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  1. #1
    KLF
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    Rotors for LONG slow descents

    The rotor reviews are sparse and sometimes vague so I'm just asking here.

    Anyone have any recommendations (pro or con) about rotors for long slow, sometimes technical descents? The "slow" part refers to long, steady application of brakes to keep speed under control.

    Instantaneous braking "power" is probably a secondary consideration. The desirable characteristics as I see them are good slow speed smoothness and modulation (no grabbiness or pulsing) along with good heat dissipation to handle constant braking that can take place for minutes at a time while creeping down extended pitches at least steep enough that brake free descending is out of the question.
    TIA

  2. #2
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    Bigger diameter should help

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    Quote Originally Posted by KLF View Post
    The rotor reviews are sparse and sometimes vague so I'm just asking here.

    Anyone have any recommendations (pro or con) about rotors for long slow, sometimes technical descents? The "slow" part refers to long, steady application of brakes to keep speed under control.

    Instantaneous braking "power" is probably a secondary consideration. The desirable characteristics as I see them are good slow speed smoothness and modulation (no grabbiness or pulsing) along with good heat dissipation to handle constant braking that can take place for minutes at a time while creeping down extended pitches at least steep enough that brake free descending is out of the question.
    TIA
    The issue with continuous braking is heat build up. Small diameter rotors may be easier to modulate if the grade is not too steep but would over heat and may warp.

    Bigger rotors will dissipate heat better and give more reliable braking. I would just fit the largest rotors that fit your bike. Might feel grabbier at first, but you will adjust.
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  4. #4
    Give it a crank
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    Quote Originally Posted by jeffscott View Post
    Bigger diameter should help
    Helps a lot, the brake pads still heat up though. Fin-cooled pads should help keeping things cool as long there's enough airflow.

  5. #5
    Spanish rider
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    I'd just add that rotors with small holes (or none at all) in the braking surface show less "pulsing" under heavy braking, so you feel a constant braking force instead of a periodically fluctuating force.

    Also make sure that the braking surface width is higher than the width of the barke pad, otherwise the rotor arms may contact the pad, making noise and fluctuating braking force.

    I usually ride that kind of terrain and I'd really like to try a rotor with no holes in the braking surface, just like cars. I'm sure that they weigh more, but who cares...
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  6. #6
    KLF
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    Quote Originally Posted by Pableras View Post
    I'd just add that rotors with small holes (or none at all) in the braking surface show less "pulsing" under heavy braking, so you feel a constant braking force instead of a periodically fluctuating force.

    Also make sure that the braking surface width is higher than the width of the barke pad, otherwise the rotor arms may contact the pad, making noise and fluctuating braking force.

    I usually ride that kind of terrain and I'd really like to try a rotor with no holes in the braking surface, just like cars. I'm sure that they weigh more, but who cares...
    That is more or less what I was thinking, but I'm just speculating. I was hoping that others would be able to provide feedback about experience with specific rotors.

    Clearly it is possible to have two rotors of different sizes and different design but with the same mass. Given similar braking, they should heat up similarly. The larger one typically will have more "relieved" areas. That may help or hinder heat dissipation. But it seems that descriptions of lightweight rotors with open areas often mention pulsing and/or heat buildup. So it is conceivable that the smaller rotor with fewer relieved areas would fare no worse for heat buildup, but might do better for smoothness.

    I didn't specifically say it, but I would not ignore weight in selecting a rotor. Around here I enjoy the climbing. Those descents I described? They are just the shortest way home.

  7. #7
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    Quote Originally Posted by KLF View Post
    That is more or less what I was thinking, but I'm just speculating. I was hoping that others would be able to provide feedback about experience with specific rotors.

    Clearly it is possible to have two rotors of different sizes and different design but with the same mass. Given similar braking, they should heat up similarly. The larger one typically will have more "relieved" areas. That may help or hinder heat dissipation. But it seems that descriptions of lightweight rotors with open areas often mention pulsing and/or heat buildup. So it is conceivable that the smaller rotor with fewer relieved areas would fare no worse for heat buildup, but might do better for smoothness.

    I didn't specifically say it, but I would not ignore weight in selecting a rotor. Around here I enjoy the climbing. Those descents I described? They are just the shortest way home.
    if rotor is not overheating then you don't need a bigger rotor......

    if it is get a bigger one, the have a lot more surface area to dissipate the heat....so they don't fade.

    The holes are there to help clean the roost off the pad face....and help cooling as well.

    A solid rotor heats up faster than a holey rotor.

    The best way to cool off hot pads is to stab at the brakes hard on for a about asec then off for as long as possible...

    Dragging pads causes rotors to overheat, cause the pad surface never gets a blast of cooling air, when the braking is removed.

