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Because when you brake your weight shifts forward so your front brake does 70% of the work. It’s the same reason cars, motorcycles etc have larger front discs. Putting a larger disc on the back will do nothing than encourage you to lock up the rear wheel.
 

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Because when you brake your weight shifts forward so your front brake does 70% of the work. It's the same reason cars, motorcycles etc have larger front discs. Putting a larger disc on the back will do nothing than encourage you to lock up the rear wheel.
A larger rear rotor will also lower your brake temperatures, thus preventing the brakes from overheating/fading/etc as easily. Depending on how fast/heavy you are, and what type of riding you enjoy, this may be a very desirable outcome.

Troy Brosnan runs larger rear rotors than front rotors on his World Cup downhill bike for this reason.

The main thing is that when you are gravity riding, the front and rear brake often end up being used very differently. The front is for actual slowing down, and thus isn't used all that often.

Because mountain bikes don't have any form of engine braking to maintain speed, and because hitting the front brakes can be more unsafe at times (washing out, endoing, etc), the rear ends up being dragged more for most riders. So if people get overheating brakes, it's almost always on the rear.

So, for any type of riding that doesn't have sustained downhills, I can agree that not all bikes need large rear rotors. But ifthe rider is larger/faster, or they Do any sort of sustained downhill riding, larger rear rotors make tons of sense to me.
 

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I used to run a larger rotor on the front. But as @ocnLogan pointed out, the rear brake is often used as a drag brake. It doesn't (at least shouldn't) skid, but it does heat up a lot. Large rotors cool better so a few years ago I upped my rear rotor size to match the front. I like it a lot. Even more than I expected to.

I know many if not most bike companies put a larger rotor on the front but doing so no longer makes sense to me for the type of riding I do (a lot of winch & plummet). But I might just be an exception to the rule.
=sParty
 

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I've kind of wondered the same thing, at least for a mtb. Luckily my bike came with 200mm at both ends. But being somewhat of a newbie and older I don't descend at breakneck speeds and the trails I ride have several hundred feet of uniterupted vertical climbing followed by descents back down the same in mostly loose rock over hard. I'm finding I rarely use the front brake and use the rear to do most of my slowing as well as a drag brake in some areas. Until last week when I barely touched the front in a very gradual turn in some really loose rock only to have the front end shoot out from under me like it was on ice. Lesson learned.
 

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I don’t know why bikes are setup that way — as a cost cutting measure?

180mm rear rotors are inadequate for real gravity riding. I don’t know why anyone would want less stopping/slowing power and easier overheating.

I ran that setup for a lot of years (203 front, 180 rear) and thought it was just my crappy Avids that couldn’t handle sustained steeps without overheating. Well, part of it was the crappy brakes, lol. But before I changed them I upgraded to a 203 in the rear and suddenly my front brake didn’t overheat near as often.

Then I got Saints.👍

With today’s extremely grippy tires, unintended skidding in the rear is most likely user error. While the front brake does provide most of the power, the rear helps the front by providing additional power.

On fall-line descents you really can’t have too much power IMO, and it’s not just heavier riders that need it. One of my friends weighs around 140 pounds and uses 220s front and rear; previously 203s. He descends gnar faster than most people could even imagine. No skidding, no loss of control.
 

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One reason is unlike cars and even motorbikes, bicycles typically do not come with different-sized calipers, so it's usually the same caliper front and rear. We don't want to lock the rear end up easier, we want more fade resistance. Unfortunately, because most bikes come with the same caliper F and R, we can't really choose, they are tied together. You can "piece together" your own brakeset, but the bike makers are probably doing it for volume production, easier to make two of the same caliper.
 

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I've been wondering this as well and I know the front brakes have about 70% of the power. With that being said, I've been running 220mm front and rear. I replace the rear pads about twice as often as I do the front.

I just changed out the fork and the fork only supports up to 203. I changed the front only to 203 and left the back at 220mm.

No issues so far in regards to the rear brakes being more sensitive. To me the brain and fingers figure out the applied finger pressure. I've not noticed any power issues from front to rear. More than enough power

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One of my bikes has 203 F/R and another bike has 203/180 F/R. I like the 203 F/R more for the reasons the other guys already stated above.

Someone once suggested to me that the smaller rotor in the rear provided more clearance to the stays (and in turn, allows shorter chainstay length and/or heel clearance). I don’t know how true that idea is.
 

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The reason, I'm certain, will have more to do with what sort of riding you do.

I don't do much (if any) heavily gravity oriented riding. A number of my rides have longer descents, and some of them do give brakes a workout. But they're not the sorts of descents where I drag my rear brake a ton, so I don't really need the extra heat dissipation of a larger rotor. I HAVE run same size rotors front and rear and I find that for the sort of riding I do where the majority of my brake efforts use both brakes, same size rotors wind up creating a braking power imbalance that DOES result in skidding in the rear. Part of this is because I use rear tires with lower profile tread that also have less braking grip, because I aim to balance rolling resistance with grip. I want the bulk of my traction up front and I sacrifice it in the rear so I get a little less rolling resistance.

For the record, I'm using Hayes Dominion A4 brakes with 203/180 rotors. I like the balanced feel I get front and rear and the powerful calipers mean a pretty light effort at the lever. Previously I was using XTR M9020 2 piston brakes and I had to squeeze them pretty hard on the same descents, so I experienced quite a lot of hand fatigue. It's actually getting to be more common to use different calipers front and rear for bikes. A number of manufacturers make it possible to run 4 pots up front and 2 pots in the rear.

I think it is important to note that riders who are heavily gravity-oriented do have different equipment needs than other riders who seek a bit more versatility and well-roundedness from their bikes and riding style.
 

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Yeah, even with 220 front and 203 rear, my rear is pretty sensitive and it's a little too easy to lock up in situations.

