From: jbrandt@hpl.hp.com (Jobst Brandt)
Newsgroups: rec.bicycles.tech
Subject: Re: Tandem Disc Brake Ideas
Date: 2 Aug 1996 22:05:29 GMT

Karl Anderson writes:

>> Does anyone have any good ideas how to fit a disc brake to our
>> front wheel. Really need stopping power due to weight (touring set
>> up) I believe there are some wire/hydraulic models around mainly
>> for MTB's..

> According to Rob Van der Plas in the sometimes sketchy _Bicycle
> Technology_, one reason that disc brakes aren't put on tandem front
> wheels is due to problems in stressing the forks.

As you may have noticed, standard steel forks on bicycles are broad in
the fore and aft direction at the crown and small and round at the
axle.  This shape is used so the fork can withstand forces of rim
braking that loads the fork blades in bending at the crown while there
is no bending at the axle.  A hub brake puts that same bending load
into the fork tip at the axle, and only on one side.

Don't put a hub brake on a front wheel with a conventional fork!  At
least, don't expect it to do much hard braking before the fork fails.

Jobst Brandt      <jbrandt@hpl.hp.com> 

From: jbrandt@hpl.hp.com (Jobst Brandt)
Newsgroups: rec.bicycles.tech
Subject: Re: Disk brakes for tandems
Date: 17 Feb 1998 17:50:30 GMT

I think there is a basic problem with disk brakes that is being
overlooked, and that is their need for a high mechanical advantage.
The pads are made of hard material that although having a low
coefficient of friction is less affected by moisture and temperature.
The high mechanical advantage required to make these hard pads work
initially caused the auto industry endless grief until it was decided
that the pads may drag when the brake released.  This was in the days
when power brakes were not accepted as necessary or desire-able.
Toady, brake pads on motorized vehicles drag most of the time.

For bicycles, that have so little excess power that racers open the QR
on their rear caliper brakes on hill climbs to avoid intermittent pad
contact, disk brakes that drag are not acceptable.  For this reason
disks on bicycles are reliving the problems of early auto disks but
even more so because the dragging brake is not only inelegant, it
materially impedes motion.  It may not be possible to make a manually
operated disk brake that does not drag and once that is accepted as a
design criterion, some of the other problems may find a solutions.

Jobst Brandt      <jbrandt@hpl.hp.com>

From: jbrandt@hpl.hp.com (Jobst Brandt)
Newsgroups: rec.bicycles.tech
Subject: Re: discs vs normal brakes
Date: 23 Mar 2000 17:49:44 GMT

Adam Who writes:

> Quick one, would anyone care to comment on the pros/cons of using a
> disc brake for fully loaded touring?  I'm presuming you'd put it on
> the front.  Anyone got any experience of these things, like
> maintenance etc?

Don't put a disk brake on just any front wheel unless you make sure
the fork can sustain the bending loads that go into the tip of one
fork blade with a hub activated brake as disks are.  Unlike a caliper
brake, whose disk is the periphery of the rim, hub mounted disks of
the kind I assume you are considering, put their load into only one
side of the fork and at the small end of a conventional road fork, a
place that does not like to be loaded that way.

To decide which brand of disk you want to use, test ride some of the
bikes on display with disks at a suitable bike shop.  I don't like the
concept of hydraulic plumbing at the hand lever and find the
hydraulics that are confined to the caliper, with brake cable between
handlebar and caliper, the more reliable kind.  Still, heat capacity
of a small disk and its high temperatures present some problems for
descending mountains.

When you see these on bicycles of professional racers in the
mountains, we'll know that they have at least matched the performance
of rim brakes.  Until then, I'm not convinced of these for road bikes
even thought hey have some obvious advantages like wet weather
performance.

Jobst Brandt      <jbrandt@hpl.hp.com>

From: jbrandt@hpl.hp.com (Jobst Brandt)
Newsgroups: rec.bicycles.tech
Subject: Re: discs vs normal brakes
Date: 26 Mar 2000 00:27:24 GMT

David Emrich writes:

> It's how brakes work - they convert the kinetic energy of your
> vehicle into heat.  There's a limit to how much energy they can
> absorb (per unit time), so there is a limit to how much braking they
> can do.  Of course you are correct that you can warp the rotors.

