[Yak] re: tandem (drum) brakes
ibike2havefun at yahoo.com
Fri Sep 12 06:04:24 CDT 2008
Greg Illes wrote:
Something I don't quite understand: the Arai drum brake is routinely
used as a third "drag" brake on tandems - - why not have it be the
primary/only rear brake? Why have three brakes?
The main reason I see for having a drum brake on a bike (I have one on each of my tandems) is really to provide long-term speed _control_ on a long descent, rather than actually trying to bring the bike to a stop. Applying that amount of drag for an extended period generates a tremendous amount of heat. A drum brake allows the heat to build up in a device that is isolated from the rim, as does a disk brake. I think that disk brakes eliminate the need for a drum brake. (They also eliminate the ability to MOUNT the drum brake, and I don't think I've ever seen a hub that was built to accommodate both types of brake.) If you use hydraulic disk brakes, you need to worry about not heating the actuator fluid to its boiling point, but with cable-activated disks that problem is eliminated. With _any_disk brake, there is the problem of heating the disks to the point where they warp- not a problem with a drum. On the other hand, the Arai drum brake is about
four pounds- a substantial fraction of the total rig.
Harvey Sachs wrote a thoughtful and articulate reply (characteristic of the man), borne of his experience (which is extensive). His second point asserted that "2) Rim brakes alone can overheat with tandem loads, and some folks claim to have blown tires on mountain grades as a result."
I am one of those people. Ten years ago I was out in Shenandoah National Park, descending from the top of Hogback (elevation somewhere over 3000 feet) to Elk Wallow (elevation c. 1100 feet) over the course of about three miles. The day was wet- we were in the midst of a three-week-long drizzle, and I had been alternating between front and rear brakes trying to keep our speen to what I felt was a safe (or at least manageable) speed- 25 MPH. I did not have a drag brake (aren't ALL brakes really "drag brakes" except, arguably, retro-thrusters?) on the bike at the time
When we reached the point where I needed to slow significantly to make the turn into the concession area where we planned to rest and eat, I applied the brakes to both wheels simultaneously. The front tire blew almost immediately, leading to one of the scariest moments I've ever experienced on a bike. I can only attribute the blowout to excessive heating of the tube through the rim.
There was some additional discussion from a couple of other responders re: activating both rim brakes, or both rear brakes, with a single lever and activating the remaining brake with the other brake lever. For various reasons, I prefer the setup where the rim brakes are cabled as normal (one on each brake lever) and the drum brake is controlled by a separate lever (usually a shift lever like a barcon). You can then "set it and forget it": the brake does its thing and keep your hands free to use the rim brakes, shift gears, and steer.
-= Keith Adams =-
Rockville, MD, USA
(Not all that far from Harvey Sachs)
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