Originally Posted by
John Hawkinson
what is the adjustment that controls the width of the arms apart ("arm spacing")?
It's the combination of the thick and thin spacer washers, as you're experimenting with here. With smooth post brake pads (as used with traditional cantilevers), you can freely slide the brake pads "in and out" of the cantilever mounting base -- to position the pads closer to or further from the brake arm. The different washer thickness with threaded post pads, as you have with the Oryx and as comes with nearly all linear pull brakes today), accomplishes the same thing. It lets you set the pads close to the brake arm (by using the thin spacers next to the pad) or further from the arm (by using the thick spacers next to the pad).
The geometry of low-profile cantilever brakes like the Oryx means that the mechanical advantage of the brakes decreases pretty rapidly as they rotate in towards the rim. This is why spacing the pads further from the arm (by using the thick spacers) is usually advantageous -- it keeps the arms splayed wider which reduces the loss in mechanical advantage as they rotate towards the rim.
I'm sure it's been posted somewhere above, but Sheldon Brown has an excellent article on this:
https://www.sheldonbrown.com/cantilever-geometry.html
Browse down to the section for special considerations for low-profile cantilevers.
Originally Posted by Sheldon Brown
Traditional good practice had been to slide the brake shoe holders all the way into the eyebolts, so that the back of the shoe butts up against the cantilever arm. This is not the case with the newer low-profile models.With low-profile cantilevers,, the shoe needs to be extended inward from the arm, increasing the effective
cantilever angle . The unsupported length of shaft connecting the brake shoe to the arm may cause an increased tendency to squeal, but that is one of the inherent trade-offs of low-profile brakes.
Originally Posted by
John Hawkinson
But maybe I'm not understanding. What does "in terms of dimensions you can measure" mean here?
I was trying to say that the brake pad wear doesn't contribute
a lot to the geometry we're discussing here...it contributes some, but not a lot. I was suggesting that the spacers you're working with are not designed to compensate for pad wear. For that, you'd just snug the barrel adjuster a little bit.