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

On 2017-10-27 17:11, John B. wrote:
On Fri, 27 Oct 2017 06:58:27 -0700, Joerg
wrote:

On 2017-10-27 01:11, John B. wrote:
On Wed, 25 Oct 2017 07:53:11 -0700, Joerg
wrote:

On 2017-10-24 17:21, John B. wrote:
On Tue, 24 Oct 2017 11:47:12 -0700, Joerg
wrote:

On 2017-10-24 07:27, wrote:
On Tuesday, October 24, 2017 at 2:19:48 AM UTC-7, John B. wrote:
On Mon, 23 Oct 2017 10:09:20 -0700, Jeff Liebermann
wrote:

On Mon, 23 Oct 2017 12:48:29 +0700, John B.
wrote:

On Sun, 22 Oct 2017 20:51:15 -0700, Jeff Liebermann
wrote:

On Mon, 23 Oct 2017 07:02:08 +0700, John B.
wrote:

But re disc brake cooling F1 car brakes appear to work with
the discs red hot. In the 1,000 degree (F) range. And they
use Carbon Fiber discs too :-) And everyone knows that CF
is better.

"Thermal Conductivity of Carbon Fiber, and other Carbon
Based Materials"
http://www.christinedemerchant.com/carbon_characteristics_heat_conductivity.html


"So...Is Carbon Fiber a good heat conductor?
As usual the answer is "it depends." The short answer is NO
not when regular carbon fiber is made up in regular epoxy and
expected to conduct heat across the thickness. IF a highly
carbonized pan fiber with graphite or diamond added, is
measured for heat transmission in the length of the fiber it
is very good and can rival and exceed copper."

On the other hand, they seem to work pretty well :-) See
https://www.youtube.com/watch?v=h5JcHAEmIYM for a visual
indication of heat dissipation. :-)

Impressive. I'll assume it's a carbon-carbon rotor, since all F1
cars seem to using them.

Undoubtedly so. But if the advantage of "carbon" bikes can be
extolled that a carbon-carbon frame must have twice the bragging
rights :-)


http://www.racecar-engineering.com/technology-explained/f1-2014-explained-brake-systems/


(4 pages)
"A typical road car uses a cast iron brake disc with an organic
brake pad. In an F1 car, though, the same material is used for
both disc and pad, and this material is known as carbon-carbon -
a significantly different material to the carbon-fibre
composites used in the rest of the car" In other words, the F1
brakes are NOT made from CF.

Some detail on Formula 1 brakes:
https://www.youtube.com/watch?v=ev6XTdlKElw

Fun destroying brakes:
https://www.youtube.com/watch?v=KslGsXMgmqg The brake starting
at 4:45 sure looks like CF but I'm not sure.

Maybe twin disk brakes would be easier?
http://nuovafaor.it//public/prodotto/75/nccrop/DOPPIO_FRENO_CROSS_ENDURO.jpg


https://i.ytimg.com/vi/Pvwj-WWlKkg/maxresdefault.jpg
https://gzmyu4ma9b-flywheel.netdna-ssl.com/wp-content/uploads/2011/09/Gatorbrake-dual-hydraulic-front-disc-brakes-carbon-rotors01.jpg


https://lh4.googleusercontent.com/-cDfAFWrGR6Q/VHKPsm-f6YI/AAAAAAAAX10/2FCyj87xs0g/s640/14%2520-%25201.jpg
https://www.minibikecraze.co.uk/wp-content/uploads/bs0978.jpg
https://endless-sphere.com/forums/viewtopic.php?f=28&t=56268

Given the coefficient of friction between a 1.25" wide rubber tire
(32mm) and a wet road probably dragging the feet will work. :-)

Joerg's experience is with full suspension MTB's. These things are
incredibly heavy and long wheelbased. He has his judgement of disks
and it is no doubt quite accurate for his experience and riding.

I have disks on a much lighter and shorter wheelbased bike. I know
the failings up close and personal. I simply cannot imagine WHY a
person would want a more complicated system than that offered by the
Campy Skeleton brakes.


The reason can be summed up in one word: Rain :-)

But last Sunday I started out my "weekend" ride in the rain. It had
been raining nearly all night and the roads had a lot of water on them
- note we have been having floods here in Bangkok lately - but it
appeared that the rain was ending so off I went.

Unfortunately my weather forecasting facility wasn't working very well
and I rode 20 Km of a 30 Km ride in light rain and flooded roads in
many places. I was splashing through water in some places and cars
were splashing through (and splashing me) in others.

