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  1. #1
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    rotor size - how much difference in real world?

    I currently have Elixir 9 front and rear with 160mm on my 29er hardtail.

    I don't have any complaints really, but like most gear-heads I'm always curious...

    If I were to up-size to 180mm rotors, how much real-world braking difference would I experience?

    thanks in advance

  2. #2
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    I ran 160's for years. Read about decreased modulation w/ larger rotors when disc brakes first came out. After I started riding really agressive trail I found the 160's overheating, a switch to 180's then 203's really kept the braking consistant. I currently run a 203 front & 180 rear on my XC / trail bike.

  3. #3
    Fat-tired Roadie
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    My new bike has a 180 on the front. My old one has a 160. Can't say I can tell a difference.

    To mess up the comparison, I did also move from 26" wheels to 29" wheels. I don't think that matters, though.
    "Don't buy upgrades; ride up grades." -Eddy Merckx

  4. #4
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    rotor size - how much difference in real world?

    Quote Originally Posted by gripper35 View Post
    I currently have Elixir 9 front and rear with 160mm on my 29er hardtail.

    I don't have any complaints really, but like most gear-heads I'm always curious...

    If I were to up-size to 180mm rotors, how much real-world braking difference would I experience?

    thanks in advance
    Makes a difference on long descents. The brake stays cooler and does not lose (as much) power.

    Reduces hand effort for the same braking.
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  5. #5
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    thanks very much for the info. I think I want to look into this. I have several really long, steep descents on my favorite local loop.

    I believe I would simply need some new 180mm (or 203mm) HS1 rotors.
    And I would need some kind of new bracket/mounting kit front and rear.
    Or am I mistaken?

    Any other advice greatly appreciated.

  6. #6
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    You'd need new adapters. It's a standard part, though.
    "Don't buy upgrades; ride up grades." -Eddy Merckx

  7. #7
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    rotor size - how much difference in real world?

    Quote Originally Posted by gripper35 View Post
    thanks very much for the info. I think I want to look into this. I have several really long, steep descents on my favorite local loop.

    I believe I would simply need some new 180mm (or 203mm) HS1 rotors.
    And I would need some kind of new bracket/mounting kit front and rear.
    Or am I mistaken?

    Any other advice greatly appreciated.
    BTW the larger rotor makes the most difference on the front.
    If you are using your brakes properly 160mm is fine on the rear for XC/trail riding.
    I use 185/160 on all my bikes: 26", 650B, 29" and even with 700x23 road tires.

    And my caliper magically aligned itself when I changed from 160 to 185! :roll eyes:
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  8. #8
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    ^What shiggy said!!!

    I'm rolling 180 up front and 160 in the rear (weight 165). I do a lot of steep downhills runs (3-8 miles) and the larger rotor up front has clearly improved both the braking power and lasting power on those runs.

    I do feel a bit more power in the front with 180 vs 160, comparatively.
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  9. #9
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    203 front 180 rear here
    Would go 203 in the rear but it doesn't physically fit.

    Found i need less pressure on the levers with the bigger discs, so my arm is less fatigues on low downhill sections, other side benifit is the brakes are now more consistent.

    On 1 downhill section on the way home it's a public road that's very steep and long, i can't release the brakes as there are pedestrians, cars, dogs and other cyclists.
    So i'm braking constantly for just over 5mins.
    With the smaller discs i found the lever was coming back further and further and after 3 mins it was tough to modulate as the brakes became very snappy.

    With the bigger discs they're pretty much the same at the top as they are at the bottom, plus i'm able to modulate and maintain a brake force just below where either wheel is locking up, this with 1 hand.
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  10. #10
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    There is absolutely a difference. Several differences actually. First, as mentioned, there is greater advantage to the larger rotor up front. Past that point:

    Larger rotors have more mass and greater friction surface. This means moment of friction is distributed over more area, with more mass to dissipate heat, means much cooler more consistent braking on your long descents.

