See also also bike pic for other bike-related technical discussions and failed parts.

Positech Dual-Leverage Bicycle Brake

Summary:

The brake starts at low leverage. When the pads touch the rim and cable tension goes up, it switches to high leverage.
Low leverage means the pads can be far from the rim, so there is no rubbing when the brake is off.
High leverage makes it possible to:
- use a pad material with similar performance wet and dry, so the brakes work well in all weather;
- which is long-wearing, so maintainance is reduced;
- and use long-wearing chromed steel or stainless steel rims, so maintainance is reduced.
The leverage switch is based on force and not on position, so the brake does not need to be adjusted as the pads wear.

Documents:

Background/motivation from David Gordon Wilson. "CPSC Proposals", HPV mailing list, 2005/05/22.
Background/motivation from David Gordon Wilson. HPV digest V2 #175, 1995/03/29.
Positech, in "The Brake That Got Away" (D.G. Wilson), Velo Vision 3 Sep 2001, p16 http://www.velovision.co.uk Velo Vision web site (2006/07). [cached]
Human Power, Technical Journal of the IHPVA, #53 Spring 2002. See pages 15-17.
Hanson's Thesis, June 1971 "Wet-weather-effective bicycle rim brake: a product-development exercise", from http://dspace.mit.edu/handle/1721.1/12572 as of 2010/03.
Wilson/Hanson/Malarkey patent 1972: US003870127.1.tiff, US003870127.2.tiff, US003870127.3.tiff, US003870127.4.tiff, US003870127.5.tiff
Amrstrong patent, 1975: US004015690.1.tiff, US004015690.2.tiff, US004015690.3.tiff, US004015690.4.tiff, US004015690.5.tiff, US004015690.6.tiff, US004015690.7.tiff
Both patents, as a PDF.
"The Positech Brake", Fred DeLong, Bicycling! magazine, 1976-11 (v17, n11), pp. 38-9. Positech-DeLong-Bicycling-1976-11.Page-38.jpg, Positech-DeLong-Bicycling-1976-11.Page-39.jpg.
Brake-related parts of Ron George's interview of David Gordon Wilson: ron-george.2009-04-27.html.
Look Patent Aplication for a force-sensitive variable-leverage brake using the same basic mechanism as the Positech.