# Disc Break Rotor Size?



## William9_17 (Jan 2, 2010)

I want to convert from V-Brakes to Disc Brakes. I have a Motobecane Cafe Sprint (Hybrid) that I use for everyday commuting. I want to use Shimano Deore XT brakes but I'm not sure which rotor size would be best for commuting or if the size makes a dif. This will be my first set of disc breaks...Can someone help me out and let me know how size makes a dif for bike rotors? My choices are 160mm, 180mm and 203mm. I can't wait to get some advice...Thanks.


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## grandsalmon (Oct 24, 2005)

Since this is for everyday commuting, 160mm will do just fine. The larger the rotor, the more "leverage" against a revolving wheel = more stopping power. Unless of _really_ significant weight in body, cargo and/or looooong hills, stay with the smaller rotor- less prone to getting knocked & accidentally bent. If you made any change it would be moving up a size only in front where your major stopping power takes place, and then only to a 180mm rotor. If you can't get separate sizes, *forget all this fuss* and worry and know 160mm front and back are going to work not only well, but a brief getting used to for the consistent power over rim brakes. Note: please, _please_ spend the time getting acquainted to the difference.
And if these are new model XTs, they have nice pucker power plus are designed for greater pad to disc gap (less opp to rub)- overall a no fuss brake in your situation with great adjustments for lever placement and engagement being separate- nice.


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## CommuterBoy (Oct 4, 2007)

160 is plenty for almost everybody on almost every bike. I went with 180 on my new 29er build, but it's really not necessary, even on the bigger wheels. generally 180 and 203 are downhill territory. I've had 6" (160) rotors on my all-mountian full suspension Kona for years and have never wanted more. I have 6" rotors on my commuter, and the back needs pads (and has for a few months). I can say with authority that one functioning 6 inch disc brake is almost overkill for your typical commute :lol:


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## William9_17 (Jan 2, 2010)

Well I usualy have a bicycle trailer behind filled with groceries and other crap so it's not unusual for me to have an etra 100+ lbs to stop. I figure better to be safe than sorry you know?


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## CommuterBoy (Oct 4, 2007)

^^ that's basically why I went with 180's for my 29er... more stopping power is rarely a bad thing...assuming you are familiar with it. But any disc brake is going to blow your mind if you're a v-brake convert.


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## bigpedaler (Jan 29, 2007)

180's are a good compromise -- more power with less effort at the lever. Extra weight is under better control, and there's no such thing as too much braking power in an emergency situation.

Unless you ride really easy, use the rule of thumb of your weight; whichever rotor size is closest to your weight is what you should run.


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## Speedub.Nate (Dec 31, 2003)

bigpedaler said:


> ... use the rule of thumb of your weight; whichever rotor size is closest to your weight is what you should run.


Rule of who's thumb? That's utter nonsense.

I suppose my tandems brakes are underpowered by a factor of 2?


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## spazzy (Aug 15, 2004)

bigpedaler said:


> 180's are a good compromise -- more power with less effort at the lever. Extra weight is under better control, and there's no such thing as too much braking power in an emergency situation.
> 
> Unless you ride really easy, use the rule of thumb of your weight; whichever rotor size is closest to your weight is what you should run.


LOL i guess i should have twin 180 rotors then

jkjk

Id say too much power is when you end up over the bars in an emergency stop...


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## William9_17 (Jan 2, 2010)

I think I'm going to go with 180 just in case...Although I am gaining weight so maybe I should go with 203mm rotors...Damn this nasty cold weather :madman: :sad:


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## skyliner1004 (Jan 7, 2010)

grandsalmon said:


> Since this is for everyday commuting, 160mm will do just fine. The larger the rotor, the more "leverage" against a revolving wheel = more stopping power. Unless of _really_ significant weight in body, cargo and/or looooong hills, stay with the smaller rotor- less prone to getting knocked & accidentally bent. If you made any change it would be moving up a size only in front where your major stopping power takes place, and then only to a 180mm rotor. If you can't get separate sizes, *forget all this fuss* and worry and know 160mm front and back are going to work not only well, but a brief getting used to for the consistent power over rim brakes. Note: please, _please_ spend the time getting acquainted to the difference.
> And if these are new model XTs, they have nice pucker power plus are designed for greater pad to disc gap (less opp to rub)- overall a no fuss brake in your situation with great adjustments for lever placement and engagement being separate- nice.


