just about to order some brakes from Merlin for my On One 456 and im going to go with Deore M615. I also want some new rotors too, so will get the Shimano RT-66 ones. Before i ordered though, I just wondered what people recommended for bike packing duties, 180mm up front/160mm at the rear? Im a fairly big lad (round the waist) and once loaded up I want to make sure I will stop
That sounds about right. Stopping does not just depend on rotor size. We always managed to stop adequately with cantilevers in the past (some of us still do ;-)) Never rely on your brakes - go at the speed that is safe for your ability and the conditions - then you'll always be able to stop! Enjoy
Yeah, 180F and 160R .. due to weight shift under braking, the rear brake is pretty useless at the best of times ... adding a bigger rotor will just make it lock up (stop working) even sooner, once your wheels are locked your brakes aren't working
Generally larger rotor size is to allow greater cooling to occur etc. You can find charts on internets where you look at mass and velocity (therefore energy) required for stopping as the velocity increases. So basically a larger rotor stop you cooking everything - that's the primary reason. You do have a bit more stopping power too, but that's not the noticeable bit is the not overheating!
EDIT: and yeah what Stu said - all about weight transfer, the rear brake has comparatively little load to the front so you can put a little-un on that.
So basically a larger rotor stop you cooking everything - that's the primary reason.
Yes and no Zippy
Because a larger rotor moves the caliper further away from the centre of the wheel, the mechanical advantage is increased = greater braking 'power'. Heat tends to have a far greater negative effect on pads and fluid than it does on the actual rotor. A bigger rotor will generally run cooler but most of the heat build up in pads / fluid is caused by friction not by conduction between pad and disc.
So basically a larger rotor stop you cooking everything - that's the primary reason.
Yes and no Zippy
Because a larger rotor moves the caliper further away from the centre of the wheel, the mechanical advantage is increased = greater braking 'power'. Heat tends to have a far greater negative effect on pads and fluid than it does on the actual rotor. A bigger rotor will generally run cooler but most of the heat build up in pads / fluid is caused by friction not by conduction between pad and disc.
Yes I totally agree, I did caveat my explanation to include that - I was talking about the primary reasoning for a larger rotor. Unless you're at maximum braking force all the time, the need for a bigger disc is governed by thermal efficiency rather than outright stopping power and mechanical advantages.
Zippy wrote:Generally larger rotor size is to allow greater cooling to occur etc. You can find charts on internets where you look at mass and velocity (therefore energy) required for stopping as the velocity increases. So basically a larger rotor stop you cooking everything - that's the primary reason. You do have a bit more stopping power too, but that's not the noticeable bit is the not overheating!
I use 180 front and rear, they overheat slightly less, but doesn't really make a huge difference, they do stop better and I find provide slightly better modulation. I'm interested that Shimano now make the Ice-tech alu-sandwich rotors in 6 bolt attachment, and cheaper than hope floating rotors etc.
I use 160 / 160
The boy scout in me wants them to match in case the front gets bent then I can swap the back on onto the front and make a better job of keeping going.
I could go 180 / 180 but never found the XT brakes lacking. They 'pump' up occasionally but 2 or 3 seconds release allows enough cooling to correct this.
So its the forks i need to check! Thanks. I believe they are RockShox Revelations, they were the standard which came with the 456 i bought back in 2011.
FLV wrote:I use 160 / 160
The boy scout in me wants them to match in case the front gets bent then I can swap the back on onto the front and make a better job of keeping going.
Good thinking boy scout. However with careful selection of the front post mount adaptor, you can generally whip the 160PM to 180PM adaptor off and make it 160 instead to accommodate your 160 rear disc on the front Thus allowing you a 180F/160R which is what I run as a general rule coz all my bikes are like it
This assumes you're fork is std 160PM and not IS or any other random standard.
Shimano do pads with cooling fins on them (F type) - they take longer to get to the point where you start getting fade. If you're worried about stopping power on long descents, it's a relatively cheap way to step up a notch.
200 front and rear as you can never have too much brake. I have a 220 on the front of the Krampus and its ace when your loaded and heading down some horrendously steep / loose / rocky etc descent as you can keep it under control with one finger whilst using the others to hang on for grim death. Sadly 220's aren't available any more.
Many thanks for the swift responses! I shall get them ordered along with some new tyres, probably 2.2 race kings! 
