I sullied the weapon today with flat bars and yucky plastic canti brake levers, not to mention the crapola plastic Shimano friction thumbies (two lefts).
But this temporary insult was merely to verify that this will shift under load:
As you can see I had to lift the DA derailer a few mm higher to clear the 50t. Works, but I might find a better fit with something else... or maybe I could dremel the cage a bit; the nickel-plated ally cage isn't long for this world anyway...
I probably could've left the drops on it if I was prepared to go the hack on my left Doubletap; the overall throw was pretty close to right, and I suspect the trim position could've been shifted to line up with the 50t... but never mind that.
This whole idea is enabled by my DIY Di2 project; there's no way I'd bother to shift this manually. Concerning the DIY Di2 thing, watch this space (I'll put the link in my tag, which dynamically updates)... I've bought nearly everything I can buy and I don't need to make much, and I have some preliminary code I can test the system with. Initially it'll be a sequential manual (ie, 'sequential' in that I won't have to worry about what ring/cog combo I'm picking, it's just up/down), and then when that's sorted I'll introduce a speed sensor for the rear wheel and a fine-grained speed and position sensor for the crank, and make it automatic, with the buttons selecting a desired cadence, basically like NuVinci control except digital and without the CVT. The half-step goes a long way to addressing the shortcomings of available ratios with a derailer setup (this physical limitation is a result of the packaging restrictions forcing us to use few rings / many cogs instead of the other way round, and the fact that a given tooth pitch becomes a larger fraction of the circumference as circumference diminishes, causing the line on the graph to curve upwards, when we'd prefer it curved the other way, due to the exponential nature of drag)...
The blue numbers (which include the green) are the ratios of the half-step triple, the red numbers are a 53/39 on the same cassette for comparison, and the green is the half-step setup short-shifting.
The X-axis on the graph is the gear number (not that that's a thing in the bike world - yet) and the Y-axis is the chainring/cog (multiply by tyre circumference for gear-inches or whathaveyou).
Looking at a standard double, I saw a fair bit of room for jamming another ring in there, and even without doing horrible things to the chainline. The big rings are about 5.5mm c-c, about the same as 6s cogs. The 52t is outboard by pretty much its own thickness... I started filing the chain-catching pin down, and then realised it was redundant so just punched it out (there isn't room for the chain between the ring and the crank arm). The 38t is inboard 2.4mm. Rather than go to the horrific expense of buying spacers, and particularly since anything over 2.5mm tended to be the wrong sort, I bought a length of 14mm rod for a few bucks and got a mate to turn some up for me on his mini lathe. We could only scrape up a 9.5mm drillbit and I had my heart set on 9.9mm (nice and snug), but it turned out I have a rat-tail file that was perfect to finish the job.
Now the packaging job on the front of the drivetrain is starting to look as impressive as the other end... moreso on a beefy carbon frame with little room to spare.
If it fits on this, it should fit on most things I bet. One caveat is there was no way it was gonna work with my RED Yaw FD, but that's understandable.... so anyway, the specs:
Inner ring: Origin8 10-hole 110/130BCD 38t (the same thing is available with some other names on it, eg Action SS)
Spaced: 2.4mm inboard from spider
Middle ring: 110mm 50t (for a 36t small ring; neither love nor money will get you one for a 38t I bet... seems to shift pretty well though)
Spaced: 3.0mm outboard from 38t (front face of the ring pretty much in line with the spider's front mating face)
Outer ring: 130mm 52t (a plain old-skool ring, since the chain won't see any shift-assist, not that there's any designed for a 50-52t shift anyway)
Spaced: 2.9mm outboard from spider
There's one set of standard chainring bolts holding the 50t on the 38t, and the bolts going through the 38t and 52t rings need to be 13mm between the heads. Such a thing is generally only available as a MTB bash guard bolt in 15mm sets of four... (if you're lucky you might a 15mm set of five, for a Birdy folder or something) or there's the Thorn SS 15mm chainring sleeves in sets of five, available from SJS Cycles. I asked about the last set of red bashguard bolts Moove had on their site, and a nice bloke called Ronan threw in a fifth bolt for free : ) Many filings later, I had 13mm bolts...
There'd be some room to tighten this up a tad; although the gap between the 38t and the 50t is 0.5mm narrower than usual and they're inboard by a chainring's thickness, there's also a smaller diameter difference and the chain can reach the 8th cog on the 38t without catching the 50t, when I only need it to reach the seventh.
A note on the ratios selected: these were chosen of course with my own preference in mind (I'm a light guy on fairly flat terrain) and in consideration of what's commonly available, but IIRC that nice flat curve I was able to obtain on the graph was a happy accident - as I recall (it was a few months ago now) there was nothing else in the generally obtainable range that gave as flat a spread. I'd be delighted if anybody feels like firing up Excel and proving me wrong, though.
A chunk more top-end range could be had by going 11s and using an 11t (bah) or going the other way with a 25t...
Roboshifting allows the indexing to be pretty arbitrary, so maybe you could do a 12s setup using unmodified big cogs on a 10 or 11s carrier, with the other 8 or 9 being separate cogs spaced closer...
And the fact the FD here is hardly optimal can be compensated for by integrating overshifts into the code, and mitigated by phasing the shift with the crank angle. Front and rear shifts will be timed to coincide, using cadence and chain position info, by controlling the delay on front shifts following rear shifts.