160 will be good, but i dont think a larger rotor will have more stopping power, unless he's overheating his rotors on his commutes. the larger rotor is just used to dissipate heat better in situations where the smaller rotor will overheat and fade.


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## AlexJK (May 2, 2009)

i vote for a 180 up front with a 160 rear, that gives primo stopping power up front with good modulation in the back


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## CommuterBoy (Oct 4, 2007)

skyliner1004 said:


> 160 will be good, but i dont think a larger rotor will have more stopping power, unless he's overheating his rotors on his commutes. the larger rotor is just used to dissipate heat better in situations where the smaller rotor will overheat and fade.


My motorcycle experience tells me that this is hogwash...sorry man.

The closer the caliper is to the outer edge of the wheel, the greater the stopping power. This is because of the rotating mass of the wheel... simple physics says that if you apply stopping power near the hub, it's harder to stop the rotating mass...if you apply the same pressure further out on the rotating mass, it results in greater stopping power.


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## Speedub.Nate (Dec 31, 2003)

CommuterBoy said:


> My motorcycle experience tells me that this is hogwash...sorry man.
> 
> The closer the caliper is to the outer edge of the wheel, the greater the stopping power. This is because of the rotating mass of the wheel... simple physics says that if you apply stopping power near the hub, it's harder to stop the rotating mass...if you apply the same pressure further out on the rotating mass, it results in greater stopping power.


This assumes the 160s are inadequate and can't lock up the front wheel.

Once a rider has a rotor size that will easily lock up the front wheel (or result in an endo), max braking has been realized.

After this point, larger rotors will make max braking easier, and less modulated, but will not result in "greater stopping power," unless rotor overheating is a factor.

This, of course, gives consideration to "normal" weights and speeds.

As weights and speeds increase -- such as with motorcycles -- then we have something to talk about.


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## CommuterBoy (Oct 4, 2007)

^^ very true. There is an argument to be made for greater stopping power when wheel speed increases, but it's a pretty weak argument when you're talking about bicycles. Downhill guys like the heat dissipation and increased power with more speed and heavier bikes...the OP might consider bigger rotors when thinking about hauling a trailer, but I agree that 160 mm rotors can stop a spinning 26" or 700c wheel with equal authority, when you're talking about normal bike weights and speeds.


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## ataylor (Dec 28, 2009)

One other thing that may play in the mix is what level of brakes you are using. My cheap ones have less modulation and I think a larger rotor may compound this. FYI I am just below the dual 160's in weight..


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## CommuterBoy (Oct 4, 2007)

I will be able to answer this question with authority an a few months... Hayes 9's with 160 rotors on my Kona, and Hayes 9's with 180 rotors on the current project bike... If you want to contribute to my build project so that I can do some brake testing sooner, I take paypal.


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## Gary the No-Trash Cougar (Oct 14, 2008)

I have 185mm rotors on my 29er and I'd like to go up to 203mm, mainly because I'm a wuss and the 185mm rotors overheat quickly (I'll also probably switch to BB7s, as the BB5s kinda suck). On the street though, 160mm rotors should be more than sufficient (maybe even overkill).


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## grandsalmon (Oct 24, 2005)

This thread has become pretty funny. Many misapplied- some incorrect, absolute understandings. I wouldn't know where to start, nor find the energy.

I love the "Rule of Thumb" theory ONLY for reminding me of the movie "Boondock Saints", great scene.

Um, let's get this guy moving; I'll let me first post stand, the combo seconded by AlexJK.
Roll on brother!

(I just want to say I mean not to be strict- I LOVE this forum, dig the regulars here and their enthusiasm and support for posters. And yes, I velo commute heavily.)


