Bicycle Mechanics - Chain line calculations.

I recently got Lennard Zinn's book The Art Of Mountain Bike Maintenance. Not a bad book and the lad seems to know his stuff. Anyway, he comments on chain line calculations where you do a number of measurements between down tube and chainset, and between rear drop outs and cassette, etc. After a bit of math and spooky magic, you end up with a number, that providing it's a number of millimeters in a certain range will tell you if the chain line is correctly set or not.

That got me thinking about that. I suppose carefully built bikes at the top-end of the market would be correctly setup, but does that imply that bikes at the lower end of the price range would have a chain line that's sloppily setup?

Further, does it actually matter? I suppose the purist would strive towards that but us lesser mortals would not be too bothered?

Finally, if the chain line calculation is not correct, how do you go about setting it? As I see it, you can't do much at the rear wheel, and the only thing I can see at the bb you could tinker with is whether you can adjust the bb left or right to get the desired chain line calculation.

What idead do you folk have on this?

Being off a bit does net matter except to anal retentive types. Sometimes a person will cob together a combination that is way off and biases the chainline one way or the other to favor large or small cogs.If one uses the BB that is recommended for the crank being used,it's usually ok and that's how its done with all Factory bikes or even shop assembled ones using stock components.One can play with chainline by using different spindle lengths or an adjustable BB. Playing with spindle length is difficult in cases where the shimano splined system gives you 1 choice for a double and 1 choice for a triple in road systems. current campy does not give much choice either, but at least with the ISO taper there are some aftermarket choices available.Shimming the BB is often another possibility for small changes on the drive with Cartridge BB.

As long as the inner chainring barely clears the chainstay under heavy load/flex and the chain barely clears the chainstay in the highest gear, you should have a near-optimal chainline, particularly if you have a triple chainring. Most chainline problems I have seen arise because the chainrings are too far from the centerline of the bicycle, and/or the cogset is too close to the centerline, making the large - to - large combinations run very roughly. (On most bikes, you should avoid using the extreme cross-chain combinations, anyway.)

Your chainline can also be set by way of shimming your chainrings.

There are also some "reality factors" that may call for fudging the chainline. One of them is present on my own mountain bike: I have a Ritchey Z-Max 2.35" rear tire, and if I shift into my inner chainring, the tail of the derailleur rubs lightly on the tread knobs sometimes, due to the size of the tire. Shimming the bottom bracket out a few millimeters and readjusting the front derailleur accordingly would fix this, although I haven't seen the need.

Another "reality factor" is that some derailleurs, particularly when mounted to a large-diameter seat tube, swing inboard as far as they can and scissor shut on themselves, or hit the frame, and still are not in far enough to get the chain onto the inner ring. Again, scooting the BB/crank out is a solution.

Personally, I don't fret about chainline beyond a practical level.