Can Disk Brakes flip you over the handle bars?

Lou Holtman

Nate Nagel wrote:
> jobst.brandt@stanfordalumni.org wrote:
>> Nate Nagel wrote:
>>
>>
>>>>>>>>> Lots of cars didn't used to have servo assistance on the
>>>>>>>>> brakes. All the force and work required to brake came from
>>>>>>>>> your leg. And yet you could slow down a tonne of car from
>>>>>>>>> 70mph like that.
>>
>>
>>>>>>>> Drum brakes used massive self servo action to achieve useful
>>>>>>>> braking. Because that effect is highly unpredictable, they had
>>>>>>>> both lock-up and fade. That is why we use disks today. You
>>>>>>>> may recall that this was discussed here at great length.
>>
>>
>>>>>>> I don't understand why you consider that action "highly
>>>>>>> unpredictable" it's simply speed dependent. Sure they lock up
>>>>>>> if you stomp on 'em but so will boosted discs. The fade comes
>>>>>>> from the inability of the shoes and drum to shed heat as
>>>>>>> effectively as discs where all the parts are out in the open and
>>>>>>> in some racing applications even have forced air ducted over
>>>>>>> them for additional cooling.
>>
>>
>>>>>> I think you don't know the history of the drum brake. It's self
>>>>>> servo action is laid out about a specific friction coefficient
>>>>>> such that when it is a bit high, brakes lock up (even after
>>>>>> taking the foot off the pedal)
>>
>>
>>>>> Which is not an issue on any automotive application, as if
>>>>> anything, current non-asbestos linings have an even lower
>>>>> coefficient of friction than the original linings around which the
>>>>> brake system was designed. I have seen some high-performance
>>>>> linings made available for various vintage drum brake setups
>>>>> (presumably for vintage racing) and have not ever heard of this
>>>>> happening in practice.
>>
>>
>>>>>> and when it is a bit low from heating, there is essentially no
>>>>>> brake at all, resulting in run-aways on mountain roads.
>>
>>
>>>> The servo effect is what makes the drum brake respond poorly and
>>>> why, if you read the Wiki article, no RR ever used drum brakes,
>>>> skidding a wheel being a major loss.
>>
>>
>>>>> That much is true. Of course this can also happen with discs, if
>>>>> you abuse them enough, but discs cool down *much* faster than
>>>>> drums given that the friction surface is in direct contact with
>>>>> cooling air, the discs have a higher surface area to mass ratio,
>>>>> and that the calipers and at least portions of the linings are
>>>>> also in closer proximity to cooling air than in a drum brake.
>>
>>
>>>>> It cannot happen with disks. The relation of the friction
>>>>> coefficient and brake retardation is linear so the 5% change in
>>>>> friction that formerly cause total brake failure, the disk sees
>>>>> a 5% change. It's much like boiling engines in cars. That
>>>>> they don't boil today is not because they have better
>>>>> radiators, but because they have a non-leaking water pump seal
>>>>> and don't lose coolant.
>>
>>
>> http://en.wikipedia.org/wiki/Engine...were_air-cooled
>>
>>
>>
>>> a 5% change in brake friction will not cause a "total brake failure"
>>> on any decent automotive brake system. Now it may cause more than
>>> 5% loss on a servo-action system, but not "total."
>>
>>
>>>>>>> Old manual drum brakes are actually quite pleasant to drive on,
>>>>>>> so long as you're not racing or driving fast through the
>>>>>>> mountains. IMHO they have a much better pedal feel than all but
>>>>>>> the best new disc brakes; certainly more direct as there is a
>>>>>>> direct hydraulic connection between foot and shoes with only a
>>>>>>> very few, simple mechanical parts in between.
>>
>>
>>>>>> Oh nostalgia for things that weren't. Read:
>>
>>
>> http://en.wikipedia.org/wiki/Brake_..._in_drum_brakes
>>
>>
>>>>> Seeing as I actually own and (occasionally) drive a '55 Studebaker
>>>>> coupe equipped with the (excellent) factory equipment brakes, with
>>>>> the only upgrade being the addition of finned drums from a later
>>>>> model car, I can say that they certainly aren't anywhere near the
>>>>> death-dealing devices that you seem to be making them out to be.
>>>>> The three major shortcomings of drum brakes in general are 1)
>>>>> brakes often undersized for the vehicle, exacerbating other
>>>>> problems (not an issue on my car, but, say, the 9" front drums on
>>>>> an old Dart are an insult to common sense.) 2) Cooling - drum
