# Does Rotor Size Make A Difference?



## xxSquig (Mar 18, 2014)

I am 220 and 5'10 and I have always wondered about rotor size. I am building my first bike and I would like to know if having a 203mm rotor as opposed to 160mm makes a difference. I mean, is having a larger rotor purely aesthetic or is their a science behind it. " Bigger Belly, Bigger Rotor"?


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## tigris99 (Aug 26, 2012)

Rotor size in the front does matter, bigger than 160 in the rear is overkill. I'm 6'1 265lbs and run 180mm front and 160 rear, perfect lvl of braking balance.

Be careful though if u try to go above 180mm front can cause issues and your forks may not support them.
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## icecreamjay (Apr 13, 2004)

Not purely aesthetic, bigger rotor = more stopping power. I'm no cylde, but I prefer 180 mm front and rear. That's just my preference though, most people have more rotor in front, as most of your stopping power is generated from your front brake.

There can be compatibility issues so check with frame/fork manufacturer or LBS.


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## Joules (Oct 12, 2005)

you really think there's a chance it's just aesthetic? Torque - force x radius. Bigger rotor = more stopping torque. 

I see no reason to go less than 200/180. 160 rotors are for little kid bikes.


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## debaucherous (Jul 2, 2004)

Joules got it. Larger rotor is more stopping power when all other things are equal.
Think using a cheater bar on a stuck nut. Longer wrench = more torque. It's the same with brakes. Just make sure the fork is rated for a large rotor...


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## JHH (Jul 4, 2013)

Before you get all crazy on rotors... I'd try composite vs. steel disc pads. Composites have more grab meaning you stop faster. Cheaper and less weight than more rotor.........They may wear out quicker. I got 900 miles on mine before changing to steel and I want to go back to composite.


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## Guest (Jun 30, 2014)

Larger rotors also mean lighter lever effort. Too light a brake lever reduces control, that's why some riders prefer NOT to use too big a rotor. They are also heavier and more prone to damage.

The size rotor you use is also subject to wheel size. Smaller rotors make perfect sense on smaller wheels because smaller wheels spin faster for a given speed.


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## berny2435 (Feb 21, 2009)

there's a heat capacity side to it as well

the larger rotor has a larger heat sink as well(sort of depends on rotor design as well). Do a lot of fast techy stuff on the way down and you can get the brakes pretty hot.

If you have 160s with good tires, brakes, pads and fluid now and you can't lockup your rear brake when you are ballanced within the bike, you probably should consider 180s.

If your current Organic pads tend to smear onto the rotor, might want to look elsewhere.

Brake pad buying guide
Mountain Bike Disc Brake Pads - Reviews, Comparisons, Specs - Vital MTB

Brake Pads - A break down of what you need to know - Pinkbike

Fluids
DOT Brake Fluid vs. Mineral Oil - and the Winner is.. | Epic Bleed Solutions

That Shimano stuff looks good. handles the heat and keeps the water out too.


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## leeboh (Aug 5, 2011)

@ 235 lbs I like 180 front , 160 rear.


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## kestrel242 (Jul 11, 2008)

craigsj said:


> The size rotor you use is also subject to wheel size. Smaller rotors make perfect sense on smaller wheels because smaller wheels spin faster for a given speed.


Not exactly. The reason you can use smaller rotors on smaller wheels is torque, not rpm. The contact patch to the hub is the lever arm you're working against, and the rotor diameter is the lever arm working for you. For a given braking effort, you want to keep the ratio consistent.


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## Guest (Jul 1, 2014)

Frankly I think one needs to look at the tires/speeds/terrain as well. Using big rotors on a cyclocross front end may allow the rider to brake harder than the tires just like using a big rotor on back may not be appropriate. I like a lot of brake, but I want it to take considerable pressure to lock-up a wheel so I don't crash with an inadvertant panic brake (like I did in 1988 on my FXR-SP).


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## gsmith11 (Jun 15, 2014)

berny2435 said:


> If you have 160s with good tires, brakes, pads and fluid now and you can't lockup your rear brake when you are ballanced within the bike, you probably should consider 180s.


