Re: Unsprung weight ?

SuperSlinky said:

However, rotating weight is more
important than static weight, so reducing the weight of tires, tubes and
rims is the best place to shave grams for pure performance. Other than
that, a pound on you is going to have the same effect as a pound on the
frame. Having the pound on the frame might actually be better, because
the pound on you results in a higher center of gravity.

1) The bicycle is only a small percentage of the bike + rider system
weight. And weight at the perimeter of the wheel must be accelerated
to a maximum of twice the bike's linear speed-- mass inside the outer
diameter get accelerated less. And bicycles spend only a tiny
fraction of their time, and an even tinier fraction of the rider's
energy, accelerating.

So even of the small amount of energy that a cyclist uses to
accelerate, it's a miniscule portion that gets allocated to the
wheels. For instance, assume a 180lb rider on a 23lb bike with 5lb
wheels. Assume that the wheels require 1.6x the energy to accelerate
that an equal non-rotating mass would. That means that only 1.4% of
just the work required to accelerate is used to spin the wheels.
Considering that the work required to accelerate is no more than a
single-digit percentage of the rider's output even in stop-and-go city
riding, that isn't very much.

Then any weight you save in the wheels will be at most a very small
percentage of the previous wheel weight.

If you had, say, a 400g chunk of cheese in one hand, and a 400.04g
chunk of cheese in the other, could you tell which one was lighter?
(Answer: the one with the carbon fiber wrapper. :^D ) That's more
or less the proportional difference in output required between the
hypothetical bike I mentioned, and one with 10% lighter wheels.

2) High center of mass helps handling in a bike. The reason is that
the bike must be moved between the center of mass and the center of
mass's impact point on the ground in order for the system to remain
balanced. For bikes of equal overall weight, the one that carries its
mass closer to the rider (that is, higher) will handle better.

Low C of G is car-think. Single-track vehicles (bikes) benefit from
having the mass centralized for better rotation about the longitudinal
axis.

Chalo Colina

Chalo said:

Low C of G is car-think. Single-track vehicles (bikes) benefit from
having the mass centralized for better rotation about the longitudinal
axis.

I've seen a theory that high-up weight is better on a bike because the
bike balances like an inverted pendulum, and extra weight increases the
period of oscillation. That may be true in theory, but anyone who's
ridden with an occupied child seat will disagree that high-up weight
alone is beneficial for handling!

Jim Blackburn suggested that the weight should lie around a diagonal
line roughly between the front hub and the top of the rear rack - so
front lo-riders are better than a front rack, but it's OK to have the
load carried higher at the rear (so you can use the top of the rack with
impunity).

"Zog The Undeniable" wrote: I've seen a theory that high-up weight is
better on a bike because the bike balances like an inverted pendulum, and
extra weight increases the period of oscillation. (clip)
^^^^^^^^^^^^^
Nope, not even in theory. An inverted pendulum does not have a period. It
falls over. For a pendulum to hav a period, gravity must act to OPPOSE the
swing of the pendulum. An inverted pendulum, or a bicycle, falls over
because gravity aids the swing.

Chalo said:

...
Low C of G is car-think. Single-track vehicles (bikes) benefit from
having the mass centralized for better rotation about the longitudinal
axis.

This rotation about the longitudinal axis will occur more rapidly with a
low CG. Whether or not this responsiveness is a benefit is subjective,
but after riding a bicycle with a very low CG, some will not want to go
back to higher bicycles. 🙂

--
Tom Sherman

Tom Sherman said:

Chalo said:

...
Low C of G is car-think. Single-track vehicles (bikes) benefit from
having the mass centralized for better rotation about the longitudinal
axis.

This rotation about the longitudinal axis will occur more rapidly with a
low CG. Whether or not this responsiveness is a benefit is subjective,
but after riding a bicycle with a very low CG, some will not want to go
back to higher bicycles. 🙂

That's not exactly true. The systems's mass remains centered in the
rider, and the bike in effect revolves around that point. So the
closer the bike's particular Cof G is to the rider's (assuming the
bikes height remains the same), the faster it will be able to transit
through a range of lean angle, because the parts furthest away from
the center of rotation will have less inertia.

I think you may be confusing this effect with the effect of higher vs
lower BB height, but that's not what I'm talking about. Making the
whole bike taller increases its rotational inertia and slows its
response to steering forces.

