How hard is it to find a good mechanic??
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From: atec7 7 ""atec77" on 28 May 2010 08:06 John_H wrote: > atec7 7 <""atec77\"@ hotmail.com"> wrote: >> the rule is the coefficient of friction of a known area and a known >> applied force hence if you increase the swept are without modification >> of the other two braking force as in resistance to travel increases so >> it brakes better ,same applied to any friction situation > > Whose rule would that be? Dunno bil has left but they way he explained it made sense as I see it if you apply the a constant x per area and increase the area with all else remaining constant you have more friction > > Presumably whatever was meant by it has also been lost in translation! > either way is adamant that's it's correct and he's the bloke with the match degree
From: Noddy on 28 May 2010 08:16 "atec7 7" <""atec77\"@ hotmail.com"> wrote in message news:htobkl$3v5$2(a)news.eternal-september.org... > Dunno bil has left but they way he explained it made sense > as I see it if you apply the a constant x per area and increase the area > with all else remaining constant you have more friction No, you don't. To get more friction you need to apply more force to overcome the drop in pressure as the surface area rises. > either way is adamant that's it's correct and he's the bloke with the > match degree Presumably his degree is by mail order from the University of Nantucket. -- Regards, Noddy.
From: Jason James on 28 May 2010 15:55 "John_H" <john4721(a)inbox.com> wrote in message news:2e0vv55ltgpqhvvffi3o7o4lsle8btobem(a)4ax.com... > 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. Yup,..the force or work, provided by the hydaulic system (from Master-cylinder) couldn't care less what the force/work dissappation is engineered like,...however there is one exception? IE if the disc was covered in grease? > > As for contact area, disc brake examples that come to mind are those > fitted to the Triumph TR3 and Mk I Jaguar from 1956 (first road cars > to use them). They had massive pad areas compared to modern brakes > but didn't work any better (or worse). Only difference was the life > of the components. I can recall it as being common to see 25 y.o.cars > that still had their original pads. Rotors never got machined (or > replaced). Good point,..and I used to blame the lousy brakes on the '74 POS Passatt on the pissy little pads :-) Jason
From: Jason James on 28 May 2010 16:00 "Sylvia Else" <sylvia(a)not.at.this.address> wrote in message news:4bffa457$0$7711$c3e8da3(a)news.astraweb.com... > On 28/05/2010 6:58 PM, Jason James wrote: >> "Sylvia Else"<sylvia(a)not.at.this.address> wrote in message >> news:4bff7f68$0$2122$c3e8da3(a)news.astraweb.com... >>> On 26/05/2010 10:54 PM, Clocky wrote: >>> >>>> 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. >>> >>> It is hugely counterintuitive that friction should not be a function of >>> area in contact (for non-adhesive surfaces). But it's true nevertheless, >>> and if you think you know otherwise, you know wrongly. >> >> M'dear,..scoring by nature does not present the pad with more parallel >> wearing surface. The sides of the scores are angular. God knows what >> extra >> friction that produces,...but it aint a function of the additional area. > > A score that manages to go right around the risk and form a circle would > tend to lead to the pad acquiring a corresponding ridge where the pad > isn't worn down by contact with the disk. I can't see this producing extra > friction. > > In any other case, I'd expect the pad to undergo rapid extra wear. The > mechanism would be that where the pad material is not supported by the > disk because of missing metal, the material would deflect slightly into > the groove. The end of the groove would then cut into the deflected > material. This effect would be rapid, but limited to cutting off the pad > material only to the extent that it deflects into the groove. The force > required for the cutting would be small compared with the overall > friction. In consequence, I would expect a non-circumfrential groove to > produce a very modest and short duration increase in the apparent > friction. I like your writing/info style Sylvia,..point taken ! Jason.
From: Brad on 28 May 2010 17:49
"jonz" <fj40(a)deisel.com> wrote in message news:4bff06f4$1(a)dnews.tpgi.com.au... : On 5/27/2010 9:44 PM, Clocky wrote: : > 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. : ~~~~~~~~~~~~~~~~~~~~~~~ : if you can`t dazzle `em with brilliance.......baffle `em with : bullshit.............. : : > : > : : : -- : 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 I can see some points but not all. More brake pad / rotor contact means that there is less force (Pedal pressure) required to slow a vehicle. More often than not effective braking especially in motor is to do with heat dispersal. Larger surface area of the rotor gives greater dispersal of heat. Add ventilated discs and the cross drill them just to be safe and you end up with a vehicle that stops with a greater reliability not so much on the first application of the brakes but on the 2nd 3rd etc.. Greater grip by the pads doesn't mean much until things get out of hand. By that I mean that in a standard car with standard tyres a great percentage of (non ABS) vehicles will lock their brakes long before the pedal is on the floor. Once they're lock they are almost useless. Add monster diameter rims with low profile tyres and width and tyre compounds that when at operating temp have the grip of Velcro the situation changes. With stiff suspension and low centre of gravity you can make use of better brakes. It's funny seeing these guys with 18" rims clad with tyres looking like rubber bands on old hoydens etc with their discs and callipers made to fit inside a 14" rim. As to tyre pressures, the low pressures are great in a straight line but when out of shape it can get nasty. Low is always a relative thing. Dragsters don't run high pressures at all in their tyres and they do over 300km/he. Get a landcruiser with the old style 11.5 X15 tyres lower the pressure to lapis and below and you're good for soft sand at low speeds. Same setup and add a tone in the rear (like 6 mates, water, food and camping gear for 2 weeks) and do 110km/he, this is a common situation on Frazer, and when you swerve things do not go as well as they expect. Splattered tourists usually blame the equipment or somebody else. -- Brad Leyden 6� 43.5816' S 146� 59.3097' E WGS84 To mail spam is really hot but please reply to thread so all may benefit (or laugh at my mistakes) > > |