Bicycle Mechanics - Chainrub with compact crank
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08-13-10, 01:29 PM
I'm pretty much sure what the answer is but here goes.
All 105 (9)sp components with a 105 external 50-35 Crank.
I ride a smaller bike so the chainstays are short. On the 34-12 rings (small, small) the chain catches on the pins on the 50 ring. If I shift up to the 13 the issue goes away. Obviously the chainline has changed.
Has anyone had any luck with shimming the crank out a mm or two?
You will get better resluts not using the 2 extreme gears, small, small and large large. 2 mils might work well.
As you imply knowing already, it's a combination of the chain coming from the extreme outside to the inner. Generally it's best to avoid riding crossed over, but there are situations where you might want to for a while, and IMO a bike should be set up so you can.
You have two choices.
You can modify the chainline but I'd check first to see on which side of dead center it is already. If it's already toward the outside of the cassette's center, you might not want to move it farther out, lest performance on the inner cassette sprockets suffers.
You might also use small shims to increase the chainring separation a hair, but be careful not to add so much that the chain hangs on shifts to the 34t chainring.
In your shoes, I'd start by checking the chainline. If it's a bit inboard, and was easy to correct I'd go ahead and move it out, but otherwise I'd probably just live with things as they are.
08-13-10, 03:54 PM
Its the nature of the beast to get chain rub on the small/small with that set up. Its the 15 tooth difference between the 50 and the 35 that makes in inevitable that you will have the chain rub. I have a 53/39 and mine does the same thing and I've got a great chainline. Just don't use that gear combo.
08-14-10, 04:02 AM
thanks for the replies. I pretty much stay away from the 34-12. I'm not worried about the chainline hurting anything but I hate the noise. It has got to be causing wear on the pins as well.
It is a ratio that I like and the extra shifting to get to a similar gear on the 50 is often a nuisance.
Comapct cranks have their uses but I sometimes think I would be better going back to a 53-39 and just getting a cassette with a few cogs with higher teeth counts.
08-14-10, 07:28 AM
Switch to a cassette that starts with an 11 and/or quit using the small/small. Shimming the crank is not a proper solution, except in the rare case where it's really off.
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