I'll try to explain again to help others from wasting their money.
Originally Posted by
Jipe
As I wrote and as showed on the Sheldon Brown website, the chainline is the distance between the center of the bike and the chainring.
"Chainline" has at least two meanings. Both meanings are common. Sheldon Brown uses both, on the page I linked to. I'll explain with examples:
1. "This bike has bad chainline" means: "The chain is not running straight between front and rear sprockets".
2. "The chainline on this bike is 42 mm" means: "The lateral centre of the chain is 42 mm from the lateral centre of the bike".
Originally Posted by
Jipe
It doesn't change when you increase or decrease the chainring.
As I explained before, on the Birdy you will sometimes have to move the chainring outwards when you increase the chainring size, to avoid chainstay rubbing. This affects chainline.
Originally Posted by
Jipe
Most Birdy are factory equipped with a 52t chainring and the Birdy Touring has a 9-32 cassette.
I have a 52-39t front and -32 rear and there is space for more. I took 9-32 because my rear derailleur (Shimano Ultegra now) is limited to 32t rear.
The cassettes I have shown are all road cassettes including the 9-34 and 9-36, they fit on my rear wheel as my 9-32.
I explained that with mtb cassettes, 10s and 11s has the inner cog in a different position - the chainline is different. It's more narrow on 11s, closer to the centre of the bike.
The Birdy Touring 2018 edition, which I have, originally has a Sunrace 10s 9-32 cassette, yes. I upgraded to the Sunrace 11s 9-36 cassette. This is an mtb cassette, meaning it uses mtb sprocket positioning. Which means in 1st gear, the chain is more inwards than on the original 10s. And that can lead to chainstay rubbing, as it did for me. To fix it, I had to move my single chainring outwards, which lead to a bad chainline. The chainring aligned with the 8th or 9th gear, which meant that in 1st gear the chainline was very bad (not straight). It worked, but not
well.
The problem is not so much about the number of teeth on the 1st sprocket. It's mainly about the 1st sprocket
position on 11/12-speed mtb cassettes, which is too narrow to work well on the Birdy.
I'll try to be as clear and specific as possible:
On the 2018 R&M Birdy Touring,
with a single chainring 45t or larger,
and using the Sunrace 11-speed 9-36 mtb cassette,
or any other 11 or 12 speed mtb spaced cassette 36t or larger,
it is not possible to achieve a good chainline (chainring aligned with 6t sprocket) because the chain will rub on the chainstay near the suspension pivot.
This I have verified in practical testing, not just theoretically. If you have a different result, then either you have a different bike, or a road cassette, or something else is different. If you managed to find a Sunrace 11s 9-36 cassette with road spacing, then good for you!