Page 1 of 2 12 LastLast
Results 1 to 50 of 65
  1. #1
    mtbr member
    Reputation:
    Join Date
    Aug 2008
    Posts
    428

    New question here. Are bigger discs better?

    I just bought a used Trek 4900 (hardtail) that the previous owner had put Hayes discs on. It came with an extra set of discs because he was switching to 8" from the stock discs. He had the rear mounted up but the front is still stock.

    I'm just wondering if there is any real advantage to the bigger disc or if it (and the required adapter) adds weight.

    Another thing that I wonder about...it seems like the extra leverage of having the caliper held further from the mounting points would increase the likelihood for tweaking a seatstay. Is this something to be concerned about? I ask because I've seen some threads on Treks with cracked or broken seatstays near the disk mounts.

    By the way, I'm migrating the brakes/wheels over to a 2001 Trek 8000 hardtail. The mounting setup is the same between the two bikes.

    Thanks!

  2. #2
    mtbr member
    Reputation:
    Join Date
    Aug 2008
    Posts
    298
    The weight is negligible and it is a good idea in my opinion to have larger disks unless you weigh 160 lbs.

  3. #3
    bike tester
    Reputation: syl3's Avatar
    Join Date
    Apr 2008
    Posts
    1,100
    i'm just building a new bike, went over to the lbs and weighed every brake on the shelf.

    for example the hayes nine rear caliper with lever and hose weighs 430g with 160mm rotor and 480g with a 180mm rotor.

  4. #4
    mtbr member
    Reputation:
    Join Date
    Aug 2008
    Posts
    428
    Quote Originally Posted by syl3
    i'm just building a new bike, went over to the lbs and weighed every brake on the shelf.

    for example the hayes nine rear caliper with lever and hose weighs 430g with 160mm rotor and 480g with a 180mm rotor.
    Don't forget the required adapter and bolts. The one on my bike is pretty meaty. I don't mean to nit-pick but if you are concerned with weight...it's something to factor in.

  5. #5
    Double-metric mtb man
    Reputation: Psycho Mike's Avatar
    Join Date
    Apr 2006
    Posts
    4,483
    Ok...couple things. You will pay a minor (60-90g) weight penalty for the adaptors and larger rotors and you'll lose a bit of modulation. On the other hand, you will gain more power and heat resistance/dissipation.

    My bike came stock with Hayes 9's (6" rotor F/R) and some of the local trails are moderately downward pointed stuff (I typically ride a river valley...not super steep or long, but some decent runs that will want to push you to the 45 mph point if you don't work the brakes). I'm a bigger rider (235+gear) and I was able to routinely fade (boil) the front. I upgraded to a 8" rotor on the front and never had the problem again. I noticed the power but also the slight loss of modulation. On a ~30 lb bike, the 3 oz of extra weight for the adaptor and rotor was not noticeable.

    I ended up getting a better brake and going 8" front and 7" rear...overkill for a long travel XC bike, but the brakes have far better modulation for larger rotors than the stock brakes did. Heck, I even went to the local bike park and wasn't able to fade the brakes on a lot more demanding terrain (and I even had the rotors pinging as they were getting nicely heated up).
    As if four times wasn't enough-> Psycho Mike's 2013 Ride to Conquer Cancer Page

    Moran? Let your opinion be free -> F88me

  6. #6
    banned
    Reputation: Jerk_Chicken's Avatar
    Join Date
    Oct 2005
    Posts
    16,466
    Quote Originally Posted by gotboostedvr6
    The weight is negligible and it is a good idea in my opinion to have larger disks unless you weigh 160 lbs.
    Before dispensing such advice, it's adviseable to sort of know what you're saying, as in putting larger than approved rotors on a fork.

  7. #7
    local trails rider
    Reputation: perttime's Avatar
    Join Date
    Aug 2005
    Posts
    12,228
    A larger disk may have negatives too: too much torque for frame and fork (check what the manufacturer says).

    Some difference in weight.

    A larger disk may be more likely to get bent out of true, or develop irritating noices from vibration.

  8. #8
    banned
    Reputation: Jerk_Chicken's Avatar
    Join Date
    Oct 2005
    Posts
    16,466
    A larger disc can also present modulation issues.

  9. #9
    mtbr member
    Reputation:
    Join Date
    Aug 2008
    Posts
    428
    Quote Originally Posted by perttime
    A larger disk may have negatives too: too much torque for frame and fork (check what the manufacturer says).
    That's what I'm mainly concerned with. It's an older frame (2003 Trek 8000 SLR) and I'm not sure if I can even find that information.

