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  1. #1
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    Road disks... 140mm vs 160mm rotors. And why?

    Iím using Shimano RT-99ís, 140mm front and back. I weigh 200lbs.

    I havenít noticed any trouble stopping. No brake fade. Nothing. Iím wondering why 160mm are becoming more commonplace now.

    What do you folks use, and why?

  2. #2
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    My road bike has rim brakes.

    When it came time to build my new MTB frame, I decided to go with new "stronger" 4-piston brake calipers and gave my old brake set to my wife to replace her lower end discs. I think she ended up with a 160 mm rotor up front whereas the old one was 140 mm.

    Funny thing, I always felt my old set up worked great. She felt the same about her's. After the swaps, we both had the same reaction, "wow, I didn't think it would be a big improvement but it is".

    I'm not saying any brake system is inadequate, but sometimes you don't know what you're missing until you make a change.

  3. #3
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    You use larger disk rotors when you need more stopping power.

    As in steeper/longer inclines (say MTB descents) or higher loads (as in say touring or on tandem bikes). My gravel/touring bike is spec'd with 160s. Which is nice when I'm hauling 13kg of bike, 78kg of myself, and another 30kg of panniers/stuff.
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  4. #4
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    In the end, how much stopping power you have all comes down to the tire's contact patch with the road. You can throw 500mm brakes on your wheels and it won't matter once the tire starts to lose it's grip.

    That aside, if you're not having any issues with larger rotors, then there's no need to go larger. If you find that you often can't stop even when your tires have traction, perhaps that's a reason to go bigger.

    A larger rotor puts the caliper further from the center of the wheel, giving it more leverage to actually stop the wheel from turning. If you're a particularly heavy rider or have particularly heavy wheels (like on a fat bike), larger diameter discs might be necessary.

    If you live in a particularly hilly or mountainous area, you may consider larger rotors so that there is more metal to dissipate heat (as well as more leverage to compensate for the fact that all your mass is traveling downhill).

    I think this would be a good topic for the GCN Tech show on YouTube... do a test of a road bike to see when or if one ever actually 'needs' the larger 160mm rotors.

    I went with the smaller rotors on my CX bike build because I'm a weight weenie at heart. So far, I haven't had a reason to go bigger, but I only weigh 167 lbs. and don't live in the mountains.

  5. #5
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    I've read elsewhere that the 160mm discs show an improvement over the 140's when used with mechanical discs. I'm a rim brake guy so no experience, but that makes sense to me.
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  6. #6
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    Just generally speaking, a 160mm rotor will brake a little better and run a little cooler than a 140mm. There might also be some marginal gains in pad wear for the reasons stated previously.

    The thing about this sort of stuff is, you generally don't need it... right up until you do. You haven't had any problems with 140mm rotors, and might never, but that one time you are bombing down a long fast decent and your pads and rotors overheat will be very thrilling.

    I cooked a 160mm rotor (and pads) on my first disc brake bike. Mostly due to lack of knowledge and experience (i.e. user error), but let's just say I haven't been down that hill since that day.

  7. #7
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    Plain and simple - more surface area, more heat dissipation. If your fluid starts to boil going down a steep hill, there goes your braking power. Some mountain bikes have even larger rotors now.
    "With bicycles in particular, you need to separate between what's merely true and what's important."-- DCGriz, RBR.

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  8. #8
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    Quote Originally Posted by Waspinator View Post
    Iím wondering why 160mm are becoming more commonplace now.
    The technical aspects of rotor size are pretty simple, and mentioned. But the vast majority of people who buy a bike won't push their brakes to the limit where they will notice a difference.

    Marketing reasons are also simple. Big rotor looks bigger on the showroom floor, so even noobs will believe that a bike with bigger rotors has "better" brakes.
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  9. #9
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    Quote Originally Posted by QuiQuaeQuod View Post
    The technical aspects of rotor size are pretty simple, and mentioned. But the vast majority of people who buy a bike won't push their brakes to the limit where they will notice a difference.

    Marketing reasons are also simple. Big rotor looks bigger on the showroom floor, so even noobs will believe that a bike with bigger rotors has "better" brakes.
    Admittedly, on a road bike, I like the look of smaller (140mm) rotors.

    On my mountain bike, I like the look of larger rotors. I run 203mm rotors on it.

  10. #10
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    Are tandems starting to run dual calipers/rotors on the front wheel full moto style?

