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Thread: Uneven rotor wear... how much is fucked?

  1. #41
    Member SomeBloke's Avatar
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    But the force isn't quite the same.

    The force from the piston onto then pad is a normal compressive force.
    The compressive force on the rotor is the same as the piston force.
    The brake force (i.e. what becomes the brake torque) is the compressive force*mu
    Yes the pressures are different but they all resolve back to a force over an area. Friction is proportional to force, not pressure.

    I guess another way to think about it is if you swapped callipers to one with a smaller or larger piston but remained using the same pad. For a given lever force, you would expect the braking performance to change, even though the pad surface area is the same.

  2. #42
    Member Joe Shabadoo's Avatar
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    for a given lever force.. yes
    but put some larger pads in that caliper and can you not squeeze harder, transferring more force to rotor, resulting in more friction?

  3. #43
    Member sixofthebest's Avatar
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    I thought we were talking about a constant lever force? Of course, if you squeeze harder you get more braking, regardless of the surface area of the pads. The point is that it's the force that causes the braking, not the surface area.

  4. #44
    Member Chuck Steak's Avatar
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    Quote Originally Posted by sixofthebest View Post
    I thought we were talking about a constant lever force? Of course, if you squeeze harder you get more braking, regardless of the surface area of the pads. The point is that it's the force that causes the braking, not the surface area.
    But if the force is applied over a greater area the total effect of the braking is better. Fuck me roan son - it's not fucking rocket surgery!

    Go and jump onto a single disc 860GT and try to brake. Then bolt the extra disc on, and come back and tell me that there's no difference.

    Here's a quiz question for you: 80 series Landcruisers, when they first came out, had notoriously shithouse brakes. In 1993 the upgraded the system. Here's your question. Were the discs

    a. Smaller
    b. The same size
    c. Bigger

  5. #45
    Member Chuck Steak's Avatar
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    Quote Originally Posted by Ryder View Post
    I think I'm done here after this. I've not only explained it mathematically but linked a video that physically demonstrates it. I'm happy to discuss physics concepts because I find them very interesting and it's an area where there are lots of misconceptions; these things are often not intuitive. But please at least read and watch what's already up first.

    Twin disk bikes have more stopping power because they have more brake piston area. Simply put, if you put the same force on the lever for a single disk bike as you put on the lever for a double disk bike, it will create the same brake system pressure. BUT you now have twice the piston area. Force=pressure*area, so if you double the area, you double the normal force, which doubles the stopping power.

    It's got nothing directly to do with the brake pad to disk contact area.

    Yes there's other little things going on it there that have effects too but unless you're being ridiculous about it to make a point or discussing extreme applications like racing in the MotoGP they will be small, and will only make the concepts more confusing to take in.
    More brake piston area. Obviously. Greater SA of contact with the disc. Bigger discs mean you can have bigger calipers, bigger pistons, and therefore bigger contact area. Scored discs, with grooves, have graeter SA than smooth discs, therefore better braking

  6. #46
    Member AZAZL's Avatar
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    Quote Originally Posted by Chuck Steak View Post
    [...] Scored discs, with grooves, have graeter SA than smooth discs, therefore better braking
    We now have gone full circle. Well trolled Sir.

    How much greater SA? < 1%
    TV's got them images
    TV's got them all
    It's not shocking!

  7. #47
    Member sixofthebest's Avatar
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    Quote Originally Posted by Chuck Steak View Post
    But if the force is applied over a greater area the total effect of the braking is better. Fuck me roan son - it's not fucking rocket surgery!

    Go and jump onto a single disc 860GT and try to brake. Then bolt the extra disc on, and come back and tell me that there's no difference.

    Here's a quiz question for you: 80 series Landcruisers, when they first came out, had notoriously shithouse brakes. In 1993 the upgraded the system. Here's your question. Were the discs

    a. Smaller
    b. The same size
    c. Bigger
    I owned an 80 series standard wagon back in the day and remember changing the brake discs in my driveway as the originals had warped, courtesy of a previous owner, and the generally shit design, as you point out.