If you can't lock up your rear wheel, something is wrong with your braking system... even if you are big and heavy. Usually it's harder NOT to skid.


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## berny2435 (Feb 21, 2009)

gsmith11 said:


> If you can't lock up your rear wheel, something is wrong with your braking system... even if you are big and heavy. Usually it's harder NOT to skid.


and that's why I said what I said.

Gotta pick a happy medium with a little extra brake for the trails you ride most.

Here's some of my feelings braking
If skidding into a corner to get rotation isn't your thing, you can probably get away with less brake.

If you aren't very good at modulating, you probably don't want big brakes with metalic pads.

IF you run skinny XC tires that can't brake worth crap, you probably don't need a ton of brake. Probably why many Cyclocross bikes come with mechanical brakes unless you spends tons of money.

If you are big guy riding big tires that brake well and you do frightening fast decends that have some oh sheeeet areas where you have to slow down for features, you might want to consider some 205/180s with good pads and learn to modulate like a pro.


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## TooTallUK (Jul 5, 2005)

berny2435 said:


> Probably why many Cyclocross bikes come with mechanical brakes unless you spends tons of money.


Nope. disc brakes have only recently been made legal for CX and for road racing. Given that CX is far closer to road than mountain bikes for the tech (drop bars etc) then you'll find disc brakes are a new thing, so they cost more - exactly the same as when mountain bikes got them.


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## Tystevens (Nov 2, 2011)

Depends what type of riding you are doing. I had 160's front and rear on my last bike. Fine for regular trail riding, but not so great for long DH runs.

203/185 set up now. Overkill for many situations, but sure nice for a long 5500 ft descent like I did last night!


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## berny2435 (Feb 21, 2009)

TooTallUK said:


> Nope. disc brakes have only recently been made legal for CX and for road racing. Given that CX is far closer to road than mountain bikes for the tech (drop bars etc) then you'll find disc brakes are a new thing, so they cost more - exactly the same as when mountain bikes got them.


I probably should have said Mechanic DISC like BB5. 
even the lower end cyclocross bikes are getting BB5s now. Some of the expensive ones are still mech. disc. Might have something to do with wheel strength as well.


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## Dawgprimo (Mar 7, 2004)

Like a lot have said.....
The bigger the rotor the more control you have on the steep stuff.
Just want to add that one more variable in your issue with disc size.
It also depends on what type of brakes you have also........
2 piston compared to the 4 piston. 4 Piston has more power but are also heavier and $$.
Just thought I would throw that out there.......

I myself use Shimano Saints on 203 discs (Front and back) and they are amazing!
But not light in weight, however on steep rock faces they stop me on a dime and are quiet.
Best brakes I ever bought but they are not cheap.....


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## NYrr496 (Sep 10, 2008)

My Stumpjumper has 203 front and 180 rear. Last February, when I finished my fatbike, my thinking was that the bike was going to be a crawler and 160 front and rear would be fine. I couldn't have been more wrong. 
When going downhill, they slowed me a little. There was no way I was stopping. I switched the front to a 180. Slightly better but still not enough and noisy as hell. I was about to give up on Shimano brakes when I switched to 203, 180. The MTB gods smiled upon me. I can one finger brake like my other bike. 
BTW, I weigh about 255.


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## BuickGN (Aug 25, 2008)

I'm 220-240lbs and I run 203mm Zees front and rear. The way I see it, there's no reason not to. If you need less brake.......don't squeeze as hard. Modulation is excellent, much better than any 160 or 180mm brake I've used. Instead of handicapping the brake system to make it hard to get maximum power, work on modulating the brakes. Again, I have all the modulation I could ever want and I can send myself over the bars with one finger. 

You gain two things. Power, not having to squeeze the lever as hard for a given decel rate. Heat rejection, the larger rotors run cooler so there's less chance of fade. 

Also, 4pot brakes are not automatically stronger than 2 pot. It's total piston area that matters and there are some 2 piston brakes with more piston area than some 4 pistons. The 4 pistons can use a pad with more pad area. This gives a little better fade resistance and longer pad life. Larger pads do not affect power. 

At my weight I automatically use 200-203mm rotors on any bike I buy. I feel like modulation is better and I have a lot of power in reserve. Even if it doesn't see downhill I like knowing the tires determine my stopping distances and not the brakes.


