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Why are brake discs frequently replaced?

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  • 14-03-2018 11:40pm
    #1
    DaveyDave
    Registered Users Posts: 6,185 ✭✭✭


    I know this might sound like a stupid question but a few people I know mentioned the need for new discs with their pads. I've seen it mentioned here a few times too.

    What causes the need for the discs to be replaced? It doesn't sound like they really last too long on some cars. Is it the heat warping them or is it just pads wearing down to nothing and destroying them?

    The only experience I've had was on the mountain bike. I know not quite the same but I never needed to replace a disc after a fair bit of mileage and different types of pad materials used. I think once the rear pad wore down and was grinding the disc but I just took it off and sanded it. Same when the disc was warped a little. There's actually a tool that slots onto it for you to bend it back out (or just use your hand).

    Can the same not be done on a car? As I said I've no first hand experience with it, but it doesn't sound like refurbishing is an option on them. Just curious as it seems quite common.


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    #2
    dr.fuzzenstein
    Banned (with Prison Access) Posts: 16,635 ✭✭✭✭dr.fuzzenstein


    It's a mixture of warping and wear.
    You could get them machined, but you have to be careful not to go beyond minimum thickness.
    But in the end it's not worth it, since discs don't cost a whole lot. It would make no sense to spend as much on machining an old, worn disc as a new one would cost.
    Good luck bending them back by hand. :D


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    #3
    Toyotafanboi
    Registered Users Posts: 12,917 ✭✭✭✭Toyotafanboi


    Yeah it's just a big lump of metal used to slow down an even bigger lump of metal so it's going to have a finite lifespan.


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    #4
    punisher5112
    Registered Users Posts: 24,647 ✭✭✭✭punisher5112


    Discs tend to last quite some time on a car but all depends on usage and driver.

    If someone is easy on the brakes the longer pads and discs will last.

    If someone is braking hard and using them a lot they will wear much quicker and always the possibility of warping but as mentioned above that is the end of them and new ones needed.
    No hope in re shaping.

    I've yet to replace mine and the car has covered 176k miles.
    Pads a few times.


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    #5
    rex-x
    Registered Users Posts: 1,893 ✭✭✭rex-x


    The reason they are replaced is because they have reached their mimimum safe thickness. On most cars you can only wear down 1mm per side or 2mm overall on a disc that is 26-30mm thick so not alot. As a rule no car will get more than 2 sets of pads to 1 disc and still be within spec but it doesn't stop many people slapping pads on them and ah sure it will be grand.... If there is any lip at all then you need new discs.
    Also over time the internal cooling vanes rust out so they do not offer much if any cooling and this is dangerous in heavy braking situations as you may find the pedal fades to nothing.

    Here is a disc that went just that bit too far, not too much of a lip but clearly should have been replaced years before:
    15rbdad.jpg

    And here is an example of internal rust that you cannot see without removing the disc:
    24bj6vr.jpg


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    #6
    Mr.David
    Registered Users Posts: 1,701 ✭✭✭Mr.David


    A brake disc is basically just a heat sink. When you want to slow the car you are effectively converting the kinetic energy of the vehicle into heat energy which must be transferred to the brake disc and then dissipated to the surrounding air.

    The thicker/more material on the disc the greater the capacity it has as a store of heat energy. So as it wears down it will heat up more quickly and you will experience fade sooner too as braking becomes less effective. The storage capacity of the disc is reduced. And of course the risk of the disk breaking increases too.


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    #7
    DaveyDave
    Registered Users Posts: 6,185 ✭✭✭DaveyDave


    Thanks for the explanations guys, never thought of a disc as a heatsink that would lose capacity! :)


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    #8
    gooner99
    Registered Users Posts: 2,036 ✭✭✭gooner99


    Do you think genuine discs tend to last longer than spurious ones?


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    #9
    biko
    Registered Users Posts: 81,223 ✭✭✭✭biko


    OEM vs spurious is an old debate.
    It very much depend on the actual brand.

    If in doubt go with manufacturer's recommendation = OEM


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    #10
    CJhaughey
    Registered Users Posts: 14,906 ✭✭✭✭CJhaughey


    gooner99 wrote: »
    Do you think genuine discs tend to last longer than spurious ones?

    I think a lot depends on the manufacturer. Not all manufacturers use the same quality in their parts


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    #11
    Ardennes1944
    Registered Users Posts: 9,238 ✭✭✭Ardennes1944


    Discs are so cheap and I rarely keep a car more than 2 years I have almost changed them any time I have changed pads first time on a car.


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    #12
    Fann Linn
    Registered Users Posts: 7,772 ✭✭✭Fann Linn


    The lad who services my car says I don't use my brakes enough as I don't do much milage,less than 8000 km last year. He has advised me on the odd occasion, to go heavy on the brakes every now and again, when safe to do so of course.

    Why would this be?


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    #13
    jsd1004
    Registered Users Posts: 712 ✭✭✭jsd1004


    Ardennes1944 wrote: »
    Discs are so cheap and I rarely keep a car more than 2 years I have almost changed them any time I have changed pads first time on a car.

    Was behind a new A4 the other day sitting well back as there was no point in overtaking as there was a line of cars ahead. He sat on the bumper of the car ahead and we counted him using his brakes about 200 times over a 10km stretch. I never used mine once.


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    #14
    CJhaughey
    Registered Users Posts: 14,906 ✭✭✭✭CJhaughey


    Fann Linn wrote: »
    The lad who services my car says I don't use my brakes enough as I don't do much milage,less than 8000 km last year. He has advised me on the odd occasion, to go heavy on the brakes every now and again, when safe to do so of course.

    Why would this be?

    Probably getting rusty and the pad is wearing unevenly.
    A few stops from 100+ to 5 or 10 in a safe place will clean them off well.
    Just don't do them repeatedly or you could boil the fluid.
    Let them cool between applications.


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    #15
    Paudee
    Closed Accounts Posts: 488 ✭✭Paudee


    jsd1004 wrote: »
    Was behind a new A4 the other day sitting well back as there was no point in overtaking as there was a line of cars ahead. He sat on the bumper of the car ahead and we counted him using his brakes about 200 times over a 10km stretch. I never used mine once.

    I've basically made some sort of weird ongoing game out of similar when on long journeys. The amount of people on the roads who leave next to no braking distance between cars is incredible.


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    #16
    bucketybuck
    Registered Users Posts: 18,103 ✭✭✭✭bucketybuck


    Fann Linn wrote: »
    The lad who services my car says I don't use my brakes enough as I don't do much milage,less than 8000 km last year. He has advised me on the odd occasion, to go heavy on the brakes every now and again, when safe to do so of course.

    Why would this be?

    Brake discs are open to all the road dirt and surface water, and like any metal will rust if left sitting. Rusted discs will break up when used and of course affect braking performance. Using your brakes regularly can clean the surface of the discs, for want of a better word.

    More of an issue is actually the pads, if not used enough they can start to glaze over on the surface of the pad and that is a bigger problem.


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