. It seems to
only help if the riders can get close to terminal velocity on the
straights, which IME is far from the case for most steep descents in
the UK. Can anyone show different?

I have always viewed this as because the thermal disippation of the rims
is super-linear with temperature. Therefore to disippate the same
energy, it is better to do so in a short burst which creates higher
temperatures that disippate the energy quickly than in long slow periods
which gives more opportunity for the heat to conduct through the rims.

Rims are very thin and conduct very well: it seems unlikely to me that
heating them to a _higher_ temperature, even if intermittently, can be
much less likely to cause a tyre/tube problem than maintaining a lower
but steadier temperature. At least, it would only work for some set of
circumstances, and could surely increase the likelihood of failure in
others. But I'd certainly be open to any evidence that supports this
theory - has anyone ever seen any experimental results?

Remember that the "problem" isn't that the rim gets hot - you wouldn't
have to worry about that until it started melting. The problem is
that the TIRE gets hot and/or the air in the tire gets hot.

Since the tire will resist heating due to its mass, and the "thermal
connection" between the rim and tire is very small, it stands to
reason that short, intense braking would transfer less overall heat to
the tire than long, sustained braking.

Think of two alternate ways to boil water on an electric stove.

1) Put the pot on the burner turned to "medium" and wait for it to
boil.
2) Turn the burner to "high" and put the pot on the burner for 15
seconds every minute.

Participant #1 will be drinking his tea / eating his boiled egg long
before participant #2.

I was taught similar braking techniques on hills for cars from the days
where brakes were not as good as today and tended to fade with overuse.

I have seen what seems to me to be a plausible explanation for this
advice, which may help explain the origin of the myth (if indeed it is
a myth). Light application of a drum brake means that only a small part
of the pad is in contact with the shoe, with the result that it can
overheat and glaze even for light braking. In contrast, intermittent
firm braking spreads the load more widely. Of course this explanation
does not apply to bicycles with rim brakes, but that wouldn't
necessarily stop the advice from spreading.

There are other advantages as well...

First, assume that there are a given number of "watts to dissipate"
going down a given hill. If you keep your speed down to under 20mph,
you are losing only a few of those watts to aerodynamic drag.
However, if you "let it roll" between corners and achieve speeds of
double that (40mph), a HUGE amount of kinetic energy "disappears"
before you reach that corner. I always sit upright on steep descents
on my tandems to maximize this "aero braking".

And as stated by another poster, the hotter the rim, the more energy
it will radiate into the ambient air.

So no, it's not really a myth, IMHO. Anecdotally, I've ridden tandems
for 20+ years, braking hard into corners, have never had a drag brake,
and have never had any issues with overheating rims/tires.

Mark Hickey
Habanero Cycles
http://www.habcycles.com
Home of the $795 ti frame

On Sun, 21 Aug 2005 11:16:05 +0100, Tony Raven wrote:

James Annan wrote:

The second claim is the old chestnut about going fast on the straights
and braking hard on the corners. In principle this means a greater
dissipation of energy to air resistance (for the same average
descending speed), due to the nonlinearity of this effect. However, I
cannot see how this can be "much better" than a steady speed as DD
claims - any time I have tried what I believe to be reasonable
calculations, I find the effect to be close to negligible. It seems to
only help if the riders can get close to terminal velocity on the
straights, which IME is far from the case for most steep descents in
the UK. Can anyone show different?


I have always viewed this as because the thermal disippation of the rims
is super-linear with temperature.

What is it about a rim that makes it immune to Newton's law of cooling?

--

David L. Johnson

__o | Deserves death! I daresay he does. Many that live deserve
_`\(,_ | death. And some that die deserve life. Can you give it to
(_)/ (_) | them? Then do not be too eager to deal out death in judgement.
-- J. R. R. Tolkein

Mark Hickey wrote:

So no, it's not really a myth, IMHO. Anecdotally, I've ridden tandems
for 20+ years, braking hard into corners, have never had a drag brake,
and have never had any issues with overheating rims/tires.


That's the "hills" of Florida for you ;-)

--
Tony

"I did make a mistake once - I thought I'd made a mistake but I hadn't"
Anon

David L. Johnson wrote:


What is it about a rim that makes it immune to Newton's law of cooling?


http://www.madsci.org/posts/archives...0375.Ph.r.html

--
Tony

"I did make a mistake once - I thought I'd made a mistake but I hadn't"
Anon

James Annan writes:

David Damerell has a letter in the recent UK Tandem Club Journal on
this subject. He makes 2 claims that I think are very controversial.

The first is this: it is widely accepted that there is a "worst
speed" to descend, at which the rims are most strongly heated (go
faster, and air resistance dissipates the potential energy; go
slower, and the rate of conversion of gravitational potential energy
to heat drops). DD claims that this worst speed is "around walking
pace, maybe lower". I'm sure I have seen calculations presented on
tandem@hobbes (where I have also posted this message), and have done
calcs myself, indicating that the worst speed is likely to be in the
region of 10-20mph. Of course, it depends on gradient and load, but
walking pace seems implausible. Were that to be the case, tyres
would never be under threat, because the energy dissipation would be
too low.

My experience is that for about 10% grade 10-15mph on a single bicycle
is effective in causing tire blow-off on roads where this is otherwise
not a problem.

