Singlespeed & Fixed Gear - chainline using middle ring of road triple crank?
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01-19-06, 12:57 PM
Hey all, I've got an old 10-speed Schwinn that I'll be converting to a fixed, and one of my crank options is to use a Campy Mirage triple crank. My brother shredded the chainrings riding with an old chain, but the crank and 115.5mm BB are both in good shape. I'll just need to buy one chainring - a non-ramped Salsa or something - and I'm set.
I'm planning to use a rear track hub with 120mm spacing; fits the frame spacing that was originally designed for a 5-speed freewheel.
What I'm wondering is: if I put the chainring where the middle chainring would have gone on the crank, then with the 115.5mm bottom bracket the chainring would be lined up with the middle of the cassette on a 130mm road hub. Will this chainring then also line up with a typically-placed track cog mounted on a 120mm track hub? Or, would I need a bottom bracket with a shorter (or possibly longer) spindle? (Or, would it look a lot nicer if I put the chainring on the outside of the crank, where the large chainring would have gone, and used a much shorter spindle bottom bracket?)
Thanks for any help here. I've searched but haven't found anything along these lines. Even Sheldon's site isn't much help on this sort of question.
01-19-06, 01:01 PM
It's not really an internet question unless someone did the exact same thing using the exact same components, or if you have the exact dimensions of both hubs, the freewheel, and the cog. The best thing to do is just try it and see where you need to go from there.
01-19-06, 01:09 PM
Well, my question could be phrased more generally, to be web-friendly. I think it's pretty straightforward actually.
Would the middle chainring of a road triple and its intended bottom bracket (intended to line up with the middle of the cassette on a 130mm freehub) line up properly with a track cog on a 120mm track hub?
I'm assuming that most 120mm track hubs have the cog in about the same place.
01-19-06, 01:11 PM
According to this page (http://www.sheldonbrown.com/bbsize.html#chainline) Road triples usually have chainline of 45mm measured at the middle ring.
Most track hubs have 42mm chainline.
Therefore to get proper chainline you'll want a slightly shorter BB a 111 should be pretty close, or use a hub with 45mm chainline.
01-19-06, 01:18 PM
are you shimming the 120 mm hub? I don't understand. The middle chainring lines up with the middle cog of a 130mm spaced hub, and you need to know how this correlates with a 120 mm track hub?
FWIW, when I did my GFs conversion on a 130mm spaced bianchi we had a 120mm hub with a really long axle. I used a road crankset, a 110 or 109mm shimano cartridge BB to replace the 120mm+ loose bearing OEM unit, and ran the chainring on the outer position of the crank. Then the hub was shoved over to the driveside with spacers to get perfect chainline before I re-dished the wheel to center it in the frame.
01-19-06, 01:27 PM
Thanks for the link, Mattface. That's helpful. And for me to figure out how chainline is actually measured.
AfterThisNap: As noted above, I'll be using a frame with 120mm rear spacing. So I won't need to shim a 120mm hub.
My point about the middle chainring of the campy road triple being designed to line up wtih the center of the cassette on a 130mm-spaced road hub was just specifying the chainline for which the crank was designed, and asking how that would compare to the chainline on a track hub.
matt is correct about 45 being the typical middle ring measurement. this, of course, assumes that you are using the crankset paired with its intended bottom bracket. you cannot really assume much about a particular setup. without digging into the depths of my databases of various bottom bracket and crank chainline information, i can suggest something simple: measure the actual chainline to the middle ring. subtract 42.5 from that (42.5-43mm is a more realistic measurement for a track chainline once the cog is on). assuming that your current spindle is symmetrical (and the one that you plan on buying is, as well), multiply the number that you just got by 2 and subtract that from the length of your current spindle. this will be what length spindle/bb you will need to get.
example: say you measure the chainline to be 45mm. subtract 42.5 and you get 2.5mm. multiplied by 2 (since your current bottom bracket is symmetrical), you have 5mm. you say that your current bottom bracket is 115.5 (again, assuming symmetry. very minor and obvious adjustments to this procedure need to be made if it's not symmetrical), so you subtract 5 and get 110.5 as your ideal length. of course, that's a strange size, so you might want to go with 110.
as a note, the 110 shimano un53 and un73 are not symmetrical and are effectively 107mm (only difference is on the non-drive side).
01-19-06, 01:49 PM
110.5 is actually a common size for campy bottom brackets though, and you'll want an ISO taper which Campy has, and Shimano doesn't.
01-19-06, 01:52 PM
those measurements never worked out perfectly for me in the geared racebike world for some reason, though luckily most customers bought the entire gruppo. I'm still a fan of putting things together and buying parts to accomodate what doesn't fit (excepting wheelbuilds)
01-19-06, 02:09 PM
Yeah, I've got some UN-7X's sitting around, and noticed that the 110 was asymmetrical and that it's a 107mm BB for chainline purposes.
If I've gotta get a new BB, I may just sell the Campy crank and BB on eBay, and cobble something else together. I've got lots of Shimano-compatible (JIS) bottom brackets, but no other Campy-taper (ISO) bottom brackets.
01-19-06, 06:50 PM
Worrying about chainline on a conversion is pointless until you actually try it out. Waaay too many variables to take into account. Just throw it together, measure, and figure out how much spindle you need to lose.
Check your LBS for a spindle too, if you need it. They usually have all kinds of old parts laying around.
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