  10. #10
    banned
    Reputation: Jerk_Chicken's Avatar
    Join Date
    Oct 2005
    Posts
    16,466
    I'm really surprised the rear of that frame can clear a 203. However, I wouldn't recommend it. There are even people on FR rigs that don't run 203's. I'm heavier, on a mid-weight FR rig and I actually do 3000+ foot descents that take close to a couple hours and I don't use, nor need a 203 in the back.

  11. #11
    mbtr member
    Reputation: scottzg's Avatar
    Join Date
    Sep 2006
    Posts
    3,785
    Quote Originally Posted by Jerk_Chicken
    I'm really surprised the rear of that frame can clear a 203. However, I wouldn't recommend it. There are even people on FR rigs that don't run 203's. I'm heavier, on a mid-weight FR rig and I actually do 3000+ foot descents that take close to a couple hours and I don't use, nor need a 203 in the back.
    +1. If you NEED a 203 in the back, you need to reevaluate either your braking technique or your diet. The big rotor makes the rear end lock up very easily, even with xc brakes. This 230lb fella ditched his 185 rotor for a 160 for that reason, haven't had a problem with fading.
    I like cheap stuff that works great and is very sturdy.

  12. #12
    mtbr member
    Reputation:
    Join Date
    Aug 2008
    Posts
    428
    I think the reason the previous owner may have swapped out the disk was because of damage to the original. I looked at the spare smaller disk and it has some wear on it. I laid it on a glass tabletop and it doesn't lie perfectly flat. I suspect it is warped or bent. So, he probably just opted to go big when he got new rotors.

    Maybe I'll just buy a new smaller rotor. I'll ride this one for a while first to see how it goes. I haven't even been able to ride this bike yet. The weather has sucked and I need to get tires.

    Thanks for all the feedback.

  13. #13
    I am a pathetic rider...
    Reputation:
    Join Date
    Sep 2007
    Posts
    644
    I am going to say you most likely don't need 203mm, I just put a 203 on my bb7's, and if the trail had been dry, they would have been an amazing brake, but with the wet leaves and roots, they were just way to grabby and I was barely able to save the front end on numerous occasions. so if you have great traction, go for the biggest, but if you are riding in any sort of adverse conditions, they are way too much power.
    Save the Earth, Ride a Cyclist

  14. #14
    mtbr member
    Reputation:
    Join Date
    Dec 2006
    Posts
    56
    Coming off my experience with cars- but the general principles should still apply...

    Simply having a bigger disc won't provide any more stopping power unless the brake pad has a larger contact area with the disc. What it WILL do is provide more surface area to absorb and dissipate heat to avoid brake fade during prolonged braking. Since any decent bike brake setup should have no problem locking up the wheels, you should only be concerned with fade and not stopping power.

    Therefore, if you are doing prolonged downhills, a big rotor will help keep the brakes stopping well as you descend. If your downhills are short, all the big rotor will do is add unnecessary weight.

  15. #15
    mtbr member
    Reputation:
    Join Date
    Aug 2008
    Posts
    298
    Since when does larger rotors not produce more stopping power>?

  16. #16
    mtbr member
    Reputation:
    Join Date
    Dec 2006
    Posts
    56
    If the larger rotor has more pad contact, it will produce more stopping power. Whether that is the case will depend on the specifics of the setup. However, stopping power alone is almost never an issue on a bike unless the brakes are fading from a long downhill. If you can lock up the tires, you have already used all the available stopping power. This is simple physics. Short of having a drag chute, you can't get more stopping power than the friction of the tires on the ground provides.

    Stopping power as opposed to the effort needed to squeeze the levers is a separate issue. As long as modulation is o.k., most would prefer less effort needed to produce a given amount of stopping power.

  17. #17
    mtbr member
    Reputation:
    Join Date
    Aug 2008
    Posts
    298
    While I'm not saying your 100% wrong, we can agree to disagree.

  18. #18
    I am a pathetic rider...
    Reputation:
    Join Date
    Sep 2007
    Posts
    644
    I can speak for bigger rotors and same pads = more braking power. I think it is not because of more friction between pads and rotors, but because of the longer lever arm the disc now has on the hub.
    Save the Earth, Ride a Cyclist

  19. #19
    mtbr member
    Reputation:
    Join Date
    Dec 2006
    Posts
    56
    While I'm not saying your 100% wrong, we can agree to disagree.
    I think I am being a bit misunderstood...