Of course, Sunday is much lighter traffic then on work days but still,
Bangkok is rated as one of the cities with the most chaotic traffic
in the world, and I did have to stop suddenly several time, on flooded
roads with wet wheels and brakes.

My brakes worked just as they do in the dry. Back brake stops me
somewhat slowly and front brake stops rather suddenly, both brakes
together provides best stopping. No long wait after grabbing a brake
lever although I did think of you with your stopping problems and I
have the feeling that the brake lever pressure might be a tiny bit
more to stop in the rain but if it was it was so little that it
couldn't be quantified.

But of course I am using quality brake pads. Why it costs me US$12.12
a wheel just for pads alone.... but they do last a year or more.


It seems Californian rain and Thai rain aren't the same. When it rains
heavily and I have to do a surprise emergency stop after not having used
the brakes for a while there is 1-2sec of nada, absolutely nothing. It
makes no difference whatsoever whether I use $17 high-falutin Koolstop
rain-rated pads or $4 Clarks pads. The experience of other riders around
here and in this NG is similar.

Which, to be honest, I find a little mystifying as I've had pretty
constant success with conventional brakes.

Frankly, I can't believe this is solely because I'm somehow so
uniquely skilled or that y'all are all in the awkward squad

I do see a number of people here and many who are not here who seem to
have ridden for years using conventional brakes without complaint and
some of the blogs I read don't even talk about brakes. Dave Moulton,
for example. An old fellow, used to race bikes, came to the U.S. in
about 1979 and built frames commercially for years, now retired, has
one entry in his blog about brakes - "centering side pull brakes".

Another blog from the long distance side of the bicycleing world, The
Blayleys, who are into Audex's and who apparently each ride in the
neighborhood of 10,000 miles annually, mentions Vee brakes in
reference to a Tandem while a photo of them on a tandem on their web
page shows disc brakes. On the other hand, when she discusses a "good
brevet bike she simply says that the "brakes must clear the fenders
and probably long reach caliper brakes will suffice".

In short, it seems that brakes just don't seem to be a hot subject in
much of the cycling fraternity.


To a large part that is because most cyclist will not ride in driving
rain. Some do and those know exactly how that delay with rim brakes
feels. Occasionally it is called "free fall" because that's how it feels
like.

Well, the Blayleys state that the husband, John, has ridden 10 - 17
thousand miles a year for the past 25 years and the wife, Pamela, has
ridden from 10 - 14 thousand miles a year for the past 20 years, or
another way to put it might be that together they have ridden from 20
- 30 thousand miles a year for the past 20 years.

Somehow I suspect that they may have encountered rain in that period.


And grandpa has driven his cars without safety belts yet survived ...

For people who do not shy away from unpaved roads or use a lot of
singletrack and ride in the rain there is a much more extreme issue: Wet
mud. You may have never encountered it but I have many times. You reach
in and, after a second or two of nothing, the rim brakes come on but let
off an awful grinding noise. You can literally hear the rim being
tortured but because of a rapidly approaching curve you can't let go. As
I have mentioned before the rims on my old MTB are only 1000mi old but
the front rim is almost shot from all that. Deep grooves.

I stand by my opinion that rim brakes are fair weather brakes. Then they
are fine but not when the going gets tough. Like this kind of weather:

https://www.youtube.com/watch?v=mX_EKybzK4Y


I might comment that I've ridden coaster brakes, drum brakes, rod pull
brakes, cantilever brakes, side pull single pivot caliper brakes,
double pivot caliper, Vee brakes and for one short ride a cable disc
brake. and at the time I rode them I found all the brakes to give
acceptable service. Well with one exception, rim brakes and chrome
plated steel rims were sometimes a bit iffy :-)


Yes, those were the worst. It got a little better with aluminum rims but
not a lot. In the world of automotive such a brake "system" would not
stand the slightest change of being legal.

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?

Well, when I worked on airplanes I remember that the F-4 had multi
plate disc brakes which provided a tremendous amount of stopping power
in a very small package.


Some tandems have that as well, and of course motorcycles: Two discs up
front. But not stacks of discs.


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?


I've thought about it :-)

--
Regards, Joerg

http://www.analogconsultants.com/

On 2017-10-27 17:18, John B. wrote:
On Fri, 27 Oct 2017 09:26:57 -0700, Joerg
wrote:

On 2017-10-27 07:11, jbeattie wrote:

[...]


OT, I was riding my son's 29er up Emigration Canyon a while back and
encountered a mud flow and sprayed crap all up the back of the seat
tube. We didn't bother rinsing that off, and when I came to visit
the next time, it was still there, hardened like cement. It was some
sort of clay or adobe. Some mud is super-tough and should be rinsed
off immediately.


