Increasing brake power by increasing rotor size

If I want a good estimate of how much extra power I would get by increasing the size of my disc rotors, is it just a case of taking a ratio of the radius of the rotors? Or is there more to it than that? Assuming everything else stays the same, 180mm to 200mm diameter would be about a 10% increase?

(I think)

absolute power prob won’t change that much – most people could lock up with a 160 if they really grabbed the lever

main difference is staying cooler on those long alpine descents you’re planning

the moment (?) would be bigger, but the rotor will be travelling a bit quicker through the calliper which means, err, … better modulation

or worse

or else pretty much the same

Can’t say that precisely but I’ve gone up from a 160 rotor to 183 on the front of my chameleon and I’ve noticed quite a lot of difference.
However, I also put a new set of pads in as well to start from “as new”.
But saying that they feel it feels much better, more power and control.

Bigger discs will give more power, the same as putting bigger discs on a car.

I’m asking because the difference between my XT M775 and M785 brakes is noticeable (both with 180/160mm rotors). I was wondering if larger rotors would close the power gap. I think Shimano claim the new XT have 25% more power from their various new technologies.

Couple of years ago i went from Hayes Sole single pots on 160 discs to Hayes nine’s on 180 discs.

Changed over because the sole’s were feeling unreliable at stopping 16 stone of me.

The nine set up made a huge difference. Probably partly due to the two pot design but i think mainly due to the 180 discs.

The 180s just stopped the bike. No fuss and no fade on any of the Stanes.

Do wish i had gone for Formulas in the same size. The Hayes do seem less muck resistant in the long run. Sticky pistons etc.

Yes, it’s just the ratio, assuming the same calipers.

A hand-waving calculation would be:

Deceleration = braking force x (rotor diameter / wheel diameter)

The braking force is constant (same caliper), the wheel diameter is the same, so if you increase rotor size you increase the deceleration.

Larger diameter discs will reduce the leverage effort effect from the wheel.
If the caliper and master cylinder still have enough reserve force available.
The available grip is a factor a larger disc will cause lock up to occur in low friction circumstances.

In a word yes to a point don’t upspec too much without matching calipers and master cylinders or there will be problems

Not sure how figures help explain power but if you fit a bigger rotor it will increase breaking power, reduce overheating on long downhills etc.