Chalo Colina

Chalo said:

Tom Sherman said:

Chalo said:

...
Low C of G is car-think. Single-track vehicles (bikes) benefit from
having the mass centralized for better rotation about the longitudinal
axis.

This rotation about the longitudinal axis will occur more rapidly with a
low CG. Whether or not this responsiveness is a benefit is subjective,
but after riding a bicycle with a very low CG, some will not want to go
back to higher bicycles. 🙂

That's not exactly true. The systems's mass remains centered in the
rider, and the bike in effect revolves around that point. So the
closer the bike's particular Cof G is to the rider's (assuming the
bikes height remains the same), the faster it will be able to transit
through a range of lean angle, because the parts furthest away from
the center of rotation will have less inertia.

I was under the impression that a single-track vehicle for the most part
rotates about the line connecting the tire contact patches. It is
possible to initiate a turn by leaning without the tire contact patches
moving very far in the opposite direction, so the combined center of
mass is moving through an arc. The closer the center of mass is to the
ground, the shorter the arc length will be for a given lean angle (which
determines the rate of directional change). Therefore, the bicycle with
a lower center of gravity should respond more quickly.

Quoted message said:

I think you may be confusing this effect with the effect of higher vs
lower BB height, but that's not what I'm talking about. Making the
whole bike taller increases its rotational inertia and slows its
response to steering forces.

For what it is worth, on my bike the BB (front bracket?) and combined CG
are both about 18 inches (~45 cm) off the ground. And yes, it changes
direction with very little input required.

--
Tom Sherman

Tom Sherman said:

Chalo said:

Tom Sherman said:

Chalo wrote:

> ...
> Low C of G is car-think. Single-track vehicles (bikes) benefit from
> having the mass centralized for better rotation about the longitudinal
> axis.

This rotation about the longitudinal axis will occur more rapidly
with a low CG. Whether or not this responsiveness is a benefit is
subjective, but after riding a bicycle with a very low CG, some will
not want to go back to higher bicycles. 🙂

That's not exactly true. The systems's mass remains centered in the
rider, and the bike in effect revolves around that point. So the
closer the bike's particular Cof G is to the rider's (assuming the
bikes height remains the same), the faster it will be able to transit
through a range of lean angle, because the parts furthest away from
the center of rotation will have less inertia.

I was under the impression that a single-track vehicle for the most part
rotates about the line connecting the tire contact patches. It is
possible to initiate a turn by leaning without the tire contact patches
moving very far in the opposite direction, so the combined center of
mass is moving through an arc. The closer the center of mass is to the
ground, the shorter the arc length will be for a given lean angle (which
determines the rate of directional change). Therefore, the bicycle with
a lower center of gravity should respond more quickly.

Quoted message said:

I think you may be confusing this effect with the effect of higher vs
lower BB height, but that's not what I'm talking about. Making the
whole bike taller increases its rotational inertia and slows its
response to steering forces.

For what it is worth, on my bike the BB (front bracket?) and combined CG
are both about 18 inches (~45 cm) off the ground. And yes, it changes
direction with very little input required.

A bike allowed to passively fall over with no correcting steering
input rotates about its contact patches. Its angular acceleration
of falling over is the sine of its current angle times its weight
times the height of the COG, divided by the moment of inertia of
the bike/rider combination around the contact patches. Simply moving
the rider up increases moment of inertia faster than it increases
height of the COG, so the bike falls over more slowly, allowing
more time for the rider to make corrections.

Steering moves the line connecting the contact patches under the
bicycle. For small movements when the bike is upright, the
instantaneous center of rotation is the bike's center of percussion,
or center of oscillation, which will be higher than its center of
gravity. The less the bike's moment of inertia about its center
of percussion, the less sideways force needed at the contact patches
to control the bike. To some extent the contact patch forces also
depend on the bike's moment of inertia about its yaw axis, since
the bike also accelerates around that axis with steering.

For what (little) it's worth, a fully-recumbent very-high-racer
style bent and a fully-upright bike with the same masses and heights
to the center of gravity would behave differently. The bent would
have lower moment of inertia around its contact patches, and would
fall over faster, but would also have a lower center of percussion
and lower moment of inertia about that roll axis, tending to make
control forces lower.