Dirt on an MTB is a badge of honor. So far I've never cleaned my MTB
because it'll be dirty again 10 miles later.

http://www.analogconsultants.com/ng/bike/Muddy3.JPG

After a free bike wash due to rain:

http://www.analogconsultants.com/ng/bike/Muddy3.JPG

The remainder is caked in so hard that the soft side of a green-yellow
kitchen sponge won't get it off. The scrub pad of it might but then the
paint job is going to be scuffed and it'll collect dirt like a magnet.

Urban dwellers can buy spray-on dirt so their ride looks more manly. No
kidding.

https://www.theguardian.com/science/2005/jun/14/uknews

A rather revealing post. Do you bathe? Once a year on New Year's,
perhaps?


I tend not to take my bicycles into the shower with me.

--
Regards, Joerg

http://www.analogconsultants.com/

On 2017-10-27 17:15, John B. wrote:
On Fri, 27 Oct 2017 06:59:56 -0700, Joerg
wrote:

On 2017-10-26 17:16, John B. wrote:
On Thu, 26 Oct 2017 17:08:10 -0400, Radey Shouman
wrote:

jbeattie writes:

On Wednesday, October 25, 2017 at 8:59:47 AM UTC-7, wrote:
On Tuesday, October 24, 2017 at 5:22:03 PM UTC-7, John B. wrote:
On Tue, 24 Oct 2017 11:47:12 -0700, Joerg
wrote:


[ ... ]


The reason can be summed up in one word: Rain :-)

But last Sunday I started out my "weekend" ride in the rain. It had
been raining nearly all night and the roads had a lot of water on them
- note we have been having floods here in Bangkok lately - but it
appeared that the rain was ending so off I went.

Unfortunately my weather forecasting facility wasn't working very well
and I rode 20 Km of a 30 Km ride in light rain and flooded roads in
many places. I was splashing through water in some places and cars
were splashing through (and splashing me) in others.

Of course, Sunday is much lighter traffic then on work days but still,
Bangkok is rated as one of the cities with the most chaotic traffic
in the world, and I did have to stop suddenly several time, on flooded
roads with wet wheels and brakes.

My brakes worked just as they do in the dry. Back brake stops me
somewhat slowly and front brake stops rather suddenly, both brakes
together provides best stopping. No long wait after grabbing a brake
lever although I did think of you with your stopping problems and I
have the feeling that the brake lever pressure might be a tiny bit
more to stop in the rain but if it was it was so little that it
couldn't be quantified.

But of course I am using quality brake pads. Why it costs me US$12.12
a wheel just for pads alone.... but they do last a year or more.

This is something I just can't understand. Indeed there is a
millisecond or two delay for the brake shoe to excise the water
directly in front of the initial application of the brake but after
the brake shoe touches the rim it pushes any water in the way off
without floating the shoe above it.

I see no reason whatsoever for disk brakes and their complications
even on most MTB's since a good V-Brake is longer lasting, just as
effective, cheaper and doesn't require special wheels and frame and
fork changes.

If you ride a lot in the rain and use a rim brake, you might go
through a front rim every 1-2 years -- at least based on the
experience of one of my commuter cohorts who just switched to discs.

And the slowly dying rims generate some really messy black sludge, at
least in my experience. I'm not sure why the "wax your chain to stay
clean" cohort hasn't noticed.

It is probably heresy to mention it, but some people wash their bikes,
particularly after riding in the rain. Amazing how easily all that
black sludge washes off with soap and water :-)


Oh yeah, now we have to wash out bikes after each rain ride. Standing
out there in the rain with sponge and shampoo. Great.

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?


Bingo!

--
Regards, Joerg

http://www.analogconsultants.com/

John B. wrote:
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.

Theoretically, the mechanical advantage of a brake system and the length of
your fingers aren't related, but practically, in this example of the
mechanical advantage of a bicycle brake system, the two quantities are
chained together at the ankles. You can't have high brake pad travel and
high mechanical advantage unless the travel of the brake lever is very
large, and human hand dimensions just won't allow that.

On 2017-10-27 19:17, Joy Beeson wrote:
On Fri, 27 Oct 2017 06:59:56 -0700, Joerg
wrote:

Oh yeah, now we have to wash out bikes after each rain ride. Standing
out there in the rain with sponge and shampoo. Great.

You don't have a garden hose?

Back when I could ride after the outside faucets had been turned off
for the winter, I used to hose salt and ice off my bike by filling my
water bottles with hot tap water, reserving the first squirt from each
bottle to get under the fenders. I don't think I ever had to make
more than two trips into the house for more water.

