Understanding rear chainline - math does not compute on a modern road bike

[musiclover]

Hi,

A standard rear road thru axle is 142mm, a standard cassette width (with a lockring) is around 44mm. A distance between the lockring and inner side of the drop out is pretty standard as well - around 2.5mm
That equals to 142/2=71-44/2=49-2.5=46.5.
To bring that to a road standard (43.5mm which should normally be observed at the front), the gap between the cassette and the drop out needs to be 3mm more, i.e. 5.5mm which does not really make sense on modern frames.
So, my Cannondale Supersix has 43.5 front and 47mm or so at the rear... Which should not be the case. Is there an error in my measurements/calculations? At the same time the only non standard item that should be measured is that gap between cassette and drop out. But it can not / should not be much more than 3mm?..

With all that stated I have to say that if I try to pull a straight rope between the middle of the chainrings and the cassette it lands around the middle of the cassette, but as per the chainline should land 3mm out.

 

[BikeGremlin]

Hi,

As far as I know, 142 mm hubs are often aligned as 135 mm (MTB) hubs with 3.5 mm extra groove for alignment inside the dropout inner parts (see the booster hubs section for a bit more details).

So, if you missed to take into account the 3.5 mm from the paragraph above, your rear chainline might in fact end up at 46.5 - 3.5 = 43 mm (or so).

If you wish to check on the spot so to speak, I would use a straight firm ruler (or a similar steel flat sheet). Align it against a front chainring and see where it ends up at the rear. The procedure is shown here at 46:27 (the link should open there - the narration is in my native Serbocroatian, but the demonstration should be clear):

https://youtu.be/1MBSX4vyGfU?si=d7iXtT9QSHjLP5On&t=2787

Let me know if I've missed something or if you have other questions. Off to the party now.

Happy New Year

 

[musiclover]

Hi,

As far as I know, 142 mm hubs are often aligned as 135 mm (MTB) hubs with 3.5 mm extra groove for alignment inside the dropout inner parts (see the booster hubs section for a bit more details).

So, if you missed to take into account the 3.5 mm from the paragraph above, your rear chainline might in fact end up at 46.5 - 3.5 = 43 mm (or so).

If you wish to check on the spot so to speak, I would use a straight firm ruler (or a similar steel flat sheet). Align it against a front chainring and see where it ends up at the rear. The procedure is shown here at 46:27 (the link should open there - the narration is in my native Serbocroatian, but the demonstration should be clear):

https://youtu.be/1MBSX4vyGfU?si=d7iXtT9QSHjLP5On&t=2787

Let me know if I've missed something or if you have other questions. Off to the party now.

Happy New Year

Thank you! Yes, now the numbers make sense. Oh and Happy New Year to you too!