Woha!
Originally Posted by Shaman
"The rear brake is also used as a 'drag' brake for long or fast downhills. Trice provides this option. They too use a frame mounted shifter lever for setting this brake. It sounds like you plan on carrying payloads and if hills are in the territory, a drag brake may be a good safeguard."
Originally Posted by BlazingPedals
"For a drag brake on hills, I would think you'd want a drum brake that won't be susceptible to overheating the way rim brakes are."
Your both right, but rim and disk brakes should be avoided for use as a drag brake on any loaded bicycle. Both will over heat when used as a drag brake. The rim brake can over heat the rim causing the tire to fail, the disk brake will also over heat warping the rotor and eating up the pads. For a drag brake use a Aria drum brake mounted to a tandem hub. For more information see
http://www.precisiontandems.com/arai.htm
The bike in the pirates is set up with drum brakes (Great photo Rodger!) The brakes have been modified to use stub axles, just like many recumbent trikes that use drum brakes.
The christiania "light" weighs in around 77 pounds, then add the rider 160 pounds, and the two kids at 50 pounds. 337 pounds... Can two disk brakes bring this mass to a quick safe stop? Yes, I've tested them in the real world (Organic Engines Tandem trike, two adult riders and 80 pounds of camping gear) About 530 pounds from 32 mph during a down hill run on tour in VT. And you can be sure I caught hell from the stoker for that test!
As for your disk brake question. Get the largest rotor available for the brake your going to use, Avid offers 203mm rotors for their mechanical disk brake. The larger the rotor the better it can dissipate heat. And you will want to protect little fingers from touching the rotors as they do get hot (300+) when used for heavy loads.