Originally Posted by
oldacura
My guess is that if Shimano or any of the other companies migrating to discs ever did an objective test or calculation to determine the heat capacity of discs -vs- rims and it came out that rims had a higher heat capacity, they would likely bury the results because the company had already decided to migrate to discs (as long as the heat capacity of discs was adequate).
Since you asked about a comparison of the "'best' caliper rim brakes" vs the "'best' disc brakes" under the "most taxing conditions", let's look at specific options.
For rim brakes, your best options are the Santana long-arm V-brakes (or Shimano Dura Ace BR-9000 dual pivot brakes). You may also throw in a boutique rim brake like the THM Fibula.
Based on my personal experience, this is my ranking of tandem disc brakes:
1. Shimano R785 11sp shifters (mechanical or Di2 version) with the Shimano Saint 4-piston hydraulic calipers (w 180mm or 203mm rotors). The 2017 Dura Ace ST-R9170 shifters would be even better.
2. Shimano R785 11sp shifters (mechanical or Di2 version) with R785 2-piston calipers. The R785 caliper is essentially an older XT design.
3. Shimano/SRAM/Campy mechanical shifters with TRP Hy/Rd 2-piston caliper. The Hy/Rd uses a cable to actuate the hydraulic caliper.
4. Shimano/SRAM/Campy mechanical shifters with Bengal M700T or Avid BB7 single-piston mechanical calipers..
Since disc brakes have been widely used on mtn bikes since the 1980s, there are thousands of results from sanctioned races and timed events. Everyone from World Cup racers to weekend warriors choose disc brakes because they offer more power, modulation, and reliability in all conditions. Shimano has discontinued XTR/XT v-brakes because of low demand, and only makes v-brakes at the Alivio/Acera level. SRAM/Avid also limit their v-brakes to the entry market.
There is no conspiracy because Shimano and SRAM continues to offer both types of brakes. The competitive reality is that no racer or serious rider would choose an Alivio v-brake over a XTR disc brake on a grueling downhill when money, ranking points, or pride are at stake.
Before you say "it's different for road bikes", read this article about disc vs rim brakes on a 17km descent of Mount Etna. I think it's fairly representative of most disc brake reviews:
https://roadcyclinguk.com/gear/shima...UwVv6o8S3LY.97
Still not convinced? Why don't you do your own road test. Do several big descents on two tandems, one with rim brakes and the other with hydraulic discs. Borrow a 2nd tandem if you have to and do 3-5 shuttle runs in quick succession. Bike fit is not important because you're not doing much pedaling. Use Garmin or Strava to measure time & distance. Find out which brake is faster and more reliable for your toughest descents.
A rim brake is fine for most century rides and tandem rallies, but we would go with dual hydraulic discs for loaded touring or epic descents (eg. Mt Ventoux).