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## CommuterBoy (Oct 4, 2007)

I've always said that as disc brakes continue to evolve, the rotors will keep getting bigger and bigger, until some day we'll get smart and design a brake that actually just grabs on to the rim itself. Imagine the stopping power you could get with those!


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## david8613 (May 31, 2005)

no one mentioned this, but ive been doing research on brakes and one thing you get with bigger rotors is you can brake way later, thats why i went with 203mm and i also went with 4 pots instead of just 2 for my niner wfo 9...


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## Speedub.Nate (Dec 31, 2003)

david8613 said:


> no one mentioned this, but ive been doing research on brakes and one thing you get with bigger rotors is you can brake way later, thats why i went with 203mm and i also went with 4 pots instead of just 2 for my niner wfo 9...


Nobody mentioned it because it's not true.

It's the kind of bullshit a salesman would spew to get a noobie to spend more.


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## scottzg (Sep 27, 2006)

This thread is fkn awesome.

First, on a commuter, i would rather run rim brakes. You don't do exxxxxtreme extended braking commuting, and ease of service/maintenance is nice, and there's no rotor to be constantly readjusting because you knock it against hallways, pedestrians, and stop signs. I don't mind a little rotor rub on my mtb, but listening to it on my commuter is MADDENING.

Not one to follow my own advice, i have bb7s on my commuter. It's a disk-only frame and fork. 160/160 was plenty of power for all the commuting i do, but since i take the bike down a 2500' 6 mile descent every other week or so, when a 185mm front rotor came into my possession i mounted it up. I'm 225lbs.


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## AlexJK (May 2, 2009)

http://www.hygia.com.tw/

cheap and everybody that uses them is happy, they are quiet(apparently even in the wet), powerful, cheap, they look good. theres a thread about them here on MTBR. Parts are supposed to be easy to source (i think they use shimano XTR pads)


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## William9_17 (Jan 2, 2010)

scottzg said:


> This thread is fkn awesome.
> 
> First, on a commuter, i would rather run rim brakes. You don't do exxxxxtreme extended braking commuting, and ease of service/maintenance is nice, and there's no rotor to be constantly readjusting because you knock it against hallways, pedestrians, and stop signs. I don't mind a little rotor rub on my mtb, but listening to it on my commuter is MADDENING.
> 
> Not one to follow my own advice, i have bb7s on my commuter. It's a disk-only frame and fork. 160/160 was plenty of power for all the commuting i do, but since i take the bike down a 2500' 6 mile descent every other week or so, when a 185mm front rotor came into my possession i mounted it up. I'm 225lbs.


Hey not to bash anything you've said...I think I'd prefer dealing with the hassle's of disc brakes because the drivers around here suck...But it doesn't shock me...I don't think hlf the people over here have thier licenses...


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## scottzg (Sep 27, 2006)

William9_17 said:


> Hey not to bash anything you've said...I think I'd prefer dealing with the hassle's of disc brakes because the drivers around here suck...But it doesn't shock me...I don't think hlf the people over here have thier licenses...


How bad the drivers are doesn't matter. If your brakes can't stop you as fast as you can move your weight back it's not because of the style they are, it's because there's something wrong with them. If the brakes have always been like that, i'd take the bike to the shop you got it from and ask whats up. Disk brakes won't stop a bike any faster than rim brakes will.


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## CommuterBoy (Oct 4, 2007)

Hassle? They are easier to adjust, less maintenance and upkeep, easier to change pads, you don't have to mess with them when you remove a wheel...am I missing something? I've been running discs on 2 bikes since 2004 and I've never bent a rotor. Of course I will bend one today now that I've said that, but I find discs to be way less of a 'hassle' than any rim brake I've ever had.


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## Bulldog (Jan 13, 2004)

What kind of commuting are you guys running full-on DH 203mm or even 185mm rotors doing exactly?! Holy crap. I love discs but find my average rim brakes (Avids) can EASILY overpower my skinny, smooth, high-pressure commuting tires. Having to panic-stop and grabbing a handful of 203mm discs on my commuter would be suicide. If/when I move my commuter to discs I hope I can downsize to a 140mm rotor in the back, and keep a 160mm up front. Can't see how I would need more. Oh btw - I weigh around 230 too.


