How hard is it to find a good mechanic??
in [Aussie Drivers]
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From: jonz on 27 May 2010 21:09 On 5/28/2010 10:23 AM, Noddy wrote: > "jonz"<fj40(a)deisel.com> wrote in message > news:4bff06f4$1(a)dnews.tpgi.com.au... > >> if you can`t dazzle `em with brilliance.......baffle `em with >> bullshit.............. > > Which is the credo you live your life by, huh? ~~~~~~~~~~~~~ nup.......but it is definitely something that a few around here majored in......... > > -- > Regards, > Noddy. > > -- jonz "Usenet is like a herd of performing elephants with diarrhea - massive, difficult to redirect, awe-inspiring, entertaining, and a source of mind - boggling amounts of excrement when you least expect it." - Gene Spafford,1992
From: John_H on 28 May 2010 02:51 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. What possible difference could a few score marks in the disc make? IIRC Coulomb made his observations back in sixteen hundred and something using blocks of wood. It's not hard to come up with far more complex situations but provided the coefficient of friction remains a constant (as it will for the same materials under the same conditions) the equation still holds for both static and kinetic friction. -- John H
From: John_H on 28 May 2010 02:51 D Walford wrote: >On 27/05/2010 6:06 PM, John_H wrote: >> D Walford wrote: >>> >>> 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 knew that was the case but I threw it in just to confuse the issue:-) You also provide the clue to the difference further down. Unlike a conventional car tyre they don't work by friction! >They get so sticky with heat that you often have to clean the rubbish >that's stuck to the tyres before the next run. Racing slicks work the same way as adhesive tape does (the penny should've dropped earlier). And for the trivia department... that stuff they used to call friction tape wasn't. :) -- John H
From: Jason James on 28 May 2010 03:45 "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 > news:hbqtv59khu67vhiioh1iqd8ijm0ereje90(a)4ax.com... > >> Almost, but not quite... and I _never_ have brake rotors machined. >> Either reuse them or replace them. > > Agreed. > >> 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. > > 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? :-) Jason
From: D Walford on 28 May 2010 04:10
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 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. > 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. Daryl |