>>>>> brakes do retain heat far more so than do discs, resulting in
>>>>> earlier fading. 3) single circuit master cylinders can result in
>>>>> complete brake system failure with one single hydraulic failure.
>>>>> Not an issue with the drums themselves, but legal until the late
>>>>> 60's and therefore common on most vehicles you'll find with four
>>>>> wheel drum brakes.
>>
>>
>>>> I take it you are saying that the auto companies of the world are
>>>> barking up the wrong tree when they dumped the drum brake. That's
>>>> a bit far fetched. Drum brakes are non-linear and unreliable.
>>
>>
>> http://tinyurl.com/jhiu
>>
>>
>>> No, I'm saying that while discs are definitely an improvement, drums
>>> are not nearly as dangerous and unacceptable, at least in the better
>>> automotive implementations, as you make them out to be.
>>
>>
>>> How do you explain, if the servo-action is responsible for the
>>> effects that you claim, that some automotive drums did *not* use
>>> servo-action brakes and still exhibited similar fade? It's the
>>> heat, plain and simple. That was the biggest downfall of drums, and
>>> it's the biggest advantage of discs. To a lesser extent the
>>> increased linearity of brake torque vs. line pressure that you did
>>> mention. That is not to say that there aren't downsides - the
>>> biggest *disadvantage* of discs is the necessity to install a power
>>> booster on all but the lightest cars, as you need significantly more
>>> line pressure to develop the same brake torque as the same size drum
>>> brake. This is what I was alluding to when I mentioned the improved
>>> pedal feel of drums - the vacuum booster commonly used does make for
>>> a far less direct feel on the brake pedal.
>>
>>
>> When the brake application force is in the direction of drum rotation,
>> as it must be with drum brakes, then there is a self energising
>> effect.
>
> Actually, it doesn't matter which direction, on most brakes. The servo
> effect works both ways, but the leading shoe is smaller for reasons of
> even wear.
>
>> Proof of that is the rum brakes lock up at times when the
>> friction coefficient is slightly higher than the design level.
>
> Ok, so? that's not generally a problem.
>
>> All
>> deum brakes have servo effect.
>
> No, there are most definitely non-servo drum brakes. Early Studebaker
> drums for example, or some of the odd dual wheel cylinder designs.
>
>> If you have worked on drum brakes you
>> should have noticed that they are designated leading shoe brakes, that
>> is that the activating hydraulic piston is pushing in the direction of
>> drum rotation.
>
> And that is for reasons of even shoe wear. also the wheel cylinder
> pushes both ways in the normal drum brake setup, so the servo action can
> work in both directions.
>
>>
>>>>>>>>> The difficult part is getting rid of all the heat that the
>>>>>>>>> brakes are converting the bike's kinetic energy into.
>>
>>
>>>>>> That's why heavy trucks still use drum brakes, drums having large
>>>>>> surface to reject heat by forced convection. Highways have long
>>>>>> dual tired skid marks because the response to brake application
>>>>>> cannot be well controlled with drum brakes.
>>
>>
>>>>> I believe that they still use drums because they're easier to work
>>>>> with in an air brake application. They still don't shed heat
>>>>> nearly as well as to discs. I suspect many of those skid marks
>>>>> are also the result of air system failures, not panic stops gone
>>>>> bad.
>>
>>
>>>> You are guessing. Brakes are used for performance and disks large
>>>> enough to control a large truck will not fit inside standard truck
>>>> wheels. That's why disks are not used there.
>>
>>
>>> I actually *have* seen discs on heavy trucks... not nearly as many
>>> as drums, but they are out there.
>>
>>
>> You may have seen them on tractors of semi trailer rigs, but not on
>> the tandem dual dive axles or the semitrailer where heavy braking
>> occurs.
>>
>>
>>>>>>>> Apparently the bicycle industry is not addressing that part of
>>>>>>>> the problem when the surface area and thermal mass their
>>>>>>>> gossamer rings of steel have. I have asked brake manufacturers
>>>>>>>> why their disks are mostly air with a thin pattern of thin
>>>>>>>> steel between. For cooling is the answer. Maybe they should
>>>>>>>> tell the automotive and railway people about their theory.
>>
>>
>>>>>>> ??? most automotive disc brakes are "vented," that is, the