I think most reasonably astute designers solve the warpage problem by
suspending the disk as a free ring (all motorcycles do this) but I
don't believe that the wafer thin gossamer <4oz disks that abound can
dissipate nor store the energy necessary for a road bike descent of a
steep curvy mountain road.  The ratio of vehicle mass to front brake
disk mass is out of reasonable proportions in my estimation.  I'm sure
this is also true for off road bicycling on continuously descending
trails, where little potential energy of descending goes into wind
drag.

> The solution for bikes is to have brakes that convert some of the
> energy into squeal.  Disks and V brakes are easy to set up this way.

Actually, squealing brakes are worse because they are in a stick/slip
mode where part of the sliding motion takes place under a lubricious
mode and the other under elastic deformation mode.  I don't know what
tests have been done, but I suspect that frictional drag drops with
the onset of squeal.

> While I was kidding, actually the only canti brake pads I've ever
> used that stopped much better than others were a very soft set of
> Winwoods that screamed when you applied the brakes.  Alignment was
> pointless as they deformed too much at the rim.

Although some riders complain of incorrigible squeal with them, I find
Kool-Stop salmon colored pads the best I have used.

Jobst Brandt      <jbrandt@hpl.hp.com>

From: jbrandt@hpl.hp.com (Jobst Brandt)
Newsgroups: rec.bicycles.tech
Subject: Re: Magura GustavM on tandem (was: Re: Drum Brake on Tandem)
Date: 11 May 2000 21:59:39 GMT

Tony Raven writes:

>> I'd be curious about the same test with 20+min of descending.
>> Descents in mountain passes (at least where I live) are usually a
>> lot longer then 5 minutes.

> I'd anticipate that it would reach thermal equilibrium within 5
> minutes which means it won't be any different for 20 mins.

Disk temperatures reach equilibrium in about 10 seconds on a descent.
You can arrive on that because there is so little cooling area that
the disk always reaches high temperature immediately, relying on a
large temperature difference between air and disk for convection and
near glowing temperatures for radiant cooling.

What doesn't come up to steady state as quickly is the caliper and its
fluid, that is separated from the hot disk by the brake pads that are
moderate insulators themselves.

Jobst Brandt      <jbrandt@hpl.hp.com>

From: jobst.brandt@stanfordalumni.org
Subject: Re: Why do discs work better?
Newsgroups: rec.bicycles.tech
Date: Sat, 01 Sep 2001 05:05:22 GMT

JRE writes anonymously:

>> Developing the disk brake for cars was not easy because it doesn't
>> operate well as a non power assist brake.  In cars, they must work
>> even without power.  Therefore they generally are dragging brakes
>> when released, in order to keep free travel (that takes up much
>> pedal travel) to a minimum.  These problems apply to bicycle disks
>> as well.

> I believe that the caliper seals on automotive disc brakes are
> designed to retract the piston a few thousandths of an inch and get
> the pads off the disc.  Disc brakes should not normally drag on the
> disc (and if they did they would wear faster).

That was once a theoretical feature that never got far because
elasticity of the caliper, pistons, and pads always got in the way of
a predictable retraction.  Today no one bothers with it because pads
naturally ride with no practical pressure on the disk, it having
enough lateral motion to push pads away enough to achieve this.

Jobst Brandt    <jobst.brandt@stanfordalumni.org>

From: jobst.brandt@stanfordalumni.org
Subject: Re: Why do discs work better?
Newsgroups: rec.bicycles.tech
Date: Wed, 31 Oct 2001 23:13:30 GMT

Daniel Lauring writes:

> On disc pads in vehicles the retracting pedal releases pressure
> allowing the pads to back up a minute amount.  On the pads on a
> mechanical system like Avids the rushing air forms a bearing between
> the pad and the disc...no retraction mechanism is necessary.

On cars, there is no retraction effort.  In early disks this was a
design feature that never worked reliably and soon it was recognized
that brake drag of a disk is insignificant.  Air bearing from "rushing
air" is not effective on surfaces as rough as automotive disk brakes,
and even if disks were smooth enough the "flying height" of the slider
over the disk would be inconsequential, the clearance between pads and
disk being far smaller than axle flex in curves, where on pad advances
as the other is pushed back by disk misalignment.

