From: John_H on
Jason James wrote:
>"Noddy" <me(a)home.com> wrote in message
>news:4bff0e6c$0$11949$c30e37c6(a)exi-reader.telstra.net...
>> "John_H" <john4721(a)inbox.com> wrote in message
>>>
>>> What I'm disputing is the existence of any credible theory that says
>>> the increased surface area due to scoring affects the performance in
>>> any way.
>>
>> I also tend to agree, however the theory that increased contact via larger
>> surface area seems to make sense.
>
>Except, the angled sides of scoring dont present the pad with as an
>effective friction surface as parts of the disc surface that are parallel
>with the pad surface. Make sense? :-)

No. Because the contact area is irrelevant to their performance, so
is the shape. Two sheets of corrugated iron will behave exactly the
same as two flat sheets if you were to slide them apart under the same
load.

As for contact area, disc brake examples that come to mind are those
fitted to the Triumph TR3 and Mk I Jaguar from 1956 (first road cars
to use them). They had massive pad areas compared to modern brakes
but didn't work any better (or worse). Only difference was the life
of the components. I can recall it as being common to see 25 y.o.cars
that still had their original pads. Rotors never got machined (or
replaced).

--
John H
From: John_H on
D Walford wrote:
>On 28/05/2010 4:51 PM, John_H wrote:
>> Noddy wrote:
>>> "John_H"<john4721(a)inbox.com> wrote in message
>>> news:hbqtv59khu67vhiioh1iqd8ijm0ereje90(a)4ax.com...
>>>>
>>>> What I'm disputing is the existence of any credible theory that says
>>>> the increased surface area due to scoring affects the performance in
>>>> any way.
>>>
>>> I also tend to agree, however the theory that increased contact via larger
>>> surface area seems to make sense.
>>
>> But it doesn't make any sense at all. The basic theory of friction
>> found in any physics textbook (attributed to Coulomb) states that the
>> force of friction equals the coefficient of friction multiplied by the
>> force pushing the two objects together (Ff = COF x Fn) . No mention
>> of surface area.
>>
>> Imagine a 3m diameter brake disc with a 1m diameter brake pad. You
>> operate it by pushing on the centre of the pad with your finger.
>>
>> Now change the 1m pad for one 10cm in diameter, of the same material,
>> located at the same centre position. Operate it with the same finger.
>>
>> Which setup do think will provide the greatest braking force?
>>
>> The answer is neither. Assuming you apply the same force with your
>> finger, the braking force (due to friction) will be exactly the same
>> even though the second disc only has a thousandth of the contact area
>> of the first.
>
>Are you sure about that?

Absolutely... unless a good part of my formal education has suddenly
been rendered obsolete! :)

Google will find no shortage of proof. Wiki has a reasonable
article.... http://en.wikipedia.org/wiki/Friction

Note particularly (under History).... "The force of friction is
independent of the apparent area of contact. (Amontons' 2nd Law)".
Coulomb's equation leads to the same conclusion but the logic might be
a little harder to grasp.

>Another way of looking at it which as far as I can work out uses the
>same physics is the pressure exerted by a woman wearing stiletto heels
>on a surface compared to the pressure exerted by the same woman wearing
>flat shoes, the stilettos exert far greater pressure on the surface they
>are contacting because all the pressure is concentrated on a smaller area.
>The pressure on the disc exerted by the smaller pad would be greater so
>the braking force initially would be greater but in a very short time
>the temp in the smaller pad would rise dramatically, because of its
>smaller size it wouldn't be able to transfer as much heat to the disc so
>its braking force would be reduced very quickly.

Part of the reason I used a finger as the force is because it wouldn't
be capable of producing enough friction to generate large amounts of
heat. The assumption is that conditions remain the same. In any case
a lot of brake materials, especially the better ones, work better when
hot. Metal Kings being an example. However the design criteria for
brakes is that the coefficient of friction will remain constant (or
very close to it) over the pressure range generated. It's also what
limits the minimum pad size manufacturers can use (see my reply to
Jason).

The stiletto example also supports the case that contact area doesn't
matter. In fact it's pressure (force per unit area) that generates
the friction but if you double the contact area you halve the pressure
so the end result is the same (area cancels out when you do the sums)
..
>> What possible difference could a few score marks in the disc make?
>
>Not a lot because they don't reduce the contact area by a great amount.

Correct answer is absolutely none whatsoever since contact area is
irrelevant! :)

--
John H
From: John_H on
atec7 7 <""atec77\"@ hotmail.com"> wrote:
>
> the rule is the coefficient of friction of a known area and a known
>applied force hence if you increase the swept are without modification
>of the other two braking force as in resistance to travel increases so
>it brakes better ,same applied to any friction situation

Whose rule would that be?

Presumably whatever was meant by it has also been lost in translation!

--
John H
From: Clocky on
Jason James wrote:
> "Sylvia Else" <sylvia(a)not.at.this.address> wrote in message
> news:4bff7f68$0$2122$c3e8da3(a)news.astraweb.com...
>> On 26/05/2010 10:54 PM, Clocky wrote:
>>
>>> That's a great theory. All I know is that bedding (or wearing as
>>> you put it)
>>> the pads to the rotor results in more friction material being in
>>> contact with the rotor surface and braking performance increasing
>>> as a result.
>>
>> It is hugely counterintuitive that friction should not be a function
>> of area in contact (for non-adhesive surfaces). But it's true
>> nevertheless, and if you think you know otherwise, you know wrongly.
>
> M'dear,..scoring by nature does not present the pad with more parallel
> wearing surface. The sides of the scores are angular. God knows what
> extra friction that produces,...but it aint a function of the
> additional area.

Given that it took Holden about 25 years to engineer a disc rotor that
didn't produce brake shudder in short space of time I'm not going to get
into an arguement of theories.

The theory is probably pretty simple, but putting it into practice proved to
be anything but.


From: Sylvia Else on
On 28/05/2010 6:58 PM, Jason James wrote:
> "Sylvia Else"<sylvia(a)not.at.this.address> wrote in message
> news:4bff7f68$0$2122$c3e8da3(a)news.astraweb.com...
>> On 26/05/2010 10:54 PM, Clocky wrote:
>>
>>> That's a great theory. All I know is that bedding (or wearing as you put
>>> it)
>>> the pads to the rotor results in more friction material being in contact
>>> with the rotor surface and braking performance increasing as a result.
>>
>> It is hugely counterintuitive that friction should not be a function of
>> area in contact (for non-adhesive surfaces). But it's true nevertheless,
>> and if you think you know otherwise, you know wrongly.
>
> M'dear,..scoring by nature does not present the pad with more parallel
> wearing surface. The sides of the scores are angular. God knows what extra
> friction that produces,...but it aint a function of the additional area.

A score that manages to go right around the risk and form a circle would
tend to lead to the pad acquiring a corresponding ridge where the pad
isn't worn down by contact with the disk. I can't see this producing
extra friction.

In any other case, I'd expect the pad to undergo rapid extra wear. The
mechanism would be that where the pad material is not supported by the
disk because of missing metal, the material would deflect slightly into
the groove. The end of the groove would then cut into the deflected
material. This effect would be rapid, but limited to cutting off the pad
material only to the extent that it deflects into the groove. The force
required for the cutting would be small compared with the overall
friction. In consequence, I would expect a non-circumfrential groove to
produce a very modest and short duration increase in the apparent friction.

Sylvia.