>>>>>>> braking surfaces are separated by a webbing of cast iron
>>>>>>> intended to have air pass through. High-performance rotors are
>>>>>>> also cross-drilled and/or slotted.
>>
>>
>>>>>> I think you'll find that they are not vented but rather solid
>>>>>> disks with no holes.
>>
>>
>>>>> Not true at all.
>>
>>
>>>> OK! You keep making these claims and show no reason or reference
>>>> for them. Just inspect BMW cars and note that even their large
>>>> "sport cars"do not have holes in the disks.
>>
>>
>>> But they are still vented.
>>
>>
>> Look at all the high performance (BMW, Mercedes, Caddillac, Chrysler,
>> Cheverolet, Buick,...) cars and you will see they have no holes. You
>> must be assuming car companies are taking safety lightly if they sell
>> the largest part of their cars with, as you claim, inferior disks that
>> have no holes.
>
> They may not be cross drilled, but they are all vented.
>
>>>>>> The cross drilling is found only on vehicles tauted to be high
>>>>>> performance while no passenger sedan has cross drilling that has
>>>>>> no technical advantage but is a relic of the days when brake fade
>>>>>> was believed to be caused by out-gassing of brake shoes to
>>>>>> generate a gas bearing. The holes allow that imaginary gas to
>>>>>> escape.
>>
>>
>>>>> True, but all but very light and/or inexpensive cars still use
>>>>> vented rotors, at least on the front. you are confusing
>>>>> cross-drilling and/or slotting with simple venting, which is a
>>>>> common feature of nearly all automotive disc brakes. Look at a
>>>>> disc edge-on, you will see that it is not solid but is essentially
>>>>> two solid braking surfaces connected by a webbing and with lots of
>>>>> air space in between. That is what is commonly known as a
>>>>> "vented" rotor.
>>
>>
>>>> Name a few please.
>>
>>
>>> Any modern car equipped with disc brakes. In my driveway, a Chevy
>>> Impala (icky company car) a Porsche 944 and a Ford F-150. All have
>>> vented discs, at least on the front.
>>
>>
>> Not the new ones aold today. Exactly these are proof that there is no
>> benefit to the cross drillings in the disks of "sports cars".
>
> Yes, all of the new ones sold today. They are vented, as I defined
> above (and as is common usage.)
>
>>
>>> The last car I saw *without* vented front discs was my mom's '86 VW
>>> Golf. It did have solid front discs.
>>
>>
>> You use the term "vemnted" without definition. What do you mean?
>
>
> See above, I defined the common usage of the term when referring to brakes.
>
>>
>>>>>> I worked in brake design with Girling and ATE in the years when
>>>>>> the auto industry was changing to disks and was amazed at the
>>>>>> amount of myth and lore surrounding brakes.
>>
>>
>>>>> I don't mean to be insulting, but that surprises me given the
>>>>> inaccuracies in your post. Although that would explain your
>>>>> unfamiliarity with vented rotors, as AFAIR at least the Studebaker
>>>>> applications of the first Girling brakes did in fact use solid
>>>>> rotors. I would assume that other applications of those brakes
>>>>> did as well as I'm not aware of any significant variations on the
>>>>> caliper design.
>>
>>
>>>> You are not trying hard enough to not be insulting. Your
>>>> unsupported claims are in themselves insulting in facer of evidence
>>>> to the contrary. The subject here was to cross drilling with holed
>>>> through the disk faces. RR disks are all internally radially
>>>> vented in the disk casting.
>>
>>
>> http://www.merriam-webster.com/dictionary/disk
>>
>>
>>> At this point, I can't even determine if you understand the
>>> difference between venting and drilling. If you can't keep your
>>> terminology straight there's no point in continuing this discussion.
>>
>>
>> At this point I suggest you heed your request for definition of disk
>> form. I detect you are trying to make a technical educational course
>> about disk brake, not understanding the basic difference between disks
>> and drums. I am not ready to outline a course on disk brakes here.
>
> I'm afraid I don't need your "technical educational course" - not to do
> the credential dick-waving thing, but I too have worked professionally
> with braking systems (albeit specifically with ABS software) and
> apparently quite a bit more recently than you. You're confusing venting
> with cross-drilling, or you're not understanding the terminology that
> I'm using which is standard and has been since the late 60's.
>
> nate
>


