The German bike-mag "Mountain Bike" published another test report of current disk brakes in their current issue (Jan 2001). They tested 16 disks in the lab and on the trail. They only tested XC disks, no DH or "tandem" versions, but since many tandemists have recently expressed interest in the newer cable-operated disks, I'll sum up the results here. I'll concentrate on the part testing performance on an extended downhill, as this is what interests tandemists most. The only disk brakes capable of slowing down a tandem safely on long descents are still the Magura Gustav M and the Hope DH-4 (neither of them included in this test).
Avid ball-bearing disk, 165mm diam: well-modulated for a mechanical brake. Downhill performance: heavy fading very early, brake stops functioning at 900W as plastic parts melt.
Hayes HMX-1, 160mm: modulation quite good. Downhill performance: heavy fading early during 900W stage, very rapid pad wear, brake stops functioning at 900W as plastic parts melt.
Shimano Deore mechanical, 160mm: very poor modulation, lots of hand strength needed. DH test: very rapid pad wear (needs to be readjusted several times during test), heavy fading during 1100W stage and pad worn out.
Formula Evoluzione 9.5, 160mm (THIS IS THE BRAKE USED BY SANTANA WITH A LARGER 203MM ROTOR): powerful, poor wet weather performance, good modulation, extremely soft feel at the lever. DH test: tube melts off at 1100W, until then no problem.
Hayes FH, 160mm (HAYES WARNS AGAINST THE USE OF THEIR DISKS ON TANDEMS but used by some teams): very good performance wet and dry, feel at the lever a bit too hard, very noisy. DH test: heavy fading early on (700W), loss of brake power at 900W, can be regained by pumping the lever.
Hope Pro, 185mm (Hope does not market specifically to the tandem crowd, but does approve of the use of their brakes on tandems in private communication): perfect lever design, excellent modulation and stopping power, little pad wear, almost impossible to improve. DH test: lever moves out a bit due to (semi-)closed design, disk warped temporarily after 1100W stage, but goes back to round after cooling off.
Hope XC-4, 150mm (same caliper as DH-4, same "semi-open" lever as Pro): less power than Pro, but otherwise excellent. DH test: No fading even at 1100W, but needed to be adjusted at the lever as it would have locked locked up otherwise. THIS LETS ME ASSUME THAT THE DH-4 WITH ITS LARGER DISK AND EXPANSION CHAMBER ("open design") MIGHT INDEED BE THE PERFECT TANDEM DISK BRAKE!
Magura Julie, 180mm (rear 160mm) (NOT TANDEM RATED, BUT AT WAY UNDER $100 A STEAL FOR ANY SINGLE - I've got one on my fould-weather MTB): good modulation and power, not for small hands (lever too far away from bar). DH test: slight fading, no other problems even at 1100W!!
Shimano Deore XT, 160mm (NOT TANDEM RATED; INCLUDED BECAUSE IT'S PROBABLY THE MOST POPULAR DISK): gooid powre and excellent modulation, very noisy. DH test: heavy fading at 700 W, loss of brake power, lever needs to be pumped to keep it working.
Other brakes tested: Bassano Grimeca System 8, Formula B-4, Magura Clara, Magura Louise, Shimkano Deore hydraulic, SR Suntour 700 HD, Tektro Gemini.