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## Gary the No-Trash Cougar (Oct 14, 2008)

Bulldog said:


> What kind of commuting are you guys running full-on DH 203mm or even 185mm rotors doing exactly?! Holy crap. I love discs but find my average rim brakes (Avids) can EASILY overpower my skinny, smooth, high-pressure commuting tires. Having to panic-stop and grabbing a handful of 203mm discs on my commuter would be suicide. If/when I move my commuter to discs I hope I can downsize to a 140mm rotor in the back, and keep a 160mm up front. Can't see how I would need more. Oh btw - I weigh around 230 too.


I'm running 185mms on my 29er hard tail (strictly off-road), on my commuter I'm still using cantis.

BTW, I don't think they make rotors in 140mm but I could be wrong.


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## bigpedaler (Jan 29, 2007)

Yup, they make 140's -- Avid has 'em.

Bulldog -- I went to larger discs because a year on 160's proved them inadequate (and I weight 235). A front 203 put me on my head simply because I hadn't yet upgraded my fork spring, and got some serious dive in the front. 185 front/160 rear is only marginally adequate for my riding, I'll be bumping up to 185/185 next month.

What kind of commuting am I doing? Energetic. I also have to routinely dodge idiots who barely know what's going on 11 feet in front of their windshields. So hard stops are a way of life.

Natehawk, it's not utter nonsense, it's empirical knowledge and experience. Just because YOU haven't experienced it doesn't mean jack to me. There's a fair-sized group of people who come to me for advice on rides AND equipment, and they keep coming back because I don't steer them wrong. So bite me.


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## CommuterBoy (Oct 4, 2007)

160's on my commuter, 160's on my dual suspension trialbike, 180's on my 29er build in progress (trail use). 

...but your statement reinforces my original point = bigger rotor, more stopping power.


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## Speedub.Nate (Dec 31, 2003)

bigpedaler said:


> 185 front/160 rear is only marginally adequate for my riding, I'll be bumping up to 185/185 next month.


This move makes sense if you can't lock up your rear 160, or are frequently overheating it because you drag it. Otherwise, all a 185 rear will do is skid more easily.

And look hella cool.


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## Gary the No-Trash Cougar (Oct 14, 2008)

CommuterBoy said:


> 160's on my dual suspension trialbike....


You do observed trials on a dual suspension bike?!?


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## Bulldog (Jan 13, 2004)

bigpedaler said:


> Yup, they make 140's -- Avid has 'em.
> 
> Bulldog -- I went to larger discs because a year on 160's proved them inadequate (and I weight 235). A front 203 put me on my head simply because I hadn't yet upgraded my fork spring, and got some serious dive in the front. 185 front/160 rear is only marginally adequate for my riding, I'll be bumping up to 185/185 next month.
> 
> What kind of commuting am I doing? Energetic. I also have to routinely dodge idiots who barely know what's going on 11 feet in front of their windshields. So hard stops are a way of life.


I know there ARE 140mm rotors. It's my understanding though that 140's can create a fitment issue with certain frame/caliper combos since the caliper is tucked in pretty tight to the frame and hub. That's what I will have to figure out when the day comes. My frame manufacturer (Cotic) is pretty helpful so I will email them that question some day.

What tires are you running where you can't easily overpower them with 160's?? I just can't imagine that amount of traction is available from a road-going tire. And BTW - what make/model of disc brake are you using?


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## CommuterBoy (Oct 4, 2007)

Gary the No-Trash Cougar said:


> You do observed trials on a dual suspension bike?!?


Yes. I'm freaking awesome too. I use it for training, since it's 20 pounds heavier than my competition trials bike.


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## AlliKat (Apr 28, 2006)

I wouldn't convert to disc on your commuter. It is extra weight and expense that won't be used. I love my XT disc on my trail bike but have no need for them on my commuter. 