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## Alias530 (Apr 1, 2013)

240lbs here and running 203 front/180 rear. XT Ice Tech rotors. Metallic pads up front, resin in the rear.

I notice much more difference with metallic vs resin and tires for sheer power. Larger rotors dissipate heat better, but that's it in my experience. Although that may be because I'm not needing to use them as much. But, again, tires and pads make more of a difference.


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## kestrel242 (Jul 11, 2008)

Alias530 said:


> 240lbs here and running 203 front/180 rear. XT Ice Tech rotors. Metallic pads up front, resin in the rear.
> 
> I notice much more difference with metallic vs resin and tires for sheer power. Larger rotors dissipate heat better, but that's it in my experience. Although that may be because I'm not needing to use them as much. But, again, tires and pads make more of a difference.


Larger rotors have a little more surface area so technically they dissipate heat better, but the real story about heat is that they don't generate as much in the first place because of their superior leverage.

Because of that same increase in leverage, you will get more braking power on tap for a given setup regardless of whether the brakes were running hot or cold before.


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## Guest (Jul 7, 2014)

kestrel242 said:


> Larger rotors have a little more surface area so technically they dissipate heat better, but the real story about heat is that they don't generate as much in the first place because of their superior leverage.


No. Braking is the process of converting kinetic energy into heat. The amount of kinetic energy is the same so the heat generated is the same. The larger rotor spins faster AND has greater surface area so it transfers heat to the air better.

Leverage is a silly way of looking at it as well. The larger rotor has greater speed past the brake pad. Regardless, the math is the same.


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## dgrolem (Jun 29, 2013)

I switched from 180 to 203 on the front and I feel like I get a bit more stopping power. Things like this are hard to quantify, but maybe 15-20%. Biggest improvement was the reduction in brake fade on long downhills. On one trail (4 miles long) with the steepest part at the end, I'd have a lot of trouble stopping with the 180. With the 203, I now have enough reserve to stop in an emergency.


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## DSFA (Oct 22, 2007)

At my weight (super clyde) I love my Hope floating rotors 203/185 with Shimano XT Ice Tech brakes. Had a 160 ice tech rear rotor and it did not have near the feel or power of the 185 Hope.


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## Surfdog93 (May 30, 2005)

Also depends on how / where you ride and design of rotor. A big rotor is heavier and takes a bit more to get to speed, so on XC bikes/trails, you typically have smaller rotors. On my FL trails, I can easily get away with 160's.


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## thesenator (Jul 26, 2007)

"The larger rotor spins faster AND has greater surface area so it transfers heat to the air better."

Larger rotors spin MORE slowly. Larger rotor=greater circumference. Larger circumference=more time to complete a revolution.


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## gsmith11 (Jun 15, 2014)

thesenator said:


> "The larger rotor spins faster AND has greater surface area so it transfers heat to the air better."
> 
> Larger rotors spin MORE slowly. Larger rotor=greater circumference. Larger circumference=more time to complete a revolution.


This isn't right. Different rotors rotate at the SAME angular velocity but the linear velocity at the outer edge is different. So saying a big rotor rotates slower or faster is at best confusing. Lets look at an extreme example, where distance from the axis of rotation goes to 0 (so, middle of the hub). LINEAR velocity goes to zero at this point, and gets bigger the farther from the axis of rotation you go.

So yes, the linear velocity of the braking surface with respect to the bike frame is larger on bigger rotors. However, before we all get too excited remember the whole bike is going forward and we care about the linear velocity of the rotor with respect to the (presumably) still air you are riding through - this is what would cool the rotor.

Lets say we're going forward at 10m/s. The linear velocity of the braking surface with respect to the bike frame is 10 m/s *(rotor diameter)*(wheel diameter) = 2.4 m/s. That's for a 180mm rotor on a 29er.

So, now we know the relative velocity of the rotor braking track with respect to the (still) air around you.

