From: Noddy on

"John_H" <john4721(a)inbox.com> wrote in message
news:8pmuv59ebradiq5sbt5kjuf81dq1nj1nf1(a)4ax.com...

> 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.

Agreed.

> 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?

I'd imagine them to be exactly the same.

> 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.

Absolutely.

However, I'd add that in increasing the size of the pad you reduce the
pressure between the pad and rotor for a given amount of force holding the
two together. The amount of braking force applied to the rotor will be
exactly the same as you mentioned, but the pressure between the rotor and
pad will reduce as the size of the pad goes up, and in direct proportion to
the size of the pad since pressure equals force divided by the area of
contact. If you were to keep the pressure between the pad and rotor the same
for both large and small pads, then the friction applied by the larger pad
would be greater and have a stronger braking effect.

> What possible difference could a few score marks in the disc make?

Nothing. It's just one of those things where you look at it at face value
and think that increased surface area *has* to make some kind of difference
but the reality is that it doesn't unless other forces are also altered.

--
Regards,
Noddy.


From: Noddy on

"John_H" <john4721(a)inbox.com> wrote in message
news:h64vv5h4t21mcr7aor6lih6mt0q7h24jat(a)4ax.com...

> Whose rule would that be?

Atec's second law of vehicle engineering.

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

See Atec's first law of vehicle engineering :)

--
Regards,
Noddy.


From: Sylvia Else on
On 28/05/2010 6:10 PM, 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?
> 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...

in the exact inverse of the change of area. So if the area is halved,
the pressure is doubled. Multiply the two together, and the changes
cancel out, and you find that reducing the area has had no effect on the
total friction.

so
> the braking force initially would

not

> 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.

The disk is a much better conductor of heat than the pad. I would think
that fading was a consequence of the disk getting hot as a whole, not of
the pad getting hot.
>
>> 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.
>

There we agree.

Sylvia.
From: D Walford on
On 28/05/2010 7:42 PM, John_H wrote:
> 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)

Which is what I said, the brake with the smaller contact area with the
same pressure applied will be more affective since the "force per unit
area" is greater.
For the contact area not to matter the "force per unit area" would have
to remain constant and it isn't in your original example.
> .
>>> 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! :)
>
Contact area won't make any difference to the friction coefficient but
it does make a difference to how effective a cars brakes are.
Since the way brakes stop a vehicle is to convert kinetic energy into
heat energy getting rid of lots of heat quickly is advantageous and a
larger contact area would be better at doing that.



Daryl
From: Sylvia Else on
On 27/05/2010 10:55 PM, atec7 7 > wrote:
> John_H wrote:
>> D Walford wrote:
>
>>
>> The groove story for F1, or at least the explanation given, doesn't
>> stand up if only because grooving the tyres won't reduce the area of
>> the contact patch as claimed.... Because the contact patch adjusts
>> itself to match the load and tyre pressure.
>>
>> It's an accepted fact that slicks have more grip on a hard dry surface
>> than treaded tyres but the reason isn't likely to be due to any change
>> in the size of the contact patch. Nor have I ever been interested
>> enough to look for the real story. A possible explanation might be
>> that the voids in the contact patch lower its coefficient of friction
>> (air is much, much lower than rubber) or that the grooves reduce the
>> tread temperature.
>>
> Experience tells me if you have a contact patch of x area and y friction
> /c then doubling the patch increases the amount of available grip hence
> wider tyres in drag racing
> or do you fail to agree ?

I fail to agree.

Some dragsters will be using sticky tyres, to which the normal friction
laws don't apply.

Another consideration is that even where the friction is constant, the
tyre surface experiences a shear force per unit area that is inversely
proportional to the total area in contact. That shear force per unit
area mustn't be enough to remove the tyre surface from its substrate.

Sylvia.