From: John McKenzie on
John_H wrote:
> 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.

yep. I suppose one could also discuss whether increased pressures also
contribute to more stabilty of each 'chunk' of tread. It certainly must
count for something, because in categories wth a mandated treaded tyre,
they get better handling, and longer life by buffing them down to near
min tread depth.

> 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.

I'm wondering if one of the gains might be from situations where (for a
worse case scenario)some of the tread goes over a section of road with
inconsistent grip - perhaps the painted lines on the road. If most of
the tread happened to be contactig such, but a slick would still have
more contact with grippier bitumen, then the slick might win there -
essentially the friction coefficient of the road is not consistent (for
want of better terminology)

John McKenzie

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From: Milton on

"Clocky" <notgonn(a)> wrote in message
> Noddy wrote:
>> "Milton" <millame23(a)> wrote in message
>> news:4bfd70fd$0$8775$c3e8da3(a)
>>> 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.
> Agree with who?

Thanks for asking, I couldn't get his drift either other than a foot in both
camps. FWIW, as I already stated I agree with you in this instance.



From: Milton on

"Clocky" <notgonn(a)> wrote in message
> John_H wrote:
>> 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! :)
> Throwing physics textbooks under the wheels of the car may improve braking
> performance also.
Yep, nothing like reality. These damn theorists who go by the book just
don't get it in the real world do they! I just don't understand why he of
all people would be arguing the point with you when it's as simple as
understanding very basic physics.



From: Milton on

"Noddy" <me(a)> wrote in message
> "Clocky" <notgonn(a)> wrote in message
> news:4bfe5af3$0$27823$c3e8da3(a)
>> Agree with who?
> You and John.
> It may look like you're disagreeing, but as I read it you're essentially
> saying the same thing.
> --
> Regards,
> Noddy.
Come on Noddy, let's see you get out of this one. Explain yourself, please!
Oh you can't? Just do your usual then and throw some diversionary tactics at
me by calling me names again, that'll do to big note yourself.



From: John_H on
Noddy wrote:
>"Clocky" <notgonn(a)> wrote in message
>> Agree with who?
>You and John.
>It may look like you're disagreeing, but as I read it you're essentially
>saying the same thing.

Almost, but not quite... and I _never_ have brake rotors machined.
Either reuse them or replace them.

I agree that grooved discs don't matter (as do most car
manufacturers). I neither agree nor disagree that they work better
than ungrooved rotors after the pads are worn in to conform with the
grooves (some might, some might not).

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.

Clocky might like to try this....

Throw away the crappy OEM pads and replace them with something decent,
preferably Bendix Ultimates. Using a suitable location do three
consecutive crash stops from 150kph or so, don't lock 'em up and don't
brake to a complete stop. The rotors will be glowing and the pads
will be smoking!

They're now bedded in and the brake performance will *probably* be as
good as it's ever likely to get.

After they're cooled down inspect the rotors. It's highly unlikely
the pads will be sufficiently worn to fill the grooves at that point
in time, nor is the brake performance likely to improve once they
have. If perchance it does I'd be looking for other explanation than
surface area... brake dust collecting in the grooves perhaps.

John H