Has anyone out there tried running 2 chain rings on a single speed crank? I’m looking at the possibility of using an Origin8 square taper 155mm (or shorter) crank with double chain rings on a training bike I’m building up. Note: I currently use Cobb 145mm cranks on my tri bike (It works well for me because I’m 5’6" with short legs). Just don’t want to spend $300, or more, for a crank on a bike that may only cost a little more than that in total - I’ve got a garage full of bike parts, but no suitable cranks…
That’s a pretty wide-open question. The first answer - does the crank have guiding shoulders for the inside chainring? If it doesn’t the only thing supporting it will be the chainring bolts, which might break eventually as the loads on the inside ring are usually higher. If it doesn’t have a support shoulder I wouldn’t do it. Other than that, the other big question is the centerline-to-centerline distance between the rings. Whatever shifter/front derailleur combo you’re running should match the crank - i.e. 9 sp, 10 sp, 11 speed. Finally, where the chainring sits laterally relative to the FD will also be important. The travel of the FD side-to-side will be matched to its corresponding manufacturer’s crank, in that the inner position of the FD will be lined up with where the inner ring on the crank should be. There is a little forgiveness built in to the derailleur to allow for some shifting of the chainring centerline, but not much.
The other option would be to forego the front inside ring and just use a wider-spaced cassette in the back. For a training bike, combine something like a 47T front with an 11:32 rear cassette and you get a good usable span. It is about the same as a 38/52 front combo with an 12:25 rear cassette, but the gaps between shifts will be wider. You might want to use some sort of chain keeper with it though.
I did see that. What I’m really interested in is the chain ring mounting. That is putting two chain rings on a spider meant for only one (That is shoulder on only one side of the spider arm). It appears that some kind of spacer system will be needed. I’m really wondering if someone has successfully fitted two rings on a single ring spider. Also, custom spacers will also allow for some variation is chain alignment. I have both a lathe and a mill - spacers are easy.
Hadn’t really thought about just using a single chain ring. It’s really a great option, as I live in Florida (Home of the “Flat Earth Society”) I build and sell chaincatchers, so a guide would be pretty easy. I wonder if it would really be necessary though - how often does a chain come off a front chain ring during a rear shift?
you simply use a double chain ring bolt. that’ll hold the other ring in place. it would be nice if you also had a ledge to hold the chain ring there, but i would imagine you’d have to produce an awful lot of power to shear those bolts off. i would imagine it would be important to have pretty good chain ring bolts, maybe err on the side of steel versus aluminum, otherwise those bolts ought to hold your chain ring in place.
Structurally, no problem, even with aluminum chain ring bolts and nuts. There are successful examples of this from big crank makers, most notably Shimano’s Dura-Ace triple FC-7703.
No inner shoulder:
As others have mentioned, check the various dimensions to get good shifting.
If you run a single ring, you want a chain guide. It usually doesn’t come off. Unless the race is the Tour de France prologue (David Millar), or the Elite National Championships (Rebecca Twigg), just to name two high-profile examples.
Thanks Damon. It looks like much less of a problem than I first imagined.
This would be less an issue if there were more really short cranks available. This may happen when the short crank “trend or fad” question is answered. Right now most of the inexpensive short cranks are square taper (vintage or perhaps BMX style) and are intended for a single chain ring. Currently Cobb, Rotor and some others do have cranks in shorter lengths for more modern BB’s, but they make for pretty expensive experiments…
