From: Clocky on
John_H wrote:
> Clocky wrote:
>> John_H wrote:
>>> Clocky wrote:
>>>>
>>>> In theory braking performance should increase with grooves because
>>>> you are effectively increasing the surface area of the disc.
>>>
>>> In theory, the braking performance should be exactly the same, with
>>> or without being bedded in, unless heat transfer is an issue (which
>>> would be improved by the scoring).
>>
>> Braking performance is reduced if the pads are not bedded into a
>> scored disc as the pads are only in contact with the upper ridges
>> which reduced the effective surface area of cotact between the pad
>> and the disc surface.
>
> No it isn't. The frictional force between the pad and the rotor is
> proportional to the pressure acting between the surfaces. Under
> normal design pressures and equal working temperatures the coefficient
> of friction is a constant and the total friction (braking force) is
> the coefficient of friction multiplied by the contact pressure.
> Double the contact surface and you halve the pressure (or vice versa)
> but the overall braking force remains the same.
>
> The exception is at very high pressure, which will increase the
> coefficient of friction as the materials approach the point of
> seizure. If brakes operated under those conditions (which they don't)
> decreasing the pad area would *increase* the braking force applied to
> the rotor... the exact opposite of what you're claiming.
>
>>>
> <snip>
>>>
>>> Force applied to the pad multiplied by the coefficient of friction
>>> is the braking force applied to the rotor, irrespective of the
>>> contact area.
>
> My previous statement is incorrect... I should've written *Force per
> unit area* (ie pressure) multiplied by the coefficient of friction is
> the braking force applied to the rotor....
>
>>
>> Sure, the force may be the same but the amount of friction is
>> reduced by having less contact between the disc and the pad which
>> results in poorer brake performance, until the pads are bedded in
>> that is.
>
> Nope. Exactly the same principle applies between tyres and road. If
> what you're claiming is correct you'd increase the grip by reducing
> the tyre pressures (which increases the area of the contact while
> weight of the car remains the same).
>
> In fact the opposite applies, same as it would for brake materials
> operating above their design pressures.

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.


From: Milton on

"Clocky" <notgonn(a)happen.com> wrote in message
news:4bfd19cc$0$8786$c3e8da3(a)news.astraweb.com...
> John_H wrote:
>> Clocky wrote:
>>> John_H wrote:
>>>> Clocky wrote:
>>>>>
>>>>> In theory braking performance should increase with grooves because
>>>>> you are effectively increasing the surface area of the disc.
>>>>
>>>> In theory, the braking performance should be exactly the same, with
>>>> or without being bedded in, unless heat transfer is an issue (which
>>>> would be improved by the scoring).
>>>
>>> Braking performance is reduced if the pads are not bedded into a
>>> scored disc as the pads are only in contact with the upper ridges
>>> which reduced the effective surface area of cotact between the pad
>>> and the disc surface.
>>
>> No it isn't. The frictional force between the pad and the rotor is
>> proportional to the pressure acting between the surfaces. Under
>> normal design pressures and equal working temperatures the coefficient
>> of friction is a constant and the total friction (braking force) is
>> the coefficient of friction multiplied by the contact pressure.
>> Double the contact surface and you halve the pressure (or vice versa)
>> but the overall braking force remains the same.
>>
>> The exception is at very high pressure, which will increase the
>> coefficient of friction as the materials approach the point of
>> seizure. If brakes operated under those conditions (which they don't)
>> decreasing the pad area would *increase* the braking force applied to
>> the rotor... the exact opposite of what you're claiming.
>>
>>>>
>> <snip>
>>>>
>>>> Force applied to the pad multiplied by the coefficient of friction
>>>> is the braking force applied to the rotor, irrespective of the
>>>> contact area.
>>
>> My previous statement is incorrect... I should've written *Force per
>> unit area* (ie pressure) multiplied by the coefficient of friction is
>> the braking force applied to the rotor....
>>
>>>
>>> Sure, the force may be the same but the amount of friction is
>>> reduced by having less contact between the disc and the pad which
>>> results in poorer brake performance, until the pads are bedded in
>>> that is.
>>
>> Nope. Exactly the same principle applies between tyres and road. If
>> what you're claiming is correct you'd increase the grip by reducing
>> the tyre pressures (which increases the area of the contact while
>> weight of the car remains the same).
>>
>> In fact the opposite applies, same as it would for brake materials
>> operating above their design pressures.
>
> 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.

You're right as far as I'm concerned Clocky. I'm sure John has his
"theories" mixed up with reality. In layman's terms, the more contact the
pad has with the rotor, would "naturally" create more area to grip. If there
are high ridges on the rotor, once the new pads were installed, of course
the first point of contact is going to be with the high ridges, until those
ridges have grooved into the new pads allowing them to then make contact
with the bulk surface area of the rotor.

Interesting to note Darrin has kept out of this "discussion" between two of
his favourites. He either has no idea or he doesn't want to "upset" one of
you.