Dave Lehnen

Dave Lehnen said:

Tom Sherman said:

Chalo said:

Tom Sherman <[email hidden]> wrote:

> Chalo wrote:
>
>> ...
>> Low C of G is car-think. Single-track vehicles (bikes) benefit from
>> having the mass centralized for better rotation about the longitudinal
>> axis.
>
>
>
> This rotation about the longitudinal axis will occur more rapidly
> with a low CG. Whether or not this responsiveness is a benefit is
> subjective, but after riding a bicycle with a very low CG, some will
> not want to go back to higher bicycles. 🙂

That's not exactly true. The systems's mass remains centered in the
rider, and the bike in effect revolves around that point. So the
closer the bike's particular Cof G is to the rider's (assuming the
bikes height remains the same), the faster it will be able to transit
through a range of lean angle, because the parts furthest away from
the center of rotation will have less inertia.

I was under the impression that a single-track vehicle for the most
part rotates about the line connecting the tire contact patches. It is
possible to initiate a turn by leaning without the tire contact
patches moving very far in the opposite direction, so the combined
center of mass is moving through an arc. The closer the center of mass
is to the ground, the shorter the arc length will be for a given lean
angle (which determines the rate of directional change). Therefore,
the bicycle with a lower center of gravity should respond more quickly.

Quoted message said:

I think you may be confusing this effect with the effect of higher vs
lower BB height, but that's not what I'm talking about. Making the
whole bike taller increases its rotational inertia and slows its
response to steering forces.

For what it is worth, on my bike the BB (front bracket?) and combined
CG are both about 18 inches (~45 cm) off the ground. And yes, it
changes direction with very little input required.

A bike allowed to passively fall over with no correcting steering
input rotates about its contact patches. Its angular acceleration
of falling over is the sine of its current angle times its weight
times the height of the COG, divided by the moment of inertia of
the bike/rider combination around the contact patches. Simply moving
the rider up increases moment of inertia faster than it increases
height of the COG, so the bike falls over more slowly, allowing
more time for the rider to make corrections.

Steering moves the line connecting the contact patches under the
bicycle. For small movements when the bike is upright, the
instantaneous center of rotation is the bike's center of percussion,
or center of oscillation, which will be higher than its center of
gravity. The less the bike's moment of inertia about its center
of percussion, the less sideways force needed at the contact patches
to control the bike. To some extent the contact patch forces also
depend on the bike's moment of inertia about its yaw axis, since
the bike also accelerates around that axis with steering.

This is true for steering used to keep the bike upright, but I was
referring to turns initiated by weight shift.

Quoted message said:

For what (little) it's worth, a fully-recumbent very-high-racer
style bent and a fully-upright bike with the same masses and heights
to the center of gravity would behave differently. The bent would
have lower moment of inertia around its contact patches, and would
fall over faster, but would also have a lower center of percussion
and lower moment of inertia about that roll axis, tending to make
control forces lower.

My experience indicated that this is true - recumbents with normal
geometry have light control forces and a high "roll rate".

--
Tom Sherman

Chalo said...

Quoted message said:

1) The bicycle is only a small percentage of the bike + rider system
weight. And weight at the perimeter of the wheel must be accelerated
to a maximum of twice the bike's linear speed-- mass inside the outer
diameter get accelerated less. And bicycles spend only a tiny
fraction of their time, and an even tinier fraction of the rider's
energy, accelerating.

I don't know of a nice way to put it, but that is all wrong. The rider
is constantly accelerating even if he is maintaining the exact same
speed on level ground. Why? Because there are forces acting to
decelerate him at all times (friction, wind resistance), therefore he
must accelerate in the opposite direction to maintain speed. We all know
that we must always apply force to maintain speed on level ground. Force
= mass * acceleration. Acceleration = force/mass. As the bike gains
speed, the primary force opposing it, wind resistance, increases
exponentially, therefore acceleration must increase exponentially to
maintain or increase speed.

I'll snip the rest of your arguments and just say that I never claimed
that shaving a few grams from tires and rims would work miracles. Just
that it is the best place to start for pure performance, something which
even you agreed with, even though you tried to make it look like you
didn't.

Quoted message said:

2) High center of mass helps handling in a bike. The reason is that
the bike must be moved between the center of mass and the center of
mass's impact point on the ground in order for the system to remain
balanced. For bikes of equal overall weight, the one that carries its
mass closer to the rider (that is, higher) will handle better.