  11. #11
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    The physics of it have already been well covered, but what you need really comes down to how much you weigh, and the terrain you ride. I find that road bike discs get the worst abuse compared to mtbs because while the grades aren't always as steep, they are much more sustained, and the speeds are higher. Where I live in Colorado, you can come into a switchback at 45-50mph on the road, and repeat that over and over. On a mtb, you brake more often, but in shorter bursts and at half that speed. Accordingly, I run the biggest rotors I can fit on a road bike and can still easily turn them purple and get the pads hot enough to start to lose friction. Which, is easily remedied by opening them up for a second, but still. I'd happily run 185mm rotors up front if I could get them to fit.

    If you live somewhere with rolling terrain and no long sustained descents, 140s would be ok if you're light and 160s perfect for everyone else.

  12. #12
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    I run 180 front and 160 rear on my commuter. The only thing I can add to the other comments is that the mfr won't recommend anything it hasn't tested.

  13. #13
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    Quote Originally Posted by harryman View Post
    The physics of it have already been well covered, but what you need really comes down to how much you weigh, and the terrain you ride. I find that road bike discs get the worst abuse compared to mtbs because while the grades aren't always as steep, they are much more sustained, and the speeds are higher. Where I live in Colorado, you can come into a switchback at 45-50mph on the road, and repeat that over and over. On a mtb, you brake more often, but in shorter bursts and at half that speed. Accordingly, I run the biggest rotors I can fit on a road bike and can still easily turn them purple and get the pads hot enough to start to lose friction. Which, is easily remedied by opening them up for a second, but still. I'd happily run 185mm rotors up front if I could get them to fit.

    If you live somewhere with rolling terrain and no long sustained descents, 140s would be ok if you're light and 160s perfect for everyone else.
    ^This.

    Road bikes can really get up there in speeds. One could really put a lot of heat into a braking system on the road, MUCH more compared to mtb, reason is because mtb tires will lose traction way before the brake system gets hot, where as on the road, the road tires (in spite of their skinnier size) can really allow a lot of braking fore before they will slide out.

    Also, one thing I haven't seen mentioned in here is that a larger rotor will alow for a better modulation (everything being equaled).

    However, a larger rotor does have its drawback too, it's easier to warp and get distorted (thus causing pad rubbing that you will not be able to get rid off unless you buy a new rotor). I find that when the rotor gets to the 180mm range and above, that's when the noise that it makes (when it makes it) also tend to get amplified compared to a smaller rotor.

    Another reason to go 160mm front and 140mm rear is that the UCI is standarizing to these sizes. This will force manufacturers to concentrate their developements and offerings in these sizes.

  14. #14
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    Quote Originally Posted by aclinjury View Post
    However, a larger rotor does have its drawback too, it's easier to warp and get distorted (thus causing pad rubbing that you will not be able to get rid off unless you buy a new rotor). I find that when the rotor gets to the 180mm range and above, that's when the noise that it makes (when it makes it) also tend to get amplified compared to a smaller rotor.
    Warping is a concern and is even more of an issue with rotors which have an aluminum spider. The purpose of an aluminum spider is to dissipate more heat - aluminum is an awesome conductor next to copper and silver. But aluminum bends more easily and can get hot enough to actually melt. Solve one problem, create another.
    "With bicycles in particular, you need to separate between what's merely true and what's important."-- DCGriz, RBR.

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  15. #15
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    Quote Originally Posted by aclinjury View Post
    ^This.

    Road bikes can really get up there in speeds. One could really put a lot of heat into a braking system on the road, MUCH more compared to mtb, reason is because mtb tires will lose traction way before the brake system gets hot, where as on the road, the road tires (in spite of their skinnier size) can really allow a lot of braking fore before they will slide out.

    Also, one thing I haven't seen mentioned in here is that a larger rotor will alow for a better modulation (everything being equaled).

    However, a larger rotor does have its drawback too, it's easier to warp and get distorted (thus causing pad rubbing that you will not be able to get rid off unless you buy a new rotor). I find that when the rotor gets to the 180mm range and above, that's when the noise that it makes (when it makes it) also tend to get amplified compared to a smaller rotor.

    Another reason to go 160mm front and 140mm rear is that the UCI is standarizing to these sizes. This will force manufacturers to concentrate their developements and offerings in these sizes.
    While I agree that road bikes will normally see higher speeds and generate a lot of head in braking events mountain bike brake systems definitely see a LOT of heat. 180mm Ice Tech rotor shown:

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  16. #16
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    If your rotors look that bad, might I suggest a parachute?


