How do I measure the chainline on my bicycle? (re: ebike conversion; pictures inside)

[sofakng]

I’m installing a Bafang BBS02 ebike conversion on my Specialized Expedition but I want to verify my existing chainline so I can determine if I need an offset chainring.

I’ve read that you should measure from the middle of the bottom bracket shell to the center of the chain but this doesn’t seem possible on my bike? (see pictures)

Also, do I need to measure the rear chainline? This seems even more difficult (or impossible?) to measure from the center of the rear hub to the middle (4th) gear.

Thanks for any tips!

Pictures:

https://imgur.com/a/nMuYcUg

(and see attached files)

 

[RunForTheHills]

You measure from the centerline of the frame to the center of the chain. You could tie a string between your seat stays and mark the center on the string.

 

[tomnlo]

youtube Park tool you'll find what you need

 

[Apartunis]

In my experience you will be best served with a 68/73mm version of BBS02 and the stock steel chainring 42t or larger.

Chainline can be very negotiable on bikes that do not get ridden in all their gears. In that case it would make sense to center the chainring on the sprockets that get used frequently rather than the whole stack of gears. Are you going to be riding on very steep slopes? If so you will probably need the entire 14 to 34 tooth range of your freewheel.