A vented disk is like a rotor of a radial ventilator. Maybe Jobst
understands now what you mean.

Lou

Ben C

On 2008-05-02, Nate Nagel <njnagel@roosters.net> wrote:
[...]
> How do you explain, if the servo-action is responsible for the effects
> that you claim, that some automotive drums did *not* use servo-action
> brakes and still exhibited similar fade? It's the heat, plain and
> simple.

I used to have a car with four drum brakes (still have it actually long
story).

There were three distinct kinds of fade. The first was "pedal drop",
which was easily cured by pumping the pedal a couple of times. I believe
this was most likely to be caused by drum expansion. But I'd be
interested to hear other theories.

The second was fluid fade, where the fluid boils, resulting in a
distinctive "spongy" feel to the pedal. That was cured by changing the
brake fluid: modern fluid in good condition seemed more than capable of
surviving the highest temperatures that could be produced by such a
braking system.

The third kind was when you had the pedal pumped up, the fluid wasn't
boiling, but you'd find yourself pushing very hard on the pedal without
the car slowing down nearly as much as you were hoping. It didn't go
spongy or drop, just didn't seem to do much.

I took it that was some kind of reduction of friction coefficient caused
by too much heat, but don't claim to be able to explain it completely.

> That was the biggest downfall of drums, and it's the biggest advantage
> of discs. To a lesser extent the increased linearity of brake torque
> vs. line pressure that you did mention. That is not to say that there
> aren't downsides - the biggest *disadvantage* of discs is the
> necessity to install a power booster on all but the lightest cars, as
> you need significantly more line pressure to develop the same brake
> torque as the same size drum brake. This is what I was alluding to
> when I mentioned the improved pedal feel of drums - the vacuum booster
> commonly used does make for a far less direct feel on the brake pedal.

Another downside of drums was that if you had to brake suddenly there
was a greater chance of the brake snatching on on one side before the
other.

This I took to be because they aren't "self-adjusting" in the simple way
disks are, so the pads on each side are likely to start off a more
variable distance from their target, and because of the leading-shoe
servo action which makes them snatch on as soon as they make contact.

Mine weren't self-adjusting at all, you had to keep adjusting them. Some
other cars had self adjusting mechanisms of various kinds but they
weren't as simple as the way a disk caliper works because they need
springs to retract the pads.

I don't miss drum brakes at all.