Top: 12.4 m/s
Bottom: 7.6 m/s
Front and back edges: 10.3 m/s

So you gain a little on top and lose the same amount on the bottom. Does this yield (noticeably) better cooling? My intuition says no, because the top and bottom should more or less cancel each other out, the deviation from the nominal forward velocity is relatively small, the changes at the front and back are negligible and (in the back) the caliper is on top anyway blocking the air flow. These last statements are of course just my best guess and could be wrong.

So it looks like the cooling benefits of larger rotors are purely from having a greater surface area and not from any airflow changes.

(Yes, I'm an engineer or at least an engineering student)


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## thesenator (Jul 26, 2007)

*Isn't that what I said?*

"the linear velocity at the outer edge is different."

Isn't the outer edge, or the braking surface, the part with which we are concerned?


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## Guest (Aug 2, 2014)

I think the debate is circling (pun intended) around the wrong variable. All rotors turn exactly at the same RPM as the wheel. This means that any given point on the surface of the rotor comes under the brake pads with the exact frequency (once per revolution). The difference is that more rotor surface is moving under the pad every revolution and with a larger moment arm. As a result the amount of friction (given the same caliper pressure and brake pads) is greater for larger rotors and that friction is applied against a larger moment arm (lever), inversely, it takes less pressure to generate the same braking force. Less friction required to effect the same decrease in velocity equals less heat, and equal friction results in more braking power. Dropped Mechanical Engineering before I graduated Motorcycle Tech followed by a year working on a Meteorology degree and finally finishing my Business Management and Operations Management degree, none of which is pertinent to this discussion so ignore everything after "braking power" and view the rest with a high degree of skepticism.


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## Guest (Aug 2, 2014)

thesenator said:


> "The larger rotor spins faster AND has greater surface area so it transfers heat to the air better."
> 
> Larger rotors spin MORE slowly. Larger rotor=greater circumference. Larger circumference=more time to complete a revolution.


Good lord.

The rotor is bolted to the hub. It doesn't get to choose how long it takes to complete a revolution. Every rotor spins at the speed of the wheel.

The speed at which the braking surface passes the pad is greater with a larger rotor. That means more braking power per unit pressure on the pad, or lighter lever pressure for equivalent braking.

The larger rotor has more mass to absorb heat and more surface area to transfer it to the air.


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## Guest (Aug 2, 2014)

Forster said:


> I think the debate is circling (pun intended) around the wrong variable. All rotors turn exactly at the same RPM as the wheel. This means that any given point on the surface of the rotor comes under the brake pads with the exact frequency (once per revolution). The difference is that more rotor surface is moving under the pad every revolution and with a larger moment arm.


You can do the math using moment arms or you can do the math using linear velocities under the pad. You'll get the same answer either way. What you can't do is say that both apply because then get a double credit (which you're just about to do...)



Forster said:


> As a result the amount of friction (given the same caliper pressure and brake pads) is greater for larger rotors and that friction is applied against a larger moment arm (lever), inversely, it takes less pressure to generate the same braking force. Less friction required to effect the same decrease in velocity equals less heat, and equal friction results in more braking power.


This part is wrong.

A bike moving a given speed has a certain amount of kinetic energy. To stop that bike requires all that kinetic energy converted into some other form...heat in the case of braking. You cannot stop the bike while producing less heat from some manipulation of "leverage" because that suggests you can stop the bike with no energy conversion at all. Hopefully it's clear why that can't be possible just as perpetual motion machines aren't possible.

The most intuitive way for me is use linear velocity under the pad. The rotor/pad speed times pad pressure produces a rate of heat generation. That heat generation comes out of the kinetic energy of the bike. Simple as that, no leverage concerns at all.

Fact is, for a given rate of deceleration the torque at the hub MUST be the same. With the same torque at the hub, the larger rotor has less clamping force but a longer moment arm. This is, in fact, true since the larger rotor needs less pad pressure which IS less clamping force. You can say it's the moment arm that matters or the extra speed under the pad that does but they are two sides of the same coin. You can't apply it twice.


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## thesenator (Jul 26, 2007)

*Erroneous*

The larger rotor has more mass to absorb heat and more surface area to transfer it to the air.[/QUOTE]

If this was primary importance, manufacturers would produce a smaller rotor with two braking surfaces, and cooling channels in the center, like on an automobile.