Regards

Milton

From: John_H on
Clocky wrote:
>John_H wrote:
>> Clocky wrote:
>>> John_H wrote:
>>>> Clocky wrote:
>>>>>
>>>>> In theory braking performance should increase with grooves because
>>>>> you are effectively increasing the surface area of the disc.
>>>>
>>>> In theory, the braking performance should be exactly the same, with
>>>> or without being bedded in, unless heat transfer is an issue (which
>>>> would be improved by the scoring).
>>>
>>> Braking performance is reduced if the pads are not bedded into a
>>> scored disc as the pads are only in contact with the upper ridges
>>> which reduced the effective surface area of cotact between the pad
>>> and the disc surface.
>>
>> No it isn't. The frictional force between the pad and the rotor is
>> proportional to the pressure acting between the surfaces. Under
>> normal design pressures and equal working temperatures the coefficient
>> of friction is a constant and the total friction (braking force) is
>> the coefficient of friction multiplied by the contact pressure.
>> Double the contact surface and you halve the pressure (or vice versa)
>> but the overall braking force remains the same.
>>
>> The exception is at very high pressure, which will increase the
>> coefficient of friction as the materials approach the point of
>> seizure. If brakes operated under those conditions (which they don't)
>> decreasing the pad area would *increase* the braking force applied to
>> the rotor... the exact opposite of what you're claiming.
>>
>>>>
>> <snip>
>>>>
>>>> Force applied to the pad multiplied by the coefficient of friction
>>>> is the braking force applied to the rotor, irrespective of the
>>>> contact area.
>>
>> My previous statement is incorrect... I should've written *Force per
>> unit area* (ie pressure) multiplied by the coefficient of friction is
>> the braking force applied to the rotor....
>>
>>>
>>> Sure, the force may be the same but the amount of friction is
>>> reduced by having less contact between the disc and the pad which
>>> results in poorer brake performance, until the pads are bedded in
>>> that is.
>>
>> Nope. Exactly the same principle applies between tyres and road. If
>> what you're claiming is correct you'd increase the grip by reducing
>> the tyre pressures (which increases the area of the contact while
>> weight of the car remains the same).
>>
>> In fact the opposite applies, same as it would for brake materials
>> operating above their design pressures.
>
>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.

Go back to to your original post... the one that says "In theory
braking performance should increase with grooves because
you are effectively increasing the surface area of the disc."

The theory, according to the physics textbook, doesn't support your
claim.

What happens in practice *might* be something else again, since
factors other than the surface area of the disc apply. Perhaps you
forgot to mention them! :)

--
John H
From: Noddy on

"Milton" <millame23(a)yahoo.com> wrote in message
news:4bfd70fd$0$8775$c3e8da3(a)news.astraweb.com...

> Interesting to note Darrin has kept out of this "discussion" between two
> of his favourites. He either has no idea or he doesn't want to "upset" one
> of you.

It may not have occurred to a simpleton like you Woger, but I actually agree
with what's being said.

--
Regards,
Noddy.


From: John_H on
D Walford wrote:
>On 27/05/2010 2:44 PM, John_H wrote:
>> jonz wrote:
>>> On 5/27/2010 10:48 AM, John_H wrote:
>
>>>> "Formula One tyre for dry surfaces is a colossus of 660 mm in external
>>>> diameter and 350 mm wide, containing four
>>>> longitudinal grooves of at least 2.5 mm imposed by the Depth
>>>> Regulations. These grooves are symmetrically placed from the centre of
>>>> the tyre tread and spaced 50 mm apart. Far from being just an altered
>>>> slick, the dry surface tyre is a completely new concept,
>>>> _introduced to F1 with the sole aim of reducing the size of the
>>>> ground contact area, i.e. the surface which ensures grip_, "
>>> ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>>>
>>> and here`s the bit you left out.....: "resulting from the contact of
>>> the rubber compound and track. The aim of the
>>> regulations: to reduce the speed of the cars on corners... even if this
>>> means lost sleep for the manufacturers' design offices". ...... in other
>>> words, *less* traction resulting from *decreased* contact patch thus
>>> achieving the "powers that be" aim of *reducing* speed in corners.....
>>
>> Very astute... perhaps you're not quite as stupid as you pretend to
>> be. Score one point. :)
>>
>> Now for Mensa award.... If the explanation is correct (which I'd
>> seriously doubt) why don't the competitors simply reduce their tyre
>> pressures and increase the contact area?
>>
>That would depend on how low they already run their tyre pressures, most
>Clubman racers use 16-18psi in their tyres because the lower pressure
>improves grip, any higher and you can't keep the car on the road.
>The most popular tyre on racing Clubmans at club level is Avon ACB's
>which are Formula Ford tyres.

They also use entirely different technology to road tyres (it's not
comparing apples with apples). I used the car tyre example purely to
demonstrate that coefficient of friction can increase with pressure at
some point (ie a smaller surface area can result in more friction if
the pressure is high enough). The same example is used in some texts
on the topic of friction and I would've thought anyone who's ever used
a tyre gauge would be familiar with the effect. :)

I'd suspect that if racing cars do get more grip at lower pressures
it's due to the higher running temperatures (and more rapid warm up
times) produced by those pressures (the coefficient of friction will
increase with temperature). The penalty is a very short tread life
which wouldn't be acceptable on a road car.

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.

--
John H