Bicycle Mechanics - No such thing as a stupid question. Right?
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09-20-11, 09:50 AM
I have a chain minder / watcher thing on my seat tube that helps guide the chain onto my granny gear. I'm dropping from 42 to 26 right now and the plastic shows some wear so I assume it's working from time to time.
My question is do they make a ring or a granny with a ring that would do the same thing? It wouldn’t be fixed to the frame but would rotate with the chain rings. Something like a bash guard on the outside but on the inside. It seems to me something larger than the granny traveling at the same speed and completely surrounding the granny with a slight taper / conical would bring the chain into place better.
I have searched around for something like this and am about to fire up the lathe and make one to find out as I haven't seen one for sale. Or am I missing something as to why it won't work?
09-20-11, 10:01 AM
I've never seen what you describe so I assume there is no commercial version. The only problem I can see is clearance between the bash guard and the drive side chainstay. You have to off-set the guard ring enough to let the chain fit between it and the granny ring and there may not be enough room for it to rotate.
09-20-11, 10:15 AM
Also, as it is rotating at the same speed as the chain, I could see the potential for the chain to try to shift to it instead of the chainring unless it had a diameter significantly larger than the ring which would, as noted above, cause clearance issues.
09-20-11, 10:50 AM
As long as the chain cannot get trapped between it and the granny gear, it sounds good. I like the idea. Seems like we would of seen it somewhere by now.
09-20-11, 11:03 AM
errrrrrrmmmmm I thought this is why triples existed?
09-20-11, 11:05 AM
I'm not sure if you understood the significance of HillRider's comments. This inside-chain-guard needs clearance between the crankset and the right chainstay. On most bikes, they already have major clearance issues and chain alignment challenges on the frame so they add a recess on the drive side of the right chainstay for the crank to clear, and they add another one just an inch further back on the inside for the tire mud-room. I've broken 2 frames right in this area because the deformation focuses stress here. So if there was enough clearance for an inside chain guard, it's unlikely you have proper chain alignment, unless you're using a 2-ring compact mtb crank. But clearance might still be an issue then for any ring over a certain tooth size (e.g. 28t). This is aggravated by composite and aluminum frames that require thicker walls and larger diameters to equal the strength of equivalent steel in older frames.
I don't know if they make such a thing, but one argument against one is that it would use up critical chainstay clearance, and probably force the entire crankset out 4or more millimeters.
09-20-11, 01:25 PM
I know it's very close on some bikes and that is most likely why it hasn’t been done. I just put a triple mtn crank on an old Schwinn le tour for a friend and the heads slightly rubbed the chain stay. I had to tap the tube in ever so slightly to make clearance. So in that case I answered my own question. I know there was quite a bit of space on the double as the clearance was such to clear a 42t so if it was a 30t or something I think it would work on a double on that bike anyway.
I don’t have anything to look at here right now but when I get home I will take a look at my tour bike and a couple mtn bikes and see what clearance there is. Seems like if the frame was being made with something like this in mind then it would work. But that’s not most likely about to happen.
Thanks for the replies.
You usually need to run a longer bb to handle a triple. The rub was a symptom of having too short a spindle on your bb.
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