It sounds like you are saying that a rotor's effectiveness has nothing to do with diameter.

"The rotor is bolted to the hub. It doesn't get to choose how long it takes to complete a revolution. Every rotor spins at the speed of the wheel.

The speed at which the braking surface passes the pad is greater with a larger rotor. That means more braking power per unit pressure on the pad, or lighter lever pressure for equivalent braking."

You contradict yourself, first paragraph said all rotors are moving at the same speed, in your second paragraph you state, "The speed at which the braking surface passes the pad is greater with a larger rotor."


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## gsmith11 (Jun 15, 2014)

You are confusing angular and linear velocity. Brake rotor size doesn't change the angular velocity of the rotor but it does change the linear velocity of the braking track through the caliper.



> If this was primary importance, manufacturers would produce a smaller rotor with two braking surfaces, and cooling channels in the center, like on an automobile.


Shimano did more or less this with their IceTech rotors with the big aluminum cooling fins inside the braking track. It works great (but is a little heavy and expensive).

Larger rotors with more surface area and mass and fancy solutions like IceTech rotors make a big difference in keeping the rotor, pads and brake fluid cooler during longer descents to prevent brake fade. But until you start overheating the brakes and getting brake fade, rotor size is the main factor that affects how much braking power you get for a given lever pull (assuming pads, rotor surface, etc all stay constant).

If you brake moderately and continuously on long downhills and often fade your brakes you'd probably be better off with a smaller IceTech rotor than a larger "normal" rotor. This would resist overheating but (due to the smaller moment arm) the brakes would have a little less "bite" under cool conditions. On the other hands if you ride aggressively and brake late and hard bigger rotors without fancy cooling technology may be better since they'll have time to cool down between braking.


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## thesenator (Jul 26, 2007)

I follow what you're saying, and do not disagree. However, I was stating that (when all other things are equal) heat absorption/dissipation are of secondary importance to the diameter of the rotor.


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## Guest (Aug 2, 2014)

thesenator said:


> It sounds like you are saying that a rotor's effectiveness has nothing to do with diameter.


The rotor's "effectiveness" has everything to do with its diameter. Don't know how you could possibly conclude anything else.



thesenator said:


> You contradict yourself, first paragraph said all rotors are moving at the same speed, in your second paragraph you state, "The speed at which the braking surface passes the pad is greater with a larger rotor."


Only because you don't understand the difference between angular velocity and linear velocity. I, and others, have explained this several times.

The rotor spin at the same rpm. That is determined by the wheel. The larger rotor has a greater braking surface speed at the pad. because of its larger diameter.


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## Guest (Aug 2, 2014)

thesenator said:


> However, I was stating that (when all other things are equal) heat absorption/dissipation are of secondary importance to the diameter of the rotor.


Heat absorption is of secondary importance. Heat dissipation is most important. Larger rotors improve both of these things.

Most disc brakes have enough power to lock a wheel off-road, so larger rotors only serve to lessen the lever force required. Too much lever force is fatiguing on sustained descents. Too little lever force causes brake control problems. This factor is, IMO, less important for general trail riding since rotors come in increments of 10% or so. It seems to be what everyone yaks about.

I have no problem locking a 29er wheel with a 160mm rotor but I have overheated one on a long descent. The argument for a larger one in that case is dissipation, not power. I brake with one finger regardless. On long descents I use two to ease the fatigue.

It should also be said that most FS bikes have a lot of rear brake dive and that causes momentary loss of traction with sudden applications of the brake. I would like to switch my rear rotor to 140mm to lessen this but the frame won't take it. I hate rear stutter skidding when descending and large rotors aggravate that.


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## gsmith11 (Jun 15, 2014)

thesenator said:


> I follow what you're saying, and do not disagree. However, I was stating that (when all other things are equal) heat absorption/dissipation are of secondary importance to the diameter of the rotor.


Maybe. I think it's a matter of opinion, riding style and terrain.

I run a normal steel 160mm rotor in the back (on a 29er) with an SLX brake. This gives more than enough braking power but I do get a little bit of fade after descending a couple thousand feet. So I'd benefit more from a finned rotor or finned pads than from a 180mm rotor.


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