[...]
>> OK! You keep making these claims and show no reason or reference for
>> them. Just inspect BMW cars and note that even their large "sport
>> cars"do not have holes in the disks.
>>
>
> But they are still vented.

I also haven't seen a disk that wasn't vented in the way you described
for years. Cross-drilled disks are rarer. You see them on Porsche 911s.

Ben C

On 2008-05-03, Nate Nagel <njnagel@roosters.net> wrote:
> jobst.brandt@stanfordalumni.org wrote:
[...]
>> All
>> deum brakes have servo effect.
>
> No, there are most definitely non-servo drum brakes. Early Studebaker
> drums for example, or some of the odd dual wheel cylinder designs.

Just to clarify he means the servo effect of leading shoes, not of a
manifold vacuum operated brake servo or "power booster".

>> If you have worked on drum brakes you should have noticed that they
>> are designated leading shoe brakes, that is that the activating
>> hydraulic piston is pushing in the direction of drum rotation.
>
> And that is for reasons of even shoe wear.

No Jobst is right about that: it is for the servo action.

> also the wheel cylinder pushes both ways in the normal drum brake
> setup, so the servo action can work in both directions.

Front drums usually have two cylinders because they're twin leading-shoe
for best braking going forwards.

Rear drums often have one cylinder with a piston coming out of each end
to give you one leading and one trailing shoe. That's so you have some
braking in reverse. Also because you might park facing up a hill.

Ben C

On 2008-05-03, Tom Sherman <sunsetss0003@REMOVETHISyahoo.com> wrote:
> jobst.brandt@stanfordalumni.org aka Jobst Brandt wrote:
>> [...]
>> That's why heavy trucks still use drum brakes, drums having large
>> surface to reject heat by forced convection. Highways have long dual
>> tired skid marks because the response to brake application cannot be
>> well controlled with drum brakes.[...]
>>
> Herr Brandt must not have driven a 2002-2005 Honda Civic, which has rear
> drum brakes but exhibits none of the ills that Brandt claims are
> inherent to drums.

Drums are fine on the back. The back brakes do very little in normal
driving, and are mostly useful for when you're parked, for which the
drum is a good design as it shrinks "on" when it cools.

You don't want drums on the front.

A cable operated parking brake on rear disks is possible but you usually
get a self-adjusting threaded oddity in the rear slave cylinders which
is a bugger when you come to change the pads because you have to twist
it back in.

Some cars with rear disks have little drums as well, just for the
parking brake.

Ben C

On 2008-05-03, TBerk <bayareaberk@yahoo.com> wrote:
> On May 2, 7:49 pm, Tom Sherman <sunsetss0...@REMOVETHISyahoo.com>
[...]
>> Be sure to get a system with a through-bolt or one of the rare forks
>> that has the dropouts and/or caliper mounts arranged so the brake is not
>> trying to eject the wheel:
>> <http://www.ne.jp/asahi/julesandjames/home/disk_and_quick_release/inde...>.
>>
>> --
>> Tom Sherman -
>
>
> Now, see? While as yet unsubstantiated it seems like the kind of help
> I started this thread for in the 1st place.

The wheel ejection problem is correct in theory but in practice it
doesn't happen if you do your QRs up reasonably tight.

Nate Nagel

Ben C wrote:
> On 2008-05-03, Nate Nagel <njnagel@roosters.net> wrote:
>
>>jobst.brandt@stanfordalumni.org wrote:
>
> [...]
>
>>>All
>>>deum brakes have servo effect.
>>
>>No, there are most definitely non-servo drum brakes. Early Studebaker
>>drums for example, or some of the odd dual wheel cylinder designs.
>
>
> Just to clarify he means the servo effect of leading shoes, not of a
> manifold vacuum operated brake servo or "power booster".