I agree there is a ton of misinformation in this thread. A larger rotor will take less force/friction from the caliper to apply the same stopping force because the rotor is a longer lever arm. The net result is less heat generated for the same amount of stopping power because less friction is required to generate the same amount of torque at the axis; An additional benefit is a larger rotor surface area to distribute the friction further reducing the heat. This mostly will result in a reduction in brake fade (reduction in braking power due to heat affect on component) 

Keep in mind, power is of little value. You need enough power at a given time but that is rarely an issue unless you experience too much brake fade. More important than power is modulation. That is the ability to apply the right amount of power for the given need. Usually the issue is poor modulation which results in skidding, going over the bars etc.

Have you noticed road bikes are still using rim brakes? It is because they are lighter and adequate for the road.


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## CommuterBoy (Oct 4, 2007)

I use discs on my commuter because of road slop and gunk accumulation from my dirt road in the winter. I'd rather not have the satisfying feeling of destroying a set of wheels every time I apply the brakes between November and March :lol:

Roadies still use rim brakes because they're like baseball players...they won't come out and play if the weather is bad.


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## jeffscott (May 10, 2006)

AlliKat said:


> I wouldn't convert to disc on your commuter. It is extra weight and expense that won't be used. I love my XT disc on my trail bike but have no need for them on my commuter.
> 
> I agree there is a ton of misinformation in this thread. A larger rotor will take less force/friction from the caliper to apply the same stopping force because the rotor is a longer lever arm. The net result is less heat generated for the same amount of stopping power because less friction is required to generate the same amount of torque at the axis;Nope for the same amount of stopping energy the same amount of heat is generated An additional benefit is a larger rotor surface area to distribute the friction further reducing the heat.The larger rotor provides more surface area to allow the heat to transfer to the atmosphere This mostly will result in a reduction in brake fade (reduction in braking power due to heat affect on component)
> 
> ...


While rim brakes are certainly adequate for the road, and a lot of mountain biking, commuting is a little different...

Firstly I ride every day, rain or shine, the hazards are there all the time....

I for one like the better performance of disk brakes to help reduce the risks. Last thing I need is to have a poor brake when I merge into traffic and the bottom of my hill, on a rainy or snowy day.


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## sanjuro (Sep 29, 2004)

BTW, braking is very dependent on tire size as well.

If you were running a 2.5 inch Hookworm, then using an 7 or 8 inch rotor maybe useful. But if you are running a 25-28mm front tire, I am going to guess you are more likely to lock up the wheel than get any usable modulation.

BTW, I disagree that disc brakes are not useful for commuter bikes. I don't like rim brakes in the wet conditions, and they are severely diminished in the snow. 

I agree with the brake fade comments, but unless your commute starts on top of a 2500' hill, I don't think you have worry. I live on top of a 500' hill and I descend at 25mph+ on rim or disc brakes...


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## sanjuro (Sep 29, 2004)

AlliKat said:


> Have you noticed road bikes are still using rim brakes? It is because they are lighter and adequate for the road.


Just to clear this up, UCI has banned disc brakes on cyclocross bikes for no other reason but Luddism. While there is no demand for discs on road bikes, the UCI will not allow it. This influences all road bike specs.

Adequate is a funny word to use when it comes to braking. If I was descending at 45 mph, would I want excellent brakes or adequate brakes?

Now that the top end road bikes are under 15lbs, the UCI limit, moving to disc brakes seems like an excellent idea. One thing to consider is that all the pros use tubulars and most use carbon rims as well. A disc brake will solve any overheating problems (which could cause the glue to come loose) from rim brake friction, and while a disc brake system will be heavier than rim brakes, the wheels will become even lighter because the rims won't need a braking surface.


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## Speedub.Nate (Dec 31, 2003)

AlliKat said:


> I agree there is a ton of misinformation in this thread. ...The net result is less heat generated for the same amount of stopping power because less friction is required to generate the same amount of torque at the axis; An additional benefit is a larger rotor surface area to distribute the friction further reducing the heat.


My physics prof told me to disagree with this part of your assessment (though I'm on board with the rest). Here's why:

Braking, boiled down to its core, is the conversion of speed into heat.