    Your brakes work harder so you don't have to. Modulation stays pretty good, even with the largest rotors, it just happens easier. You don't have to apply as much force to the brake lever to get the same amount of braking. As pointed out, this leads to less fatigue at the bottom of the hill.

  11. #11
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    this is great info. thanks very much. I'm gonna start by upsizing to 180 on front and keep the 160 rear.

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    what has been said.

    besides, look carefuly in the forks operation manual, some cant handle the 203mm torque.

    i switched from 160's to 180's and even with bb5 it made lots of difference in braking power. i didn't need that much because the route was pretty flat, but being able to emergency stop my 90k mass faster than other vehicules when on road is priceless.

    the most outside the wheel the brake bites, the less torque you have to apply to slow the wheel the same amount, hence why some cantilever users are so happy with them.

    too big can also lead to more on-off brakes if you haven't good enough modulation in the brake system.

  13. #13
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    I did check my fork documentation, and it appears that my Marzocchi Corsa SL 29er fork supports up to a maximum disc size of 185mm.

    thanks again

  14. #14
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    I notice a little difference, not a lot. If you don't currently have problems with fade or overheating, you probably won't notice a lot of difference either. I've done the Monarch Crest decent (6000' elevation loss) with 160/160 rotors. No big deal.

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    i guess as i notice the difference mostly on road, the fact that the road tyre to road grip is much better and regular than mtb tyre /loose terrain, it should be normal as on loose surface your wheel will "stop" earlyer in the lever pull due to bad grip?

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    Quote Originally Posted by smilinsteve View Post
    I notice a little difference, not a lot. If you don't currently have problems with fade or overheating, you probably won't notice a lot of difference either. I've done the Monarch Crest decent (6000' elevation loss) with 160/160 rotors. No big deal.
    What sort of distance is that elevation loss on? And how much do you weigh? Both will factor heavily in how successful one would be in making that descent with those rotors. I'm fond of telling people that if they're over 190lbs they should probably be running a 180 up front, just for the heat dissipation.

  17. #17
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    There's a lot of good info in the posts above.

    Adding to that, rotor size is dependent on the brakes you use. Some are more vulnerable to heat induced brake fade, some are more powerful or offer less modulation. Terrain makes a big difference also. Rotor type makes an equally big difference. These factors go into determining the right rotor size.

    For example, I just recent switched to Magura MTS brakes. They offer a bit more power and initial bite than the Hygia SLPs I used previously. For XC racing we have dry, sketchy conditions. As a result I switched to a 140mm rear rotor as the 160mm with the grabby rear brake was making it harder in the heat of battle to modulate the rear. The 140mm has made locking the rear less likely. However, when I go bikepacking in the mountains where there are often descents of 1000m vertical at a time and I'm carrying 10kg of extra gear I use a 180mm rotor up front and 160mm rear. The braking power isn't greatly increased but the capacity to deal with the heat involved in dragging the brakes to control speed on long descents in greatly improved.

  18. #18
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    Quote Originally Posted by Cotharyus View Post
    What sort of distance is that elevation loss on? And how much do you weigh? Both will factor heavily in how successful one would be in making that descent with those rotors. I'm fond of telling people that if they're over 190lbs they should probably be running a 180 up front, just for the heat dissipation.
    I weigh 190 and that total ride is up and down around 25 miles (mostly down), and downhill sections can go on for a long long time.

    Of course, people could bomb down that trail twice or 3 times as fast as I do. Some people will have overheating and brake fade on that ride, no doubt.

    Everything people have pointed out about large rotor advantages is true. I was just trying to put into perspective how much it matters. My normal ride has 180/160 rotors which just makes sense for me. But when I ride my second bike with 160/160 rotors, I have no problems. If you live in a place where there are no big downhills, then large rotors really aren't going to give you much, if any, benefit, other than a lighter touch at the lever, which can be reason enough to like them I guess.

  19. #19
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    Quote Originally Posted by smilinsteve View Post
    I weigh 190 and that total ride is up and down around 25 miles (mostly down), and downhill sections can go on for a long long time.

