Wow, Bill. That is an inspiring (and, for those like me who lack the imagination/skills to conceptualize such a thing, humbling) winter project. Pics along the way, please, please, please. And good luck!
From: rbw-owners-bunch@googlegroups.com [mailto:rbw-owners-bunch@googlegroups.com] On Behalf Of Bill Lindsay Sent: Thursday, January 08, 2015 11:31 PM To: rbw-owners-bunch@googlegroups.com Subject: [RBW] Re: What's your winter project? OK, this might be the weirdest winter project I've ever taken on, but I did put in some serious design work today, and the concept is actually taking shape. This Winter project is that I am going to build my own desmodromic rear derailer. What the heck is that? A desmodromic mechanism is something that is actively driven in all directions. The most well known desmodromic mechanism, and what you'll see if you Google the word "desmodromic" , is Ducati valves. A rotating cam throws the valve open and another rotating cam throws it closed. There is no return spring. It's driven open and closed. Some classic old French derailers had a loop of cable that, like a push-me-pull-you, would drag the derailer to the left and to the right. The parallelogram had no return spring. Much more recently, White Industries made a desmodromic shifting system, called the LMDS (Linear Motion Derailer System). Look it up, it's pretty swank. So, what I've got scoped out, is that I'm going to take my favorite stock rear derailer (RD-M760-GS low normal), remove the spring, and set it up for a secondary cable. The actuating mechanism of our modern derailers is a parallelogram with a pretty complex geometry. Those of us used to friction shifting with lots of gears can attest to the fact that the derailer responds differently at different places in the sweep of the shifter. For me the incredibly sensitive spot is dropping from the biggest cog on the cassette to the next one. You just touch the shifter and it moves. It's super sensitive. I did all the measurements and calculations and now the data tells me why. The geometry of the parallelogram demands it. So, I need a shifter with a profile that isn't round. It needs to be a profile that complements the geometry of the parallelogram of my derailer. Let's say you want to move the shifter 120 degrees to sweep all the way from cog 1 to cog 9. Then, ideally, you want every one of those 8 steps to be exactly 15 degrees of movement. With the right shifter profile, that's no problem, if you can do the math, then do the design, and then have a manufacturing method that can produce one of them. This is where 3D printing comes in. The really complex math exercise that gets you to the shifter profile that pulls the RD-M760-GS in this direction gets repeated once you figure out where to put a secondary cable to pull the derailer in that direction. Those profiles need to also have the property that they counterbalance each other so you never get even a bit of slack in the cable, allowing you to actuate both cables with one shiftlever. Push me pull you. That's why White Industries made the LMDS on sliders. They at least made the derailer motion linear, and the cable motion linear, so the math wouldn't be so GNARLY. Me, I don't plan on building a derailer, and I don't mind the math. Today I worked out the GNARLY math and have both cam profiles. I now need to strong arm my brother in law to translate my numbers into a solidworks model. Once that's done, it's trivial to 3D print this weird cammish shifter, and try to hook up a DIY desmodromic rear shifting system. The real key compnent is to modify an existing shifter design that has adjustable and equal friction in both directions and fits into a really small space. I'm going to base my design on this Cheapo Simplex model. French AND Cheapo<http://www.ebay.com/itm/like/380734533172?lpid=82&chn=ps> If I can emulate that plastic inner shifter piece, but implement my optimal cam profile, and figure out a cable fixing mechanism to prevent slipping at the shifter, then getting it 3D printed is practically free. Why do it? Well, Jan goes on and on how much the rest of the world wastes energy pushing against a spring to shift in this direction and having nothing to do with the shift in that direction because only the spring is doing it. I just wanted to see what it is he's fussing about. I don't want to buy a 1930s Singer or Herse for $15000, and I don't want to build a totally custom $20000 bike like Jan did, but I do want to explore. I don't even want to spend the $400 it takes to buy a WI LMDS on Ebay. But, if I can spend $50 and a number of hours exploring something in detail that I had previously just taken for granted, that amounts to a Winter project for me. On Wednesday, December 17, 2014 at 2:37:11 AM UTC-8, ascpgh wrote: Riding, besides my commutes, get a bit scant from here on out through winter. The season, the holidays, more deliberate prep for a trip in the conditions, plenty of things distract from just a nice ride but I realize that at this time every year I always seem to contemplate a bike project to go with the anticipated springtime, fruition or not. It's biking fun for the extra bandwidth. My project is a low trail, 650b wheeled, all-around bike made with a lively tube set (versus sturdy for touring) with drop bars, center pull brakes, generator hub, LED lighting. All on a budget recognizing the value of experience, unlike the box bike/mass market interpretation, handmade wheels versus machine made ones as an example. I am reading and including many posting subjects and items in this project and admit that may not reach reality, but it's fun to have on the drawing board. Andy Cheatham Pittsburgh -- You received this message because you are subscribed to the Google Groups "RBW Owners Bunch" group. 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