  8. #8
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    Quote Originally Posted by KLF View Post
    That is more or less what I was thinking, but I'm just speculating. I was hoping that others would be able to provide feedback about experience with specific rotors.

    Clearly it is possible to have two rotors of different sizes and different design but with the same mass. Given similar braking, they should heat up similarly. The larger one typically will have more "relieved" areas. That may help or hinder heat dissipation. But it seems that descriptions of lightweight rotors with open areas often mention pulsing and/or heat buildup. So it is conceivable that the smaller rotor with fewer relieved areas would fare no worse for heat buildup, but might do better for smoothness.

    I didn't specifically say it, but I would not ignore weight in selecting a rotor. Around here I enjoy the climbing. Those descents I described? They are just the shortest way home.
    Rotor weight differences are minor. You will not notice any difference while climbing.

    A drilled or slotted (and/or wavy) rotor also cools quicker while helping pull cooling air through the caliper and pads.
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  9. #9
    KLF
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    Quote Originally Posted by jeffscott View Post
    if rotor is not overheating then you don't need a bigger rotor......

    if it is get a bigger one, the have a lot more surface area to dissipate the heat....so they don't fade.

    The holes are there to help clean the roost off the pad face....and help cooling as well.

    A solid rotor heats up faster than a holey rotor.

    The best way to cool off hot pads is to stab at the brakes hard on for a about asec then off for as long as possible...

    Dragging pads causes rotors to overheat, cause the pad surface never gets a blast of cooling air, when the braking is removed.
    As far as surface area is concerned, a holier rotor has greater surface area. But, everything else being equal, a holier rotor has less mass and therefore heats up more quickly than a solid one from the same amount of energy input (braking heat).
    So we have "dueling" characteristics. Without some "real world" input about a specific rotor, it probably is not appropriate to speculate or generalize about how it will behave.
    That's why I was asking for comments about specific rotors, pro or con. Maybe I could have worded that more clearly.

    I would also speculate that rotors with more arms and/or short paths between arms and braking surfaces are better at heat dissipation.

    And the pad temperature is dictated almost entirely by what the rotors are doing. The heat of the pads is primarily cooled by being dissipated into the rotor. Even the technique you describe relies upon this. What matters most is the air cooling of the rotor not the pads.

  10. #10
    KLF
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    Quote Originally Posted by shiggy View Post
    Rotor weight differences are minor. You will not notice any difference while climbing.

    A drilled or slotted (and/or wavy) rotor also cools quicker while helping pull cooling air through the caliper and pads.
    We could beat this to death, but while the weight of any one specific component is probably not significant, the combined weight of numerous components is. So all else being equal I choose the lighter stuff and have a lighter bike that I enjoy as a result.

    But to the extent that you discount small things, calculate the mass of air (and its relative heat holding ability) moved through the caliper and pads by rotor slots/waves, especially at slow speeds.

  11. #11
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    Quote Originally Posted by KLF View Post
    As far as surface area is concerned, a holier rotor has greater surface area. .
    Whoop, Whoop, Whoop!
    This is the math police.
    Step away from your computer and put your hands where I can see them.

    That's not a slide rule in your pocket, is it?

    Since I"m here to help dispel myths, I'll give you the data and let you work it out. I just went down to the garage and measured the round holes in the front rotor on my favorite bike. They're 3/16" diameter. The thickness of that rotor is 0.070".

    HINT:
    a. Figure out the area of a 3/16" circle and multiply by two because there are two sides to the rotor. That would be the surface area of the rotor that is missing due to the hole.

    b. Figure out the circumferential surface area of the cylinder described by the diameter of the hole and the thickness of the rotor. That is the surface area of the inside surface of the hole.

    Which is greater, a or b?
    Is it the top and bottom of that stumpy little cylinder, or the sides?


    EXTRA CREDIT:
    As the hole becomes larger or elongated, what happens to the ratio of a/b?

    As the thickness of the rotor increases, what happens to the ratio of a/b?


    POINT TO PONDER:
    The thermal conductivity between a brake pad and the minute pockets of air contained in the rotor holes is negligible compared to the conductivity between the pad and the rotor. So why are there holes in rotors?

    YOUR ORIGINAL QUESTION:
    A larger diameter is certainly going to help with the cooling. A continuous, solid surface will probably be best for eliminating slow speed vibration.

  12. #12
    Spanish rider
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    Ok, if you're asking for specific models I can tell you one you should NOT try, which is Avid Cleansweep G2. The braking surface is narrow, it's noisy with most pads and they vibrate sometimes because of the cutouts in the braking surface.

    BUT... I know that there's another version of this rotor which has no cutouts: Avid Solid Sweep G3



    It's not perfect because the braking surface is narrow as well, but probably better than the original. I haven't personally tried it.

    However, I heard that there was a guy at this forum which manufactured tailor-made rotors. Maybe he can do something specific for your needs.
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  13. #13
    KLF
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    Quote Originally Posted by RustyIron View Post
    Whoop, Whoop, Whoop!
    This is the math police.
    Step away from your computer and put your hands where I can see them.

    That's not a slide rule in your pocket, is it?
    Nope, just happy to respond to you

    Quote Originally Posted by RustyIron View Post
    Since I"m here to help dispel myths, I'll give you the data and let you work it out. I just went down to the garage and measured the round holes in the front rotor on my favorite bike. They're 3/16" diameter. The thickness of that rotor is 0.070".

    HINT:
    a. Figure out the area of a 3/16" circle and multiply by two because there are two sides to the rotor. That would be the surface area of the rotor that is missing due to the hole.

    b. Figure out the circumferential surface area of the cylinder described by the diameter of the hole and the thickness of the rotor. That is the surface area of the inside surface of the hole.

    Which is greater, a or b?
    Is it the top and bottom of that stumpy little cylinder, or the sides?


    EXTRA CREDIT:
    As the hole becomes larger or elongated, what happens to the ratio of a/b?

    As the thickness of the rotor increases, what happens to the ratio of a/b?
    Let's jump right to the point-
    Given a cylindrical hole of radius R and depth D
    The area of the missing circular cylinder ends is 2 x Pi x R^2
    The area of the cylindrical wall of the hole 2 x Pi x R x D

    Those areas are equal when R = D

    So, the new surface area resulting from drilling a hole is greater than that of the non-drilled material if the radius of the hole is less than the thickness of the material.
    So, using simpler figures, to increase surface area of a 2mm thick rotor, a hole needs to be less than 2mm in radius = 4mm in diameter = ~5/32".
    For comparison, note that this is roughly 3/4 of the diameter of a typical bolt hole on a 6-bolt rotor.
    Hmm .... How big are the holes in an Avid HS1 rotor? (HS, as in "Heat shedding," right?)

    POINT TO PONDER:
    The thermal conductivity between a brake pad and the minute pockets of air contained in the rotor holes is negligible compared to the conductivity between the pad and the rotor. So why are there holes in rotors?
    Good question. A few answers. Weight, aesthetics, contaminants. The most important ones for this discussion are that the rotor has to dissipate accumulated heat to the air. Adding holes can (1) increase surface area as described above, and (2) enhance turbulence around the rotor to increase the volume of air coming into contact with the rotor and thereby increase heat dissipation. Significantly? I don't know.

    YOUR ORIGINAL QUESTION:
    A larger diameter is certainly going to help with the cooling. A continuous, solid surface will probably be best for eliminating slow speed vibration.
    OK, So why not a smaller rotor with LOTS of tiny holes? Cool AND smooth. Or more surface area in the form of more arms?
    Last edited by KLF; 10-27-2012 at 07:59 AM. Reason: clarity

  14. #14
    gran jefe
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    rotors with holes also wear out pads faster (at least on cars) but are probably better at shedding dirt that gets on them.

  15. #15
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    Quote Originally Posted by KLF View Post
    We could beat this to death, but while the weight of any one specific component is probably not significant, the combined weight of numerous components is. So all else being equal I choose the lighter stuff and have a lighter bike that I enjoy as a result.

    But to the extent that you discount small things, calculate the mass of air (and its relative heat holding ability) moved through the caliper and pads by rotor slots/waves, especially at slow speeds.
    Fine. Maybe you should just use rim brakes.
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  16. #16
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    Holes in rotors have *nothing* to do with allowing the rotors to shed heat. Holes in rotors allow better maintaining of braking force when the pads heat up enough to begin to "evaporate" for want of a better way to describe the problem. For example, take a piece of dry ice, and place it on a smooth, flat rotor. Give it a tap. It'll fly off that puppy. The gas creates a cushion between the solid surface and the dry ice - same thing happens with a hot brake pad. Now put the dry ice on a drilled or slotted rotor. Notice there's significantly more friction there. See where this is going?

    Stop focusing on drilling or slotting as a way to allow the rotor to shed heat or be lighter. That's simply not why it's there. It's there to give super heated air and brake pad contaminates or gasses someplace to go to get out of the way of the pad, and still allow it to make contact with the rotor.

  17. #17
    KLF
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    Wink

    Quote Originally Posted by shiggy View Post
    Fine. Maybe you should just use rim brakes.
    I was, but ran into problems when I drilled holes in my aluminum rims to lighten them up (and improve heat dissipation).

    But I learned my lesson, I won't drill the carbon fiber replacement rims. But I am having trouble getting the rim pads adjusted properly.

    So, do YOU have any experience to share with specific rotors that have worked particularly well or poorly for long brake-dragging descents? Or do you find that there is no difference from one rotor to another, aside from size?

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