    I'm not saying that a bigger rotor setup will always not have more stopping power, but to the extent that is does have more stopping power it is due to ancillary factors other than the rotor size.

    More brake pad contact or more leverage due to a longer arm would both be reasons why the larger disc setup could have more stopping power, but it's not directly due to the bigger disc itself. It would be theoretically possible to design a brake setup with a smaller rotor with more instant stopping power by changing leverage characteristics, brake pad shape/design, etc.

    However, what I am saying is that stopping power by itself is not the biggest challenge in a brake setup on a bike. It does not take much force to lock up a bike wheel because there is not that much weight involved. Because of the precision necessary in mountain biking, modulation is far more important. By resisting fade, a bigger rotor could help keep modulation consistent over the course of a big downhill. Fade, if it was bad enough, could also cause enough of a loss in stopping power that it would be no longer possible to create enough clamping force to stop the wheel from turning. This is why downhill bikes need big rotors- they need to be very fade resistant because a long downhill will create a serious amount of heat.

    More braking force can also be nice in that it will take less finger strength to produce the force necessary to lock up the wheels- that is good up to a point- but it could go too far and cause modulation issues and oversensitivity.

    For the OP: The correct choice in braking depends on the use. If you are doing 1000 ft+ vertical descents, you probably are going to generate enough heat to cause brake fade over a long descent. In that case, a big rotor is going to be very useful. However, if your descents are short and quick (say more like 50 vertical feet) there is no way enough heat is going to be generated to cause fade. A bigger rotor will just slow you down by adding weight. This is why downhill bikes have huge rotors and XC bikes have small ones.

    As long as you are generating enough force to lock the tires up, a bigger brake setup won't stop you one bit sooner. That much is not really debatable. What is debatable is the effect of overall stopping power on brake feel, etc., which is often a matter of preference and subject to debate.
    Last edited by nealric; 11-10-2008 at 07:52 PM.

  20. #20
    mtbr member
    Reputation:
    Join Date
    May 2007
    Posts
    47
    Quote Originally Posted by nealric
    I think I am being a bit misunderstood...

    I'm not saying that a bigger rotor setup will always not have more stopping power, but to the extent that is does have more stopping power it is due to ancillary factors other than the rotor size.
    No, you arn't understanding at all.
    The greater braking power from larger discs is ALL about the size. Due to the moment of force being radially further out in space the leverage is multiplied for the given force applied to the disc. Its a pure application of mechanical advantage, in a rotational format. Just think of it as a second-class lever. Increase the lever arm, and you increase the moment of force.

    Also, if you think its easy to lock up a front wheel at speed, then think again. I have Avid Codes on my DH bike, and i regularly struggle to slow on the local trails, and thats with a 203mm front rotor, and a 185mm rear.

  21. #21
    Nightmare on Lyrik st. VI
    Reputation: amrgb's Avatar
    Join Date
    May 2007
    Posts
    818
    Quote Originally Posted by nealric
    Simply having a bigger disc won't provide any more stopping power unless the brake pad has a larger contact area with the disc.

    I'm not saying that a bigger rotor setup will always not have more stopping power, but to the extent that is does have more stopping power it is due to ancillary factors other than the rotor size.
    You're missing one thing. The larger the disc the larger its circumference. Therefore, for every wheel revolution the pad drags over a larger area of disc, which is what gives you more stopping power.

  22. #22
    mtbr member
    Reputation:
    Join Date
    Aug 2008
    Posts
    298
    Quote Originally Posted by nealric
    I think I am being a bit misunderstood...

    I am following you but in this context your theory is a bit off.

    With all else being the same a larger rotor will produce more stopping power then a smaller rotor with the same input pressure at the handle.

  23. #23
    mtbr member
    Reputation:
    Join Date
    Dec 2006
    Posts
    56

    With all else being the same a larger rotor will produce more stopping power then a smaller rotor with the same input pressure at the handle.
    You are both correct on the bigger rotor. My bad now that I think of it. Part of the problem is I was coming from a car paradigm. Installing a big-brake kit on a car will not change the stopping distance for a one time braking maneuver.

    However, I do stand by that the real issue is not maximising one-off stopping power. If you can't lock up the tires, either your brakes are fading from heat or are not setup correctly. I suspect that is what is happening in takai's situation. I guess it could also be the case that DH tires just provide a ton more traction than the XC tires I am used to. My puny 160mm rotors with Bb7s have no problems locking up my tires at any speed when they are cool. After heating up, that's a different story.

