Disc brake 101 question
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  1. #1
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    Disc brake 101 question

    I am new to discs. On rim brake bikes, the brake pad wears out and needs replacing every now and again.

    What is the disc brake equivalent? The pads inside the calipers? The rotors themselves?

    And roughly how long do these parts last (I get that it depends on the riding and all that, but comparative to rim brakes)?

    Thanks.

  2. #2
    tlg
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    Think about the disc brakes on your car. Pads wear more frequently than the rotors.

    It's really hard to say how long. Depends on your riding conditions and the type of pads you use. I find that disc pads wear significantly faster than rim pads.
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  3. #3
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    Stock pads in wet conditions will wear much quicker. In cyclocross it is pretty common to go through a few sets of stock (organic) pads in a season. Sintered metallic pads will last longer, but may sacrifice some feel.

    I haven't run discs on road, but I imagine the same to be true.

  4. #4
    JSR
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    You have to pay attention to both the pads and the rotors. Pads should be replaced when the pad material reaches 1mm thickness. Rotors should be replaced when they reach 160mm thickness (typically). I think I got two years on my rotors and pads, replacing them at the same time.

    I have Shimano BR-RS805 brakes. Changing the pads could not be a simpler exercise. Five minutes max.

    You need to know whether your rotors are “center lock” or “ 6-bolt” type. Once again, replacement is uncomplicated.

  5. #5
    tlg
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    Quote Originally Posted by JSR View Post
    Rotors should be replaced when they reach 160mm thickness (typically).
    160mm (6.3") ? Those are some thick rotors

    Different manufacturers recommend different min thicknesses. It should be stamped right on the rotor.
    Shimano says 1.5mm
    Sram says 1.55mm


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    JSR
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    [QUOTE=tlg;5344165]160mm (6.3") ? Those are some thick rotors
    ]

    Derp. I think Shimano remmends 1.6mm. Thanks for the catch.

  7. #7
    tlg
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    Quote Originally Posted by JSR View Post
    I think Shimano remmends 1.6mm.
    No. See photo. Says right on the rotor Min.TH=1.5


    Quote Originally Posted by JSR View Post
    Pads should be replaced when the pad material reaches 1mm thickness. R
    Shimano instructions says 0.5mm
    https://si.shimano.com/pdfs/si/SI-8J50A-002-ENG.pdf
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  8. #8
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    Quote Originally Posted by eboos View Post
    Stock pads in wet conditions will wear much quicker. In cyclocross it is pretty common to go through a few sets of stock (organic) pads in a season. Sintered metallic pads will last longer, but may sacrifice some feel.

    I haven't run discs on road, but I imagine the same to be true.
    A few sets of pads for a season? I've seen them not make it through a 45min race. Pads wear very quickly in the wet. Resin pads wear more quickly but don't wear the rotor as much, metallic pads last a bit longer but are going to wear rotors out faster.
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  9. #9
    Slowski
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    Quote Originally Posted by cxwrench View Post
    A few sets of pads for a season? I've seen them not make it through a 45min race.
    Yeah, I had a set that looked pretty close to one of your hall of shame photos last year.

    Midpack Cat 4 racing in New England so mileage may vary.

    Edit: Speaking of, going to put some sintered pads to the test tomorrow. Hilly course soaked from the remnants of a hurricane. Yeah baby! It's cross season!
    Last edited by eboos; 1 Week Ago at 08:43 AM.

  10. #10
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    I have R8000 calipers and the accompanying discs. They are centerlock.

    Is there a way to measure thickness, or just look in the caliper and if they look thin, change them out?

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    tlg
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    Quote Originally Posted by billiegoat View Post

    Is there a way to measure thickness, or just look in the caliper and if they look thin, change them out?
    can you look at a piece of metal and tell the difference between 1.8mm and 1.5mm??

  12. #12
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    Quote Originally Posted by billiegoat View Post
    I have R8000 calipers and the accompanying discs. They are centerlock.

    Is there a way to measure thickness, or just look in the caliper and if they look thin, change them out?
    Get a caliper. Even a cheap digital caliper is $30 on Amazon

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    Quote Originally Posted by tlg View Post
    can you look at a piece of metal and tell the difference between 1.8mm and 1.5mm??
    Well, that's kinda my point. I will get a caliper and measure. Seems like some work (maybe not, I don't know) to remove the pads to measure just to see if they need replacement.

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    Quote Originally Posted by billiegoat View Post
    Well, that's kinda my point. I will get a caliper and measure. Seems like some work (maybe not, I don't know) to remove the pads to measure just to see if they need replacement.
    You'll start to see an indent in the rotor where the pads make contact when they start to wear. If the surface looks perfectly flat, they are most likely fine. But measure with a caliper to be sure.
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    JSR
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    Quote Originally Posted by billiegoat View Post
    I have R8000 calipers and the accompanying discs. They are centerlock.

    Is there a way to measure thickness, or just look in the caliper and if they look thin, change them out?
    https://www.harborfreight.com/4-in-d...per-63710.html

  16. #16
    JSR
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    Quote Originally Posted by billiegoat View Post
    Seems like some work (maybe not, I don't know) to remove the pads to measure just to see if they need replacement.
    It’s super simple. Remove wheel, remove Cotter pin, remove screw holding pads in place, pinch pads between thumb and and index finger, pull em out.

    The only tricky thing is to make sure you don’t squeeze the brake lever while the wheel is off the bike. The actuating pistons will drive toward one another and screw up the auto adjustment. Also, if you do replace the pads and/or rotor, you may need to drive the pistons back into the caliper housing to make some room before putting the wheel back on. A plastic tire iron and a little leverage will do the trick.

  17. #17
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    All this trouble to determine if your pads are worn? Are you kidding me? Look at the damn things...if the friction material is very thin compared to a new pad, replace them. I'd replace when when the friction material is double the thickness of the pad spring. Very easy to see.
    Rotors are easily measured on the bike, just make sure you do it at the right spot.

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    Quote Originally Posted by cxwrench View Post
    All this trouble to determine if your pads are worn? Are you kidding me? Look at the damn things...if the friction material is very thin compared to a new pad, replace them. I'd replace when when the friction material is double the thickness of the pad spring. Very easy to see.
    Rotors are easily measured on the bike, just make sure you do it at the right spot.

    This seems like what the OP was driving at. Thanks for clearing this up CX.

  19. #19
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    Quote Originally Posted by billiegoat View Post
    I have R8000 calipers and the accompanying discs. They are centerlock.

    Is there a way to measure thickness, or just look in the caliper and if they look thin, change them out?
    A caliper is probably not the best measuring tool to use. A micrometer is best, and the price differential at Harbor Freight is only about $5 more. Since the cheap micrometers are in English units, just remember that 1mm is just about .040", so a 1.5mm min thickness will be .060". As for measuring thickness of pads, a simple automotive feeler gage set works great; .5mm = .020". Just see if you can slide the .020" gage between the rotor and pad backing plate while the brake is engaged.
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  20. #20
    tlg
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    Quote Originally Posted by No Time Toulouse View Post
    A caliper is probably not the best measuring tool to use.
    Huh? a caliper is more than accurate enough. Even a cheap caliper will measure to a thousandth of an inch accuracy. A decent caliper will measure to half a thousandth.

    Quote Originally Posted by No Time Toulouse View Post
    1mm is just about .040", so a 1.5mm min thickness will be .060"
    1mm=0.039
    1.5mm=0.059
    You're worried about the accuracy of a caliper, but not the accuracy of your conversion... which is off a thousandth of an inch.

  21. #21
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    Quote Originally Posted by tlg View Post
    Huh? a caliper is more than accurate enough. Even a cheap caliper will measure to a thousandth of an inch accuracy. A decent caliper will measure to half a thousandth.
    Plenty accurate but an inexpensive micrometer would be a good choice too.
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  22. #22
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    Quote Originally Posted by tlg View Post
    Huh? a caliper is more than accurate enough. Even a cheap caliper will measure to a thousandth of an inch accuracy. A decent caliper will measure to half a thousandth.
    ....
    Not really, on 2 counts:

    1) A caliper will measure the high points over the distance of the jaws, which depending on the caliper can be between a 1"-2" length, which usually means on a disc, that includes the part on the edge which the pad does not have contact with, and does not wear. A micrometer measures over a much smaller area, which means you can measure it on the wear section, not just the outsides.

    2) A decent caliper will measure to ±.0005, but will only be accurate to about ±.002, whereas even the cheapest micrometer is good to at least ±.0001.

    BTW, I work in metrology. I have access to measuring tools 10x as accurate. I only mentioned the 1mm/.040" equivalence because it's easy to remember and easy for non-mathematical people to deal with. I doubt many people could tell you quickly off the top of their head what either 1/2 of .0039 is, or what 2x .0039 is. And in this case, a difference of .001" is trivial (as is your challenge to my post).

    BTW, not to be pedantic, but 1mm = .0394", and 1.5mm =.0591". How many significant digits do you want to take this to?
    Last edited by No Time Toulouse; 1 Week Ago at 02:53 PM.
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  23. #23
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    Quote Originally Posted by No Time Toulouse View Post
    Not really, on 2 counts:

    1) A caliper will measure the high points over the distance of the jaws, which depending on the caliper can be between a 1"-2" length, which usually means on a disc, that includes the part on the edge which the pad does not have contact with, and does not wear. A micrometer measures over a much smaller area, which means you can measure it on the wear section, not just the outsides.

    2) A decent caliper will measure to ±.0005, but will only be accurate to about ±.002, whereas even the cheapest micrometer is good to at least ±.0001.

    BTW, I work in metrology. I have access to measuring tools 10x as accurate. I only mentioned the 1mm/.040" equivalence because it's easy to remember and easy for non-mathematical people to deal with. I doubt many people could tell you quickly off the top of their head what either 1/2 of .0039 is, or what 2x .0039 is. And in this case, a difference of .001" is trivial (as is your challenge to my post).

    BTW, not to be pedantic, but 1mm = .0394", and 1.5mm =.0591". How many significant digits do you want to take this to?
    I'm guessing you didn't read (or even look at the photo) post #19. I give specific instruction where to measure so you don't have this problem. I'm surprised someone w/ your obvious intelligence doesn't understand how to do that.
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    Quote Originally Posted by cxwrench View Post
    I'm guessing you didn't read (or even look at the photo) post #19. I give specific instruction where to measure so you don't have this problem. I'm surprised someone w/ your obvious intelligence doesn't understand how to do that.
    works on all rotors with a saw tooth outside edge, but not all have that - Campagnolo has at least a couple current rotors that do not, a micrometer is a better choice for those to get away from the outside edge or you have to use a feeler gauge and caliper to see what if anything you can slide between the caliper edge and the rotor away from the edge.

    Quote Originally Posted by No Time Toulouse View Post
    BTW, not to be pedantic, but 1mm = .0394", and 1.5mm =.0591". How many significant digits do you want to take this to?
    Well, you set yourself up on significant digits when you posted 1mm was approximately .040" and 1.5mm = .060", should have said .04 and .06 inches......
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  25. #25
    tlg
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    Quote Originally Posted by No Time Toulouse View Post
    Not really, on 2 counts:

    1) A caliper will measure the high points over the distance of the jaws, which depending on the caliper can be between a 1"-2" length, which usually means on a disc, that includes the part on the edge which the pad does not have contact with, and does not wear.
    You don't need to use 1-2" of the jaw to reach "the part on the edge" that does not wear.


    And in this case, a difference of .001" is trivial (as is your challenge to my post).
    If .001" is trivial, you don't need a measuring device capable of .0005"


    BTW, I work in metrology. I have access to measuring tools 10x as accurate. I only mentioned the 1mm/.040" equivalence because it's easy to remember and easy for non-mathematical people to deal with.
    Because who doesn't have a google machine to just do the conversion.


    BTW, not to be pedantic, but 1mm = .0394", and 1.5mm =.0591". How many significant digits do you want to take this to?
    Well.... to be pendentic.... who in metrology uses .0394" as a conversion? When the actual exact conversion is 1"=25.4mm. A 25.4 conversion is more accurate and easier to remember.
    25 years as an engineer (working in metric and imperial), everyone I know in the field uses 25.4.


    This is all just silly and putting waaaay to much thought into something so simple. Park tool (who makes a tool for everything) doesn't make a special rotor micrometer. Go figure.

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