abrinton

Hi everyone-

I've recently built up an old Peugeot frame with parts from my old commute bike. It's got a 105 double drive train with an 8 speed cassette.

It rides great, but shifting performance is not so good. In the small ring, everything is fine across the 8 speeds in the back. In the big ring, things are ok in the 3 smallest cogs. When I get to the 4th cog, I get some chatter in what sounds like the rear derailleur. Going up from there it gets worse. More noise, and sometimes the chain slips down a cog in the 4th and 5th smallest. While I know cross chaining is bad, on my old frame I could cross chain it all the way up to the big-big combo without any issues. Does this sound like a chainline issue?

So I read the sheldonbrown.com page on chainline. I measured 45 in the front and 40 in the back. So does that mean I have to move the cranks in by 5mm? What's needed, a new BB? Also, the rear spacing is 126mm, I've got a 130mm hub in it. I've read this is no big deal, but will it make a difference to the chainline?

Thanks for any help!

Adrian

DMF 

Yes.


Yes, unless you happen to have an adjustable one.


Yes, but it should be accounted for in your measurement. (You did measure with the wheel on?)

gmason

Teach me something about chainlines please.

If one does install, say, a 111 instead of a 115.5 BB with a triple, that moves the chainline in, I think. But whichever direction it is displaced, doesn't that only become an issue of which cogs can "safely" be used with which rings? If so, can you simply adjust that thinking and just ride? In fact, wouldn't it only create a one cog offset from normal operations?

abrinton

Yup, I measured it at 130, the spacing of the hub. I read another post where someone said that the same cranks, bb, cassette and wheel should give you the same chainline on every frame. Is the 5mm the difference in rear spacing between the old frame and new?

Also, does the issue with shifting sound like what would be caused by the chainline being off?

San Rensho 

First, check your chainline. Tie a long piece of sewing thread to the front of the big CR, pull it back towards the cassette. With the string taught, slowly move it so it just touches the back of the CR. You now have a straight line up of the big CR with the cogs on the cassette. If it lines up with the fourth cog starting from the smallest, your chainline is ok.

If you have to move your chainline in, you can also remedy the problem by respacing your hub to move the cassette out, closer to the dropout. Switch the hub spacers/washers from the cassette side to the non drive side to adjust. You will also have to re-dish the wheel. But many times, this is easier than messing with BB spindles.

DMF 

As far as I understand it, yes. But "riding on" might not be as convenient as one might imagine. Do you want to give up the little cog on the big ring, or the big cog on the little ring? Also realize that there is a rear dérailleur in between the two that is expecting certain chain angles. I'm not sure I can explain why, but in cases where I've had the front chainline misplaced by as little as 2.5mm, the rear shifting was noticeably degraded.

DMF 

Seems me that this is okay as a quick check, but that the measurement method is considerable more accurate. Have you compared the two?

Otoh, the string method would be independent of the frame alignment. Hmm. Good idea. I like that.

DMF 

They should, if everything is square and the dropouts are centered on the frame. It's possible that the extra 4mm spread you're getting is all from one side, or that the dropouts just aren't square. Try the string method to check dropout alignment without, then with, the wheel in place. Run a string from one dropout around the head tube to the same place on the other dropout. Then measure the distances between the string and the seat tube. Both sides should be identical.

Yes.