I'm wondering how much of an effect switching from a 203mm to a 185mm rear disc brake rotor will have. My search has only revealed the common knowledge that bigger rotor = more heat capacity, but has not revealed any actual quantities showing what the real-world difference is. Does anyone know a source for such data?

The reason I'm asking it that we have an Avid BB7 disc brake with 203mm rotor on the rear of our tandem. Because it's a 2008 Co-Motion Speedster, the disc mounting bracket is on the seat-stay, and so the disc brake interferes with the rack (they've moved the bracket to the chain-stay on newer models, which solves this problem). I had managed to find a random rear rack that didn't interfere with the brake when I mounted it with an extra kit from Tubus. However, the rack was a temporary solution - pretty low quality and not that strong - so I want to upgrade to a fancy Tubus rack. Unfortunately, I've found no way to mount a Tubus Logo rack without interfering with the disc brake. I've tried using two different-shaped mounting brackets from Tubus, but using all possible orientations of those parts puts the rack in various positions, none of which provides a solution. The one thing that would solve the problem would be to use a 180mm or 185mm disc rotor because then the disc brake would mount nicely between the Logo rack's stays when using a certain combination of mounting brackets.

So, it seems that we can have a 203mm rotor and a crappy rack or a 185mm rotor and a super-nice rack (or a 203mm rotor with the super-nice rack mounted in a really precarious way, making it no better than the crappy rack, so I'm not considering this as an option).

Since I couldn't find objective data on the difference in heat capacity, I did some calculations to compute the difference in mass and surface area of different rotor sizes. I assumed that the braking surface is always 15mm wide, the rotors are 1.9mm thick, that rotor density is constant (so mass is proportional to volume), and for simplicity I ignored holes in the braking surface. If my geometry skills are sufficient, the 185mm rotor has 9.6% less mass than the 203mm, the 180mm has 12.2% less than the 203mm, and the 160mm has 22.9% less than the 203mm. The formulas on this Wikipedia page suggest that heat conduction is a linear function of surface area, and the differences in surface area are the same as the percentages above.

Therefore, my best estimate is thatassuming zero heat loss, the 203mm disc would absorb 10% more heat (i.e., braking power) than the 185mm disc before they both reach the same temparature (due to the difference in mass). However, when factoring in heat loss we must consider the surface area difference also, so the larger rotor should actually handle more than 10% more braking power than the 185mm rotor, but I don't know how much more - I am at the limit of my skills here

There are probably other factors that I haven't considered, so some real test data would be very nice to see. Also, our current 203 mm rotor is just the stock Avid Roundagon. I'm wondering whether I could get a fancier 185mm rotor that has better cooling properties than the stock 203mm rotor, which might cancel out the difference in mass and surface area. Can anyone recommend something?