    The answer is c - the discs were bigger. This allowed both better braking due to the additional torque on the wheel, to stop the lack of braking issue and better heat rejection, to stop the fading / warping / melting the pads issues.

    So we can conclude that doing the following improves braking performance, all other things being equal:
    - Additional disks
    - Bigger disks
    - More pistons / bigger pistons
    - Bigger / better master cylinder to increase hydraulic pressure
    - Braided lines to also keep the pressure higher for the same lever travel
    - Anything with "Brembo" written on it...
    Particularly so if there are issues with rejecting enough heat, a la 80 series. Scoring the disks / drilled rotors is helpful to reject more heat and preventing fade etc.

    We can also still conclude that increasing pad surface area by itself (ie without doing any of the above) does fuck all... as per Ryder's video. It just spreads the same force over additional area, achieving nothing by itself, unless the pads are getting too hot and need the additional area to achieve the required heat rejection.

    It must be pointed out that drilling holes in the rotors actually reduces the contact surface area between pad and disk, while also increasing braking performance (by better heat rejection and more stable expansion = less warping). Explaining why pretty much every modern bike now has drilled rotors. Which only further proves your theory is bollocks... although this has already been well proven by others in this thread.

    I blame the general lack of physics education in high school... BTW there's no point arguing with engineers - we have the facts on our side. If you want an argument, politics and religion are a safer bet. This is just making up for the aforementioned lack of education...

    But I'm sure you already know all this... this is more for others who may be interested.

  8. #48
    Member SomeBloke's Avatar
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    sixofthebest
    You have far more patience than I do.

  9. #49
    Member Chuck Steak's Avatar
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    Quote Originally Posted by sixofthebest View Post
    I owned an 80 series standard wagon back in the day and remember changing the brake discs in my driveway as the originals had warped, courtesy of a previous owner, and the generally shit design, as you point out.

    The answer is c - the discs were bigger. This allowed both better braking due to the additional torque on the wheel, to stop the lack of braking issue and better heat rejection, to stop the fading / warping / melting the pads issues.

    So we can conclude that doing the following improves braking performance, all other things being equal:
    - Additional disks
    - Bigger disks
    - More pistons / bigger pistons
    - Bigger / better master cylinder to increase hydraulic pressure
    - Braided lines to also keep the pressure higher for the same lever travel
    - Anything with "Brembo" written on it...
    Particularly so if there are issues with rejecting enough heat, a la 80 series. Scoring the disks / drilled rotors is helpful to reject more heat and preventing fade etc.

    We can also still conclude that increasing pad surface area by itself (ie without doing any of the above) does fuck all... as per Ryder's video. It just spreads the same force over additional area, achieving nothing by itself, unless the pads are getting too hot and need the additional area to achieve the required heat rejection.

    It must be pointed out that drilling holes in the rotors actually reduces the contact surface area between pad and disk, while also increasing braking performance (by better heat rejection and more stable expansion = less warping). Explaining why pretty much every modern bike now has drilled rotors. Which only further proves your theory is bollocks... although this has already been well proven by others in this thread.

    I blame the general lack of physics education in high school... BTW there's no point arguing with engineers - we have the facts on our side. If you want an argument, politics and religion are a safer bet. This is just making up for the aforementioned lack of education...

    But I'm sure you already know all this... this is more for others who may be interested.
    Good answer

  10. #50
    Member dwillia's Avatar
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    Quote Originally Posted by AZAZL View Post
    We now have gone full circle. Well trolled Sir.

    How much greater SA? < 1%
    and the additional lack of cooling across the face of the disc ~0.05c
    Likes AZAZL liked this post

  11. #51
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    Quote Originally Posted by SomeBloke View Post
    sixofthebest
    You have far more patience than I do.
    Same here!

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