  24. #24
    mbtr member
    Reputation: scottzg's Avatar
    Join Date
    Sep 2006
    Posts
    3,785
    Quote Originally Posted by nealric
    You are both correct on the bigger rotor. My bad now that I think of it. Part of the problem is I was coming from a car paradigm. Installing a big-brake kit on a car will not change the stopping distance for a one time braking maneuver.
    cars don't really compare that well to bikes. With cars the center of mass and brake balance is fixed. You add a larger rotor and caliper bracket to a car and the front biased brake system will simply lock the front end up even sooner- braking distance will be worse because the rear brakes are under used. Stick a big brake kit on and it gets even worse, the fluid displaced in the new calipers may more or less what the rest of the system is designed around. It's silly to add a big brake kit to a car without an adjustable brake proportioning valve, and on a street car even then it's stupid because you can't set the balance from light to heavy braking.

    On a bike, none of that is true. Bigger rotor = more power = shorter stopping distance, given infinite traction.
    I like cheap stuff that works great and is very sturdy.

  25. #25
    banned
    Reputation: Jerk_Chicken's Avatar
    Join Date
    Oct 2005
    Posts
    16,466
    ....

  26. #26
    mtbr member
    Reputation:
    Join Date
    May 2007
    Posts
    533
    Quote Originally Posted by scottzg
    ...given infinite traction.
    As I understand it, this is the key point. If you can lock the tire up with a small rotor you're not going get any more "stopping power" with a big one on a car or a bike. More brake torque yes, shorter stops no. At least until things get hot anyway. In the end it's the tires that stop the car or bike.

  27. #27
    mtbr member
    Reputation: vdubz's Avatar
    Join Date
    Sep 2008
    Posts
    210
    You may want to ask this question in a womans forum or your riding buddy who always seems to be behind you. Ohhhh you said "discs" sorry haha

  28. #28
    mtbr member
    Reputation: hitek79's Avatar
    Join Date
    Oct 2008
    Posts
    1,302
    Can someone define "modulation" for me?

  29. #29
    Reviewer/Tester
    Reputation: Rainman's Avatar
    Join Date
    Apr 2004
    Posts
    6,191
    It's all about lever pressure.
    The difference between a 160mm and a 185mm on the front end of the bike is that you need less lever pressure to slow the bike down, all things being equal.
    If you brake correctly, ie: use the front brake for 85%+ of your stopping/slowing power then there is no need for a large rear disc, simply because when braking most of your weight is transferred to the front of the bike, which means that it takes only a minute amount of braking to lock the rear wheel. A locked rear wheel isn't doing your overall braking much use at all...not to speak of the loss of control of the rear end.

    Most bikes benefit from a change from 160mm to 180/185mm on the front end, but little benefit from changing to a larger than 160mm on the rear.

    R.
    It is inevitable ...

  30. #30
    mtbr member
    Reputation:
    Join Date
    Aug 2008
    Posts
    1,430
    Quote Originally Posted by gotboostedvr6
    Since when does larger rotors not produce more stopping power>?
    What I think he originally meant was stopping power is ultimately determined by traction (of the tire to the road).

  31. #31
    mtbr member
    Reputation:
    Join Date
    Aug 2008
    Posts
    1,430
    Quote Originally Posted by Jerk_Chicken
    Before dispensing such advice, it's adviseable to sort of know what you're saying, as in putting larger than approved rotors on a fork.
    I've never understood this statement. Assuming stock 160s could lock the wheel, and you upgrade to 203s which will also lock the wheel, with traction not changing, how can the big brakes put more load on the fork? If anything, moving the caliper outwards and increasing the torque arm distance, you would have less load at the caliper mounting bolts for the big brakes.

  32. #32
    Vaginatarian
    Reputation:
    Join Date
    Oct 2005
    Posts
    5,686
    Quote Originally Posted by amrgb
    You're missing one thing. The larger the disc the larger its circumference. Therefore, for every wheel revolution the pad drags over a larger area of disc, which is what gives you more stopping power.
    doesn't make any difference, the amount of pad touching the rotor is the same, if the rotor was straight and the brake pads gripped it and stopped in say 4" would they stop any faster because the distance was longer? no they would still stop in 4"
    as mentioned above heat dissipation is what you gain from bigger rotors

  33. #33
    Vaginatarian
    Reputation:
    Join Date
    Oct 2005
    Posts
    5,686
    Quote Originally Posted by b4 stealth
    I can speak for bigger rotors and same pads = more braking power. I think it is not because of more friction between pads and rotors, but because of the longer lever arm the disc now has on the hub.
    not so fast
    wouldnt the bigger rotor also have more torque against the pads

  34. #34
    Vaginatarian
    Reputation:
    Join Date
    Oct 2005
    Posts
    5,686
    Quote Originally Posted by takai
    No, you ain't understanding at all.
    The greater braking power from larger discs is ALL about the size. Due to the moment of force being radially further out in space the leverage is multiplied for the given force applied to the disc. Its a pure application of mechanical advantage, in a rotational format. Just think of it as a second-class lever. Increase the lever arm, and you increase the moment of force.