I understand that. There are drum brakes that were put into production
that did *not* use the typical servo-action design.

nate

--
replace "roosters" with "cox" to reply.
http://members.cox.net/njnagel

Tom Sherman

Ben C wrote:
> On 2008-05-03, TBerk <bayareaberk@yahoo.com> wrote:
>> On May 2, 7:49 pm, Tom Sherman <sunsetss0...@REMOVETHISyahoo.com>
> [...]
>>> Be sure to get a system with a through-bolt or one of the rare forks
>>> that has the dropouts and/or caliper mounts arranged so the brake is not
>>> trying to eject the wheel:
>>> <http://www.ne.jp/asahi/julesandjames/home/disk_and_quick_release/inde...>.
>>>
>>> --
>>> Tom Sherman -
>>
>> Now, see? While as yet unsubstantiated it seems like the kind of help
>> I started this thread for in the 1st place.
>
> The wheel ejection problem is correct in theory but in practice it
> doesn't happen if you do your QRs up reasonably tight.

If there is any motion between the axle and the dropout, this could
cause the QR to loosen. In addition, the skewer could fail or the QR
could be loosened by a minor impact while riding off-road.

While wheel ejection is obviously a rare occurrence, considering the
likely severity of the accident if wheel ejection does occur, NOT
designing the wheel retention system to eliminate the possibility is
irresponsible.

--
Tom Sherman - Holstein-Friesland Bovinia
The weather is here, wish you were beautiful

Michael Press

In article <slrng1odji.i35.spamspam@bowser.marioworld>,
Ben C <spamspam@spam.eggs> wrote:

> On 2008-05-03, TBerk <bayareaberk@yahoo.com> wrote:
> > On May 2, 7:49 pm, Tom Sherman <sunsetss0...@REMOVETHISyahoo.com>
> [...]
> >> Be sure to get a system with a through-bolt or one of the rare forks
> >> that has the dropouts and/or caliper mounts arranged so the brake is not
> >> trying to eject the wheel:
> >> <http://www.ne.jp/asahi/julesandjames/home/disk_and_quick_release/inde...>.
> >
> > Now, see? While as yet unsubstantiated it seems like the kind of help
> > I started this thread for in the 1st place.
>
> The wheel ejection problem is correct in theory but in practice it
> doesn't happen if you do your QRs up reasonably tight.

Depend on what you mean by ejection. Many riders reported
a loose quick release at the bottom of the hill that was
tight at the top. Braking reaction force in the direction
of the open fork tip combined with the short, hard forces
of a fast, steep down hill run combine to loosen the
quick release nut.

--
Michael Press

Peter Cole

Ben C wrote:

>>> OK! You keep making these claims and show no reason or reference for
>>> them. Just inspect BMW cars and note that even their large "sport
>>> cars"do not have holes in the disks.
>>>
>> But they are still vented.
>
> I also haven't seen a disk that wasn't vented in the way you described
> for years. Cross-drilled disks are rarer. You see them on Porsche 911s.

Cross-drilled seems to be more about looks than anything else. "Vented"
or "ventilated" rotors are another matter. They appear to be reasonably
effective, but it's not a *huge* effect (+40%, rather than 100%).

The best paper I have found:
http://adt.lib.rmit.edu.au/adt/uplo...lic/02whole.pdf

Conclusions: The most important thing is air flow to the brake area
(open wheels, etc.), second importance is vehicle speed, third is rotor
venting (radial), etc.

I had thought one of the reasons that "vented" rotors were used was to
make the disk stiffer in the face of thermal distortion, but it's not
clear that is the case. I doubt that they cost much more to make, so
with a significant improvement in dissipation, it's probably a
no-brainer. Outside of the track, it seems like modern front wheel
disks, vented or not, seem to have plenty of capacity, so I don't know
what the big deal is. That's all I've driven for the past 30 years.

JG

My physics-fu is feeling a little weak.

While it is true that the limits on grip strength and size will limit
how much force you can put into the brake pads, it does seem that
that force on the pads at a 28" rim is going to be four times as
effective as pads on a 7" rotor, and that a disk brake will be
inherently less effective than a rim brake. Or am I missing
something?