Low C of G is car-think. Single-track vehicles (bikes) benefit from
having the mass centralized for better rotation about the longitudinal
axis.

Chalo Colina

I see there is a good discussion of it further down the thread. I'll let
them hash it out.

Tom Sherman <[email hidden]> wrote

Quoted message said:

Dave Lehnen said:

Steering moves the line connecting the contact patches under the
bicycle. For small movements when the bike is upright, the
instantaneous center of rotation is the bike's center of percussion,
or center of oscillation, which will be higher than its center of
gravity.

This is true for steering used to keep the bike upright, but I was
referring to turns initiated by weight shift.

Even then it is the bike that moves with respect to the rider, and not
the other way around.

In order to turn, you must first "countersteer" the wheels out from
underneath you to initiate a lean angle. That is, to turn left, you
first move the wheels out from directly underneath you to the right,
which causes you to "fall over" towards the left until the desired
lean angle is established. At that point you stop steering the wheels
out to the right of your body, and the forces balance as you describe
an arc-shaped path. To stop turning, you steer the wheels back
underneath you.

Chalo Colina

SuperSlinky said:

Chalo said...

Quoted message said:

1) The bicycle is only a small percentage of the bike + rider system
weight. And weight at the perimeter of the wheel must be accelerated
to a maximum of twice the bike's linear speed-- mass inside the outer
diameter get accelerated less. And bicycles spend only a tiny
fraction of their time, and an even tinier fraction of the rider's
energy, accelerating.

I don't know of a nice way to put it, but that is all wrong. The rider
is constantly accelerating even if he is maintaining the exact same
speed on level ground. Why? Because there are forces acting to
decelerate him at all times (friction, wind resistance), therefore he
must accelerate in the opposite direction to maintain speed. We all know
that we must always apply force to maintain speed on level ground. Force
= mass * acceleration. Acceleration = force/mass. As the bike gains
speed, the primary force opposing it, wind resistance, increases
exponentially, therefore acceleration must increase exponentially to
maintain or increase speed.

I'll snip the rest of your arguments and just say that I never claimed
that shaving a few grams from tires and rims would work miracles. Just
that it is the best place to start for pure performance, something which
even you agreed with, even though you tried to make it look like you
didn't.

Quoted message said:

2) High center of mass helps handling in a bike. The reason is that
the bike must be moved between the center of mass and the center of
mass's impact point on the ground in order for the system to remain
balanced. For bikes of equal overall weight, the one that carries its
mass closer to the rider (that is, higher) will handle better.

Low C of G is car-think. Single-track vehicles (bikes) benefit from
having the mass centralized for better rotation about the longitudinal
axis.

Chalo Colina

I see there is a good discussion of it further down the thread. I'll let
them hash it out.

Dear Slinky,

Yes, A = F/M.

But acceleration is the rate of change of velocity, which
has two components--direction and speed.

But A goes to 0 when the forces (all of them) balance.

That is, 0 feet per second per second = 0 / 100 kg

When the bicycle reaches a speed of 20 mph and stays at that
speed in a straight line, its velocity is steady and the
forces acting on it (pedal forward, wind drag backward) are
balanced and there is no rate of change of velocity (which
is what acceleration is).

If the bicycle at a steady speed of 20 mph turns, then there
is an acceleration--the velocity is 20 mph north and changes
to 20 mph west. If you magnify this to the earth orbiting
the sun, we are accelerating toward the sun at just the
right rate of change to velocity to maintain a slightly oval
orbit. A speedometer would show the same speed of one solar
year, varying slightly with the ellipse (faster in the
middle)--only the direction component of the velocity
changes as we turn around the circle.

If the bicycle slows from 20 mph down to 10 mph in a
straight line, there is a negative change of velocity, which
we call a deceleration (but it's all acceleration really,
postive, negative, or direction).

If the bicycle speeds up from 15 to 16 mph in a blazing hill
climb, that's also an acceleration. The direction component
of the velocity stays the same (slightly uphill east), but
the speed has increased. If it takes an hour for the speed
to rise from 15 to 16 mph, then the acceleration is a rate
of increasing one mile per hour--per hour.

