Chainline Adjustment
 

I just had a tuneup and freehweel/chain replacement done on my road bike, and when I was riding it home, I noticed a bit of chain noise when I was on my smaller chainring (39T) and smallest sprocket (14T). The bike is a double chainring with a six-speed freewheel.
When I got it home, I realized that the problem was that the chain, a Shimano PC-870, was rubbing on the inside of the larger chainring. I know that, strictly-speaking, I am not supposed to be riding on my smaller chainring and my smallest sprocket, but I have done this all my life with no problem. My question is: Are there spacers available that will bring my larger chainring out and away from my chain when it is in this extreme position? The crank/chainrings are Campy Veloce.

LeTour makes 0.6mm chain ring spacers for converting 10 speed rings to 9 speed spacing. Branford Bike sells them.

You answered your real question, don't ride in that combo. Understand that too many spacers between the chain rings will potentially allow the chain to ride up on top of the small ring's teeth in a vertual "neutral". Andy.

If you didn't have this problem before, nothing that the shop did should have created it, except maybe replacing the chain. Chain line is built into the system, by the relative positions of the crankset and rear cassette, neither of which get moved in a tube up.

I call your problem "shadowing" because it's comparable to a short person being in the shadow of a taller one if he stands too close when the sun is behind the tall one. You can separate the rings slightly, but run the risk of the chain getting trapped between the rings or riding the top of the inner while leaning on the face of the outer.

If you only get a bit of slight shadowing on the smallest sprocket, living with it is the smartest action. If shadowing is affecting more than one rear sprocket, then a narrower chain, or more chainring separation might make sense.

Thanks Drew & Andy. F.B, I also has the freewheel replaced. I had a Suntour Alpha on it before. Now, it is a Shimano MF-TZ20. I don't know if the Shimano might be a tad wider or the smallest sprocket located somewhat to the right of where the Suntour was. I don't have the old freewheel to contrast it to. The Suntour components were original equipment on the bike, but I have used other Shimano freewheels on this bike before with no problem. This model is something new to me. Oh, I made a mistake earlier: The PC-870 chain is not Shimano, but SRAM. I'll see what another bike shop says. I can't believe they missed this noise. I bet they did not bother test riding it. I appreciate the advice.

"I can't believe they missed this noise."

It probably would not have occurred to them to test it by riding cross-chained.

They might have road tested, and heard the rub. But shadowing on the outermost sprocket is par for the course these days, so they would have discounted it

The standard cure for this type of chainrub is a bottom bracket spacer that goes between the bottom bracket and the frame on the drive side of the bike. These are available in 1 mm, 1.5mm, 2mm etc. thicknesses and will move the crankset outward by the thickness of the spacer. These spacers are inexpensive and very easy to install if you have the tools to remove the crankset and boom bracket. They work on conventional bottom brackets, not sure about outboard bearing bottom brackets.
I agree with the others that if you did not have this problem before the tune-up you should not have it now.

Unless you have a need to ride in the small-small combo, I'd just stop doing it. Problem solved!

It's not a matter of permission (not supposed to) but of mechanics. You say you've done it all your life without problems, but if you rode that combo very much you would have worn out chains and that cog significantly faster. In addition there's likely a combo with the large chainwheel that is close to the same gear ratio as the small/small. Finally, adding spacers between chainwheels, as with any change to the original specs) is not a one-dimensional proposition. Yes, it will avoid the rub on the chain. It will also increase the distance the front derailleur has to push the chain, increase the chain angle on the large-large combo and as previously mentioned may cause the chain to slide in an in-between position (or even jam between chainrings).

Thanks again to all. I'll try to find some better gear combinations and "escape the grind".:)

:thumb:

Beyond avoiding "the grind", efficiency and chain wear/life are other good reasons to use the bigger ring(s) with bigger sprockets in the back when possible*. If you don't find yourself using the big ring as much because the gears are too high, try substituting a smaller ring. I'm currently loving the heck out of this 48/39 crank -- I can spend a lot more time in the 48T than I would in a 52/53T, and shifting is easy despite the lack of ramps and pins.

http://i36.photobucket.com/albums/e2...nchi/48_39.jpg

* The big-big combination is somewhat of a no-no, although generally not fatal.

applies here too..

OK, not that you've posted a picture and mentioned the new chain, I can ell you what changed causing the problem.

The new chain has bulged outer plates making it a bit wider than chains with flat outer plates, which your original chain probably had.

I'm not as big a bear on riding crossed over as others here, and often do it (both ways) when the situation makes it convenient, such as approaching a steep changing climb from a flat or shallow grade. Here I prefer shifting the front, and tiding small/small, so I'm ready for progressive downshifts as I climb.

Odds are that if you switch back to a flat plate 8s chain the rubbing will be eliminated.

Was the ST freewheel a standard width or an ultra width? The Ultra width was a method of cramming an extra cog into the (almost) same width as a FW with one less cog but "standard" spacing. So an Ultra 6 was about the same width as a standard 5 and an U7 about the same as a standard 6. Shimano 6 speed freewheels are/were all standard widths. So it is possible that you old FW was narrower then the new one.

Add the wider chain (good call Francis) and that mechanics tend to test ride a bike so it works well so avoid cross over combos and it's no surprise that nothing was mentioned. Learn to use the gears better and you'll do fine. Andy.

Thermi: your chainring combo is wild! I had never seen anything like that!

F.B: The side plates on the SRAM plates are indeed "bulged". Thank you for pointing that out. The chain I was using before had flat plates (a Sachs/Sedis Powerchain).

Andrew, it was a regular, old Suntour Alpha, not ultra. (I am not sure how close the tolerances are with the Chinese Shimano, though. It is not even a hyperglide.)

Thanks to all.