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From: John_H on 28 May 2010 19:46 Clocky wrote: > >Given that it took Holden about 25 years to engineer a disc rotor that >didn't produce brake shudder in short space of time I'm not going to get >into an arguement of theories. > >The theory is probably pretty simple, but putting it into practice proved to >be anything but. Except the example you've cited demonstrates no such thing. The Holden brake problem wasn't due to faulty design. It was a materials problem, which reflects mainly on the bean counters. The problem was also greatly exacerbated by the servicing procedures adopted to try and rectify it (notably by machining the affected rotors and the failure to fit better pads). All of which goes to show that the GMH Customer Service Department probably doesn't ever talk to the design engineers, and almost certainly doesn't have any of its own. :) -- John H
From: John_H on 28 May 2010 20:11 Noddy wrote: > >The point I'm trying to make is that while I agree with you that changing >the size of the contact area doesn't affect the coefficient of friction and >won't change the braking ability for the same amount of applied force, it >*does* change the pressure between the pad and rotor which effectively makes >the smaller pad work harder than a larger one. 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. 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. 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. 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. :) -- John H
From: D Walford on 28 May 2010 20:32 On 29/05/2010 9:46 AM, John_H wrote: > Clocky wrote: >> >> Given that it took Holden about 25 years to engineer a disc rotor that >> didn't produce brake shudder in short space of time I'm not going to get >> into an arguement of theories. >> >> The theory is probably pretty simple, but putting it into practice proved to >> be anything but. > > Except the example you've cited demonstrates no such thing. > > The Holden brake problem wasn't due to faulty design. It was a > materials problem, which reflects mainly on the bean counters. The > problem was also greatly exacerbated by the servicing procedures > adopted to try and rectify it (notably by machining the affected > rotors and the failure to fit better pads). Except that until recently there hasn't been a pad on the market that claims to cure the problem, if it was that simple the problem would have been eliminated long ago. Bendix have recently come up with a pad that is supposed to fix the problem, I fitted a set to my son's EL a couple of weeks ago. http://www.bendix.com.au/GeneralCt.aspx My guess its a combination of pad choice and the rotor material used although more expensive after market rotors don't completely cure it either. Interesting that I've never experienced the problem with any of the Toyota's or Subaru's I've owned. Daryl
From: Noddy on 28 May 2010 21:38 "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.
From: John_H on 29 May 2010 01:46
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. :) -- John H |