Hi Peter!
I realize this isn't an answer to the (now very old) question you asked, but I've played extensively with self-energizing cantilevers and will hasten to say I've found their performance depends on many variables. When everything aligns optimally, I've found them wonderful and prefer them. However, when things don't "click", then I've found their performance falters -- sometimes badly -- compared to more conventional cantilever designs.
For those unfamiliar with them, this brake design was largely applied to cantilevers, though there was a U-brake model was also produced briefly. The brakes bolt solidly to the bicycle's cantilever bosses, and the arms turn on their concentric threaded helix pivotsthat allow the arms to move forward as they move inward. When the pads contact the rim, the forward motion of the wheel helps draw the arms further inward, helping apply the brakes. The result is a bit like "power-assisted" brakes on a car, but is progressive and non-linear in action, which can lead to unpredictable results especially if one is not familiar with this action. The helix grooves were non-linear as well (pitch angle varied between 20°-70°), and the self-energizing action depended on a number of factors including how much return-spring preload was applied. Adding to the complication, the brakes were position-specific (i.e. Front and rear had opposite helixes) and the brakes didn't always play well with certain fork combinations, as seen here:
http://www.johnforester.com/Articles/BicycleEng/sebrake.htmThe patent drawings are here:
http://www.google.com/patents/US4974704I've owned numerous examples of four generations of the Scott/Pedersen self-energizing cantilevers, and how well each worked depended on a number of factors. The all-metal, long-arm, wide-profile early version remains my favorite cantilever of all time and is still mounted on my blue rando bike. The later versions (including those with composite resin helix covers) became much less effective as they were retooled with reduced leverage to compete against low-profile versions of other brakes. Arm length varied from version to version as well as the helix angle and its pitch (on my samples, anyway), which determined the amount of self-energizing behavior.
SunTour licensed the design for rear brakes (
http://www.flickr.com/photos/steel-is-real/3428197708/sizes/l/in/photostream/ ), but never produced a front version under their name out of liability concerns, fearing a front-wheel lockup might occur. On my tandem, I run the SunTour version on the rear and the Scott/Pedersen version on the front. Careful tuning and optimization has prevented front lockup in my use, but I can see it as a possibility.
As mentioned, a lot of factors influence how well these brakes work in a given application. Most of these apply to the SunTour versions as well as the Scott/Pedersens:
• Brake boss post width (can't do anything about this except switch to a version with different arm angle/leverage)
• Rim width, when combined with a given brake boss post width (affects angle of approach)
• Helix rotation as a byproduct of spring preload (a matter of setup adjustment)
• Arm angle (low-, medium-, or high/wide profile -- depends on the model selected)
• Helix pitch, which varied between models on the examples I own (again, depends on the model selected)
• Straddle cable length and resultant pivot/arm/straddle cable angle/variations in leverage (can be adjusted during setup)
• Pad/rim friction coefficient
• Pad mounting stud offset (how far the brake pad mounting studs are placed in the arm mounting hardware during setup)
• Amount of pad toe-in (adjustable in setup
• Presence of a brake booster. These self-actuating nature of the brake can really stress canti-bosses, and using a booster can help. I bent-and-brazed my own for the tandem using a section of arc'd and flattened seatstay tubing. It really increased braking efficiency on the Big Bike.
My best success came when mounting a pair of early all-metal Scott/Pedersen SEs front and rear on my blue rando bike. I used low-profile Mathauser salmon finned-back bonded (glued) pads and the bike had closely-spaced brake mounting bosses. I'm running narrow Mavic MA-2 rims, and the bike was converted from 27" wheels to 700C, which are 4mm smaller in radius, allowing the pads to sit closer to the brake pivots (I reversed the mounting hardware on the rear and notched the pads to get them even closer to the pivots on the front). Straddle cables are set for 90°.
My next-best success was on the tandem with wide-set brake bosses, Sun CR18 rims, and Mathauser salmon cartridge pads with low-profile Scott/Pedersen SEs and straddle cables set at 90°.
My worst-performing example was on the Miyata 1000LT, using medium-profile Scott/Pedersen SE's running Kool-Stop salmon pads near the top of their brake-mounting slots with straddle cables set at 90°.
Self-energizing brakes are really fiddly and time-consuming to set-up and require even more time to set-up optimally. As Freddered mentioned, they are horrible to adjust in the field. I can see why they are no longer made, especially when simpler solutions gave largely equal or better results. That said, when the stars align, there's nothing like the feel of a "perfect" self-energizing setup. They're easy on the hands, producing great braking force with little finger pressure, and the progressive response is positively addictive -- especially with a heavy touring load.
Peter, I'm guessing yours may be a case where things just didn't come together to give the best results, but if you want to try making adjustments in one of the bulleted areas noted above, the improvement can be remarkable. Just one factor can be "off" enough to make all the difference in the world. Finding which one is the real challenge!
Did you stay with the self-energizing SunTours or did you change? If so, what brake did you choose?
All the best,
Dan. (...who is usually self-energized for a mechanical challenge)
