Why is chainline so important on retro bikes?

[Rob H]

A bit of a newbie question but i like to understand what I'm doing and why.

I've been doing a bit of reading around whilst collecting parts for my retro build and I've found alot of references to getting the chainline spot on. Now i can understand getting this spot on for a single speed but given alot of modern bikes use 1 by 10 gears which have shocking chain lines and seem to get away with it, so why is it so critical on a 3 x 8 especially as most riders don't cross the block whilst riding. Is it jusy a case of good engineering practice? or are older 7 & 8 speed chains less able to cope than modern 10 speed chains?

 

[legrandefromage]

buggerall to do with just retro - its all bicycles. if the chainline isnt good it buggers up the shifting and prematurely wears the chain, rings, cassette, jockey wheels and so on.

 

[CAPIN]

I never totally got chainline thing.
Of course ideally straight chain wears the least and so on, but on mountain bike with 3x8 gears chain is all over the place all the time.
So it always seemed to me that the cassette/crankset position against each other shouldnt matter that much.
And if it does, than it shoud be a different from what we have. As its now, chain is most of the time heavily bent to one side towards the wheel.
Logically i would design it the way that chain is straightest at gear where im the most, or where are biggest forces
But on a mountain bike when you are on the small chainring, the chain still have to be bent a lot to reach to bigger cogs on the cassette.
More so when you are on the middle chainring where we spend most of the time..
Current evolution to systems such as 1x11 is just proof to me that you can get away with a lot and that having bb 1mm longer or shorter is really no issue.. (ofcoure new chain is different, thinner and probably designed to be more flexible, than HG but still..)

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.
The picture at the bottom of your posts has it totally wrong. It should be like THIS!!

cheers
jan

 

[RadNomad]

Yes agree with LGF. Chainline is only going to be perfect in one combination of chainring and sprocket anyway. As soon as you shift one or two gears your chainline is already compromised. As you say, sensible selection of ratios front and back helps to minimise extreme misalignment.

7/8 speed chains are a bit wider than 9+ speed narrow chains which from an engineering point of view would be logical to suggest the 7/8 speed chain be prone to more wear due to the increased off-axis leverage loads on a wider chain roller, but i'm surmising a bit and the difference in width isn't much anyway (roughly 1mm difference in max chain width).

Below are 3 photos showing 3 different bikes using 3 different groupsets, all of which have the chain on the innermost ring and sprocket (lowest possible gear).
Orange bike: Retro LavaDome running original 7 speed STX.
White bike: Modern full susser running it's original 2009 9 speed XT.
Silver bike: Retro HeiHei test build with 2013 10 speed XT, bottom bracket only about 1mm away from being fully seated.

Of the three, the 7 speed kit has the best chainline in lowest gear. The modern and retro frames running narrow chain XT kit have chainlines which look similarly extreme and both need to be in a medium rear sprocket to achieve ideal chainline. Not a scientific comparison and the photo angles are not identical, but what i learned from this (and making chainline calculations and measurement comparisons) is that modern 10 speed groupsets fit just as well on a retro frame as they do on a modern frame. So like LGF suggested, it aint the bike, it's the way you use it...

 

[cce]

there's actually more possible variations of chainline in a 3x8 than there is in a 1x10. a 10s cassette is the same width as an 8.

 

[Antfox]

So is there an easy way to find out what's best ? I was given a link to a chart but it made no sense to me .

Would it be worth all the guys getting a list together stating :

1) frame / year
2) b/b shell width
3) splined or tapered
4) cranks inc arm length
5) how many rings ? ( not sure if this makes a difference )
6) cassette speed
7) b/b axle width

This may be wrong but I am just waiting for my wife to come out the changing rooms

This could help people out who need a quick bit of info like me when I need a new b/b for the zaskar

Cheers

Ant

 

[RadNomad]

If your wife reappears in anything other than cycling garb, you're in the wrong shop.

 

[cce]

recommended chainlines for mountain bikes are between 47.5 and 50mm.

 

[Rampage]

It's because most of us spend more time fiddling with bike than riding them.

Personally I don't give a **** about chainline. If the gears work, then they work.

 

[FMJ]

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.