Come to think of it, I don't think I ever hosed it in the summer
(except when cleaning the braking surfaces, of course), because I rode
only on pavement. There was one mile of gravel road in an adjacent
county, but the grader wore out and it was cheaper to pave the road
than to buy a new grader.


I try to milk the most time out of a my rides and come home shortly
before dinner. Then I only have to quickly park the bike in the garage,
run inside and take a quick shower.

What I still don't get is why should a bike be washed so often? What is
the benefit of that other than for show? Salt I understand, that can
corrode stuff. We don't have that out here.

--
Regards, Joerg

http://www.analogconsultants.com/

On 10/28/2017 2:19 AM, Ralph Barone wrote:
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.

Good job! Taking your two posts as a whole, I think you've pretty much
covered the situation.


--
- Frank Krygowski

On 10/28/2017 11:12 AM, Ralph Barone wrote:

Theoretically, the mechanical advantage of a brake system and the length of
your fingers aren't related, but practically, in this example of the
mechanical advantage of a bicycle brake system, the two quantities are
chained together at the ankles. You can't have high brake pad travel and
high mechanical advantage unless the travel of the brake lever is very
large, and human hand dimensions just won't allow that.

Can we go into exhaustive detail on that point? (Of course we can! This
is Usenet!)

The mechanical advantage that really matters relates to the relative
forces once the friction material contacts the disc or the rim. In
theory, the ratio of the forces is the inverse ratio of the relevant
distances, but there are complications, as always.

As has been stated, one complication is clearance when the brakes are
off. Here discs have an advantage, because discs don't go out of true
nearly as much or as often as rims do. To give sufficient travel to
clear a slightly out-of-true rim, a rim brake has to sacrifice some
mechanical advantage.

Except there was a never-marketed design of caliper brake that avoided
that. In one edition of the book _Bicycling Science_ by Wilson, an
illustration showed a caliper brake that moved in quickly to the rim
(i.e. low mechanical advantage) until it contacted; then part of the
mechanism locked, transitioning to a high mechanical advantage.

The reason? They had tested a bunch of brake shoe materials for wet vs.
dry coefficient of friction, and found that harder materials with lower
coefficients had less wet vs. dry difference. To take advantage of that,
they needed more mechanical advantage when force was applied, but less
mechanical advantage when force was off, to give clearance. (Discs take
advantage the hard pad friction consistency in a different way entirely.)

More on lever vs. pad motion: My first "good" bike (1974?) came with
long reach center pull brakes. They were not great. There was
considerable lost motion in that system: flex in the brake arms
themselves, flex in the mounting yoke, some compression in the cable
housing, etc. I eventually brazed on cantilever bosses and used (still
do!) Shimano 600 short-arm cantilevers. Those are far more rigid and
feel much better to me.

But the cantilevers on my touring bike and on my tandem are even better
because of their cable hanger, the thing that attaches at the headset
and stops the housing while letting the cable pass through. Many of
these hangers are thin (maybe 2mm thick) aluminum or steel, and flex
mightily. The touring bike and tandem have very rigid aluminum hangers,
thickness about 5mm or more.

That flex does cause some force loss and some lack of modulation, since
the cable, under tension, has to slide to make up for the lost motion.
That causes cable friction losses and changes the ratio between caliper
force and lever force. Hydraulics don't suffer that particular loss.
Neither do direct pull cantilevers, since they do away with the hanger
entirely.

Here's one last little bit, something I've mentioned before, since it's
good to get rid of lost motion in a rim brake. Classic center pull
cantilevers have one more source of lost motion, and that's the
straightening of curves in the straddle cable. The straddle cable should
run in a straight line from the saddle to the canti arm, as in the
illustration at http://www.sheldonbrown.com/images/canti.gif

But unless you pre-form the cable to get it to run that way, it will
have a curve as it leaves the saddle, and some motion will be consumed
in straighening it. Pre-forming it gives noticeably better feel to the
brake.

--
- Frank Krygowski

On 10/28/2017 11:10 AM, Joerg wrote:

I tend not to take my bicycles into the shower with me.

And you call yourself a cyclist??

;-)


--
- Frank Krygowski

On Sat, 28 Oct 2017 13:57:05 +0700, John B.
wrote:
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/

Yep. How about a pyramid shaped drag chute? 4 sides of the pyramid
are independent and hinged at the base of the pyramid. The 4 points
of each side come together and are controlled by shroud lines. Pull
on the shroud lines, the 4 peaks come together, and the pyramid acts
like a drag chute. Release the shroud lines, the 4 sides open up, and
the drag is greatly reduced.

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

That's not really so bad as long as one uses the drag chute only for
an emergency stop. Kinda like that use-once air bag.

As long as you're willing to install a parachute, how about a wing,
parasail, or similar airfoil? Instead of using it to stop the
bicycle, it would provide sufficient lift for the bicycle to go over
obstructions and road hazards.
https://www.google.com/search?q=flying+bicycle&tbm=isch
Only $45,000:
https://www.hammacher.com/Product/12187
It's a bird. It's a plane. It's a biycle?

Maybe just add some permanent aerodynamic drag? At low speeds, it
won't have much effect, but will put an upper limit on the air, err...
ground speed. Something like this:
http://boxercycles.com/wp-content/uploads/2015/01/BN_RocketElectricTrike_014.jpg
http://boxercycles.com/product/rocket/
Nice name. Nice crumple zone.
Only £4,950.00

If you really want to stop quickly, what you need is a retro-rocket.
Normally, the rocket is used to propel the bicycle in the forward
direction. However, for this contrivance, the rocket is used to slow
down the bicycle. Simply attach one of my "Instant Stop Retro Rocket
Brake" systems onto the frame. If you find yourself speeding out of
control in the general direction of an immovable obstruction, merely
press the "stop" button. The rocket will fire forward and bring the
bicycle to a sudden halt. It might also incinerate the obstruction,
but that can be dealt with in court. Soon, everyone will be using
"Rocket Brakes".


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

Frank Krygowski wrote:
On 10/28/2017 11:12 AM, Ralph Barone wrote:

Theoretically, the mechanical advantage of a brake system and the length of
your fingers aren't related, but practically, in this example of the
mechanical advantage of a bicycle brake system, the two quantities are
chained together at the ankles. You can't have high brake pad travel and
high mechanical advantage unless the travel of the brake lever is very
large, and human hand dimensions just won't allow that.

Can we go into exhaustive detail on that point? (Of course we can! This
is Usenet!)

The mechanical advantage that really matters relates to the relative
forces once the friction material contacts the disc or the rim. In
theory, the ratio of the forces is the inverse ratio of the relevant
distances, but there are complications, as always.

As has been stated, one complication is clearance when the brakes are
off. Here discs have an advantage, because discs don't go out of true
nearly as much or as often as rims do. To give sufficient travel to
clear a slightly out-of-true rim, a rim brake has to sacrifice some
mechanical advantage.

Except there was a never-marketed design of caliper brake that avoided
that. In one edition of the book _Bicycling Science_ by Wilson, an
illustration showed a caliper brake that moved in quickly to the rim
(i.e. low mechanical advantage) until it contacted; then part of the
mechanism locked, transitioning to a high mechanical advantage.

The reason? They had tested a bunch of brake shoe materials for wet vs.
dry coefficient of friction, and found that harder materials with lower
coefficients had less wet vs. dry difference. To take advantage of that,
they needed more mechanical advantage when force was applied, but less
mechanical advantage when force was off, to give clearance. (Discs take
advantage the hard pad friction consistency in a different way entirely.)

More on lever vs. pad motion: My first "good" bike (1974?) came with
long reach center pull brakes. They were not great. There was
considerable lost motion in that system: flex in the brake arms
themselves, flex in the mounting yoke, some compression in the cable
housing, etc. I eventually brazed on cantilever bosses and used (still
do!) Shimano 600 short-arm cantilevers. Those are far more rigid and
feel much better to me.

But the cantilevers on my touring bike and on my tandem are even better
because of their cable hanger, the thing that attaches at the headset
and stops the housing while letting the cable pass through. Many of
these hangers are thin (maybe 2mm thick) aluminum or steel, and flex
mightily. The touring bike and tandem have very rigid aluminum hangers,
thickness about 5mm or more.

That flex does cause some force loss and some lack of modulation, since
the cable, under tension, has to slide to make up for the lost motion.
That causes cable friction losses and changes the ratio between caliper
force and lever force. Hydraulics don't suffer that particular loss.
Neither do direct pull cantilevers, since they do away with the hanger
entirely.

Here's one last little bit, something I've mentioned before, since it's
good to get rid of lost motion in a rim brake. Classic center pull
cantilevers have one more source of lost motion, and that's the
straightening of curves in the straddle cable. The straddle cable should
run in a straight line from the saddle to the canti arm, as in the
illustration at http://www.sheldonbrown.com/images/canti.gif

But unless you pre-form the cable to get it to run that way, it will
have a curve as it leaves the saddle, and some motion will be consumed
in straighening it. Pre-forming it gives noticeably better feel to the
brake.


Good stuff Frank. Thanks.