Why is chainline so important on retro bikes?

[gtRTSdh]

This is what's actually important about chainline.
a) front mech has to be able to get close enough to the seat tube to move the chain onto the smallest chainring.
b) chainrings must not rub on the frame
c) cranks must not hit the seat stays or swingarm

The wider the BB, the worse the Q angle is of the riders legs, the bottom bracket is less stiff and the balnce of gears where the chainline is good is moved outboard of the cassette.

Realistically 10sp chains suffer incredibly reduced life spans due to their reduced width.

The chainline does not change with regards 8/9/10sp as the cassettes are all the same physical width.

 

[NeilM]

FMJ got there first.

If the chainline is too small, then you risk grinding the granny ring on the frame, although you will be able to get the chain onto all three (or two if you're so inclined) front chain rings. If the chainline is much beyond 50mm, then chances are you won't be able to get the chain onto the big ring, as you don't have enough cable pull on the shifter and because of the action of the front mech.

 

[Rob H]

Thanks for all the comments you've given me a fair bit to think about.

But Rob, there is a another thing.. since you are quite new to the site, its good to adjust your priorities and values to be more in line with retrobike mindset.

Jan, you've not meet my misses, she's got more bikes that I have and is currently toying over a retro Peugeot and spending her wedding fund on a full-susser.

 

[FluffyChicken]

of course it is otherwise they'd not give it as a specification.

 

[Antfox]

So is there a guide that's easy to follow we can use to get this all correct ?

 

[CAPIN]

Thanks for all the comments you've given me a fair bit to think about.

But Rob, there is a another thing.. since you are quite new to the site, its good to adjust your priorities and values to be more in line with retrobike mindset.

Jan, you've not meet my misses, she's got more bikes that I have and is currently toying over a retro Peugeot and spending her wedding fund on a full-susser.

Wow. Its great and disaster at the same time! If shes so much into bikes, she will know how much you spend on it!!

 

[NeilM]

So is there a guide that's easy to follow we can use to get this all correct ?

Have a look on Sheldon Browns site. There is a chart that covers a lot of the popular chain sets.

 

[Rampage]

There is a way to measure it on Sheldon Brown site.

 

[FluffyChicken]

MTB Square Taper
1) Stick crank on correct length axle. Use it's profile as a estimation
Pre '92 then look at 122 to 127
'92 and after then add
Super low profile (like M900) then 107 to 113
Low Profile then 113 to 118

2) If it's 68mm shell and the front mech is small band size then aim for the smaller axle length.
If it's 73mm shell and widest band size then aim for longest length of the range.

Anything in between go for smallest, though most specified the longer for a 73mm shell.

Other than that the tolerance on a square taper is poor, it can easily be 1mm different each tighten and between bottom bracket brands and some spec for grease on taper (raceface) some don't mention so this contributes to the distance on the taper.
If it is a mm out is not going to kill you

The guide though and ultimately one the crank is fitted in the correct range for it's profile then
1) it does not hit the frame
2) measure the chainline (given in an above post)

3) Go read Sheldon Brown for anything more.
It is just a measurement, not voodoo.

 

[We_are_Stevo]

It has more to do with the arc the cage of the front mech moves in. (rear, not so much. the much bigger distance between the dual parallelogram points in a rear dearailleur {sort of} solves this) The derailleur cage moves up and out in an arc. It's a compromise, and works best with the crankset is a specific distance from the centreline of the seat tube (aprox. 48-50 mm CL of middle ring to CL of seat tube). The farther the optimum chainline setting of the crankset is outboard, the less liner travel there is at the end of the throw. it messes up the shifting, especially middle to big ring, more so on standard (retro) drivetrains than newer compact.

This reply is so good it should have its own 'Sticky...'.