Personally I’ve never understood why you’d want the same size rotor out the back as up front, unless you’re a serial brake dragger and have issues with overheating. Physics dictates that the front brake is going to do the bulk of the work when slowing down, so makes sense to keep a larger disc up front IMO.

Most vehicles have more powerful front brakes as that is where the weight / grip is concentrated during braking. Big disc on an unweighted rear wheel is pointless.

Tandems and cargo bikes may differ.

I’ve never found anything less than 203mm to be enough on a dh bike, even on the rear. Heavier bike, heavier wheels and tires, higher speeds and more braking for me = bigger rotors.

Overheating does become an issue, and you do need to be able to lock up as and when.

Other than weight saving why not have bigger rotors? 180mm on a trail bike is fine. More power, brake later, brake harder, brake less. Or just have extra bite when you need it.

70% of braking force should come from the front wheel. you don’t need a big rear disc if your brakes are working properly.

That’s an oddly specific bit of info. Where are you getting it from?

Tandems don’t We can just about get the whole weight on the front wheel of ours under hard braking – it bends e forks rather frighteningly tho

Sounds about right given decent grip maybe even more – when doing a stoppie its 100% front wheel- its obviously less when grip is lower. the reason you can do this is weight transfer – under braking more of the weight goes onto the front wheel so it has n more grip so can brake harder – thats why you need to load the tyres gradually not just grab a handful which will lock the wheel

Sounds about right given decent grip maybe even more

I think this percentage is largely, as TJ has pointed out, grip dependant. On an MX bike, you’ll see normally the rear brake rotor is roughly the same size as the front, but they race solely on very lose dirt with not a huge amount of grip, and they have powerful engines powering the rear wheel (which is also wider, and heavier, and can give nasty gyroscopic effects if not brought under control) so a largish rotor on the rear makes sense in that application.

The other end of the spectrum is Superbike racing, or MotoGP. Carl Fogarty famously never used to use his rear brake ever, unless it was wet. Didn’t do him any harm on his way to 4 Superbike World Titles! But these bikes run on very grippy tarmac surfaces, with big sticky slick tyres and very stiff chassis to get the best out of the brakes. Rear brakes really are a token effort though on circuit racing bikes, usually being of approx 220mm diameter with only a single piston caliper, whereas up front you would normally have 2x320mm diameter discs with a 6 piston caliper per disc.

As the old Pirelli advert used to say, “Power is nothing without control” and it is your tyres ability to convert the brakes performance into retardation on the chosen surface you are on, that will be the limiting factor.

Everyone seems to think that the front brake will give more braking traction/power…and they would be right, but only in theory. In reality, off road) you use the rear brake a whole lot more as there are few circumstances in which strong front braking power can be used without losing traction. Its easier to control a bike that’s losing traction from the rear than the front.

I remember seeing Steve Peat’s Orange 224 at a bike show ages ago with a Mono 6Ti on the front and a Mini on the rear….

Personally I use 203’s front and back as it’s easy and they don’t overheat.

I have a 180/160 combo. Over several years of 2-3 weeks each summer in the alps, it proved slightly less than desirable, but there is no-where in the UK I’d need any larger rotor on the back (for aggressive xc that it – obviously a good full-face DH-er would need more).

It was only repeated high speed hooning down black DH runs in the Portes de Soleil which boiled the back brake. That doesn’t meet anyone’s description of aggeressive xc.

..that made my morning coffee time )

And what’s all this I read about ‘attacking the trail’ while we’re at it? Why can’t we just work it out and go smoothly on our way? Surfers talk about being at one with a wave, some of us talk about attacking and shredding the trails.. ffs )

(apologies to OP, not a dig at you, just funny terms)

When I run MTB skills courses I start by doing a braking test. Experienced riders roll their eyes as everyone knows how to brake, right?

Generally riders estimate that the front brake is up to around 50-70% more effective than the rear.

So we roll down a fireroad at jogging speed. Use the back brake only and stop as fast as you can. Everyone locks the rear wheel. Stopping distances are usually around 12-15m. Repeat the test with the front brake only, no-one locks the wheel and stopping distances are around 3m. With both brakes stopping distances only drop to around 2.5m.

We play about getting used to modulating the brakes and adding a few other techniques and we get the distance with the front brake down to around 1.5m – one bike length.

Then we move to a steep greasy chute and repeat. Everyone goes for the rear brake and slides down it with the back wheel either locked or using cadence braking – lock/roll/lock/roll etc but they certainly cannot stop. Generally people are scared of the front brake in case it locks so they slide the rear. We move to the front brake only and repeat. Using the techniques I have taught them they can stop the bike on the chute with just the front brake.

Most people really have no idea how to use the brakes effectively – they just grab a handfull on the front and drag the rear. They rip up the trail, overshoot corners and look like an accident waiting to happen. Using the brakes right means faster, smoother, less crashes and a trail that doesn’t get braking bumps. Riding standards in this respect have deteriorated since we all moved from v brakes to discs.

Lets just say my front brake pads last about 3-4 months and my rears last 3-4 years. I don’t brake as often as most, and my rear is rarely used.

So to answer the OP,go for 160 rear unless you can buy them in 140. And if it feels like the rear is not powerful enough then go on a skills course! 😀

That’s not aggresive XC

That’s agressive XC

ASR-5 rider here.

I recently replaced 2011 XT 180 rotor on the rear of mine for a 160. 180 is overkill just locked up the rear a lot.

Maybe I’m heavy handed but I much prefer the 180/160.

The “Steve Peat and co only use 140mm brakes” thing doesn’t translate into the real world. They have to complete one run with a perfectly set up bike, and they are the world’s best riders.

You or I are ham-fisted duffers, and will likely be riding something that’s not perfectly set up, many many times in a row if on an uplift day or in the Alps etc.

Anecdotally the bigger the brakes the better, front and rear.

Edit: I thought you were talking about DH

I just sling a gert big **** off 205 on the front. Doesn’t matter what you shove on the back then

covert: 200/180
575: 180/160
blizzard: 160/160
evo2 (singlespeed): 160/160

all used for various flavours of aggressive-xc / trail riding, with the covert also doing mincecore-DH and huck-lite-wobbly-landings.

Dave

sure doesnt seem to slow him down

Superstar pads?

Didn’t the guy said going down hill fast was not a priority so small rear disks it is.

Interesting conversation this, I have 2012 XT brakes with 203 up front and just upgraded my rear from 180 to 203 as well 😯 I have a faily burly Five at 29lbs and weigh 16st, I also try to ride as fast as I can on technical decents. I found that whilst the front was perfect the rear needed quite a bit of ‘pull’ on the lever before I hit wheel lock*, especially when my weight was well over the rear tyre. Saturday’s ride was great, I found the locking point on the rear was much earier, I have basically tightened the modulation and find it much easier to work with.

* I’ve read the posts about breaking skills with interest and look forward to my yet to be arranged schooling. But IMO whilst there is good advice from experianced riders, there is no standard to keep to, just use the brakes that allow you to ride the way you want.

180/160 on an ASR5 sounds fine unless like me you’re heavy and find you need tighter modulation.