    Also, if you think its easy to lock up a front wheel at speed, then think again. I have Avid Codes on my DH bike, and i regularly struggle to slow on the local trails, and thats with a 203mm front rotor, and a 185mm rear.
    you're leaving out one thing, the larger rotor is also directing higher torque to the pad because its spinning faster than a smaller rotor
    just as a bigger gear on your cassette moves you slower it has gobs of low end torque

    as far as your DH analogy, I bet you can lock up the front wheel before everything heats up
    but after, its another story. while not DH, my brakes stop fine until sustained downhill braking, with a bigger rotor they don't fade

    I think were both right here its just a matter of which forces are more powerful, the longer leverage or the higher torque, or the cooler big rotor vs. the hotter small rotor
    the truth is probably somewhere in the middle

  35. #35
    mtbr member
    Reputation:
    Join Date
    Jan 2004
    Posts
    8,014
    Didn't read the all post's but a larger rotor will offer more raw stopping power but the modulation will change slightly. A 160mm rotor and a 203mm rotor share the same pads and contact area but the larger rotor is further from the rotor, torque... blah blah blah. The added power changes the modulation, I diddn't say reduces because the master cyl. & caliper are the same - the additional power is more to harness @ your finger tips.

  36. #36
    mtbr member
    Reputation:
    Join Date
    Aug 2008
    Posts
    1,430
    Quote Originally Posted by dan0
    not so fast
    wouldnt the bigger rotor also have more torque against the pads

    A larger rotor has less torque against the pads. Or another way of looking at it is the pads have more torque on the rotor. Think of the rotor as a lever. The larger disk is a longer lever.

  37. #37
    I think we should go back
    Reputation:
    Join Date
    Nov 2008
    Posts
    583
    Why do they make larger front rotors than backs? I have rim brakes and the wheels never get warmer than about 100 degrees F

  38. #38
    R.I.P. DogFriend
    Reputation: jeffj's Avatar
    Join Date
    Jan 2004
    Posts
    6,547
    http://www.minimania.com/web/SCatagory/BRAKES/DisplayType/Calver's%20Corner/DisplayID/933/ArticleV.cfm

  39. #39
    Vaginatarian
    Reputation:
    Join Date
    Oct 2005
    Posts
    5,686
    Quote Originally Posted by drain bamage
    Why do they make larger front rotors than backs? I have rim brakes and the wheels never get warmer than about 100 degrees F
    as mentioned earlier, when you hit your brakes your weight shifts to the front, so thats where most of your stopping power is. larger rotors ( like your wheel) have a bigger area to dissipate heat so they stay cooler ( plus on a rim you have a couple of inches of air between the 2 brake shoes)

  40. #40
    mtbr member
    Reputation:
    Join Date
    Aug 2006
    Posts
    14
    Neilric, you shouldn't have conceided so early. the complete story of fade has yet to be told. you can get pad fade and fluid fade which will both decrease your stoping power. if your still heating your brakes up too much then you have no choice but to switch to a brake with higher thermal capacity. a larger caliper is what is going to decrease fading, not increasing brake torque. switching to a larger rotor is not really addressing your fading issues, if it is solved just by adding brake torque you didn't need a very substantial increase in thermal capacity to solve your fading in the first place (fluid and pads would have worked). switching to a pad and fluid that can take more heat would have been a better route and you would have maintained your modulation.
    to even talk about brakes with absolute tire grip is completely impractical (and a red herring). why not talk about the suspension advantage of hover bikes or other such impossible things? they have no practical application in the real world.

