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do big rear rotors really make sense?

dexterq20

Turbo Monkey
Mar 6, 2003
3,442
1
NorCal
Does it make sense for mountain bikes to have the same size rotor in the rear as in the front? If you look at other wheeled vehicles, such as street bikes, they have two massive rotors in the front and one tiny little rotor in the rear. Cars are the same way - some (cheaper) cars only have drum brakes in the rear. So why do so many mountain bikes have matching rotors front and rear? Is there an actual reason for this? I'm thinking of downsizing the rear rotors on both my DH and trail bikes, but wanted to hear some arguments for or against this idea first. Discuss.
 

blackspire

Monkey
Jul 19, 2007
115
0
Does it make sense for mountain bikes to have the same size rotor in the rear as in the front? If you look at other wheeled vehicles, such as street bikes, they have two massive rotors in the front and one tiny little rotor in the rear. Cars are the same way - some (cheaper) cars only have drum brakes in the rear. So why do so many mountain bikes have matching rotors front and rear? Is there an actual reason for this? I'm thinking of downsizing the rear rotors on both my DH and trail bikes, but wanted to hear some arguments for or against this idea first. Discuss.
Well it's very possible you could downsize your rear rotor, especially on your trail bikes. On your DH-bike I dunno, it depends on how much you use your brakes (your rear brake in particular) and how long runs you do.
 

ronan

Monkey
Dec 7, 2007
786
0
Toulouse, France
Does it make sense for mountain bikes to have the same size rotor in the rear as in the front? If you look at other wheeled vehicles, such as street bikes, they have two massive rotors in the front and one tiny little rotor in the rear. Cars are the same way - some (cheaper) cars only have drum brakes in the rear. So why do so many mountain bikes have matching rotors front and rear? Is there an actual reason for this? I'm thinking of downsizing the rear rotors on both my DH and trail bikes, but wanted to hear some arguments for or against this idea first. Discuss.

do you think your going to get a performance advantage/disadvantage by changing?
 

ridingsupreme

Monkey
May 12, 2008
125
0
Santa Cluas lane
it also depends on your weight, and as mentioned before your riding style has a bit of a say in it as well. bigger rotors have more power (obviously) and some riders like the extra power if you just wanna grab the rear brake to initiate a slide into a corner or load the suspension a bit through rocks or just plain slow down. bigger rotors dissipate heat better (again obviously) so on longer runs or steeper descents or if you like your rear brake for what it is then ya same size is cool.

personally i'd like to run a 7 inch rotor in the rear, however my frame doesn't allow me the choice currently, and I think the same goes for a few others. also when your ocd about your stuff matching.. lol ya
 

Sandwich

Pig my fish!
Staff member
May 23, 2002
21,031
5,919
borcester rhymes
The only way to tell if you can do it is to try. Most people run dual 8s because they provide enough power. I would bet a) if 9" rotors were commonly available, people would tend to use them in the front. b) If brakes were as powerful 5 years ago as they are now, people wouldn't feel the need for an 8 in the rear.

Keep in mind that with street bikes, you're looking at what, 400 lbs of mass in addition to the rider, vs. 40lbs on a dh bike....and you never see performance cars with drum brakes...they just don't work as well. Yes, most performance cars have larger fronts than rears, but they also have one pedal, whereas a bike rider can control front vs. rear.

I'm surprised running smaller rotors hasn't caught on with the lightweight crowd, but I think it's an area where the savings are minimal and the performance difference is rather large. To each his own, if you want to go with a smaller rotor, nobody is stopping you, and if you're not running DH runs, you probably won't notice a difference.
 

Threepointtwo

Monkey
Jun 21, 2002
632
0
SLC, UT
I have been running Codes with a 7" in the rear for a few years now. That provides more than enough power and you have the added benefit of some extra clearance.
 

Heath Sherratt

Turbo Monkey
Jun 17, 2004
1,871
0
In a healthy tension
Hey Ben, I have experienced this first hand. Here are my findings...
Long, steep courses demand more braking/modulation to keep your hands from fatiguing too quickly. It really doesn't matter how strong you are if you're doing five or more runs a day on a fast steep course your hands will fatigue MUCH faster with smaller rotors.
I run a six inch rotor for about 90% of courses on the rear as I don't use the back brake too much, but for certain tracks it simply isn't enough power. Example-Northstar, Live wire-160mm, Race run-Karp to dogbone-or whatever, the long technical course-180 to 203mm, Durango-203mm without exception, Mammoth Chainsmoke, gnarly run from nationals-203mm.
So, to summarize; Steep, technical courses=203mm. Everything else 160-180mm.
 

dhkid

Turbo Monkey
Mar 10, 2005
3,358
0
Malaysia
a few things to take into account:

similarity of parts, if you are out racing, makes more sense to carry one rotor as a spare, which can be used both end.

weight savings, if you want to save weight, the way that makes more sense is lighter xc callipers with big rotors. if you have massively powerful brakes to start with, then smaller rotors make a bit of sense. ie with codes. but the other way round is better. callipers only dissipate so much heat, rotors do the bulk of the work, more surface area = better. [peat raced champery on juicy ultimates and used to have hope minis on his orange]

in an idea world, yes, you should brake more in your front wheel. but for us mere mortals, chances are you are on the rear brake a lot more and dragging it. the front brake only comes on hard and when its needs to.
 

