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Thread: Point me to the RTFM on disc brakes

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    Default Point me to the RTFM on disc brakes

    I'm totally new to the technology. Here's the situation - brand spanking new (like 150 miles total) front caliper of a very good group that shall remain nameless unless it matters drags just a bit. The bike was assembled by somebody that certainly knows what they are doing. It looks like one pad doesn't retract. I can push on that little tiny tab on the top where the pin goes through them and it clears just fine like the other one. Clamp the brakes, release, there's a little bit of drag again. A little drag as in it comes to a stop quicker than it might at very low speed and the valve stem doesn't fall to 6 o'clock. It didn't start like that, it rolled beautifully, I just noticed it today.

    It seems as if only a few miles like that wouldn't already have met a service interval or I'm going to be stopping in the middle of rides to work on the bike. There's a little dirt and dust on it but nothing ridiculous.

    So, I'm thinking there has to be a decent write up on these things I should be checking out because I don't want to start trying this and trying that (I know some of the obvious things like don't get any grease or goo or oil on the pads, don't squeeze the brakes with nothing in there, that kind of thing) without having any idea what I am doing. Also what do I really care about? A little drag with no noise, does it matter? In my mind it sure does but I also might not know what I am talking about. Wait... I definitely don't know what I'm talking about, if I get something right it is an accident. So I definitely need to read up on these things.
     

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    Default Re: Point me to the RTFM on disc brakes

    While you are alluding to some specifics about what the brakes actually are, it would help to know what the brakes actually are.

    Are they hydro? Cable? Make? Model? New? Older?

    No reason not to just say what they are.
    Brian McLaughlin

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    Default Re: Point me to the RTFM on disc brakes

    Good point. It makes a big difference. Campagnolo Chorus H11.
     

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    Default Re: Point me to the RTFM on disc brakes

    Quote Originally Posted by Tom View Post
    Good point. It makes a big difference. Campagnolo Chorus H11.
    That helps.

    It has been many years since I worked on Magura (the company behind Campy's hydos) brakes and someone may have more/better info. But the basics are similar to Shimano's and SRAM/Avid's set-up.

    Here is an overview.

    Here is a general diagram of the caliper


    This diagram is showing what is included in a piston kit for this Avid model. That is why all the parts are numbered "2". However I labeled the parts.

    A note about the seal - the seal does three main things:
    1 - It is a seal...keep the fluid inside.
    2 - It acts as a retracting "spring" by inverting its self and then returning to normal. Think about holding a bottle cap and pressing the center in. When you take the pressure off, it snaps back to its original shape. The seal is doing this. It get pressed out a bit when under pressure but returns to its original position once the pressure is relieved.
    3 - It allows the the piston to move greater distances inside the caliper body as well. Once you have used the brakes a few times, the pistons move into a certain position. The movement required to apply the brakes once the pistons are in position is very small. If you push the piston all the way back into the body (like you would to change pads) you would then need to pump the brakes to get the piston into the position to be function normally. The seal allows the piston to travel inside the body while keeping the fluid inside. Then once the piston is in position, it does what is described in number 2 above .


    The basics (simplified)
    Applying the brakes
    - Fluid gets pushed into the body
    - This puts pressure behind the piston
    - The piston moves out
    - The outward moving piston pushes the pads

    Releasing the brakes
    - Fluid pressure behind the piston is released
    - There is a slight vacuum created by releasing the fluid pressure plus the seal doing what is described in number 2 above. This retracts the piston
    - The pads also have a spring of some type that pushes them away from each other which makes sure that the pads retract as the pistons retract.

    With that out fo the way...
    Disc that drag could have a couple of things going on:

    New discs/pads should always going through a "bed in" cycle. If unsure what that is, just ask.
    Once bedded in, it is now uncommon for new disc rotors to slightly change as they get used a couple of times and are subjected to heat cycles. They don't change much and they don't generally change again once they have gone through some heat cycles. But there isn't a lot of tolerance between the rotors and the pads, so it doesn't take much to create a slight bit of drag.
    The way to "fix" this is to simple realign the caliper body. Generally once everything is bedded in, you won't have to touch anything for a long time.
    This is likely what you have going on.

    A piston could get sticky preventing it from moving properly. This is not likely given the newness of your set-up but it is possible.
    If you remove the wheel and remove the pads, you can get a good view at the pistons. If you slightly pull the brake lever, you can see the pistons move. Pull the brake lever (slightly) and let it go a few times to watch the pistons move. It is not uncommon for the two pistons to move differently then each other. One may move slightly move. One might move first. If one moves first, the other one may not move at all. If this happens, hold the moving piston in place with a plastic tire lever while pulling on the brake lever to see if the non-moving piston moves when the other piston is held in place. If it doesn't, or is reluctant, then you have a sticky piston. They can be unstuck pretty easily.

    My guess is that you just need to readjust the caliper body on the frame/fork now that you have ridden it some. If you need help with this...it can be a little finicky just raise your hand.
    Brian McLaughlin

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    Default Re: Point me to the RTFM on disc brakes

    This morning after a cup of coffee and thinking about it a little I overcame my trepidation and removed the wheel and pads. It appeared to me that the piston on the side that dragged wasn't retracting. The one side was flush, the other protruded a bit. I checked the movement as you described, the flush one came out and returned, the other didn't move much so I held the free one with a plastic tire lever and tried again, the other moved and then seemed to retract just a tiny bit. I pressed it back flush and worked it a couple of times carefully, it seemed to retract a bit more but never really went flush. I reassembled and centered things again and it seems to be free now, no rub after applying the brakes a few times. We'll see how it behaves after a long ride today.

    I don't know if the person that built the bike bedded the brakes, they didn't mention it or I don't remember but now that I know it needs it, I suppose it can't hurt to do it now.

    Thanks for replying, one thing I figure is if I take it slow, do my reading and ask questions I should be able to keep from horking stuff up too badly.
     

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    Default Re: Point me to the RTFM on disc brakes

    @Tom you will get is figured out. In the scheme of things they are pretty simple.

    What you did by forcing the one to move back and forth is exactly right. If one is moving sluggishly, the first thing to do is what you did. The other thing you can try to use some brake fluid (what ever is used in the Campy/Magura system...mineral oil I think) and put a single small drop on the outside edge of the seal and then using the brake lever, move the piston out and in a few times like you did. Lay the bike on its side to do this so that the single drop stays in place. This can sometimes help things loosen up. If you do this, get the pads away from all of this and clean things up with dish soap when you are done. Brake fluid, oil, grease, etc can ruin the pads pretty easily.

    Also remember that the pistons will not move exactly the same. One will generally move more then the other and/or sooner than the other. That is OK. While these system are well thought out and have fairly tight tolerances, they are not scientific tools measuring atoms. For example: if you watch the pads make contact with the rotor, it is extremely likely that one pad makes contact first. There were/are some cable actuated systems that only have one piston and they just push the rotor against a static pad on the other side. They rely on the rotor flexing.

    It ultimately comes down to having the pads squeeze the rotor to brake. And to not rub when not braking. You may have to readjust the caliper on the frame/fork if it is still rubbing. No need to put up with that. Though when you ride in the rain you will likely hear some grinding because sand/dirt from the road will likely get on the rotor or pad. It will either clear its self or you can apply the brakes to clean it off.

    Once you get it the way you like it, you shouldn't have to touch it again for a long time.
    Brian McLaughlin

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