    The proper way to address fade is:
    1. if your levers still have pressure you have exceeded your brake pads heat range.
    2. if your levers get too squishy then your boiling the fluid and should switch to a higher temp fluid (ate super blue is pretty bombproof).
    3. get a larger brake caliper with a higher heat capacity (larger piston or multi piston design)
    4. if your still fading, you have no choice but to go for a larger rotor or a 2 pc floating rotor

    hydraulic brakes are hydraulic brakes are hydraulic brakes. if they are for a bike, motorcycle or car. You can't ignore the library of knowledge about how they work. If you don't want to believe this, that's fine by me. you can go plant a magic bike tree in your backyard with the magic bike tree jelly beans some old hippie gave you, water them every day and hope for something to grow. There is no mystery behind how a brake system works. it is not different for mountain bikes.

  41. #41
    mtbr member
    Reputation:
    Join Date
    Aug 2008
    Posts
    1,430
    Quote Originally Posted by BigClunke
    Neilric, you shouldn't have conceided so early. the complete story of fade has yet to be told. you can get pad fade and fluid fade which will both decrease your stoping power. if your still heating your brakes up too much then you have no choice but to switch to a brake with higher thermal capacity. a larger caliper is what is going to decrease fading, not increasing brake torque. switching to a larger rotor is not really addressing your fading issues, if it is solved just by adding brake torque you didn't need a very substantial increase in thermal capacity to solve your fading in the first place (fluid and pads would have worked). switching to a pad and fluid that can take more heat would have been a better route and you would have maintained your modulation.
    to even talk about brakes with absolute tire grip is completely impractical (and a red herring). why not talk about the suspension advantage of hover bikes or other such impossible things? they have no practical application in the real world.

    The proper way to address fade is:
    1. if your levers still have pressure you have exceeded your brake pads heat range.
    2. if your levers get too squishy then your boiling the fluid and should switch to a higher temp fluid (ate super blue is pretty bombproof).
    3. get a larger brake caliper with a higher heat capacity (larger piston or multi piston design)
    4. if your still fading, you have no choice but to go for a larger rotor or a 2 pc floating rotor

    hydraulic brakes are hydraulic brakes are hydraulic brakes. if they are for a bike, motorcycle or car. You can't ignore the library of knowledge about how they work. If you don't want to believe this, that's fine by me. you can go plant a magic bike tree in your backyard with the magic bike tree jelly beans some old hippie gave you, water them every day and hope for something to grow. There is no mystery behind how a brake system works. it is not different for mountain bikes.
    If this were true, car manufacturers would use exotic pad materials on 10" disks for their high end cars. In reality, the opposite is true, there's a noticable trend toward bigger disks on high performance cars.

    It's assumed a larger diameter disk is going to have more mass. Going from a solid 160mm rotor to something like the aligator 203mm rotor will only increase leverage but won't do much for fade resistance since the mass is about the same.

    What you left out is the larger cooler disk carries heat out of the pad. The caliper is not supposed to act as a heat sink even though it does to a certain extent. The multiple piston design is not meant to transfer heat better. Think about where that heat goes once it heats the pistons.....the fluid.

  42. #42
    mbtr member
    Reputation: scottzg's Avatar
    Join Date
    Sep 2006
    Posts
    3,785
    Quote Originally Posted by BigClunke
    Neilric, you shouldn't have conceided so early. the complete story of fade has yet to be told. you can get pad fade and fluid fade which will both decrease your stoping power. if your still heating your brakes up too much then you have no choice but to switch to a brake with higher thermal capacity. a larger caliper is what is going to decrease fading, not increasing brake torque. switching to a larger rotor is not really addressing your fading issues, if it is solved just by adding brake torque you didn't need a very substantial increase in thermal capacity to solve your fading in the first place (fluid and pads would have worked). switching to a pad and fluid that can take more heat would have been a better route and you would have maintained your modulation.
    to even talk about brakes with absolute tire grip is completely impractical (and a red herring). why not talk about the suspension advantage of hover bikes or other such impossible things? they have no practical application in the real world.

    The proper way to address fade is:
    1. if your levers still have pressure you have exceeded your brake pads heat range.
    2. if your levers get too squishy then your boiling the fluid and should switch to a higher temp fluid (ate super blue is pretty bombproof).
    3. get a larger brake caliper with a higher heat capacity (larger piston or multi piston design)
    4. if your still fading, you have no choice but to go for a larger rotor or a 2 pc floating rotor

    hydraulic brakes are hydraulic brakes are hydraulic brakes. if they are for a bike, motorcycle or car. You can't ignore the library of knowledge about how they work. If you don't want to believe this, that's fine by me. you can go plant a magic bike tree in your backyard with the magic bike tree jelly beans some old hippie gave you, water them every day and hope for something to grow. There is no mystery behind how a brake system works. it is not different for mountain bikes.
    Ummm... if you're using your calipers as heat sinks... stop riding your brakes down fire roads. Rotors are what manage the heat in the brake system, and larger rotors increase the size of the heat sink and the surface area to bleed off heat. Larger calipers are a byproduct of making a stiffer caliper (for power) or a multi-piston caliper (for modulation). Large calipers are NOT intended as heat sinks- they're designed to soak up as little heat as possible.