JG

Tom Sherman

JG wrote:
> My physics-fu is feeling a little weak.
>
> While it is true that the limits on grip strength and size will limit
> how much force you can put into the brake pads, it does seem that
> that force on the pads at a 28" rim is going to be four times as
> effective as pads on a 7" rotor, and that a disk brake will be
> inherently less effective than a rim brake. Or am I missing
> something?
>
Different materials are in contact with each other during braking in
each case.

Rim brakes are fair to poor in the wet, and lousy in mud and snow.

--
Tom Sherman - Holstein-Friesland Bovinia
The weather is here, wish you were beautiful

A Muzi

JG wrote:
> My physics-fu is feeling a little weak.
>
> While it is true that the limits on grip strength and size will limit
> how much force you can put into the brake pads, it does seem that
> that force on the pads at a 28" rim is going to be four times as
> effective as pads on a 7" rotor, and that a disk brake will be
> inherently less effective than a rim brake. Or am I missing
> something?

All else being equal, yes. But all else is not equal. Watch you calipers
flex during braking for example. And try to put a pothole rim dent on a
stainless disc for another. And so on.

They are different but IMHO neither is 'better'.
--
Andrew Muzi
<www.yellowjersey.org/>
Open every day since 1 April, 1971
** Posted from http://www.teranews.com **

Andre Jute

On May 3, 10:42*pm, JG <j...@cox.net> wrote:
> My physics-fu is feeling a little weak.

Braking is limited by the coefficient of friction between the road and
the tire. Theoretically, at least, there is no intrinsic reason for
any of the brake methods (rim, disc, drum) to be inferior to the
others. In practice the mechanics of bikes are so well developed that
any adult or child can apply enough braking force to exceed the
friction between tire and road, regardless of the type of brake
fitted. Under ideal conditions, which is what theory deals in, all
bicycle brakes are better than good enough and therefore equal.

> While it is true that the limits on grip strength and size will limit
> how much force you can put into the brake pads, *it does seem that
> that force on the pads at a 28" rim is going to be four times as
> effective as pads on a 7" rotor, and that a disk brake will be
> inherently less effective than a rim brake. *Or am I missing
> something?

If only practice could be as simple as theory! (Why, my perpetual
motion machine might suddenly burst into a couple of Hallelujah
choruses...) But, for a start, the available friction against the rim
is limited by rain, snow and just plain dirt, which is not true of
discs and roller brakes. Since the weather is known to be changeable,
that's a major functional inequality already. Also, as Andrew has
pointed out, rims are more vulnerable to becoming uneven than the
competing types of brakes, further detracting from the ideal.

It is good to think about these things. I wonder every time I see a
rear disc brake on a bike what it is *for*.

Someone kindly explain "fu" to me. TIA.

Andre Jute
--
http://members.lycos.co.uk/fiultra/...%20CYCLING.html

Clive George

"JG" <jchg@cox.net> wrote in message
news:7cb4b725-dd9b-4003-8b96-9f2f9c6a399b@y18g2000pre.googlegroups.com...
> My physics-fu is feeling a little weak.
>
> While it is true that the limits on grip strength and size will limit
> how much force you can put into the brake pads, it does seem that
> that force on the pads at a 28" rim is going to be four times as
> effective as pads on a 7" rotor, and that a disk brake will be
> inherently less effective than a rim brake. Or am I missing
> something?

That bit is entirely true. However the material for the disk rotor and pads
can be chosen for friction, not weight, the rotor can be kept true more
easily so the pad to rotor distance can be smaller, and caliper can be a lot
smaller so stiffer.
This means the mechanical advantage of the disc brake (movement of lever vs
movement of pad) can be a lot higher on a disk brake, thus making up for the
diameter-related disadvantage.

cheers,
clive