Originally Posted by
Lovegasoline
RL720 - For use with 24mm OD handlebars
RL721 - For use with 31.8 mm OD handlebars
RL726 - For use with 26.0 mm OD handlebars
I measured my handlebars’ OD with calipers at the ‘top grip area’, where the rubber grip ends and the bare handlebar is exposed: I get a measurement of about 23.62mm. However, I notice that the handlebar’s OD is slightly, but noticeably, larger at its center point where it’s held by, and makes contact with, the stem (I didn’t unscrew the handlebar from the stem and measure). Which of the two dimensions, grip area or stem area, is the one which designates handlebar size?
I believe it's grip size that they are referring to. In any case, you definitely don't have 31.8mm bars. That's a new thing. So cross that one off the list.
As fietsbob said I think it depends where you want to mount the lever. Once you remove your old grip tape you'll see that the handlebar gets noticeably smaller at the point where the grip tape ends and the center section begins. Measure the place you want to put the lever and buy accordingly.
I'd expect 26.0mm if you want it jamming up against the stem (bad placement IMO), or 24mm if you want it in the area currently occupied by your grip tape.