( Bike + rider + 25 MPH ) - ( 25 MPH ) = 0 MPH and 'X' amount of heat generated.

But as you pointed out, one can't ignore the importance of the rotor diameter -- or more specifically, rotor mass -- in spreading this heat away from the braking track, plus the increased surface area for cooling, keeping temps in check.

Not to mention that 203mm rotors are often where the rotor thickness goes from 1.8mm up to 2.0mm, and that right there accounts for a 11% jump in material available to help whisk the heat away.

But then you've got that whole "power overkill" as has been mentioned to death... god's honest truth is everyone could stick 220s front and rear on their bike and everybody would simply adapt, but that would make these back-and-forth debates no fun.


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## grandsalmon (Oct 24, 2005)

I thought a nice capstone to this thread was needed. We've garnered and entangled quite a few folks on this subject with need for deeper understanding- including myself.

It seems silly for the commuter thread somewhat, as we are generally talking about smaller contact areas from skinnier tires, but as threads go...

I do not provide this to dismiss anybody's explanation- I just found it to be a good read and "break"down-

http://forums.mtbr.com/showthread.php?t=74462&highlight=The+larger+the+rotor

Do not link if you are satisfied, or dislike the possibility of general confusion, or utter conclusion!


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## Speedub.Nate (Dec 31, 2003)

grandsalmon said:


> I do not provide this to dismiss anybody's explanation- I just found it to be a good read and "break"down-
> 
> http://forums.mtbr.com/showthread.php?t=74462&highlight=The+larger+the+rotor


Wait, it's been 5 years since that thread, AND NOTHING HAS CHANGED SINCE THEN?!?!?!?!

WTF!!!


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## CommuterBoy (Oct 4, 2007)

That thread makes good sense.... I think any perceived disagreement is a vocabulary issue.. what smart people call 'increased modulation', I call "better stopping power". Symantics. If I can do with one finger what used to require two, I call that better stopping power (granted, one finger lock-ups are a given with any size rotor, but that's beside the point). Bigger rotors will do with less effort what smaller rotors are equally capable of doing. Some people might like this, some might not. 

Bottom line, you can lock up a bike wheel with just about any tire on it whenever you want with V-Brakes, if they're tuned correctly. You can do it easier with discs, and you can do it even easier with bigger discs. There is a point where it might become overkill. That point is up to you.


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## Mr.Ice807 (Jul 15, 2009)

I run 180/160 with Avid BB7 calipers on my mountain bike. My commuter has 160/160 with Avid BB5s. I plan on switching to the other set up soon. The BB7s lock up if I want them to. They are set very close to the rotor. I like my brakes touchy. Unless your hauling a lot of stuff anything over 180 is overkill. Also be sure your frame, wheels, and fork can handle oversized rotors.


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## Guest (Jan 18, 2010)

CommuterBoy said:


> That thread makes good sense.... I think any perceived disagreement is a vocabulary issue.. what smart people call 'increased modulation', I call "better stopping power". Symantics.


Yes, the problem is that we all reserve special, personal meanings to the words. It's not a poor understanding of the subject matter, it's that none of us speak English. 

There are some real gems here, but this one went unappreciated:



CommuterBoy said:


> The closer the caliper is to the outer edge of the wheel, the greater the stopping power. This is because of the rotating mass of the wheel...


If that were true, I could improve my braking power by using lighter rims.

No, the problem isn't semantics.


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## KeylessChuck (Apr 15, 2006)

It might be nice if we quit assuming that everyone has the same kind of commute, and everyone rides skinny high-pressure tires.

I live in an area known for it's steep hills and canyons. My commute to work starts with steep, fast suburban downhill run complete with stair drops, singletrack, skinnies and a 50mph+ section. During the summer I run 2.35" Big Apples at 20psi.

I agree that bigger brakes may be overkill for some, but don't assume that if they are for you, they will be for everyone.


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## CommuterBoy (Oct 4, 2007)

craigsj said:


> If that were true, I could improve my braking power by using lighter rims.


If that weren't true, heavier vehicles wouldn't need more powerful brakes.