Jobst Brandt    <jobst.brandt@stanfordalumni.org>

From: jobst.brandt@stanfordalumni.org
Subject: Re: Disk brakes and wheel ejection - Manitou's answer?
Newsgroups: rec.bicycles.tech
Message-ID: <lAXoc.11144$Fo4.146197@typhoon.sonic.net>
Date: Fri, 14 May 2004 04:05:37 GMT

Mark South writes:

> It goes like this: rims brakes have problems for extreme offroad use
> in mud.  (So I am told. They've never failed me, but I'm not a
> hard core MTBer.)  Therefore disks were introduced as a solution.
> The tradeoff of using brakes that are more complex, heavy, require
> fork modifications and dished front wheels et etc was seen as
> worthwhile in order to achieve better brake function, which is (after
> all) one of the fundamental control dimensions of a bicycle.

I think discs are well suited for the mucky dirt market and even for
other off-road and suspension bicycles where the weight of the bicycle
has already been compromised for special functions.  For the road,
bicycles are still designed for light weight so the addition of a disc
and caliper, torque bearing fork ends and hydraulic reservoirs all
constitute additional weight and complexity over the side pull rim
brake.  STI is already a compromise of weight considerations and
complexity.

> Therefore disks are the latest thing.  Therefore they are put on all
> kinds of other bikes.  Therefore eventually all bikes will have to
> have disks, even those for which the tradeoff works in the opposite
> direction, and where rim brakes would be a superior solution - for
> example where they function just as well but are lighter and
> simpler.  That's the extension from "best in one application" to
> "best across the board".

The bicycle market, being driven by fashion, has brought us all sorts
of odd designs, most of which are justified by "spin doctors" that
make them sound useful and reasonable.  Take, for instance, saddles
with clefts of all sorts and wheels with paired spokes ala Lovelace
(pat. 1890) aka Rolf.  On our local parade route (Foothill Expy),
the latest fads are immediately on display in large numbers.

> Now I know that you know all this, because you're an engineer.  I
> know that marketroids (a) can't understand the chain of reasoning,
> and (b) wouldn't care anyway.  So I suppose all the persiflage above
> is a gloss on your version of "Disks?  We don't need no steenkin'
> disks!"

I hope that those who enjoy bicycling and reliable, useful equipment
don't fall for that approach and continue to demand functional
equipment.  I am glad that I can still fully equip myself in
reasonable parts and clothing.

Jobst Brandt
jobst.brandt@stanfordalumni.org

From: jobst.brandt@stanfordalumni.org
Subject: Re: Front drum brakes, wheel loostening?
Newsgroups: rec.bicycles.tech
Message-ID: <CDOAa.17348$JX2.1077473@typhoon.sonic.net>
Date: Tue, 27 May 2003 19:02:58 GMT

Paul Hamilton writes:

> I have put about 4,000 miles on a drum braked bike in the last year
> and a half.  The bike has nuts on the front, rather than a QR.
> There have been no problems of any kind.

> I don't like the term "design flaw."  Everything in the world has
> strengths and weaknesses.  Drums have good and bad points and their
> suitability depends upon the application. They don't give the
> leverage that rim brakes give, and this means that you have to
> squeeze harder.  They are heavy, something that may or may not
> matter to you. I have not had a brake fade problem, but I suspect
> that this would be an issue in very hilly terrain. They are
> definitely superior in wet weather.  If you ride in an area where
> the roads are heavily sanded, you are freed from the need for
> frequent pad and rim replacement.

The reason automotive brakes switched to discs from drums is not a
power problem but a control problem.  Drum brakes are non-linear in
response.  That means you cannot tell how much braking will result
from a given pedal pressure because the drum brake is self energizing.
The shoes are literally sucked into engagement and that is why it was
common to have brakes completely lock after driving through a puddle.

Disc brakes and their derivatives, bicycle rim brakes, have their
action completely decoupled, pad application force acting at right
angles to the brake force, unlike the drum brake where shoes move
in-line with drum rotation.  Disc brakes were developed on, and were
essential to aircraft to prevent them from veering off runways on
landing, because drum brakes on landing gears did not respond
identically.

> In my case, the reduced maintenance and good wet weather braking
> makes drums or disks the only alternatives I would consider.  On the
> other hand. depending upon your priorities, you might find drums
> unsuitable.

Trucks use drums because they can dissipate more power more easily and
economically.  Railroads, on the other hand, cannot used drums because
they easily cause skids and skidding a railroad wheel causes a flat
spot that is irreparable and gets worse with use.  Besides, once a
railroad wheel skids it floats on molten steel and will not stop
sliding until the brake is fully released.

