![[VARIABLE+LEV+BRAKES_WILSON.jpg]](VARIABLE+LEV+BRAKES_WILSON.jpg)
From [https://www.georgeron.com/p/my-interviews.html] as of 2025-05-05
More on the Positech brake: Positech Dual-Leverage Bicycle Brake.
See also also bike pic for other bike-related technical discussions and failed parts.
Just the Positech content from https://www.georgeron.com/p/my-interviews.html as of 2025-05-04.
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DGW: I spend a huge amount of time developing a wet weather brake. Me and my students worked on a wet weather braking system based on Raybestos, a material with a remarkable property of having nearly equal dry and wet coefficients of friction. This was at a time when we had almost zero braking in wet weather.1 My students and friends and I labored for around ten years and produced a brake that was almost perfect (I'm rather modest about it). It worked as well in the wet as in the dry, it was self adjusting,2 would fit in the same place and with the same brake levers as regular brakes, and the pads would last almost for ever. (I used a Positech brake for several years as the only brake on my first recumbent at a time when I was riding around 10,000 miles per year.3 It never needed new pads, and only one cable adjustment was required in that period) We worked with three companies to try to get it adopted. A lawyer who read about the brake in Bicycling petitioned the CPSC to have its performance specified on bicycles. The bicycle industry didn't want this, it dithered, and then suddenly switched to aluminum rims to avoid having to use our brake. So now we have rims that, if used long enough, will certainly explode4 - I have had six do so in my lifetime. If you are unlucky you could be converted into an instant vegetable for life. The bike industry knows about this but is apparently uninterested in doing anything about it.
Q. You sound very irked about this issue.
DGW : Absolutely. It irritates me that in countries where there's little regulation, the biggest changes to bicycle designs have to come through the ambulance chasers - the lawyers. The lawyer route is expensive. First someone prominent has to be killed. Then years of killed and seriously injured,and then usually the case is settled out of court. I have frequently been involved as an expert witness, and it is not pretty. Shimano's brake designs were appalling during the time. Having found my own family in peril (I have written about it here), I wrote to Shimano advising them to revise their design and consider some of my own recommendations. All I got for my trouble and concern were a whole lot of nasty letters from US enthusiasts who felt that I was damaging the sport through over-concern on safety and implied that I was a muddled academic who was somehow responsible for my own family's life-threatening experience.
Q. Could you tell me more about these brakes?
DGW : The concept for the brakes was developed and patented by Brian Hanson and me, and we built many prototypes and I hawked them around to try to get them picked up by bike or brake companies. Positech, mainly Allen Armstrong, a super designer from Arlington MA, heard about the brake and we happily came to an agreement to work on the concept together. He improved on our design, patented it and made about five prototypes. We tested them thoroughly - amazing performance, even on steel rims - and sent them to various companies for their tests. The brakes performed equally well in the companies' tests. No company wanted to take out a license or to buy the brake rights. The only result was that a prominent company tried to design something to get around the patent and called it something like a double-leverage brake. Also we spent a huge amount of time and money. I used one of the prototypes for many years on my first SWB, which had a highly loaded Moulton 16 × 1-3/8 steel wheel, and it worked beautifully wet and dry. The hard pads we used lasted for years.
A quick summary of the picture above: pulling the cable pulls on lever 9. It would pivot around 10, but the strong spring 17 resists. So instead, the whole arm 7 slides on post 8, compressing weak spring 16. For each 1 mm of cable pull, the arm slides 1 mm and thus moves the pad 1 mm closer to the rim.
When the pad touches the rim, arm 7 tilts slightly, causing it to jam on post 8 and stop sliding. Then, cable force causes lever 9 to pivot around 10. That pushes on the pad at 11, but now the cable has about 4:1 leverage. Thus, for each 1 mm of cable pull, it moves the pad only about ¼ mm, but it pushes the pad on the rim with about 4x the cable force.
Some notes on the article:
1 "Almost zero braking in wet weather"
At the time, rims on high-volume bikes were often chrome-plated steel. They were cheaper than aluminum rims, and durable — often giving decades of service in daily all-weather use. However, chromed steel rims are often heavier than aluminum rims, and wet weather braking is often much worse than with aluminum rims.
2 The phrase "self adjusting" is tricky.
For most "self adjusting" brakes, as the pads wear, the brake adjusts itself so when the brake is released, the pad holders are closer to the rim. For example, if the pads wear 1 mm, the pad holders move in 1 mm, to make up for pad wear. This is common for car brakes, and is used for some Raleigh brake levers.
For the Positech, adjustment does not change as the pads wear. Instead, the leverage change point moves. This uses some lever travel, but it is the low-leverage/fast-moving travel, so 1 mm of pad wear "feels" like (say) 0.25 mm of pad wear.
From the rider's point of view, they are similar: you can run a high-leverage brake, but without needing frequent manual adjustment.
However, they are technically different. One reason I mention it: I was initially confused by Wilson's description "self adjusting". Wilson explained (private e-mail) he simply meant the leverage change point moves as the pads wear.
3 “10,000 miles per year” deserves more explanation.
Wilson lived in the Boston, USA area, and did most trips by bike, year-round. Boston has frequent snow in the winter, but is warm enough that it is often slushy — wet, and dirty with road grit. Similarly, there are frequent summer rains.
Weather can have a big effect on rim brake wear, and on how well the brake works. Wilson's “10,000 miles per year” represents all-season riding, and not simply riding when the weather is good.
In turn, "never needed new pads, and only one cable adjustment" would compare to faster all-season wear with a conventional brake.
4 "now we have rims that, if used long enough, will certainly explode"
Chromed steel rims tend to be durable in all-weather braking, and it is rare that the brake track wears through. In contrast, aluminum rim brake tracks can wear quickly, especially with a lot of wet or muddy braking — I have known several people who wore out brake tracks in a few months from new. It is somewhat common that the bead breaks off, and the tire goes flat suddenly with a loud BANG. And sometimes with a serious wreck. For more see [http://pardo.net/bike/pic/fail-026/000.html] and scroll down to "Here are some rims with worn brake tracks".