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## Guest (Jan 19, 2010)

CommuterBoy said:


> If that weren't true, heavier vehicles wouldn't need more powerful brakes.


"Balls!" cried the Queen. "If I had two I'd be King!"

I challenge you to name one example where a heavier vehicle avoided the need for more brakes by lightening its wheels. For that to be the case, angular momentum would have to dominate the total momentum of the vehicle.

How much, CommuterBoy, of a bicycle's total momentum is held in its wheels when you include the weight of the rider?


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## CommuterBoy (Oct 4, 2007)

I'm sure you're the smartest person on the entire internet. No one is debating that. I'm just saying that the bottom line is that more weight=more need for greater stopping power. Heavier bike parts = more weight. 

Bikes are so light that it doesn't take much to stop them. We're debating over the amount of overkill 'necessary' to serve a purpose. Forget how much momentum is held in the rotating mass of the blah blah.... I'm proud to say that I don't know that. 

Heavier vehicle, more brakes. Lighten the vehicle, less brakes necessary.


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## Guest (Jan 19, 2010)

I would say that the problem here isn't vocabulary or semantics, it's a poor understanding of the issue.

A high school physics student can answer these questions. It's fine if you can't, but you should probably refrain from trying to teach it in forums.


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## fux (Oct 21, 2006)

Back on toppic, I have 160mm rotors on my commuter and 29`er.

I did have 185 on my 6 inch full susser but didnt notice the difference between any of them.

 

Oh, disks for commuting are a no brainer for me. The amount of salt used on the roads winter time here, used to mean a new set of rims every spring.


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## CommuterBoy (Oct 4, 2007)

craigsj said:


> I would say that the problem here isn't vocabulary or semantics, it's a poor understanding of the issue.
> 
> A high school physics student can answer these questions. It's fine if you can't, but you should probably refrain from trying to teach it in forums.


You caught me on a bad day. Any other day I might just ignore you.

I would like you to explain to me how adding weight to a vehicle does not mean that you need better brakes. Please. Because when I'm towing the boat to the lake, it sure seems like it's harder to stop than when I'm not towing the boat.


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## Guest (Jan 19, 2010)

Redefining the argument won't save you. You said " This is because of the rotating mass of the wheel...", you didn't say you were towing a boat with your bike.

You should determine how much the rotating momentum of your wheels is compared to your overall momentum on a bicycle. Then you'll realize why your claim was ludicrous. Your refusal to do that shows that you don't really want to know, you just want to talk.

While you're at it, learn the difference between power and control.


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## CommuterBoy (Oct 4, 2007)

Dude. I get it. Thank you. 

My point earlier was that no matter how heavy a wheel is, I would think that a tiny rotor barely bigger than the hub would require more force applied in order to create the friction needed to stop the wheel, and a bigger rotor closer in size to the wheel itself would require less force applied in order to create the friction needed to stop the wheel. If this is incorrect according to the laws of physics, I accept that. It just doesn't seem that way when I picture grabbing onto a rotor with my fingers and trying to stop the wheel. I'd rather grab on to a bigger rotor, because I think I could stop the wheel quicker that way. 

As for the difference between power and control... are you racing anywhere this season? I would jump at the chance to demonstrate my knowledge in this area...or do you just tell people how to ride bikes on the internet?


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## Guest (Jan 19, 2010)

Again, you said "The closer the caliper is to the outer edge of the wheel, the greater the stopping power. This is because of the rotating mass of the wheel...". That is not true. The larger rotor may offer more stopping power but it has nothing to do with the rotating mass of the wheel. In fact, rotating mass of the wheel has little effect on braking in any way; it's simply too small relative to other factors to matter. The larger rotor increases braking capability because the rotor travels past the pad at a higher rate of speed. End of story.

As for power and control, it was you that admitted you can't get your vocabulary straight. I realize that was simply an attempt to blame communication skills, and indirectly all of us, for your mistakes in understanding. I won't dignify your attempt to call me out on my "riding skills".


