How hard is it to find a good mechanic??
in [Aussie Drivers]
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From: Mr.T on 25 May 2010 22:55 "jonz" <fj40(a)deisel.com> wrote in message news:4bfb8ead(a)dnews.tpgi.com.au... > but...if you introduce new pads to a grooved disk the braking > efficiency *must* be reduced....coz of the hills and dales in the > rotor...yeah/?.... Why? Doesn't take long for the pads to groove themselves to match the disk. If the disk is warped OTOH, it needs replacing. MrT.
From: John_H on 25 May 2010 23:38 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. -- John H
From: jonz on 26 May 2010 00:57 On 5/26/2010 1:38 PM, 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. ~~~~~~~~~~~~~~~~~~~~~~ so, less is more.....you a politician perchance?....... > -- 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: D Walford on 26 May 2010 03:15 On 25/05/2010 10:55 PM, OzOne(a)Crackerbox-Palace.com wrote: > On Tue, 25 May 2010 16:12:33 +1000, D Walford > <dwalford(a)internode.on.net> wrote: > >> On 25/05/2010 11:21 AM, OzOne(a)Crackerbox-Palace.com wrote: >>> On Mon, 24 May 2010 23:39:20 GMT, "Crash Lander"<idont(a)think.so> >>> wrote: >>> >>>> It shouldn't be this hard! >>> >>> You won't find one in KMart... >> >> You can if you are very lucky, a mate who has owned mostly Falcons in >> the last 10yrs was pleasantly surprised that one of the mechanics at our >> local KMart was an ex Ford mechanic and believe it or not he actually >> had a clue. >> >> >> Daryl > > Wow...what a rare find that was! Seems that way:-) > Now you're gonna tell me someone you know got a fair price from Midas > :) > That would be stretching it, I priced a rear muffler from Midas for an 1989 Prelude, I ended up buying one from a Honda dealer because it was about $100.00 cheaper. Daryl
From: D Walford on 26 May 2010 03:18
On 25/05/2010 11:05 PM, Noddy wrote: > "Clocky"<notgonn(a)happen.com> wrote in message > news:4bfbc80c$0$8783$c3e8da3(a)news.astraweb.com... > >> I suppose he could be working at the only Kmart in Australia that pays >> well but I somehow doubt it ;-) > > Aren't K-mart auto's franchise outlets? He could be making very good money > indeed. Its possible, the one near me is now open on Sunday mornings and every time I've driven past its been busy so they must be doing something right. Daryl |