Here is a thread discussing on Mercedes forum (MBWorld) for upgrading from SLK 05's 340x32 to 06+ 360x32mm
rotor, which RB makes both:
The common question from users just like from any other car forums would be like this:
1. Is it available
2. How much
3. Is it worth
We usually like to address 1 & 2 from a mfgr stand point, and leave #3 to user's decision (like 365mm upgrade to 380mm in this thread). However if a user wants to know more we will then reply the advantage of a "bigger" rotor based on 'physics theory' stand point in order to keep this optional choice w/o being influence by a mfgr.
[indent][i]Bigger rotor has longer radius, and a longer radius 180mm (360mm rotor) will provide better mechanical advantage (more brake torque) vs. 170mm (340mm rotor).
Brake torque calculation:
Fn = Normal (Clamping) force is the force pressing brake pad against brake disc.
Fn = P * A (P=Hydraulic pressure; A=Piston area)
Fb = Braking force is the tangential friction force acting between brake pads and disc.
Fb = µ*Fn (µ=Coefficient of friction between pad and disc)
Tb = Brake torque is the moment of braking force about the center of rotation
Tb = Fb*Re (Re=Effective disc radius/torque arm)
Assume same braking force is applied, you get more brake torque (stopping power) with a bigger rotor, or require less pedal effort (hydraulic pressure) to stop a car than a smaller rotor.