    Oh, and floating rotors are designed to resist warping, they're marginally worse heat sinks.
    I like cheap stuff that works great and is very sturdy.

  43. #43
    mtbr member
    Reputation:
    Join Date
    Jul 2008
    Posts
    32
    Come on, you guys....

    The basic principals is known as the "moment arm"---> radius x force = constant

    When rotor size is 160mm and you apply 2N to brake lever, then you have .32NM.

    If you upgrade to 203mm, you only need to apply (.16M/.203M) x2N = 1.57N to achieve the same braking power as in 160mm. That's why a bigger rotor always have better leverage and produce bigger braking power. Heat dissipation for a bigger rotor while it may be better because of larger heat exchanging surface, it is really insignificant (for XC you never reach 60 mph, do you?). Leverage is the key.

  44. #44
    ...idios...
    Reputation: SteveUK's Avatar
    Join Date
    Apr 2006
    Posts
    5,662
    "If this were true, car manufacturers would use exotic pad materials on 10" disks for their high end cars. In reality, the opposite is true, there's a noticable trend toward bigger disks on high performance cars."

    Formula 1 cars use (relatively) small brakes made from carbon fiber, although the fact that they almost don't work at all at low temperature obviously precludes them from pretty much any other application, especially on a mountain bike.

    What use is a philosopher who doesn't hurt anybody's feelings? -
    Diogenes


  45. #45
    mtbr member
    Reputation:
    Join Date
    Nov 2008
    Posts
    16
    wow this is a little confusing?? lol
    Lol, get a step stool and put it in a doorway. Then stand on it and jump up until you slam your head on the doorway.

  46. #46
    mtbr member
    Reputation:
    Join Date
    Aug 2008
    Posts
    1,430
    Quote Originally Posted by SteveUK
    "If this were true, car manufacturers would use exotic pad materials on 10" disks for their high end cars. In reality, the opposite is true, there's a noticable trend toward bigger disks on high performance cars."

    Formula 1 cars use (relatively) small brakes made from carbon fiber, although the fact that they almost don't work at all at low temperature obviously precludes them from pretty much any other application, especially on a mountain bike.
    They're not that small when you consider the weight of the car which is about 1/3 of your average family car.

  47. #47
    ...idios...
    Reputation: SteveUK's Avatar
    Join Date
    Apr 2006
    Posts
    5,662
    "They're not that small when you consider the weight of the car which is about 1/3 of your average family car."

    If we're making comparisons, it could also be considered that it would be something of a rarity for the average family car to be braking from 185mph. An F1 brake disc will be no larger than 305mm (12in). If diameter were always relative only to weight, how small do you suppose an MTB rotor would need to be?

    What use is a philosopher who doesn't hurt anybody's feelings? -
    Diogenes


  48. #48
    mtbr member
    Reputation:
    Join Date
    Aug 2006
    Posts
    14
    Quote Originally Posted by BuickGN
    If this were true, car manufacturers would use exotic pad materials on 10" disks for their high end cars. In reality, the opposite is true, there's a noticable trend toward bigger disks on high performance cars.

    It's assumed a larger diameter disk is going to have more mass. Going from a solid 160mm rotor to something like the aligator 203mm rotor will only increase leverage but won't do much for fade resistance since the mass is about the same.

    What you left out is the larger cooler disk carries heat out of the pad. The caliper is not supposed to act as a heat sink even though it does to a certain extent. The multiple piston design is not meant to transfer heat better. Think about where that heat goes once it heats the pistons.....the fluid.
    Exotic car manufactures do use the smallest disc available on their REALLY high end race cars (to reduce weight and decrease wheel rotating mass (size)).