Jobst Brandt
jobst.brandt@stanfordalumni.org
Palo Alto CA

From: jobst.brandt@stanfordalumni.org
Subject: Re: Front drum brakes, wheel loostening?
Newsgroups: rec.bicycles.tech
Message-ID: <ZHSAa.17420$JX2.1081874@typhoon.sonic.net>
Date: Tue, 27 May 2003 23:40:41 GMT

Rick who? writes:

>> The reason automotive brakes switched to discs from drums is not a
>> power problem but a control problem.

> I don't know what really drove auto manufacturers to go to disc
> brakes.  But my recollection is that resistance to fade was the
> reason advertising and other public pronouncements gave for the
> decision to switch.  Maybe that was just marketing?

I just told you or didn't you read it.  I was in automotive
engineering at Porsche KG when this transition was made and worked on
the development.  Our first disc brakes were inside calipers grasping
a disc-ring held on the periphery by a cut away aluminum brake drum.

>> Drum brakes are non-linear in response.  That means you cannot tell
>> how much braking will result from a given pedal pressure because
>> the drum brake is self energizing.

>  ... when the activating end of the brake shoe is the leading end.
> Otherwise it's the opposite.

No one would make a trailing shoe drum brake or even one with linear
motion shoes, there being insufficient force to stop the car.  It would
require forces that could only be achieved with a strong servo system.

> IIRC, the vast majority of front drum brakes employed
> leading/leading, creating a strong servo action, while most rear
> brakes had one leading and one trailing, since less servo action was
> considered desirable to prevent the more troublesome rear wheel
> lockup.

That was only toward the end of the drum brake on large sedans before
the use of power brakes was well developed.  The poor response got
worse and braking was a hazard when peak demand was used.  Brake
companies and sports and racing car manufacturers were aware of the
advantages of disc but had no way of implementing them on cars.  The
development was slow and tedious, especially with many in the industry
who feared change, people who believed there was no problem having
never used a disk brake.

Jobst Brandt
jobst.brandt@stanfordalumni.org
Palo Alto CA

From: jobst.brandt@stanfordalumni.org
Subject: Re: Front drum brakes, wheel loostening?
Newsgroups: rec.bicycles.tech
Message-ID: <fnVAa.17451$JX2.1083947@typhoon.sonic.net>
Date: Wed, 28 May 2003 02:43:23 GMT

Rick who? writes:

>>>> The reason automotive brakes switched to discs from drums is not
>>>> a power problem but a control problem.

>>> I don't know what really drove auto manufacturers to go to disc
>>> brakes.  But my recollection is that resistance to fade was the
>>> reason advertising and other public pronouncements gave for the
>>> decision to switch.  Maybe that was just marketing?

>> I just told you or didn't you read it.

> I didn't mean to offend you, and I wasn't questioning the accuracy
> of your statement.  I meant that I pretend no knowledge of the real
> reasons vs. the marketing reasons, not that I hadn't read your post.

> Are you saying that reducing brake fade was not one of the reasons
> disk brakes replaced drums?  (Please forgive my ignorance--my
> questions are asked to increase my understanding, not to challenge
> you.)

The drum brake as a high performance passenger car brake was at its
end, but the big Detroit mfg,s didn't want to face up to it and kept
producing grabby double leading shoe brakes, with power assist mostly
with insufficient cooling to match the friction materials.  While they
were making high temperature brake shoes, the sports car people were
working on discs.  With their PR, the American public was satisfied
with the crummy brakes they had until mundane foreign sedans began to
appear here with disc brakes in competition.  The brake fade problem
was generally cured by high temperature compounds but the
non-linearity remained.  Today, highway trucks use drum brakes that
have excellent fade resistance but still have the control problem.
That is why you often see long dual skid marks on roads.  The driver
cannot assess how hard to brake with drums, especially on trailers.

Jobst Brandt
jobst.brandt@stanfordalumni.org
Palo Alto CA

From: jobst.brandt@stanfordalumni.org
Subject: Re: Front drum brakes, wheel loostening?
Newsgroups: rec.bicycles.tech
Message-ID: <nPSAa.17424$JX2.1082022@typhoon.sonic.net>
Date: Tue, 27 May 2003 23:48:35 GMT

Ryan Cousineau writes:

>> The reason automotive brakes switched to discs from drums is not a
>> power problem but a control problem.  Drum brakes are non-linear in
>> response.  That means you cannot tell how much braking will result
>> from a given pedal pressure because the drum brake is self
>> energizing.  The shoes are literally sucked into engagement and
>> that is why it was common to have brakes completely lock after
>> driving through a puddle.

> What about trailing-shoe designs? I haven't been lucky enough to
> spend any time behind the wheel of a vehicle with 4-wheel drums, but
> my father owned a 1965 Pontiac Custom Sport with mirror-finish
> drums. They had no fade resistance, apparently because when the drum
> heats up, it can flare slightly, and as soon as that happens the
> shoe contact area drops off dramatically.

> I'm not sure if that's the mechanism (could it be just that the
> expanded drum no longer conforms to the curve worn onto the shoe?),
> but I'm quite sure that fade resistance is not as good as disc
> systems that fit in a similar space.

The cause was brake pad material.  With a large contact area that a
pair of shoes had, they had to be fairly soft in order to generate
enough drag to stop the car.  Softer materials have low vapor
pressures and outgas and melt at far lower temperatures that the
friction material in disc brakes.  Disc brakes have substantially
smaller contact area and an even greater increase in contact pressure
to make up for that.  Today there are probably no disc brakes without
power assist, the mechanism being so simple and well developed.

Disc brakes on aircraft and racing cars have carbon discs and pads
that operate at glowing temperatures and produce no brake dust, that
ugly reddish-brown sludge that dirties front wheels of cars.  Their
brake wear debris is CO2, a pretty clean substance.

Jobst Brandt
jobst.brandt@stanfordalumni.org
Palo Alto CA

From: jobst.brandt@stanfordalumni.org
Subject: Re: Front drum brakes, wheel loostening?
Newsgroups: rec.bicycles.tech
Message-ID: <JTVAa.17457$JX2.1084143@typhoon.sonic.net>
Date: Wed, 28 May 2003 03:18:01 GMT

Ryan Cousineau writes:

> A quick study on the web suggests that American cars started using
> power discs in the sixties and stopped using manual discs in the
> seventies. I assume the Europeans were a bit ahead of this trend,
> but possibly some low-end (or manfully sporting; what did Porsche
> do?) Euromobiles were using manual discs for longer than that.

The power brake requirement depends on the weight of the car.  Early
Porsche 356 disc were unassisted and worked well.  They had pad
retraction features that were soon scrapped.  With pads in sliding in
contact, pedal free-travel was reduced and the mechanical advantage
could be increased.  Only later, as the 911 came along, did power
brakes become standard.  I was no longer there when that happened.

> Interesting point about drum pad material.  Couldn't they use a
> harder pad material with power-assist brakes and just dial up the
> boost? Did drum distortion play no significant role in the drum
> brake fade issues?

That's what they did and it's still the case with truck drums.

>> Disc brakes on aircraft and racing cars have carbon discs and pads
>> that operate at glowing temperatures and produce no brake dust,
>> that ugly reddish-brown sludge that dirties front wheels of cars.
>> Their brake wear debris is CO2, a pretty clean substance.

> I'm not so sure about that.  I used to watch a lot of F1 races
> (carbon-carbon discs and pads) and sometimes when those front wheels
> came off in a pit stop, there would be a cloud of black dust puffing
> out, apparently from the brakes.  Were the brakes just not up to
> temperature?

As far as I have seen recently, and in air terminals, there is no
significant brake dust debris.  I'm sure there are other components in
the pad material than pure carbon and that small component may be what
causes some solid debris.  My introduction to this was with disk
drives with carbon as a wear layer on both sliders and disks.  (In the
storage industry disc is spelled with a "k")

All sorts of more durable and harder materials were used as wear
layers and never worked.  In an atmospheric test chamber, I found that
carbon in a pure nitrogen atmosphere generated particles that reduced
friction and destroyed the magnetic layer in time from particle
contamination, just as hard materials did.  These particles are in the
several tens of nanometers in size.

If the chamber was purged with oxygen, friction increased as carbon
particles oxidized and evaporated allowing large surface contact.  A
grad student instrumented the chamber to measure CO2 production for
his thesis and proved that carbon on carbon, in disk drives, caused
benign wear debris of CO2 in an air environment.  In oxygen friction
went excessively high and with no oxygen, friction dropped from excess
particle generation in the interface, ultimately damaging the
recording medium.

Jobst Brandt
jobst.brandt@stanfordalumni.org
Palo Alto CA