The second claim is the old chestnut about going fast on the
straights and braking hard on the corners. In principle this means
a greater dissipation of energy to air resistance (for the same
average descending speed), due to the nonlinearity of this effect.
However, I cannot see how this can be "much better" than a steady
speed as DD claims - any time I have tried what I believe to be
reasonable calculations, I find the effect to be close to
negligible. It seems to only help if the riders can get close to
terminal velocity on the straights, which IME is far from the case
for most steep descents in the UK. Can anyone show different?

Because tire blow-off occurs when inflation air is heated
sufficiently, the heat transfer time is important. Braking hard into
turns and coasting at speed between reduces heating time and increases
cooling time, while at the same time dissipating much energy to air
drag. These three effects together seem to me to be the reason why
my tire blows off at 10-15mph while it presents no hazard descending
normally (fast).

Ultimately, I have no disagreement with his conclusion that an Arai
drum is a sensible choice for most teams (at least in the UK). But
I would prefer such an argument to be based on an accurate
understanding of the physics involved.

Anything that keeps the heat out of the tire helps.

Jobst Brandt

In article ,
Mark Hickey wrote:

"James Annan" wrote:

Tony Raven wrote:
James Annan wrote:

The second claim is the old chestnut about going fast on the straights
and braking hard on the corners. In principle this means a greater
dissipation of energy to air resistance (for the same average
descending speed), due to the nonlinearity of this effect. However, I
cannot see how this can be "much better" than a steady speed as DD
claims - any time I have tried what I believe to be reasonable
calculations, I find the effect to be close to negligible. It seems to
only help if the riders can get close to terminal velocity on the
straights, which IME is far from the case for most steep descents in
the UK. Can anyone show different?

I have always viewed this as because the thermal disippation of the rims
is super-linear with temperature. Therefore to disippate the same
energy, it is better to do so in a short burst which creates higher
temperatures that disippate the energy quickly than in long slow periods
which gives more opportunity for the heat to conduct through the rims.

Rims are very thin and conduct very well: it seems unlikely to me that
heating them to a _higher_ temperature, even if intermittently, can be
much less likely to cause a tyre/tube problem than maintaining a lower
but steadier temperature. At least, it would only work for some set of
circumstances, and could surely increase the likelihood of failure in
others. But I'd certainly be open to any evidence that supports this
theory - has anyone ever seen any experimental results?

Remember that the "problem" isn't that the rim gets hot - you wouldn't
have to worry about that until it started melting. The problem is
that the TIRE gets hot and/or the air in the tire gets hot.

Since the tire will resist heating due to its mass, and the "thermal
connection" between the rim and tire is very small, it stands to
reason that short, intense braking would transfer less overall heat to
the tire than long, sustained braking.

Think of two alternate ways to boil water on an electric stove.

1) Put the pot on the burner turned to "medium" and wait for it to
boil.
2) Turn the burner to "high" and put the pot on the burner for 15
seconds every minute.

Participant #1 will be drinking his tea / eating his boiled egg long
before participant #2.

I was taught similar braking techniques on hills for cars from the days
where brakes were not as good as today and tended to fade with overuse.

I have seen what seems to me to be a plausible explanation for this
advice, which may help explain the origin of the myth (if indeed it is
a myth). Light application of a drum brake means that only a small part
of the pad is in contact with the shoe, with the result that it can
overheat and glaze even for light braking. In contrast, intermittent
firm braking spreads the load more widely. Of course this explanation
does not apply to bicycles with rim brakes, but that wouldn't
necessarily stop the advice from spreading.

There are other advantages as well...

First, assume that there are a given number of "watts to dissipate"
going down a given hill. If you keep your speed down to under 20mph,
you are losing only a few of those watts to aerodynamic drag.
However, if you "let it roll" between corners and achieve speeds of
double that (40mph), a HUGE amount of kinetic energy "disappears"
before you reach that corner. I always sit upright on steep descents
on my tandems to maximize this "aero braking".

And as stated by another poster, the hotter the rim, the more energy
it will radiate into the ambient air.

Addition: the more energy and _faster_ it will radiate
into the the ambient air.

Addition: the _radiation_ into ambient air is a drop in
the bucket compared to _convection_ into turbulent air.

I completely agree with braking hard when braking, and not
"feathering" the brakes. All the way on, or all the way
off.


So no, it's not really a myth, IMHO. Anecdotally, I've ridden tandems
for 20+ years, braking hard into corners, have never had a drag brake,
and have never had any issues with overheating rims/tires.

--
Michael Press

On Sun, 21 Aug 2005 16:30:57 +0100, Tony Raven wrote:

David L. Johnson wrote:


What is it about a rim that makes it immune to Newton's law of cooling?


http://www.madsci.org/posts/archives...0375.Ph.r.html

At this site cooling is divvied up between ordinary convection,
conduction, radiation, and "free convection". The first is the classical
application of Newton's law. Conduction follows the same model, with
differing ways in which the constant is determined. Free convection takes
into account specifics of the flow of the medium, and even there has only
slightly nonlinear behavior. The only dramatically nonlinear behavior is
radiation (the infrared light that the hot rim gives off, in this case).
I doubt that the nonlinearlity of these factors could be measured in a
case like this, certainly not enough to change braking patterns.

--

David L. Johnson

__o | "Business!" cried the Ghost. "Mankind was my business. The
_`\(,_ | common welfare was my business; charity, mercy, forbearance,
(_)/ (_) | and benevolence, were, all, my business. The dealings of my
trade were but a drop of water in the comprehensive ocean of my
business!" --Dickens, "A Christmas Carol"