Die Shimano-Bremse ist im Test von MountainBike (bislang einziger Test, den ich kenne, der auch Bremsleistung (= Dauerbremsfestigkeit) bewertet hat) nicht gut weggekommen. Nur Gustav M und Hope halten, und Hope hat chronische Lieferschwierigkeiten, also Gustav M! Finger weg von allem anderen.
Ich fahre die Formula mit 185er Scheibe und kämpfe oft mit Überhitzungsproblemen (siehe meine Berichte nach Furtwangen-Marathon letzten Herbst und Gardasee-Festival 2000). Jetzt habe ich hier in Freiburg einen Spezialisten für Rennmotorräder aufgetan, der mir hoffentlich mit leistungsfähigerer Bremsflüssigkeit weiterhelfen kann. Wenn das auch nix taugt, fliegt sie wieder runter.
Wenn Du nur dort fahren solltest, wo die üblichen Scheibenbremsen halten, brauchst Du keine Scheibenbremse. Dann tun's auch Magura HS 33 oder gut gewartete V-brakes. (Es sei den, Du willst unbedingt 'ne Scheibe, weil's cool aussieht und außer uns ja keiner weiß, dass es Blödsinn ist).
Es gibt fast keine Firmen, die wirklich wissen, was Tandemfahrer im Gelände wirklich von ihrer Bremse verlangen. Santana baut ja auch ein blödsinniges Fully-Tandem, in das hinten nur 'ne 160er Scheibe reinpasst. Totaler Witz! Damit kannste nur am Deich entlangfahren.
Für alle, die genauer interessiert, hänge ich unten eine Zusammenfassung des Bremsentests, den ich mal an die amerikanische Tandemliste geschickt habe:
The German off-road magazine "MountainBike" (http://www.dirty-pages.de/ - don't know how much connection there is to "MountainBike" in the U.S.: http://www.mountainbike.com/) recently ran a test of popular disk brakes. This was the first published test I have seen which not only reported on brake force and modulation, but also on brake power (or heat capacity) - VERY IMPORTANT for us tandemists!
The test simulated a long downhill ride, on which the brake was used to maintain a constant speed of about 25 km/h (15.6 mph). The brake power was kept constant until the temperature at the mounting bolt did not increase any further, at which point the power was increased in increments of 200 W. After each stage, the brake was released completely, and a panic stop was performed. The test then proceeded with the next stage. The test started at 500 W, which according to MB is equivalent to a combined bike-rider weight of 100 kg (220 lbs) on a 7 % downgrade.
The most important piece of news up front: They found that most disks were not able to generate more brake power than a rim brake with cloth rims strip and a standard tube:
rim brake, cloth rim strip, standard (heavy) tube: "almost reached 1100 W"
rim brake, plastic rim strip, extralight tube: "midway through the 700W stage"
Bassano Grimeca Race, 160 mm disk: "brake pads fall out at 500 W (same with 2nd set)"
Bees Bee disk, 180 mm: "pressure point starts to wander at 900W, fluid leaks shortly thereafter"
Coda Expert, 171 mm: "tube fitting starts to melt at 900 W, brake fluid leaks out at 1100 W"
Eberle disk, 162 mm: "fluid leaks at the beginning of the 900 W stage, loss of brake power before proceeding to 1100 W"
Formula HD-5, 158 mm: "fluid leaks after 500 W stage, loss of brake power"
Hayes FH, 160 mm: "overheats very quickly [first stage? - test report unclear]"
Magura Clara, 160 mm: "lever needs to be 'pumped' after 700 W stage, locks up suddenly at 900 W"
Magura Gustav M, 180 mm: "very low temperatures during early stages of test, slight 'pumping' at lever needed, locks up after several minutes at 1100 W"
Shimano Deore XT, 160 mm: "heavy 'pumping' at lever needed early through 700 W stage, total loss of braking soon thereafter"
Formula MD-1, 165 mm: "total loss of brake power early in the test due to rapidly degrading pads"
Promax DC-600, 180 mm: "ONLY BRAKE IN THE TEST TO SURVIVE 1100 W STAGE! tons of force at the lever needed, though, and disk warped afterwards"
RST DX-3, 160 mm: "rapidly degarding brake pads, disk warped, survived heat well" [no wattage give]
So much on the test results. A few notes:
The Formula HD-5 is the brake used by Santana; Santana uses a 203 mm disk, however, which should increase brake forke considerably and brake power somewhat. The test confirmed the results with the Formula disk on my tandem, however, which failed during a race last September. After my report to the list, I got a few reports on similar incident with Formula brakes.
Too bad they did not test any Hope brakes. Since they are extremely hard to come by at the moment, good test results would have been almost irrelevant, however.
Very few of us have actually had a heat induced blowout since road cyclists changed form tubulars (which were dangerous because the tire would wander, causing the valve to shear off) to clinchers. Since blowouts on the front wheel are extremely dangerous, probably much more so than a total loss of braking on one brake, this is good. This leaves disks with a slight safety advantage over rim brakes, even if the numbers are the same.
The safety of rim brakes can be increased by using heavier tubes, bigger rims, deep-dish rims, cloth rims strips, and fatter tires and tubes.
However, the only solution for extreme applications (loaded touring, extremely steep downhills which do not allow high speeds due to traffic, turns or road surface) remains the trusted Arai drum backing up two good rim brakes.
Disk brakes increase the safety in wet weather and for snow riding (where rim brakes are virtually useless), and by a different failure more: The bike remains rideable and can be stopped using the remaining brake, while a blowout (esp. front) will lead to a crash in most cases. Some disk, like the Magura Gustav M (and possibly Hope's DH 04) can take a bit more heat than rims.
Another big problem cyclists face on long downhills is rapid rim wear. This is due to poor brake pads. Some Shimano pads (like the '98 LX-V) are extremely bad in this respect. Magura or KoolStop pads have increased rim life for me considerably.
Chris "I prefer riding uphill anyway" Timm