    Of course, people could bomb down that trail twice or 3 times as fast as I do. Some people will have overheating and brake fade on that ride, no doubt.

    Everything people have pointed out about large rotor advantages is true. I was just trying to put into perspective how much it matters. My normal ride has 180/160 rotors which just makes sense for me. But when I ride my second bike with 160/160 rotors, I have no problems. If you live in a place where there are no big downhills, then large rotors really aren't going to give you much, if any, benefit, other than a lighter touch at the lever, which can be reason enough to like them I guess.
    +1

    I run 160/160mm rotors on two of three 29er's over all sorts of terrain and they work fine (BB7's on the single speed and Elixir 5's on hard tail). I went to a 185mm front rotor on my FS bike after riding some crazy stuff in Pennsylvania during the summer that had very long descents. Steep and twisty enough that if you let off the brakes for just a bit too long you could get in bad trouble w/ excessive speed.

    My front brake (Elixir CR) wasn't really fading but I felt like I needed a bit more stopping power for my 195 lb. carcass
    Last edited by edubfromktown; 10-16-2013 at 10:33 AM.

  20. #20
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    I run XTs with Icetech front and back, 160/160. Had to replace the front disc and went with 180 up front. Just speaking math, going from a 160 to a 180 is a 12.5% increase in leverage, so given it's one of the two brakes in use, it'll be less effect than that overall. I suppose it's a small difference. I'd probably notice it more if I went back to 160 up front. Descents out here run about 3000 feet. If there's nothing wrong with your disc, I'd be inclined to keep the 160/160. If that's not working, I'd be more inclined to go up to a 203/180 combo. I do notice my front hub is limited to 203 (DT Swiss). Don't know about the rear.

  21. #21
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    Quote Originally Posted by MSU Alum View Post
    I do notice my front hub is limited to 203 (DT Swiss).
    You mean that with that hub you can't run all of those commonly available discs larger than 203?
    Riding Fat and still just as fast as I never was.

  22. #22
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    Quote Originally Posted by crashtestdummy View Post
    You mean that with that hub you can't run all of those commonly available discs larger than 203?
    Yes. That is what I mean. I also mean that it is labelled, so others may be as well, and they may be labelled with different restrictions. And I mean that whatever is labelled on the back hub on my bike is unknown to me. I mean all that stuff.

  23. #23
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    Quote Originally Posted by MSU Alum View Post
    Yes. That is what I mean. I also mean that it is labelled, so others may be as well, and they may be labelled with different restrictions. And I mean that whatever is labelled on the back hub on my bike is unknown to me. I mean all that stuff.
    I think the point was that 203 is about as big as they come normally, anyway.

  24. #24
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    Quote Originally Posted by 3200 View Post
    i guess as i notice the difference mostly on road, the fact that the road tyre to road grip is much better and regular than mtb tyre /loose terrain, it should be normal as on loose surface your wheel will "stop" earlyer in the lever pull due to bad grip?
    Yes, there is more available friction on paved as apposed to loose surfaces. Pad compound aside, the longer lever of a large rotor improves modulation, and will provide less effort for the same work that can definitely be felt at the fingers with a mechanical caliper. However, there is a point of diminishing returns that correlates with a typical narrow road tire; the tire becomes the limiting factor in available friction. When I doubled tire width on my road bike, stopping distance went down, and heat went up. Overcooked rotors went from straw colored to blue, and pad glazing occurred much more frequently. Current 200mm/180mm rotors with SwissStop pads have reduced overheating concerns dramatically. The SwissStop pad compound also added a degree of predictable control grabby-when-hot full metallic pads lacked.

  25. #25
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    Re: rotor size - how much difference in real world?

    Quote Originally Posted by smilinsteve View Post
    I think the point was that 203 is about as big as they come normally, anyway.
    Sounds like DT Swiss doesn't want to get sued if someone cooks up something bigger and rips the screw holes out.
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