    Big disc's are bling, drilled rotors are bling, to some extent multi piston calipers are bling (if done for the wrong reasons). it is more important to look at what they do with their race cars than doing than street driven general consumer market cars (more about marketing than performance).

    you are correct about larger discs having more mass and area to distribute heat, I didn't mean to come across as denying that, but you have to run through the first 4 steps I listed before you should really go to a larger size. you also might want to increase cooling before as well (in cars, venting and aero bits are a bit more practical than dh mountain biking, but probably should be explored to a greater extent).

    the rotors do carry heat out of the pad, but you only need to remove so much (it is just one part of the equation). you can run them hotter with a bigger caliper (more fluid to resist fade, spread heat), better fluid, and a higher temp pad.
    also, simply adding more pistons and not increasing the volume of fluid in the caliper would not help fade, agreed.
    However, most multi-piston designs are to increase the volume of fluid available in the caliper and still keep the pad contact consistent (under heavy braking with poor venting design, the fluid can heat at different rates and cause poor pad contact, brake feel, and result in irregular pad wear. that is more relevant to automotive applications though, we don't generate nearly as much heat on bikes).

    All I'd really like to get across is that most people are not troubleshooting brake fade correctly, they are loosing performance without reason and buying into the bling/hype. Some people are going to need to increase rotor size as a last resort, but we shouldn't reward lazy engineering with our hard earned dollars.

    One thing that I should address because I am sure someone else is going to bring it up, higher temp brake pads are squeaky, dusty, require a higher initial heat range (don't stop well when cold, you need to heat them up) and wear quicker.
    So, there is a balance that needs to be achieved. just as there are drawbacks to going with too large of a caliper (spongy feel), and a higher temp fluid (needs to be changed more regularly due to water absorption which decrease feel). We all have different needs and tolerances. If you would rather sacrifice modulation and can't compromise on any of those other areas, OK, go for a larger caliper without trying anything else. I'll be waiting for you at the bottom

  49. #49
    mtbr member
    Reputation:
    Join Date
    Aug 2006
    Posts
    14
    Quote Originally Posted by Harman
    Come on, you guys....

    The basic principals is known as the "moment arm"---> radius x force = constant

    When rotor size is 160mm and you apply 2N to brake lever, then you have .32NM.

    If you upgrade to 203mm, you only need to apply (.16M/.203M) x2N = 1.57N to achieve the same braking power as in 160mm. That's why a bigger rotor always have better leverage and produce bigger braking power. Heat dissipation for a bigger rotor while it may be better because of larger heat exchanging surface, it is really insignificant (for XC you never reach 60 mph, do you?). Leverage is the key.
    what does this have to do with brake fade?

    the topic of brake torque without regards to tire grip is completely irrelevant (red herring argument).

    you do not need to upgrade anything if you do not experience some sort of brake fade.
    no one needs the ability to lock their wheels more easily (that is bad). we just need to be able to stop in the conditions we ride in (if that is down fire roads with the brakes on, so be it). I think you'll see manufactures such as formula making use of larger calipers in high heat situations. you want the smallest rotor you can run, this bigger brake stuff is mostly nonsense marketing.

  50. #50
    mtbr member
    Reputation:
    Join Date
    Aug 2006
    Posts
    14
    Quote Originally Posted by scottzg
    Ummm... if you're using your calipers as heat sinks... stop riding your brakes down fire roads. Rotors are what manage the heat in the brake system, and larger rotors increase the size of the heat sink and the surface area to bleed off heat. Larger calipers are a byproduct of making a stiffer caliper (for power) or a multi-piston caliper (for modulation). Large calipers are NOT intended as heat sinks- they're designed to soak up as little heat as possible.

    Oh, and floating rotors are designed to resist warping, they're marginally worse heat sinks.
    The rotor is still the primary heat sink no matter what size rotor you run. I live in Michigan and have no problems with brake fade in this state (but If I did ride fire roads with my brakes on the whole time, a brake could still be engineered for that). However, I do HPDE and TT events in my wrx and have learned a great deal about brake fade first hand and through lots of reading/troubleshooting.
    to make a caliper stiffer you only need to add more metal not increase the volume (which manuf. do for DH brakes), multi piston designs are to maintain an even pad contact when increasing the fluid volume. the caliper is not the primary heatsink (when did I ever claim that?), but they have to function in a high heat environment, more fluid volume helps, as does fluid that resists boiling. each piece is part of the equation.

    Maybe what I have been unclear about is, I am proposing that a higher heat environment is just fine as long as you have the right equiptment (pads, fluid, large enough caliper), you don't have to gain weight and loose modulation in most cases. the MTBR.com knowledge base is troubleshooting brake fade backwards.

    on the 2pc, cars use them because the heat range is so extreme you start to fry wheel bearings. they do act as a heat buffer (worse heat sink). thanks for correcting that.

Page 1 of 2 12 LastLast

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •