Brakes Deore,XT,XTR,SLX,Hope??

[PhilipV]

Koperdraad, don't spend money too quickly. Those pads are still bedding in. If the brakes feel spongey or go completely soft after the bike is put upside down for a while then it definitely needs a bleed. If they feel firm then a bleed may not do much.

There are a myriad of other reasons why brakes feel inadequate, it could be the tyres not providing enough traction, it could be wrong technique, ie predominantly using back brake instead of predominantly front brake, grabbing brakes and locking up the back.

 

I use 615s on both my bikes, I weigh slightly less than you, but I work brakes very hard, and let me tell you, there is nothing wrong with those brakes. Upgrading the brakes itself won't make much of a difference, but better heat management may provide some improvement over long descents, but you shouldn't be dragging your brakes anyway.

[BaGearA]

Agreed with the rest, but this is incorrect. Larger rotors give you more stopping power as the moment (resultant torque) is larger due to the larger distance between the hub and the braking surface.

 

IE: A 200mm rotor is 25% larger than a 160mm rotor (radius of 100mm vs 80mm) so for a given force at the rotor (pulling the brake lever Xmm, assuming all other factors remain equal) your braking torque will be 25% higher. 

 

There is the added benefit of better cooling as well, but bigger rotors DO give you more braking power.

 

Physics

[chuckza]

Agreed with the rest, but this is incorrect. Larger rotors give you more stopping power as the moment (resultant torque) is larger due to the larger distance between the hub and the braking surface.

 

IE: A 200mm rotor is 25% larger than a 160mm rotor (radius of 100mm vs 80mm) so for a given force at the rotor (pulling the brake lever Xmm, assuming all other factors remain equal) your braking torque will be 25% higher. 

 

There is the added benefit of better cooling as well, but bigger rotors DO give you more braking power. 

 

What you say is correct, in theory at least.  In practice, the biggest limitation with stopping power is wheel traction.  The fastest possible stop will be when you apply just enough brake force on the disc so as to not drag your tires on whatever surface you are riding on (which is why we fit ABS on cars/motorbikes).  Any of the brakes mentioned here (Deore/SLX/XT or even XTR) should be able to apply sufficient brake force on a 160mm rotor to drag your wheels and therefor I argue that rotor size is not the limitation; certainly not for the "average" rider.  If you increase the mass of the rider/bike or speed substantially, you may in fact reach a point where the limitation is no longer that of mechanical grip between the wheel/trail but in fact the amount of braking torque you can apply to the wheel and in that case you may in fact see a benefit from using a larger rotor.   

[Mr lee]

You forgot about the leverage provided by the larger rotors.

 

Yes exactly. In simple layman's terms...try stopping a wheel in the center vs. the outer edge!

[Captain Fastbastard Mayhem]

What you say is correct, in theory at least.  In practice, the biggest limitation with stopping power is wheel traction.  The fastest possible stop will be when you apply just enough brake force on the disc so as to not drag your tires on whatever surface you are riding on (which is why we fit ABS on cars/motorbikes).  Any of the brakes mentioned here (Deore/SLX/XT or even XTR) should be able to apply sufficient brake force on a 160mm rotor to drag your wheels and therefor I argue that rotor size is not the limitation; certainly not for the "average" rider.  If you increase the mass of the rider/bike or speed substantially, you may in fact reach a point where the limitation is no longer that of mechanical grip between the wheel/trail but in fact the amount of braking torque you can apply to the wheel and in that case you may in fact see a benefit from using a larger rotor.   

IE larger rotors give you more braking power, or allow you to use less hand force to stop in the same distance. Yes. It's far more apparent on the front, though. And trust me - you can feel the difference pretty clearly. 

[GrahamS2]

IE larger rotors give you more braking power, or allow you to use less hand force to stop in the same distance. Yes. It's far more apparent on the front, though. And trust me - you can feel the difference pretty clearly. 

Yep, and more available braking power means better modulation, which means more traction, allows for later braking, more fun...

[chuckza]

IE larger rotors give you more braking power, or allow you to use less hand force to stop in the same distance. Yes. It's far more apparent on the front, though. And trust me - you can feel the difference pretty clearly. 

 

Totally agree with your point on modulation and hand force.  It will give you better control etc., but not make you stop any faster.  I run a set of Shimano Zee M640 with 203mm rotor upfront and 180mm rear.  It is more a function of where and how we ride though.  Never had any hassles stopping, have absolute control and have never managed to cook them either.  Interestingly enough they are not that expensive either.  Just got a set for a mate from Evan's in the UK for ~R3.5k including shipping/VAT/duties.  

[GrahamS2]

Totally agree with your point on modulation and hand force.  It will give you better control etc., but not make you stop any faster.  

Well unless you have hands that don't suffer fatigue, it sure will help you stop faster.

 

Stopping distance is a function of tyre grip, not braking force (there's no modern hydraulic discs that cannot stop a wheel). But the more braking modulation you have, the better you can maximise available tyre grip over variable terrain.