  17. #17
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    Quote Originally Posted by cxwrench View Post
    While I agree that road bikes will normally see higher speeds and generate a lot of head in braking events mountain bike brake systems definitely see a LOT of heat. 180mm Ice Tech rotor shown:

    That sure looks like gouging due to brake pads being totally worn down.
    "With bicycles in particular, you need to separate between what's merely true and what's important."-- DCGriz, RBR.

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  18. #18
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    Quote Originally Posted by Waspinator View Post
    Iím using Shimano RT-99ís, 140mm front and back. I weigh 200lbs.

    I havenít noticed any trouble stopping. No brake fade. Nothing. Iím wondering why 160mm are becoming more commonplace now.

    What do you folks use, and why?
    I switched from 140's front and rear to 160 front and 140 rear shortly after buying my disc bike in early 2014.

    I live for twisting downhills and my decision had nothing to do with over all stopping power or heat. It had more to do with making lever pressure more equal on both levers during rapid deceleration.

    It can be argued (effectively) that by adding power to the front brake you are exacerbating the loss of effectiveness of the rear--after all, we are mostly riding on the front wheel during heavy deceleration on steep downhills anyway so why make it worse? The answer relates to the willingness to move ones' ass aft and below the seat to regain some of that rear wheel contact.

    Admittedly this MTB technique is not widely accepted by roadies.

  19. #19
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    From my experience with a decade or more on MTBs with discs and just a couple of years on disc equipped road bikes, it's less about stopping power, and more about the effort required to stop.

    On my MTB I have a pair of 180mm discs, on the road bike it's 160/140 F/R. On my first real road bike it was regular rim brakes and I hit a hairpin on a fast road descent (that I'd ridden before on my MTB) and ran very wide on the corner - why? because you need to apply much more effort with rim brakes vs discs to get the same stopping power.

    What I love about discs is not that I can stop quicker, but that it takes less effort. The larger the discs, the less effort you need to apply. Light controlled 1 finger braking means I can focus instead on my line, I'm more comfortable, more confident.

    You need smaller on the rear because it locks up easier than the front, so 160 back there isn't necessary. Until a recent frame change I was 160/160 on the Defy, now 160/140 on the Supersix. Where I ride, I'd not want to drop to 140 up front.

  20. #20
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    Quote Originally Posted by zerolight View Post
    From my experience with a decade or more on MTBs with discs and just a couple of years on disc equipped road bikes, it's less about stopping power, and more about the effort required to stop.
    That's because you have more power. And more control.
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  21. #21
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    Quote Originally Posted by Waspinator View Post
    Iím using Shimano RT-99ís, 140mm front and back. I weigh 200lbs.

    I havenít noticed any trouble stopping. No brake fade. Nothing. Iím wondering why 160mm are becoming more commonplace now.

    What do you folks use, and why?
    Modulation firstly, and ultimate power level second. I ran 160/140 on road and cx for years but my new cx came with 160/160 and I liked it. Easier to feather between lockout and firm braking. Now I even run 160/160 on road so I can exchange wheelsets between bikes without changing rotors.

  22. #22
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    I prefer 700mm rotors. They've worked fine for decades, never overheat and warp, have plenty of modulation for my needs, are lighter and more aero, and are super easy to install and adjust.
    Well, you know, that's just, like, your opinion man. - The Dude

  23. #23
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    Quote Originally Posted by OldChipper View Post
    I prefer 700mm rotors. They've worked fine for decades, never overheat and warp, have plenty of modulation for my needs, are lighter and more aero, and are super easy to install and adjust.
    You'll never give up, will you?
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  24. #24
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    Quote Originally Posted by cxwrench View Post
    You'll never give up, will you?
    Sure I will! When the manufacturers stop creating more problems than they're solving. It started with the proliferation of BB standard - none of which are better in the long run that threaded - and continues with stooopid and un-needed road discs. If they would continue to make both rim and disc on high-end bikes, I'm OK. Just don't force inferior tech on me so they can make more $$$.
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  25. #25
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    Quote Originally Posted by OldChipper View Post
    Sure I will! When the manufacturers stop creating more problems than they're solving. It started with the proliferation of BB standard - none of which are better in the long run that threaded - and continues with stooopid and un-needed road discs. If they would continue to make both rim and disc on high-end bikes, I'm OK. Just don't force inferior tech on me so they can make more $$$.
    You mean back in the 1900s?
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