Why do some forks and frames have brake rotor size limits?

John B. wrote:
On Sat, 28 Oct 2017 01:13:54 +0000 (UTC), Ralph Barone
wrote:

John B. wrote:
On Fri, 27 Oct 2017 09:32:04 -0700, Joerg
wrote:

On 2017-10-27 09:25, Frank Krygowski wrote:
On 10/27/2017 9:58 AM, Joerg wrote:

Finally after many decades the bicycle industry woke up and adopted
what the automotive guys had all along, disc brakes. Why should I
accept an inferior brake system on a new bike when there is a much
better one?

sigh There are advantages and disadvantages to this equipment choice,
just as with other equipment choices. The disadvantages of discs have
been discussed. If they don't matter or apply to you, fine; but they
matter to others.


Many others just don't know any better. I have witnessed several people
riding a bike with hydraulic disc brakes for the first time and the
reaction was usually "WHOA!". Same with me, it almost sent me over the bar.


But I'll note that you're currently in a project to increase your disc's
diameter from something like 160mm or 180mm up to 200mm or more. You
seem to feel bigger diameter is better.


Because bigger is better here.


Well, even "better," why not go up to roughly 622mm? That's what lots
of us prefer, with cable actuation.


The disadvantages have been discussed ad nauseam. A rim brake is not a
disc brake. Not even close.

Care to explain the mechanical difference? I mean a rotating surface
and two friction pads that are tightened against it....
--
Cheers,

John B.

As far as I can tell, the differences between a rim brake and a 622 mm disk
a

1) The disk doesn't have to provide tire clearance, so the pads can sit
closer, facilitating higher mechanical advantage.

I'm not sure that is correct. After all some old Greek guy was
supposed to have said, "Give me a lever and a place to stand and I
will move the earth". Nothing about being close.

No. I'm pretty certain I'm right here. Let's say that you can pull 100 lbs
on your brake lever and the lever has 2" of play before it hits your bars.
You can fiddle with leverage many places in the system, but the product of
that initial 100 lbs and 2" will be constant in the system. If the final
travel of the brake pads is 1/2", then you can apply 400 lbs force to the
pads. If you tighten up your tolerances such that the pads only have to
move 1/16", then you can increase the leverage to the point where you can
apply 3200 lbs force to the pads. In disk brake systems this reduction in
pad-disk distance allow the MA to be increased to compensate for the
decreased leverage of the disk on the wheel. The increases brake pad
pressure at a given bike deceleration is what gives disk brakes misread
consistent performance in the wet.

2) The disk is not connected to the rim, so it doesn't bend and warp when
you hit a pothole. Again, the pads can sit closer and have higher MA.

3) The disk doesn't thermally couple to the tire, so no blowouts on long
descents.

4) The disk is solid, so there's less flexing when you squeeze it with the
pads.
--
Cheers,

John B.

On Friday, October 27, 2017 at 11:54:45 PM UTC-4, Ralph Barone wrote:
John B. wrote:
On Sat, 28 Oct 2017 01:13:54 +0000 (UTC), Ralph Barone
wrote:

John B. wrote:
On Fri, 27 Oct 2017 09:32:04 -0700, Joerg
wrote:

On 2017-10-27 09:25, Frank Krygowski wrote:
On 10/27/2017 9:58 AM, Joerg wrote:

Finally after many decades the bicycle industry woke up and adopted
what the automotive guys had all along, disc brakes. Why should I
accept an inferior brake system on a new bike when there is a much
better one?

sigh There are advantages and disadvantages to this equipment choice,
just as with other equipment choices. The disadvantages of discs have
been discussed. If they don't matter or apply to you, fine; but they
matter to others.


Many others just don't know any better. I have witnessed several people
riding a bike with hydraulic disc brakes for the first time and the
reaction was usually "WHOA!". Same with me, it almost sent me over the bar.


But I'll note that you're currently in a project to increase your disc's
diameter from something like 160mm or 180mm up to 200mm or more. You
seem to feel bigger diameter is better.


Because bigger is better here.


Well, even "better," why not go up to roughly 622mm? That's what lots
of us prefer, with cable actuation.


The disadvantages have been discussed ad nauseam. A rim brake is not a
disc brake. Not even close.

Care to explain the mechanical difference? I mean a rotating surface
and two friction pads that are tightened against it....
--
Cheers,

John B.

As far as I can tell, the differences between a rim brake and a 622 mm disk
a

1) The disk doesn't have to provide tire clearance, so the pads can sit
closer, facilitating higher mechanical advantage.

I'm not sure that is correct. After all some old Greek guy was
supposed to have said, "Give me a lever and a place to stand and I
will move the earth". Nothing about being close.

No. I'm pretty certain I'm right here. Let's say that you can pull 100 lbs
on your brake lever and the lever has 2" of play before it hits your bars.
You can fiddle with leverage many places in the system, but the product of
that initial 100 lbs and 2" will be constant in the system. If the final
travel of the brake pads is 1/2", then you can apply 400 lbs force to the
pads. If you tighten up your tolerances such that the pads only have to
move 1/16", then you can increase the leverage to the point where you can
apply 3200 lbs force to the pads. In disk brake systems this reduction in
pad-disk distance allow the MA to be increased to compensate for the
decreased leverage of the disk on the wheel. The increases brake pad
pressure at a given bike deceleration is what gives disk brakes misread
consistent performance in the wet.

2) The disk is not connected to the rim, so it doesn't bend and warp when
you hit a pothole. Again, the pads can sit closer and have higher MA.

3) The disk doesn't thermally couple to the tire, so no blowouts on long
descents.

4) The disk is solid, so there's less flexing when you squeeze it with the
pads.

I won't quarrel with Ralph's list of disc advantages. My main points are these:
One can also come up with a list of advantages for caliper brakes; and most
cyclists have no real need of most of the disc advantages. For them, as
throughout bicycling's history, caliper brakes are fine.

That last point is often lost on disc promoters - as in "If they are better,
why not use them?" all the way up to "I'd never buy another bike without disc
brakes."

Just for fun, here's a report on disc brakes compared to other types in an
extreme situation: mountain descents on tandems.

https://tandemgeek.wordpress.com/201...if-you-prefer/

- Frank Krygowski

On Sat, 28 Oct 2017 07:11:24 +0700, John B.
wrote:

One supposes that will be next big improvement in bicycle brakes. Or
perhaps a drag chute for those long downhill's to keep the rims from
melting?

The biycle drag chute has already been done:
https://www.youtube.com/watch?v=ztLtiyC6qMQ
https://www.youtube.com/watch?v=5Vy5Xm-Y9Ic
https://www.youtube.com/watch?v=oq2jrkGK3Wc
--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558

On Sat, 28 Oct 2017 07:15:39 +0700, John B.
wrote:

Of course not. You can ride a filthy nasty bicycle all covered over
with mud, blood and crud. In retrospect, perhaps an anti theft scheme.
After all, who would stoop to stealing such an ugly filthy thing?

In the People's Republic of Santa Cruz (CA), the bicycle thieves will
steal anything the moves. As for washing your bicycle, this video
illustrates some of the standard techniques:
"Huge Bike Jump into a Pond 35 feet in the air"
https://www.youtube.com/watch?v=B3GribQCg6c
Don't worry. It also works in winter:
"Colton rides bike into frozen lake"
https://www.youtube.com/watch?v=PEplTDlmMX0


--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558

On Sat, 28 Oct 2017 03:54:43 +0000 (UTC), Ralph Barone
wrote:

John B. wrote:
On Sat, 28 Oct 2017 01:13:54 +0000 (UTC), Ralph Barone
wrote:

John B. wrote:
On Fri, 27 Oct 2017 09:32:04 -0700, Joerg
wrote:

On 2017-10-27 09:25, Frank Krygowski wrote:
On 10/27/2017 9:58 AM, Joerg wrote:

Finally after many decades the bicycle industry woke up and adopted
what the automotive guys had all along, disc brakes. Why should I
accept an inferior brake system on a new bike when there is a much
better one?

sigh There are advantages and disadvantages to this equipment choice,
just as with other equipment choices. The disadvantages of discs have
been discussed. If they don't matter or apply to you, fine; but they
matter to others.


Many others just don't know any better. I have witnessed several people
riding a bike with hydraulic disc brakes for the first time and the
reaction was usually "WHOA!". Same with me, it almost sent me over the bar.


But I'll note that you're currently in a project to increase your disc's
diameter from something like 160mm or 180mm up to 200mm or more. You
seem to feel bigger diameter is better.


Because bigger is better here.


Well, even "better," why not go up to roughly 622mm? That's what lots
of us prefer, with cable actuation.


The disadvantages have been discussed ad nauseam. A rim brake is not a
disc brake. Not even close.

Care to explain the mechanical difference? I mean a rotating surface
and two friction pads that are tightened against it....
--
Cheers,

John B.

As far as I can tell, the differences between a rim brake and a 622 mm disk
a

1) The disk doesn't have to provide tire clearance, so the pads can sit
closer, facilitating higher mechanical advantage.

I'm not sure that is correct. After all some old Greek guy was
supposed to have said, "Give me a lever and a place to stand and I
will move the earth". Nothing about being close.

No. I'm pretty certain I'm right here. Let's say that you can pull 100 lbs
on your brake lever and the lever has 2" of play before it hits your bars.
You can fiddle with leverage many places in the system, but the product of
that initial 100 lbs and 2" will be constant in the system. If the final
travel of the brake pads is 1/2", then you can apply 400 lbs force to the
pads. If you tighten up your tolerances such that the pads only have to
move 1/16", then you can increase the leverage to the point where you can
apply 3200 lbs force to the pads. In disk brake systems this reduction in
pad-disk distance allow the MA to be increased to compensate for the
decreased leverage of the disk on the wheel. The increases brake pad
pressure at a given bike deceleration is what gives disk brakes misread
consistent performance in the wet.

Movement of the parts doesn't make any difference the efficiency is
the pressure applied to the brake lever versus the pressure applied to
the braking device, usually the pads themselves.

A lever that is 1 foot long and moves, lets say, one quarter of the
diameter of a 2 foot circle applies the same force to a load located 1
foot from the fulcrum as a 100 ft lever which moves 1/4 of the
diameter of a 200 ft circle applies to a load that is 100 ft. from the
fulcrum. The first lever moves 19 inches and the second moves 157
feet.





2) The disk is not connected to the rim, so it doesn't bend and warp when
you hit a pothole. Again, the pads can sit closer and have higher MA.

3) The disk doesn't thermally couple to the tire, so no blowouts on long
descents.

4) The disk is solid, so there's less flexing when you squeeze it with the
pads.
--
Cheers,

John B.
--
Cheers,

John B.

Frank Krygowski wrote:
On Friday, October 27, 2017 at 11:54:45 PM UTC-4, Ralph Barone wrote:
John B. wrote:
On Sat, 28 Oct 2017 01:13:54 +0000 (UTC), Ralph Barone
wrote:

John B. wrote:
On Fri, 27 Oct 2017 09:32:04 -0700, Joerg
wrote:

On 2017-10-27 09:25, Frank Krygowski wrote:
On 10/27/2017 9:58 AM, Joerg wrote:

Finally after many decades the bicycle industry woke up and adopted
what the automotive guys had all along, disc brakes. Why should I
accept an inferior brake system on a new bike when there is a much
better one?

sigh There are advantages and disadvantages to this equipment choice,
just as with other equipment choices. The disadvantages of discs have
been discussed. If they don't matter or apply to you, fine; but they
matter to others.


Many others just don't know any better. I have witnessed several people
riding a bike with hydraulic disc brakes for the first time and the
reaction was usually "WHOA!". Same with me, it almost sent me over the bar.


But I'll note that you're currently in a project to increase your disc's
diameter from something like 160mm or 180mm up to 200mm or more. You
seem to feel bigger diameter is better.


Because bigger is better here.


Well, even "better," why not go up to roughly 622mm? That's what lots
of us prefer, with cable actuation.


The disadvantages have been discussed ad nauseam. A rim brake is not a
disc brake. Not even close.

Care to explain the mechanical difference? I mean a rotating surface
and two friction pads that are tightened against it....
--
Cheers,

John B.

As far as I can tell, the differences between a rim brake and a 622 mm disk
a

1) The disk doesn't have to provide tire clearance, so the pads can sit
closer, facilitating higher mechanical advantage.

I'm not sure that is correct. After all some old Greek guy was
supposed to have said, "Give me a lever and a place to stand and I
will move the earth". Nothing about being close.

No. I'm pretty certain I'm right here. Let's say that you can pull 100 lbs
on your brake lever and the lever has 2" of play before it hits your bars.
You can fiddle with leverage many places in the system, but the product of
that initial 100 lbs and 2" will be constant in the system. If the final
travel of the brake pads is 1/2", then you can apply 400 lbs force to the
pads. If you tighten up your tolerances such that the pads only have to
move 1/16", then you can increase the leverage to the point where you can
apply 3200 lbs force to the pads. In disk brake systems this reduction in
pad-disk distance allow the MA to be increased to compensate for the
decreased leverage of the disk on the wheel. The increases brake pad
pressure at a given bike deceleration is what gives disk brakes misread
consistent performance in the wet.

2) The disk is not connected to the rim, so it doesn't bend and warp when
you hit a pothole. Again, the pads can sit closer and have higher MA.

3) The disk doesn't thermally couple to the tire, so no blowouts on long
descents.

4) The disk is solid, so there's less flexing when you squeeze it with the
pads.

I won't quarrel with Ralph's list of disc advantages. My main points are these:
One can also come up with a list of advantages for caliper brakes; and most
cyclists have no real need of most of the disc advantages. For them, as
throughout bicycling's history, caliper brakes are fine.

That last point is often lost on disc promoters - as in "If they are better,
why not use them?" all the way up to "I'd never buy another bike without disc
brakes."

Just for fun, here's a report on disc brakes compared to other types in an
extreme situation: mountain descents on tandems.

https://tandemgeek.wordpress.com/201...if-you-prefer/

- Frank Krygowski

To be frank Frank, I wasn't trying to make the case for disks being better
than rim brakes, only trying to answer the question "what would the
difference be between a rim brake and a 622 mm disk brake?"

However, if you'd like me to give equal time to the rim brake camp...

1) Disk brakes require an additional component (the disk), while the rim
has to be there anyway.

2) Tighter clearances on disk brakes can mean persnickety adjustments and
weird noises when things aren't perfectly aligned.

3) Disk brakes (but not 622 mm disks) apply more torque to the fork and
they apply it asymmetrically, while rim brakes apply a lower torque, in a
balanced manner, to a stronger part of the fork.

3a) Rim brakes apply force to the rim at right angles to the dropouts, so
there is much less likelihood of wheel ejection.

4) Rim brakes have greater thermal mass (but a lower maximum temperature).

And I'm sure there are another handful of arguments on both sides.

John B. wrote:
On Sat, 28 Oct 2017 03:54:43 +0000 (UTC), Ralph Barone
wrote:

John B. wrote:
On Sat, 28 Oct 2017 01:13:54 +0000 (UTC), Ralph Barone
wrote:

John B. wrote:
On Fri, 27 Oct 2017 09:32:04 -0700, Joerg
wrote:

On 2017-10-27 09:25, Frank Krygowski wrote:
On 10/27/2017 9:58 AM, Joerg wrote:

Finally after many decades the bicycle industry woke up and adopted
what the automotive guys had all along, disc brakes. Why should I
accept an inferior brake system on a new bike when there is a much
better one?

sigh There are advantages and disadvantages to this equipment choice,
just as with other equipment choices. The disadvantages of discs have
been discussed. If they don't matter or apply to you, fine; but they
matter to others.


Many others just don't know any better. I have witnessed several people
riding a bike with hydraulic disc brakes for the first time and the
reaction was usually "WHOA!". Same with me, it almost sent me over the bar.


But I'll note that you're currently in a project to increase your disc's
diameter from something like 160mm or 180mm up to 200mm or more. You
seem to feel bigger diameter is better.


Because bigger is better here.


Well, even "better," why not go up to roughly 622mm? That's what lots
of us prefer, with cable actuation.


The disadvantages have been discussed ad nauseam. A rim brake is not a
disc brake. Not even close.

Care to explain the mechanical difference? I mean a rotating surface
and two friction pads that are tightened against it....
--
Cheers,

John B.

As far as I can tell, the differences between a rim brake and a 622 mm disk
a

1) The disk doesn't have to provide tire clearance, so the pads can sit
closer, facilitating higher mechanical advantage.

I'm not sure that is correct. After all some old Greek guy was
supposed to have said, "Give me a lever and a place to stand and I
will move the earth". Nothing about being close.

No. I'm pretty certain I'm right here. Let's say that you can pull 100 lbs
on your brake lever and the lever has 2" of play before it hits your bars.
You can fiddle with leverage many places in the system, but the product of
that initial 100 lbs and 2" will be constant in the system. If the final
travel of the brake pads is 1/2", then you can apply 400 lbs force to the
pads. If you tighten up your tolerances such that the pads only have to
move 1/16", then you can increase the leverage to the point where you can
apply 3200 lbs force to the pads. In disk brake systems this reduction in
pad-disk distance allow the MA to be increased to compensate for the
decreased leverage of the disk on the wheel. The increases brake pad
pressure at a given bike deceleration is what gives disk brakes more
consistent performance in the wet.

Movement of the parts doesn't make any difference the efficiency is
the pressure applied to the brake lever versus the pressure applied to
the braking device, usually the pads themselves.

A lever that is 1 foot long and moves, lets say, one quarter of the
diameter of a 2 foot circle applies the same force to a load located 1
foot from the fulcrum as a 100 ft lever which moves 1/4 of the
diameter of a 200 ft circle applies to a load that is 100 ft. from the
fulcrum. The first lever moves 19 inches and the second moves 157
feet.

Sure. But the distance you can move your brake lever is limited by the
length of your fingers, and so the distance you can move at the lever end
is essentially fixed. To increase the mechanical advantage in THAT system,
you have to reduce the distance the brake pads move. No ifs, ands, buts or
maybes.



2) The disk is not connected to the rim, so it doesn't bend and warp when
you hit a pothole. Again, the pads can sit closer and have higher MA.

3) The disk doesn't thermally couple to the tire, so no blowouts on long
descents.

4) The disk is solid, so there's less flexing when you squeeze it with the
pads.
--
Cheers,

John B.
--
Cheers,

John B.

On Fri, 27 Oct 2017 21:40:26 -0700 (PDT), Frank Krygowski
wrote:

On Friday, October 27, 2017 at 11:54:45 PM UTC-4, Ralph Barone wrote:
John B. wrote:
On Sat, 28 Oct 2017 01:13:54 +0000 (UTC), Ralph Barone
wrote:

John B. wrote:
On Fri, 27 Oct 2017 09:32:04 -0700, Joerg
wrote:

On 2017-10-27 09:25, Frank Krygowski wrote:
On 10/27/2017 9:58 AM, Joerg wrote:

Finally after many decades the bicycle industry woke up and adopted
what the automotive guys had all along, disc brakes. Why should I
accept an inferior brake system on a new bike when there is a much
better one?

sigh There are advantages and disadvantages to this equipment choice,
just as with other equipment choices. The disadvantages of discs have
been discussed. If they don't matter or apply to you, fine; but they
matter to others.


Many others just don't know any better. I have witnessed several people
riding a bike with hydraulic disc brakes for the first time and the
reaction was usually "WHOA!". Same with me, it almost sent me over the bar.


But I'll note that you're currently in a project to increase your disc's
diameter from something like 160mm or 180mm up to 200mm or more. You
seem to feel bigger diameter is better.


Because bigger is better here.


Well, even "better," why not go up to roughly 622mm? That's what lots
of us prefer, with cable actuation.


The disadvantages have been discussed ad nauseam. A rim brake is not a
disc brake. Not even close.

Care to explain the mechanical difference? I mean a rotating surface
and two friction pads that are tightened against it....
--
Cheers,

John B.

As far as I can tell, the differences between a rim brake and a 622 mm disk
a

1) The disk doesn't have to provide tire clearance, so the pads can sit
closer, facilitating higher mechanical advantage.

I'm not sure that is correct. After all some old Greek guy was
supposed to have said, "Give me a lever and a place to stand and I
will move the earth". Nothing about being close.

No. I'm pretty certain I'm right here. Let's say that you can pull 100 lbs
on your brake lever and the lever has 2" of play before it hits your bars.
You can fiddle with leverage many places in the system, but the product of
that initial 100 lbs and 2" will be constant in the system. If the final
travel of the brake pads is 1/2", then you can apply 400 lbs force to the
pads. If you tighten up your tolerances such that the pads only have to
move 1/16", then you can increase the leverage to the point where you can
apply 3200 lbs force to the pads. In disk brake systems this reduction in
pad-disk distance allow the MA to be increased to compensate for the
decreased leverage of the disk on the wheel. The increases brake pad
pressure at a given bike deceleration is what gives disk brakes misread
consistent performance in the wet.

2) The disk is not connected to the rim, so it doesn't bend and warp when
you hit a pothole. Again, the pads can sit closer and have higher MA.

3) The disk doesn't thermally couple to the tire, so no blowouts on long
descents.

4) The disk is solid, so there's less flexing when you squeeze it with the
pads.

I won't quarrel with Ralph's list of disc advantages. My main points are these:
One can also come up with a list of advantages for caliper brakes; and most
cyclists have no real need of most of the disc advantages. For them, as
throughout bicycling's history, caliper brakes are fine.

That last point is often lost on disc promoters - as in "If they are better,
why not use them?" all the way up to "I'd never buy another bike without disc
brakes."

Just for fun, here's a report on disc brakes compared to other types in an
extreme situation: mountain descents on tandems.

https://tandemgeek.wordpress.com/201...if-you-prefer/

- Frank Krygowski

I was down to our local Tesco-Lotus store today and you can get disc
brake on a mountain bike costing US$227. A set of Shimano Dura-ace
R9100 Brake Caliper Set, new, on e-bay is US$256.

It is obvious that disc's are cheap stuff, think of it, an entire
bicycle, with disc brakes, cheaper then a "proper" set of brakes.
Obviously, no one, in their bright polyester jersey, would be caught
dead using them.
--
Cheers,

John B.

On Fri, 27 Oct 2017 21:45:40 -0700, Jeff Liebermann
wrote:

On Sat, 28 Oct 2017 07:11:24 +0700, John B.
wrote:

One supposes that will be next big improvement in bicycle brakes. Or
perhaps a drag chute for those long downhill's to keep the rims from
melting?

The biycle drag chute has already been done:
https://www.youtube.com/watch?v=ztLtiyC6qMQ
https://www.youtube.com/watch?v=5Vy5Xm-Y9Ic
https://www.youtube.com/watch?v=oq2jrkGK3Wc

Actually, after giving it a bit of thought, I would think that
something like the so called "air brakes" used on some aircraft might
be more practical. they could be operated either by cable or
hydraulics and could be self retracting when not needed.
https://www.preciseflight.com/genera...p/speedbrakes/

Sitting in the mud repacking your drag chute every time you touched
the brake lever might be a bit off putting :-)
--
Cheers,

John B.

On Sat, 28 Oct 2017 06:23:59 +0000 (UTC), Ralph Barone
wrote:

John B. wrote:
On Sat, 28 Oct 2017 03:54:43 +0000 (UTC), Ralph Barone
wrote:

John B. wrote:
On Sat, 28 Oct 2017 01:13:54 +0000 (UTC), Ralph Barone
wrote:

John B. wrote:
On Fri, 27 Oct 2017 09:32:04 -0700, Joerg
wrote:

On 2017-10-27 09:25, Frank Krygowski wrote:
On 10/27/2017 9:58 AM, Joerg wrote:

Finally after many decades the bicycle industry woke up and adopted
what the automotive guys had all along, disc brakes. Why should I
accept an inferior brake system on a new bike when there is a much
better one?

sigh There are advantages and disadvantages to this equipment choice,
just as with other equipment choices. The disadvantages of discs have
been discussed. If they don't matter or apply to you, fine; but they
matter to others.


Many others just don't know any better. I have witnessed several people
riding a bike with hydraulic disc brakes for the first time and the
reaction was usually "WHOA!". Same with me, it almost sent me over the bar.


But I'll note that you're currently in a project to increase your disc's
diameter from something like 160mm or 180mm up to 200mm or more. You
seem to feel bigger diameter is better.


Because bigger is better here.


Well, even "better," why not go up to roughly 622mm? That's what lots
of us prefer, with cable actuation.


The disadvantages have been discussed ad nauseam. A rim brake is not a
disc brake. Not even close.

Care to explain the mechanical difference? I mean a rotating surface
and two friction pads that are tightened against it....
--
Cheers,

John B.

As far as I can tell, the differences between a rim brake and a 622 mm disk
a

1) The disk doesn't have to provide tire clearance, so the pads can sit
closer, facilitating higher mechanical advantage.

I'm not sure that is correct. After all some old Greek guy was
supposed to have said, "Give me a lever and a place to stand and I
will move the earth". Nothing about being close.

No. I'm pretty certain I'm right here. Let's say that you can pull 100 lbs
on your brake lever and the lever has 2" of play before it hits your bars.
You can fiddle with leverage many places in the system, but the product of
that initial 100 lbs and 2" will be constant in the system. If the final
travel of the brake pads is 1/2", then you can apply 400 lbs force to the
pads. If you tighten up your tolerances such that the pads only have to
move 1/16", then you can increase the leverage to the point where you can
apply 3200 lbs force to the pads. In disk brake systems this reduction in
pad-disk distance allow the MA to be increased to compensate for the
decreased leverage of the disk on the wheel. The increases brake pad
pressure at a given bike deceleration is what gives disk brakes more
consistent performance in the wet.

Movement of the parts doesn't make any difference the efficiency is
the pressure applied to the brake lever versus the pressure applied to
the braking device, usually the pads themselves.

A lever that is 1 foot long and moves, lets say, one quarter of the
diameter of a 2 foot circle applies the same force to a load located 1
foot from the fulcrum as a 100 ft lever which moves 1/4 of the
diameter of a 200 ft circle applies to a load that is 100 ft. from the
fulcrum. The first lever moves 19 inches and the second moves 157
feet.

Sure. But the distance you can move your brake lever is limited by the
length of your fingers, and so the distance you can move at the lever end
is essentially fixed. To increase the mechanical advantage in THAT system,
you have to reduce the distance the brake pads move. No ifs, ands, buts or
maybes.

You are talking about two different things. Mechanical efficiency and
how long your fingers are.

They aren't really related.
--
Cheers,

John B.