How hard is it to find a good mechanic??
in [Aussie Drivers]
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From: Sylvia Else on 31 May 2010 02:37 On 31/05/2010 2:15 PM, John_H wrote: > F Murtz wrote: >> John_H wrote: >>> 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. >> >> do you mean two sheets of corrugated and two sheets of flat the same >> width or the same surface area? > > Either, according to the basic laws of friction. Surface area is > irrelevant to the frictional force. The force (load) acting on the > contact surfaces and the coefficient of friction are the relevant > factors. Same principle will also apply for a sheet of flat against a > sheet of corrugated, or part thereof. > > There are no shortage of exceptions however, as well as examples where > forces other than friction apply (such as the racing slicks). I > wouldn't expect brake materials to one of those exception although > building materials might be. :) > It's worth noting, for the sake of clarity, that the friction is the same for corrugated material as for flat material *when sliding along the corrugations*. Sliding perpendicular to the corrugations raises other issues because of the need for at least one sheet to move up and down. Sylvia.
From: hippo on 31 May 2010 04:00 Noddy wrote: > > > "John_H" <john4721(a)inbox.com> wrote in message > news:0hl00692l445v5f5k6mihnftmmj014egkm(a)4ax.com... > > > Precisely, which why the manufacturers tend to use the smallest pads > > they can get away with without sacrificing performance... they wear > > out quicker and require replacement sooner than larger ones. > > They do indeed. > > > You can also reach a point where sufficiently high pressure > > drastically alters the coefficient of friction (which is what happens > > immediately before metal components seize). Brakes are designed to > > operate well below that point and the assumption is that the > > coefficient of friction will be relatively constant over their > > operating pressure range... which doesn't mean it won't change with > > temperature. > > No it doesn't. > > > I'd also add that Coulomb's law has many exceptions but it's still a > > reasonable approximation for relatively smooth surfaces, which > > includes brake components. The bottom line being that any change in > > brake performance due to grooved discs won't be down to differences in > > surface area. > > The theory doesn't support it, but at face value the idea of a larger > surface area having an impact looks practical. That's not to say it is, but > it *looks* that way :) > > > Differences in surface finish is something else again... which is why > > you should _never_ lathe finish brake rotors IMHO, and why you should > > always "bed in" the friction components whenever they're replaced > > (pads or discs). "Wearing in" isn't the same thing. :) > > No, it's not. > > I always prefer disc rotors to be ground finished, but sadly more and more > after market ones come out of the box with a machined finish these days > which in my opinion is a complete waste of time. > > -- > Regards, > Noddy. > > > > Simple solution. They're machined? Drop 'em on the ground. They're finished! -- Posted at www.usenet.com.au
From: hippo on 31 May 2010 20:36 Sylvia Else wrote: > > On 31/05/2010 2:15 PM, John_H wrote: > > F Murtz wrote: > >> John_H wrote: > >>> 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. > >> > >> do you mean two sheets of corrugated and two sheets of flat the same > >> width or the same surface area? > > > > Either, according to the basic laws of friction. Surface area is > > irrelevant to the frictional force. The force (load) acting on the > > contact surfaces and the coefficient of friction are the relevant > > factors. Same principle will also apply for a sheet of flat against a > > sheet of corrugated, or part thereof. > > > > There are no shortage of exceptions however, as well as examples where > > forces other than friction apply (such as the racing slicks). I > > wouldn't expect brake materials to one of those exception although > > building materials might be. :) > > > > It's worth noting, for the sake of clarity, that the friction is the > same for corrugated material as for flat material *when sliding along > the corrugations*. Sliding perpendicular to the corrugations raises > other issues because of the need for at least one sheet to move up and down. > > Sylvia. > > Politicians are one of the major exceptions. The rate of slip *increases* in direct proportion to any increase in friction &/or pressure. -- Posted at www.usenet.com.au
From: Clocky on 2 Jun 2010 07:43 John_H wrote: > Noddy wrote: >> "John_H" <john4721(a)inbox.com> wrote in message >> news:0hl00692l445v5f5k6mihnftmmj014egkm(a)4ax.com... >> >>> I'd also add that Coulomb's law has many exceptions but it's still a >>> reasonable approximation for relatively smooth surfaces, which >>> includes brake components. The bottom line being that any change in >>> brake performance due to grooved discs won't be down to differences >>> in surface area. >> >> The theory doesn't support it, but at face value the idea of a larger >> surface area having an impact looks practical. That's not to say it >> is, but it *looks* that way :) > > It doesn't to me, but maybe that's only because I've been listening to > tribologists for far too long! ;-) > > Also taught in high school physics (which is about as basic as it > gets) back in my day. All you young folk here can probably blame > declining educational standards. :) Yeah, but you where taught that the world was flat and believed it too... ;-)
From: John_H on 2 Jun 2010 17:56
Clocky wrote: >John_H wrote: >> >> Also taught in high school physics (which is about as basic as it >> gets) back in my day. All you young folk here can probably blame >> declining educational standards. :) > >Yeah, but you where taught that the world was flat and believed it too... >;-) Even worse! I can recall a physics lecturer saying no one who wasn't a sandwich short of a picnic ever had reason to think the Earth was flat and the notion that they did is a load of bunkum. Apparently recorded proof that the Earth is a sphere goes back several thousand years. Your ancestors probably would've rejected anything that resembled scientific proof though. ;-) -- John H |