Chain line

[Blue Hills]

Must admit (be gentle) that I've never really managed to get my head round this (and my brain recoils from anything that brings back memories of O level maths. trigonometry or whatever) and maybe have been lucky in that in several years of tinkering it has never seemed to cause me any issues.

And the odd bike I have had from new, and so has presumably been designed and set up properly in the first place, has understandably had small issues with certain gear combinations in any case.

So. Does chain line really matter all that much when selecting bits?

I ask as I am building up a hybrid type bike and of course some of the bits insist on thrusting this worrying spec in front of me.

 

[Yellow Saddle]

Yes, chain line matters a lot, especially when putting together a Frankenbike.

I'll explain by extreme example. If you remove the cable from your FD and play around with the swing of it, you'll notice that the cage can only move a certain distance distance left and right. Obviously the limit screws have a say in this but imagine remove them altogether. Still, the FD cannot swing 500mm to the right. It's legs are too short. Also, if you look at the way it swivels, it works like a pendulum. Not only does it go left and right, but it also follows a curve up and down. Like a pendulum. These restrictions obviously mean that the crank spindle (BB) has to be of a length that falls within those limits. Think of a Fat Bike. A standard FD will not be able to operate 150mm from the centre of the bike.
Therefore FDs are supplied with a specific chain line in question. The chain line quoted will be a distance, in millimeters, from the centre of the bike (the centre of the seat tube usually) to the centre of the FD's optimum range. On a double-blade crank, this centre will be exactly between the two blades. On a triple crank this centre will be dead-on on the middle chainring. On modern one-piece cranks (Shimano Hollowtech etc etc) one cannot fiddle with the length of the spindle and therefore you cannot dramatically alter the chain line. However, on older cranks that rely on a separate BB, you could go and fit a stupid long BB - say 123mm on a bike designed for a 103mm BB. In this case the FD will not operate well, if it will operate at all.

Finding the relevant chain lines is another story. Shimano publishes it in its technical brochures and I'm sure other reputable companies do too.

Chain line is also relevant at the rear. You can imagine what will happen if you offset the rear wheel too far right or too far left. You'll cross-chain without even being near those two "illegal gears".

In casual speak chain line is not a quantity but a line that your chain follows from front to rear. On a triple front and a 9-speed at the back, you'll have a theoretical straight chainline when in the middle chainring and in the 5th gear at the back.

 

[Ajax Bay]

If you have a triple chainset the middle chainring needs to be in line with the middle of the cassette. If it's a double/compact then the mid point between the two chainrings needs to be in line. The middle of the cassette is a given for a particular speed cassette so the main variable you have to manage is the length of the BB axle and its interface with the right hand crank. I have seen some good diagrams of this on a previous thread and will try to find and reference it for you.

 

[Ajax Bay]

It's probably worth bearing in mind that the FC-MT60 was designed and manufactured primarily for MTB use, hence the U-brake and cantilevers in the catalogue scan, and the FC-MT60 (MounTain) product code. Obviously the improved gearing range and powerful cantilevers available in the MT range also appealed to touring bike manufacturers, although this branch of users did not need wide chainstays, and correspondingly wide bottom brackets, because they did not use wide tyres. I don't think compact chainsets (e.g. 50-34) came onto the market until much later on. Also note that when comparing chainlines between doubles and triples, the triple is measured from the middle ring, while the double is measured from midway between the two rings. Two other issues to consider are Q-factor (how far apart your feet are when pedalling) and whether the BB length causes the inner ring to rub on the chainstay. However, we're only talking ~5mm difference here, so if it was me, I'd fit whichever BB gives the best chainline for the gears I use the most.

To compare gearing between two bikes, and work out what you're most likely to use on a bike you haven't ridden yet, I recommend this link. Just drag the chainrings and sprockets to your combination (drag any unwanted sprockets/chainrings to the far left and dump them on the sprocket/chainring, or grab extra sprockets/chainrings from the same place), then click the 'compare' button, and enter your second setup.

Gear calculator

 

[Blue Hills]

Thanks all and particularly yellow saddle for that long reply.

Have long tried to ignore this, but gritted my teeth and read the Park Tools online guide - very clearly written as always - I am a great admirer of their maintenance book and have a copy.

http://www.parktool.com/blog/repair-help/chainline-concepts

The calculation showed that the front chainline is apparently near as dammit 45mm, so have ordered the front mech I was first thinking of which was indeed 45mm.

Rear mech as I have set it up is currently working fine and seems to be playing ball with the current front mech so will leave that to its own devices and not worry about it or its effective chainline relationship to the front. For now. One step at a time