Optimal Chainline Question

[davester65]

Hi, i'm restoring an old touring bike fitted with a Shimano triple 28-38-48, this was stock kit on the bike when new. The seat tubing is 28.6mm (reynolds 531ST) and the bb shell is 68mm.

I have found Shimano Tech Specs (see pic) which say 117.5mm BB or 122.5mm BB for the crankset i'm fitting (FC-MT60)

When i fit the crankset to a 117.5mm BB i get a chainline of about 45mm (ideal for a road bike) and when i fit the crankset to a 122.5mm BB (i have both) i get a chainline of about 48mm (ideal for a MTB).

Which is better for a touring bike? Road set up or MTB set up?

 

[Smurfy]

Maybe you should measure/calculate the chainline for the middle sprocket on the cassette/freewheel?

 

[Smurfy]

Have you read the Sheldon Brown article?

Possibly the chainline for MTB is only larger because the seat-tube is a greater diameter, and/or the tyres are wider. Putting this constraint aside, I think the logical thing to do is position the chainrings as per my previous post.

 

[davester65]

Maybe you should measure/calculate the chainline for the middle sprocket on the cassette/freewheel?

Did check which gave the better line when on middle-middle, both very similar and both ok, but 122.5mm set up slightly straighter/better than 117.5mm set up, not a great deal of difference though.

 

[davester65]

Will read the SB article now, Thanks.

 

[Smurfy]

Another consideration could be which end of the cassette you use most. I know I tend to use middle to small sprockets most, and I only venture onto large sprockets for steep hills (EDIT: and often when I'm on the granny ring).

I think a larger offset will increase wear and decrease efficiency. If you check this link, then you can expect efficiency to vary by about up to 10% from best to worst chainline.

 

[davester65]

Another consideration could be which end of the cassette you use most. I know I tend to use middle to small sprockets most, and I only venture onto large sprockets for steep hills (EDIT: and often when I'm on the granny ring).

I think a larger offset will increase wear and decrease efficiency. If you check this link, then you can expect efficiency to vary by about up to 10% from best to worst chainline.

Now you're making me think not good as i'm off work this week and the Rioja is going down rather well

Where i live is pan flat so i ride primarily all to the right, only dropping a rear cog or two for the slight inclines, if i need anything lower i drop to the low front cog and ride middle at the rear (on road bike 2x9) and i agree a large offset will increase wear, more so on alloy (chainrings) than steel (chain & cassette)

Speed read the hupi link, very interesting, will give more time to it when the rioja wears off , one thing that jumped out at me was "Table 4, torque measurements during a pedal stroke" this ties in with the restoration project as it has a Biopace Chainset, i've read plenty of articles both for and against elliptical chainsets and tbh i think there IS something there........why would Wiggo use them for TT rides if there was no gain, even if it is marginal......i'll look forward to doing my regular Sunday 25 mile loop on the biopace to see if there's a noticable difference.

 

[Smurfy]

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.

I only included the Human Power link because I thought you might be interested in how much efficiency is lost due to imperfect chainlines. When using Biopace, be aware that Biopace reportedly does the exact opposite of the Q-rings used by Wiggo, although perhaps it would be possible to rotate the Biopace rings to get a similar effect. Also note that Q-rings recommend 500km of riding to adapt to their rings. At the risk of dragging the thread even further off topic, for the ultimate experience in fluid pedalling and high efficiency chainlines, try fixed wheel. I ride 42-17 all the way, and it gets me up most hills without walking on rides that average 75ft/mile of ascent/descent. If I was living somewhere flatter, I'd probably increase that to 42-16, or possibly even 42-15.

 

[Ningishzidda]

Don't worry about the rear end too much.
Its a touring bike so a ROAD front triple mech should be used. Use the BB for a road triple.

 

[davester65]

Decision made....using some rather odd logic!

I'm going with the 117.5mm BB and a 45mm Chainline purely for the reason most of my riding is done on flat countryside with little or no hills, therefore most of my riding is done on the big chainring, using the set up mentioned above the chain sits straightest (truest?) on the big chainring, as opposed to being straightest on the middle ring on the other set up, which IMO will reduce wear as much as possible.
Thanks for the help and advice Tim...appreciated