Only an unbalanced force can accelerate a mass. When the
drag and pedal effort match, the bicycle is just as balanced
in terms of acceleration as it is when locked to a
fence--the rate of change of velocity at a steady 20 mph is
0, just as it is 0 when the ground resists gravity and the
bicycle fails to accelerate toward the center of the earth.

To turn a bicycle, you must change the directional component
of its velocity, so that requires a force--you scrub off
some speed when you force a bicycle to turn left or right.

Again, acceleration is the rate of change of velocity and
can easily be zero when the various forces acting on a mass
are balanced.

Carl Fogel

Chalo said:

Tom Sherman <[email hidden]> wrote

Quoted message said:

Dave Lehnen said:

Steering moves the line connecting the contact patches under the
bicycle. For small movements when the bike is upright, the
instantaneous center of rotation is the bike's center of percussion,
or center of oscillation, which will be higher than its center of
gravity.

This is true for steering used to keep the bike upright, but I was
referring to turns initiated by weight shift.

Even then it is the bike that moves with respect to the rider, and not
the other way around.

In order to turn, you must first "countersteer" the wheels out from
underneath you to initiate a lean angle. That is, to turn left, you
first move the wheels out from directly underneath you to the right,
which causes you to "fall over" towards the left until the desired
lean angle is established. At that point you stop steering the wheels
out to the right of your body, and the forces balance as you describe
an arc-shaped path. To stop turning, you steer the wheels back
underneath you.

The above describes one way of turning a bicycle.

Take a rider riding a straight line on a level surface. If the rider
slumps over to one side without changing steering angle, which moves
laterally first - the rider's CG or the contact line of the tires?

--
Tom Sherman

SuperSlinky said:

Chalo said...

Quoted message said:

1) The bicycle is only a small percentage of the bike + rider system
weight. And weight at the perimeter of the wheel must be accelerated
to a maximum of twice the bike's linear speed-- mass inside the outer
diameter get accelerated less. And bicycles spend only a tiny
fraction of their time, and an even tinier fraction of the rider's
energy, accelerating.

I don't know of a nice way to put it, but that is all wrong.

It looked pretty good to me.

Quoted message said:

The rider
is constantly accelerating even if he is maintaining the exact same
speed on level ground.

No, acceleration is defined as the rate of change of velocity over time.
So if the velocity is constant (same speed and direction) then the
acceleration must be zero.

Why? Because there are forces acting to

Quoted message said:

decelerate him at all times (friction, wind resistance), therefore he
must accelerate in the opposite direction to maintain speed.

No, if there are forces acting to decelerate him, then to maintain a
constant velocity there must be equal and opposite forces so that the
net force is zero. The force of gravity is pulling him down, but the
road is pushing up with an equal and opposite force. In the horizontal
plane wind and rolling resistance results in a force pushing back on the
rider but his pedaling acting through the tires creates an equal and
opposite force pushing him forward. When all the forces balance then
the acceleration is zero.

Quoted message said:

We all know
that we must always apply force to maintain speed on level ground. Force
= mass * acceleration. Acceleration = force/mass. As the bike gains
speed, the primary force opposing it, wind resistance, increases
exponentially, therefore acceleration must increase exponentially to
maintain or increase speed.

Wind resistance force increases with the square of the speed which is a
quadratic increase, not an exponential one. Newton's F = ma equation
applies to the net force on an object so in the case of a rider you need
to sum the forces of wind resistance, gravity, friction, pedaling, and
the upward force of the road surface. Only if the sum of the forces
isn't zero will there be an acceleration.

Quoted message said:

I'll snip the rest of your arguments and just say that I never claimed
that shaving a few grams from tires and rims would work miracles. Just
that it is the best place to start for pure performance, something which
even you agreed with, even though you tried to make it look like you
didn't.

But you implied that weight savings there would be substantially more
important than elsewhere. Since I'm not interested in participating in
short-distance drag races where acceleration is the key I would rather
save 105 grams from the frame or other parts of the bike than 100 grams
from the tires/rims if both were equally convenient to achieve.

Quoted message said:

Quoted message said:

2) High center of mass helps handling in a bike. The reason is that
the bike must be moved between the center of mass and the center of
mass's impact point on the ground in order for the system to remain
balanced. For bikes of equal overall weight, the one that carries its
mass closer to the rider (that is, higher) will handle better.

Low C of G is car-think. Single-track vehicles (bikes) benefit from
having the mass centralized for better rotation about the longitudinal
axis.

