[William9_17]

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.

 

[grandsalmon]

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.

 

[CommuterBoy]

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:

 

[William9_17]

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?

 

[CommuterBoy]

^^ 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.

 

[bigpedaler]

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.

 

[Speedub.Nate]

... 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?

 

[spazzy]

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...

 

[William9_17]

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:

 

[skyliner1004]

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.

 

[AlexJK]

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

 

[CommuterBoy]

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.

 

[Speedub.Nate]

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.

 

[CommuterBoy]

^^ 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.

 

[ataylor]

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..

 

[CommuterBoy]

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.

 

[Gary the No-Trash Cougar]

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).

 

[grandsalmon]

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.)

 

[CommuterBoy]

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!

 

[david8613]

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...