iRider

Turbo Monkey
Apr 5, 2008
5,648
3,089
There are a few pics of Minnaar running what seems to be a bigger rear than front rotor (front 7", rear 8"???). Anyone else seen this? Any idea why?
 

gratiflying

Chimp
Apr 12, 2007
70
0
ran smaller rears for ages... now running 8 F&R Elixir CRs and am way happier... not because of the power though... more because dual 8s require significantly less effort IMO and provide more control... less effort braking means faster everywhere else IMO... obviously terrain is a big factor as well though...
 

ph4se_1

Chimp
Jun 17, 2008
71
0
Dublin, Ireland
the biggest you will ever need is 8inch front and 7inch out back.this provides enough power for any length of dh track.im running this setup on codes the last year and a half and have had no problems,even on very long alpine tracks.
 

go-ride.com

Monkey
Oct 23, 2001
548
6
Salt Lake City, UT
If I was going to run one smaller rotor it would be on the front. A DH bike does not use brakes the same way a road vehicle does. Most DH rides use a lot more rear brake. For this season I'm test Elixr CRs with 185s F & R. So far so good, but our next race has a lot of continuous steeps that I may need to pull out a set of 8" rotors.
 

DirtMcGirk

<b>WAY</b> Dumber than N8 (to the power of ten alm
Feb 21, 2008
6,379
1
Oz
I run dualing 8's because I am a fat bastard. I need all the anchor I can get.
 

PatBranch

Turbo Monkey
Sep 24, 2004
10,451
9
wine country
I was using a 7" and an 8" in front with juicys. I now have elixirs and am using 7" rotors on both wheels. I may see how a 6 feels on the back, but it won't be a big weight savings like swapping an 8 with a 6.
 

Zark

Hey little girl, do you want some candy?
Oct 18, 2001
6,254
7
Reno 911
Hey Ben, I have experienced this first hand. Here are my findings...
Long, steep courses demand more braking/modulation to keep your hands from fatiguing too quickly. It really doesn't matter how strong you are if you're doing five or more runs a day on a fast steep course your hands will fatigue MUCH faster with smaller rotors.
I run a six inch rotor for about 90% of courses on the rear as I don't use the back brake too much, but for certain tracks it simply isn't enough power. Example-Northstar, Live wire-160mm, Race run-Karp to dogbone-or whatever, the long technical course-180 to 203mm, Durango-203mm without exception, Mammoth Chainsmoke, gnarly run from nationals-203mm.
So, to summarize; Steep, technical courses=203mm. Everything else 160-180mm.
That sums it up nicely. Hand fatigue isn't worth the weight savings on long steep stuff.
 

Viv92

Monkey
Jan 31, 2009
204
0
Australia
I had Juicys with 8" f 6" rear, then Codes 8" both ends, now have The Ones with 8" front 7" rear. I don't see why you would run an 8" on the back with those brakes, especially if you aren't heavy. I think 8 is worse, too easy to accidentally lock.
 

Kanye West

220# bag of hacktastic
Aug 31, 2006
3,740
470
If I was going to run one smaller rotor it would be on the front. A DH bike does not use brakes the same way a road vehicle does. Most DH rides use a lot more rear brake. For this season I'm test Elixr CRs with 185s F & R. So far so good, but our next race has a lot of continuous steeps that I may need to pull out a set of 8" rotors.
Sorry, gotta disagree with that. I've been able to put up times that are very close to my fastest on steeper trails just by running a front brake (non-functional rear brake for part of the day). Not a chance that would be possible with just a rear brake.

If you ride anything steep at all, a large front brake is absolutely critical. Between the incline and the weight shift from braking, 90% or more of the braking power is in the front if you are braking properly (read: not dragging the rear brake to slow down).
 

no skid marks

Monkey
Jan 15, 2006
2,511
29
ACT Australia
I find you can feel the tyres traction better with bigger rotors. I run 8" front and 7" rear, but I'm a fly wight. Will be changing down to 6"r and 7"front shortly, but I'm on Elixers with a Code front caliper.
My 9" rotor on my tandem is great, but even my girlfriend is light.
 

Steve M

Turbo Monkey
Mar 3, 2007
1,991
45
Whistler
Does it make sense for mountain bikes to have the same size rotor in the rear as in the front? If you look at other wheeled vehicles, such as street bikes, they have two massive rotors in the front and one tiny little rotor in the rear. Cars are the same way - some (cheaper) cars only have drum brakes in the rear. So why do so many mountain bikes have matching rotors front and rear? Is there an actual reason for this? I'm thinking of downsizing the rear rotors on both my DH and trail bikes, but wanted to hear some arguments for or against this idea first. Discuss.
It's not a reasonable analogy really, because cars always apply brakes to all four wheels simultaneously, and they always want a fairly similar brake bias too. However DH bikes involve a lot of brake dragging, particularly the rear brake, and so despite not requiring the same braking power, the rear rotor definitely benefits from being larger simply because it is being applied more often and for longer than the front brake - in fact rear brakes often suffer from overheating more than the front. The only actual benefit of going to a 6" rear rotor is lighter weight.