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What real benefit does having a larger rotor give to braking performance?

Additionally is my assumption correct that you callipers don't actually change in size, just the rotor and the adapter need to be be changed?

And for the bonus point while I'm here, what benefit does moving to a 4 pot system give over a standard (2 piston) calliper?

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3 Answers

up vote 8 down vote accepted

Simply put a bigger rotor provided better braking, and a four pot caliper provides better braking - better meaning more, and more control (Everything else being equal).

For the same force between the disc and pads, a bigger rotor generates more torque on the wheel - i.e. more stopping force. It is running though the pads faster, generating more friction for the same pressure, more stopping power, and as it has bigger surface area, is dissipating more of the generated heat, meaning the disc and pads run colder. So, for the same caliper and brake pad, a bigger rotor generates more braking for the same pressure or the same amount of braking for less pressure.

The disadvantage of a bigger rotor is weight and as its larger, more prone to warping and distortion.

This applies to all disc brakes from bicycles to aircraft......

For a bicycle, there comes a time where you have "enough", and quickly get to "too much" - with high quality equipment its a surprisingly small disk for most people. Big disks a more prone to warping and damage, which is a good reason not install them unless you need to.

Heat buildup is not normally a problem but low end pads can suffer fad. Small rotors don't typically over heat on bikes (Tandems and loaded tourers might manage it). The main advantage is more breaking for less pressure - so you get more control and feel, and your braking hand tires less (If you have ever done a 1000ft vertical descent in technical tracks on crap brakes you know the feeling)

A 4 pot caliper provides advantages of more and more even pressure, meaning better braking. Also as there are usually larger pads - more heat dissipation can be achieved.

Note: A 29'er would usually need a bigger disc than a 26" wheel for the same stopping power.

Don't forget the pad and disk material - these can make a bigger difference than size, but its not as visible. Do not go to big discs and 4 pots unless you are already at the higher end of the quality spectrum.

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"Better braking" is not a precise term. No brake can exceed the stopping power of locking up the wheel. Better brakes provide more "modulation", i.e. a better feel for getting as close to locking up the wheel as possible w/o locking it up. Like anti-lock brakes on a car this is the best you can do. In most cases you don't need the most braking power, but the appropriate braking power to slow the bike w/o loosing traction, especially for MTB bikes. –  Fred the Magic Wonder Dog Apr 1 at 14:47
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I use my bicycle for street freestyle stunting.. and from my experience, larger rotors provide better control vs. more stopping power.

I use my front brake to do stoppies (a stunt where the front brake is applied to tip the bike onto the front wheel and continue rolling). When I upgraded to a 203mm rotor, I noticed that it required less lever movement to keep the rear end up and the front wheel rolling as compared to the 160mm rotor that required considerably more lever travel to modulate. The interesting thing was, that I was able to roll stoppies for longer distances which is in the direct opposition of the statement that 203mm rotors decrease stopping distances. I am not disregarding the fact that a larger rotor increases power, I'm simply saying that larger rotors seem to affect the braking control much more than actual braking power.

On the other hand, if you are actually looking for more power, upgrading to 4 pot brake calipers really make a difference. I run an Avid Code in the rear of my bike to stop me from tipping over during wheelies. The extra piston increases the size of the pad and nearly doubles the pad's surface area, allowing it to 'grab' more of the rotor.

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The larger rotor provides a larger braking surface. A 180mm rotor has more inches of rotor per revolution. The greater mass of the bigger rotor aids in dispersing heat . A single piston caliper pushes one pad against the rotor and deflects the rotor onto the other pad. The advantage of a two piston caliper over a single piston is that each pad is squeezed against each side of the rotor. This increases the clamping force and improves brake performance. The four piston caliper also pushes both pads against the rotor. The extra piston on each side allows for a larger brake pads. Along with a larger pad the dual piston applies the clamping force over a larger area for a more balanced pressure on the pad. The percentage of the pad actually being pushed by the piston is greater.

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A larger rotor should also provide more braking power by the fact that it's further away from the axle. Assuming an equal braking force, at a longer distance, it will put more torque on the wheel, which means shorter stopping distance. –  Kibbee Jun 13 '13 at 0:53
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