3DPrinting

Had to make the cable pull measuring jig and get it repeatable. Then had to model the interior of the shifter to know where cable contacts the pulley in order to fully understand why the pulley is eccentric and not round. The only valid reference surface is the main pivot shaft and the cable housing stop location is blind.

Then it was a matter of understanding the pawls and their mechanics. This is the oddball 2012 SRAM Red 10 speed stuff that was unique from all the rest. It turns out that the pawls mechanism is designed for the 10 speed 0.120" index standard. On this system, that is around 15 degrees of rotation per index position.

My present 11spd Ultregra 6k8 stuff is very worn out but it pulls around 0.100" of cable and is more consistent than this SRAM Red in factory form, as measured.

I was thinking about burying an adaptor in my bottom bracket shell on my frame. I have space for it internally. However, actually fixing it in the index or barrel sounded more satisfying.

Someone else already made a replacement barrel model and shared it on printables/thingiverse. I printed it and measured it, but the metal index gears protrude into the cable path and disrupt the index at 5th gear. Plus, if you look up ~1.1mm stranded steel cables, they all have minimum pulley sizes that are even larger than the factory original. I've snapped cables inside the shifter with SRAM Red, so this is not my favorite method either.

While reverse engineering the pulley and mechanism, I noticed how it will all work by gravity without the springs holding tension. This counters the intuitive notion that the loads are too high or this is some kind of "tough" thing. It really is not. Almost all of the actual load comes from inside the pawls and how they slide around each other. The index gear is not loaded very much.

I went on a quest to see how much variance the pawls could handle. Like are all the shifters the same and only the barrel changes diameter. That does not appear to be the case. Each unit seems uniquely made. So I started messing with height differences and small off sets to see if I could alter the index pitch with the original pawls and find a combo that still worked with 11+ speeds.

I do not really care about the extra gear. Heck, I am more interested in running dirt cheap 8 speed cassettes and chains that last longer.

Yesterday I discovered the index trick that works. I ditched the gear tooth profile in favor of a more complex tapered and angled triangular ramp. This "tooth" profile works reliably.

Another challenge was print orientation for index tooth details. Printing on supports at a steep angle was the only way the definition was sharp enough. Today I managed to fix that using internal micro structures around each index position and added an assembled part for the spring retainer to get the print orientation optimised. This got me to actual measurable parts in the iterative regime, and it exceeded the accuracy of my mechanism replication jig.

I still want to make the thing removable in whole or part to replace the index without rebuilding the bike. In so doing, I will likely remove the requirement for the road shifting specific cable head, or any head at all. Judging by the wear on cheap prototyping PLA, this will likely last at least as long as a chain(×2)/cassette in printed form. If it can be removed in situ, that is a compromise I am willing to make in order to run any shift system combo or alternative gearing I choose. There is enough travel present to run 12 cogs at 10-speed pitch index, or 14 with 11-speed pitch. It is only a matter of rear clearance spacing... and a mor gearz iz betar disorder I guess. I have several ideas swirling at the moment for how to design an entirely new shifter made for 3d printing. A waffler stack, a bearing detent, or a star ratchet are all possible for a barrel assembly type design from scratch. The cooler option would be a cascading stage compliant mechanism that prints in place and pulls the cable forward instead of requiring rotation. Each index position is only 2.54mm for 11-speed.

I cannot eliminate or make claims about actual accuracy with my little excuse for metrology. I could certainly improve this one with a third iteration. Based on my measurements and comparisons, shifting is only around 5%-10% the same value each time even on factory stuff. I can shift a dozen times and get the same zeroed measured result when shifting back down. But every upshift, the result varies more. Some are clearly bad shifts but most are within a similar margin of precision. I am using ways inserts in the index measurement tool, with mechanisms to control backlash, and three point contact on an internal sled. I also made housing clamps to remove their flex from the equation. Chasing that one... gluing a housing into the ferrule caps would likely make a large difference in shifting performance, especially over time. Motion here has a very substantial effect.

Anyways, that is just my musing on my project so far. I have a 10 speed SRAM Red shifter with 12 speeds of index at an 11 speed pitch. All to avoid the $500+ "proper" (exploding) rear derailleur... again.