I mean, look at the extreme, it's not uncommon for cars to have bigger rear rotors even, but they never have bigger rear calipers, always smaller. There may be 4, 6 8 pistons up front, but usually a max of 4 at the rear and a much smaller caliper overall.

I think part of this is also due to expectations. Like for most XC riding, even races that have some decent descents, the braking is never that sustained or that long, so you can get away with a smaller rear rotor. For bigger steeper riding with extended downhills on steep grades, you really need some big rotors for fade resistance. But not even everyone riding "enduro" or "downhill" rides that kind of stuff. But if you ever ride something that has a sustained 45% grade where you have to control your speed for an extended period of time, even 200mm front rotors can be pretty pathetic.

At this point, it's fairly "engrained" in the industry that the calipers should be the same F to R, so I really don't see this changing appreciably in the near future. Some companies may opt to increase the rear rotor size, but even fewer will make an effort to balance the braking F to R in terms of force AND give better fade resistance by rotor size.
 
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The comparison to cars is not valid.
Cars have one brake pedal only that controls simultaneously the front and rear brakes.

You will never drag the rear brake ona car, but you might on a bike.

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One of my bikes is a decade old, and they put 203's front and back from the factory, and I still go through rear brake pads faster than the front on that bike.

Why they've gone to Mullet brakes now doesn't make sense to me. They are going backwards.
 

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200mm are just so much better and i cant imagine the weight is perceptible to anyone- so why is every bike sold with a 180mm rotor on the rear?

TL;DW;

  • Brakes work best when the wheel is rotating - locked wheel provides about half of braking of rotating one.
  • Locking the front wheel is hard because of vehicle dynamics.
  • Locking the rear is easy for the same reason.

So it makes sense to get stronger brake on the front then on the back. The issue is really that the rear usually sees more heat for longer time then the front - people tend to drag them.

Larger rotors are more capable of dissipating heat, but also make the rear that much more difficult to modulate. So optimizing the rear is finding the goldilocks size/pad/rotor combo that handles the heat the best and still does not suffer from being difficult to control.

The market emergent solution to this is using 20x/180 as a standard. There are other options to approach this, but the 20x/180 seems to be the simplest one.
 

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Counterpoint: I’ve gone back to 180/180 rotors with strong brakes and really good pads. Harder to lock up and locked up brakes are worthless. Much less susceptible to damage from rocks here in Phoenix.

I am not a pro, but generally not a slouch at getting down a mountain. I do not drag brakes.
 

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The reason not to run large rotors is they rub when the frame flexes.

They also are not really needed. Rear brakes are used a lot but they are not used particularly hard. When is the last time you thought, "I just don't have enough rear brake". Unless you are riding a fresh super tacky DH tire on a DH bike and are really good at finding traction it is unlikely that you will ever exceed the capacity of a 180mm or even 160mm rotor on the rear.
 

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The reason not to run large rotors is they rub when the frame flexes.

They also are not really needed. Rear brakes are used a lot but they are not used particularly hard. When is the last time you thought, "I just don't have enough rear brake". Unless you are riding a fresh super tacky DH tire on a DH bike and are really good at finding traction it is unlikely that you will ever exceed the capacity of a 180mm or even 160mm rotor on the rear.
This may be your experience and this is cool.

But it's not mine. I actually discovered my need for huge rear braking power on a winter road trip to Arizona, where there's traction galore in the form of desert rocks. I was descending Upper Javelina Trail which includes some hectic steps that were deep enough that I dared not use my front brake. Especially when these deep steps led into tight switchbacks. Which was often. So I used my 180mm rear rotor for all it was worth - it wasn't enough. I was crawling. Or at least attempting to. My left hand, squeezing the rear brake lever for all it was worth, ached. It wasn't long before I told myself, "Dude, when you get home you're immediately ordering a 203mm rotor for the rear!" Which I did. And haven't looked back.

That rotor is now blue. Yes I drag my rear brake. I don't skid it. But with the steep, multi-thousand foot descents we have around these parts (western Oregon), yeah, my finger is on that rear brake lever so I can constantly hug that fine line between as-fast-as-possible and one-more-tenth-of-a-mph-and-I'm-dead. Yeah, it turns blue.

Anyway I'm sold on big rotors at both ends of the bike. I won't pretend to speak for anyone else, but personally I'll keep them.
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Most riders I know think that my 203/180 rotors with 4 pot downhill brakes on my hardtail are way too much brake. I know a lot of people still on 160/160 rotors with 2 pots, and they're happy with that. IME, this seems to be the largest proportion of riders. More than once, a friend has asked to take my bike for a spin in the parking lot and my brakes have shocked them. I was pretty progressive when I put 180/160 rotors on my xc bike in the early 00's. Literally EVERYBODY I knew back then was on 160/160s.

I think I get it. They're used to using more effort to squeeze the brakes. They think it's normal. I have brakes as big as I do because I like a light touch to access the power in my brakes, and I like a reasonably light touch to modulate them. So when folks used to lots of effort to use their brakes use mine, they grab way too much and get a surprise.

It's pretty rare that I drag my brakes hard for extended periods, though. Most of the bigger downhills near me work great with pulsing technique. And I'm using both brakes with variable modulation as needed. The only time I really drag my rear on its own would be inside a descending corner where I want to maintain speed instead of accelerating through it. And that's not enough to overcook my 180 rotors.
 

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I weigh 250 lbs and ride lots of long technical descents. I can turn a 160mm rear rotor purple in one ride. I have Hope 220s front and rear now with their E-bike pads and they've been great. No warping or discoloration of any note so far. The only downside is the rear takes a little bit more time to heat up, so in the cold/wet it can feel a little underpowered at the start of the day.
 
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