What force causes a brake piston to retract, when you release the brake??

Discussion in 'The Workshop' started by maximililian, May 22, 2012.

  1. bing!

    bing! Active Member

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    In automotive applications the seal does retract with the piston away from the rotor, but its meant more to "curl" when applied and "uncurl" when released, not really pull at a 1-2 pound steel plate with braking material.

    Between Juicys and Elixirs, I have not noticed much "curling", if any, material in the caliper piston seals.
     
  2. maximililian

    maximililian You Sneaky Cork-Soaker!

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    Okay, let's go on the assumption that your explanation is correct, and that you're saying that it's vacuum (negative fluid pressure) thanks to a spring in the MC, that is retracting the piston. Then, I have a few questions.

    1. In researching different hydraulic disc braking setups and their operation, I found the following explanation of bicycle specific brakes- "Because the "motor" is small, an uncommon feature of bicycle brakes is pads that retract to eliminate residual drag when the brake is released. In contrast, most other brakes drag the pads lightly when released." Does this statement jive with your explanation? What the F is a "small motor"? The length of the piston stroke?
    2. Next, then why don't motorcycle or automobile brakes operate the same way? Pistons/pads for cars and motos do not retract, but rather rub the rotor. Why are they different?
    3. If it is mostly vacuum that is retracting the pistons, I guess if the pistons begin to not retract enough, and thus continue to rub on the rotors, one would probably suspect that there is likely some extra friction in the piston bore that is overcoming the vacuum. Thus, the piston tends not to retract. Is that about right in your view? Or could the spring in the MC be weakening, providing less vacuum?
     
  3. bing!

    bing! Active Member

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    That is what is called a sticky piston.

    Since the piston is designed to "float" on the seal for its self adjusting feature, a piston that has dirt on it is unable to slide back into the seal properly.

    Shimano deals with it by cleaning, lubricating and resetting the piston. http://bike.shimano.com/publish/content/global_cycle/en/us/index/tech_support/tech_tips.download.-Par50rparsys-0010-downloadFile.html/04) Brake Reset Procedure.pdf

    On my Avids, I just blast the calipers with brake cleaner to melt and wash away the crud.
     
  4. SeanC

    SeanC Active Member

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    I have learned nothing from this thread.
     
  5. 7pt

    7pt New Member

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    1. I assume motor means your legs. 35hp from a Moto or 130+ from a car DGAF about the amount of drag produced by a floating brake. Your couple of 10s of kilowatt at best motor in the form your legs does.

    2. No need.

    3. Spring behind mc piston will pull everything back in a good system. If the caliper seals let crud past and the piston wears on the piston bore, things might get cockeyed and bindup and stick.

    To bing!

    I think my post was trying to say that piston seals and springs don't do much to retract the pistons. It's all handled with hydraulics...
     
  6. herzalot

    herzalot Well-Known Member

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    The hobnostal defosticator puts reverse pressure on the kernble naggits using osmosis perferation. Thus when counterintuitive braking forces are applied on an opposite geometric plane, the frictional subset of inertia is compensated, causing equilibrium stasis in an outboard axis of rotation.

    DUH!!!!!!
     
  7. Racer X

    Racer X Member

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  8. SeanC

    SeanC Active Member

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    [video=youtube;hm30l8sqlQw]http://www.youtube.com/watch?v=hm30l8sqlQw[/video]
     
  9. gooseaholic

    gooseaholic Active Member

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    2 pages and one correct answer? ITS A SYSTEM, hence the name brake system.
     
  10. elab

    elab Turnbull haze

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    Thanks! now it all makes sense.
     
  11. oldschoolpl510

    oldschoolpl510 Member

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    Much like the Chrysler Turbo Encabulator
    [video=youtube;MXW0bx_Ooq4]http://www.youtube.com/watch?v=MXW0bx_Ooq4[/video]
     
  12. herzalot

    herzalot Well-Known Member

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    F-ing BRILLIANT! Best example of gobbledygook and nonsense jargon I have ever heard! :bang::beer:
     
  13. gooseaholic

    gooseaholic Active Member

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    That video is hilarious. At least the DRB is the correct scan tool. The other accurate thing: That is pretty much how a Chysler service manual reads. Its as if their not sure how to fix even their cars.
     
  14. mfoga

    mfoga Intense Whore

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    It's a magnetic, the rotor and the back of pads have same magnetic pole.:lol:
     
  15. Bryguy17

    Bryguy17 A little Shaggy

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  16. SQUIRREL

    SQUIRREL Weeeeeeeeeee!!!!!

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    ^^^ BAAAAHHAHAHAHAHA YESS!!!!! ^^^^^:clap:
     
  17. mtnbikerfred

    mtnbikerfred Super Moderator

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    Any of you idiots ever connect two syringes together with a piece of tubing in between?

    Say it with me: Fluid Displacement...

    Give me a 10cc and a couple of 50cc syringes, a "T" fitting or two, and a couple of flow-checks and of course a reservoir of some sort and VOILA!! a working model of most modern two piston disc brake caliper is born!

    When I did this (I think in 7th grade), it was tricky to get the slave piston not retract *slightly* as more fluid was being pulled into the system the problem was the elasticity of the tubing. My model was of course not for brakes, but a miniature hydraulic jack. I did successfully demonstrate mechanical advantage through fluid displacement though, which was really cool.
     
  18. 7pt

    7pt New Member

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    Bout a day late dude....
     
  19. bing!

    bing! Active Member

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    Hahahahaha! That guys amazes me. He's so full of nonsense that he's used to build a good size media business and a respectable fortune.

    It gives "making do with what youve got" a whole new meaning for me.
     
  20. Albacore

    Albacore 34x18

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    [video=youtube;TuhYd9L_d7w]http://www.youtube.com/watch?v=TuhYd9L_d7w[/video]

    The "girdle spring" at 1:35 is the explanation you are looking for.
     

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