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## CommuterBoy (Oct 4, 2007)

craigsj said:


> Again, you said "The closer the caliper is to the outer edge of the wheel, the greater the stopping power. This is because of the rotating mass of the wheel...". That is not true. The larger rotor may offer more stopping power but it has nothing to do with the rotating mass of the wheel. In fact, rotating mass of the wheel has little effect on braking in any way; it's simply too small relative to other factors to matter. The larger rotor increases braking capability because the rotor travels past the pad at a higher rate of speed. End of story.


I totally get what you are saying. I'm saying that a larger rotor has an effect on stopping power. You're saying yes, but I am stupid for thinking that it has to do with the rotating mass of the wheel, a comment I made in passing that you found error in...which made your eyes light up like a kid at Christmas because you like physics. I get it. I even reinforced your point by talking about which rotor I'd rather stop with my fingers in my earlier post.

I can't believe I'm still responding...I'm like a moth at a porch light.

So according to you, if I put a 160mm rotor on a bike wheel, and a 160mm rotor on a 45 pound freeweight with exactly the same diameter as the bike wheel, and spun them both on a hub, the weight of the wheel would not be a factor in how fast each of them stopped when I applied the brakes? This is what I'm saying, and you are saying that they would stop at exactly the same time with the same amount of effort. To me this sounds wacky (physics term).

I TOTALLY REALIZE that in terms of the differences in weight of typical bike tire/wheel applications, the difference is miniscule and applies very little to overal braking performance in real world conditions. Again, we are debating the appropriate amount of overkill, which is totally arbritrary. But you can't deny that an increase in ROTATING MASS has an affect on braking. It's not the only variable, it's not the most important variable, but you can't say that it has "nothing" to do with the rotating mass of the wheel.

And let the rest of MTBR be advised: Anything that I do not understand, I blame you.

PS: has anyone seen 'The Breakfast Club"? 
"And what do you do in the physics club?"
"..talk about physics. The properties of physics."
"So it's sort of social...DEMENTED AND SAD, but social."


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## Guest (Jan 19, 2010)

CommuterBoy said:


> So according to you, if I put a 160mm rotor on a bike wheel, and a 160mm rotor on a 45 pound freeweight with exactly the same diameter as the bike wheel, and spun them both on a hub, the weight of the wheel would not be a factor in how fast each of them stopped when I applied the brakes? This is what I'm saying, and you are saying that they would stop at exactly the same time with the same amount of effort.


Nope, I said no such thing. Nice straw man though.



CommuterBoy said:


> I TOTALLY REALIZE that in terms of the differences in weight of typical bike tire/wheel applications, the difference is miniscule and applies very little to overal braking performance in real world conditions.


Thank you.



CommuterBoy said:


> But you can't deny that an increase in ROTATING MASS has an affect on braking. It's not the only variable, it's not the most important variable, but you can't say that it has "nothing" to do with the rotating mass of the wheel.


You're right, I can't deny that but I will say it's not significant in reality and it has nothing to do with why larger rotors have more braking power.


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## Gary the No-Trash Cougar (Oct 14, 2008)

Hey Craig, I have a question; is it possible for you to contribute to this thread with your allegedly vast knowledge of physics and the application of that knowledge to braking dynamics _without_ being an a.sshole? Because you're really not proving very much, except that you can be an a.sshole when you want to be. Congratulations, you're winning an internet argument. Now can we get back to the topic at hand, the OP's original question or have we already flogged that particular horse to death?


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## Guest (Jan 20, 2010)

Gary, I believe the answer to that is no, it is not possible, at least on MTBR.

By all means, lets get back to the topic. I'm sure we'd all like to hear more about your plan to upgrade to 203s with your BB5s on your non-commuting bike. That sounds great.


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## Gary the No-Trash Cougar (Oct 14, 2008)

craigsj said:


> Gary, I believe the answer to that is no, it is not possible, at least on MTBR.
> 
> By all means, lets get back to the topic. I'm sure we'd all like to hear more about your plan to upgrade to 203s with your BB5s on your non-commuting bike. That sounds great.


They will be BB7s, actually. And don't bother trying to bait me, I'm not biting. Enjoy the rest of your week.


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