Chalo Colina

I see there is a good discussion of it further down the thread. I'll let
them hash it out.

Tom Sherman said:

Chalo said:

Tom Sherman <[email hidden]> wrote

Quoted message said:

Dave Lehnen wrote:

>Steering moves the line connecting the contact patches under the
>bicycle. For small movements when the bike is upright, the
>instantaneous center of rotation is the bike's center of percussion,
>or center of oscillation, which will be higher than its center of
> gravity.

This is true for steering used to keep the bike upright, but I was
referring to turns initiated by weight shift.

Even then it is the bike that moves with respect to the rider, and
not
the other way around.
In order to turn, you must first "countersteer" the wheels out from
underneath you to initiate a lean angle. That is, to turn left, you
first move the wheels out from directly underneath you to the right,
which causes you to "fall over" towards the left until the desired
lean angle is established. At that point you stop steering the wheels
out to the right of your body, and the forces balance as you describe
an arc-shaped path. To stop turning, you steer the wheels back
underneath you.

The above describes one way of turning a bicycle.

Take a rider riding a straight line on a level surface. If the rider
slumps over to one side without changing steering angle, which moves
laterally first - the rider's CG or the contact line of the tires?

An interesting experiment:

http://www.superbikeschool.com/us/machinery/no_bs_machine.shtml

It might be fun to build a bicycle like this.

Jim Smith said:

Tom Sherman said:

Chalo said:

Tom Sherman <[email hidden]> wrote

>Dave Lehnen wrote:
>
>
>>Steering moves the line connecting the contact patches under the
>>bicycle. For small movements when the bike is upright, the
>>instantaneous center of rotation is the bike's center of percussion,
>>or center of oscillation, which will be higher than its center of
>>gravity.
>
>This is true for steering used to keep the bike upright, but I was
>referring to turns initiated by weight shift.

Even then it is the bike that moves with respect to the rider, and
not
the other way around.
In order to turn, you must first "countersteer" the wheels out from
underneath you to initiate a lean angle. That is, to turn left, you
first move the wheels out from directly underneath you to the right,
which causes you to "fall over" towards the left until the desired
lean angle is established. At that point you stop steering the wheels
out to the right of your body, and the forces balance as you describe
an arc-shaped path. To stop turning, you steer the wheels back
underneath you.

The above describes one way of turning a bicycle.

Take a rider riding a straight line on a level surface. If the rider
slumps over to one side without changing steering angle, which moves
laterally first - the rider's CG or the contact line of the tires?

An interesting experiment:

http://www.superbikeschool.com/us/machinery/no_bs_machine.shtml

It might be fun to build a bicycle like this.

The above experiment proves that it is possible on the motorcycle for
the CG to change before the line of contact moves. However this weight
shift does not change direction of the motorcycle significantly.

I would be hesitant to extrapolate this behavior to a bicycle for two
reasons. One is that instead of 1:2 to 1:5 rider to bike mass ratios
typical of street motorcycles, the range for a rider and bicycle will
typically be 3:1 to 15:1. Secondly, there is a significant gyroscopic
effect from the motorcycle wheels, while this effect is negligible with
much lighter bicycle wheels.

It would seem to be possible, depending on inputs that a bicycle and
rider system could rotate about the combined CG, the contact line with
the road, or some point in between.

--
Tom Sherman

[email hidden] said...

Quoted message said:

Again, acceleration is the rate of change of velocity and
can easily be zero when the various forces acting on a mass
are balanced.

Carl Fogel

OK, maybe I was shooting from the hip for a minute there. You are
correct in spirit, but in the real world neither pedalling forces nor
counter forces are constant. Constant speeds, much less velocities, are
impossible in the real world, therefore acceleration is always playing a
role. Even under the best of conditions, a graph of speed (or velocity)
over time would look like a sine wave of peaks and valleys. The best you
could say is that the greater inertia of a heavy wheel decelerates less
under a given counter force, and therefore must accelerate less to
regain its speed. Whether or not the equivalence is always true in real
world conditions is probably unprovable. For example, we already know of
one unfortunate performance consequence of heavy wheels that isn't
directly related to simple kinetic equations. Ironically, it is the
original topic of unsprung weight. True, applying this to road bicycles
probably isn't very productive, but then from the very first we have
been talking about small gains. Is real world rolling resistance
constant over different speeds? Almost certainly not. But it may be that
in some cases the inertia of a heavy wheel is a performance advantage.

What we do know for certain is that whenever the brakes are applied, we
don't get to cancel out the mass from both sides of the equation. In
fact, you have to pay twice for your heavy wheels, once accelerating and
again decelerating. Your braking takes longer, generates more heat, and
wears pads and rims faster. Of course, the original point was that the
grams that count most are the ones on your tires, tubes and rims. That's
just a simple fact that everyone who is interested in the subject should
know.

SuperSlinky said:

[email hidden] said...

Quoted message said:

Again, acceleration is the rate of change of velocity and
can easily be zero when the various forces acting on a mass
are balanced.

Carl Fogel

OK, maybe I was shooting from the hip for a minute there. You are
correct in spirit, but in the real world neither pedalling forces nor
counter forces are constant. Constant speeds, much less velocities, are
impossible in the real world, therefore acceleration is always playing a
role. Even under the best of conditions, a graph of speed (or velocity)
over time would look like a sine wave of peaks and valleys. The best you
could say is that the greater inertia of a heavy wheel decelerates less
under a given counter force, and therefore must accelerate less to
regain its speed. Whether or not the equivalence is always true in real
world conditions is probably unprovable. For example, we already know of
one unfortunate performance consequence of heavy wheels that isn't
directly related to simple kinetic equations. Ironically, it is the
original topic of unsprung weight. True, applying this to road bicycles
probably isn't very productive, but then from the very first we have
been talking about small gains. Is real world rolling resistance
constant over different speeds? Almost certainly not. But it may be that
in some cases the inertia of a heavy wheel is a performance advantage.

What we do know for certain is that whenever the brakes are applied, we
don't get to cancel out the mass from both sides of the equation. In
fact, you have to pay twice for your heavy wheels, once accelerating and
again decelerating. Your braking takes longer, generates more heat, and
wears pads and rims faster. Of course, the original point was that the
grams that count most are the ones on your tires, tubes and rims. That's
just a simple fact that everyone who is interested in the subject should
know.

Dear Super,

In the real world, a coasting bicycle with no pedaling
force, steers in the same fashion on the level as one being
pedalled, even though the coasting bicycle is faintly
decelerating, while the other bicycle is maintaining a
steady speed.

Such minor accelerations of speed change have no discernable
role in steering.

The role of rotating mass is directly related to fairly
simple equations--it's a matter of linear (straight-line)
versus angular (rotary) acceleration.

Carl Fogel

SuperSlinky said:

I don't know of a nice way to put it, but that is all wrong. The rider
is constantly accelerating even if he is maintaining the exact same
speed on level ground.

Acceleration is defined as the the time rate of change of velocity. If
velocity is constant, acceleration is zero.

Quoted message said:

We all know
that we must always apply force to maintain speed on level ground. Force
= mass * acceleration. Acceleration = force/mass.

I'm exerting a force on my coffee mug as I hold it in my hand, but it's not
accelerating. The force I'm applying is resisting the force of gravity. The
force you apply to the pedals to overcome wind resistance is similar.

Art Harris

[email hidden] said...

Quoted message said:

Dear Super,

In the real world, a coasting bicycle with no pedaling
force, steers in the same fashion on the level as one being
pedalled, even though the coasting bicycle is faintly
decelerating, while the other bicycle is maintaining a
steady speed.

Such minor accelerations of speed change have no discernable
role in steering.

The role of rotating mass is directly related to fairly
simple equations--it's a matter of linear (straight-line)
versus angular (rotary) acceleration.

Carl Fogel

Actually, my point was that pedalling forces aren't constant, therefore
speeds can't be constant. Biopace chainrings, whether successful or not,
was an attempt to take advantage of the fact that pedalling forces are
not constant. There is always a small amount of acceleration and
deceleration even under the best of conditions. Yes, steering results in
a velocity change, but you brought that up, not me, and it wasn't my
intention to complicate things with that. I passed on the COG argument
if you remember correctly.

Simplified physics equations often don't cut it in real world
applications and are primarily useful for educational purposes
illustrating basic concepts. In the real world, experimentation is
usually the only way to see how a small change affects a complicated
system. I was foolish for bringing them up in the first place and even
more foolish for not thinking through how I used them.