I know the conventional wisdom is to go for cantis, V brakes or even disks for touring, but what exactly is wrong with side-pull calipers? Is it stopping power? - I thought side-pulls were meant to be more powerful than cantis etc. (I presume that the problems is not reach, as Shimano makes an Ultegra-grade long-reach caliper brake - the "BR-R600" - that will accommodate a 32mm tire, but please correct me if this is the main deficiency seen in side-pull calipers).

Thanks, Tom

Calipers limit your options for fitting fenders and getting a decent tire size.

Mostly tire width. Generally speaking, most side pulls are made for bikes that have narrow tires (<25mm). You can ride a 25 or even a 28mm tire while touring but something in the 32 to 40mm range is much more comfortable. Once you start to widen the calipers to fit something that wide, you end up with a really flexible arm, unless it's very, very beefy. As an extreme example, go ride an old Huffy with sidepulls on it. Cooked pasta is stiffer

Cantilevers, being rather stubby, don't flex as much and provide better braking with wide tires because they can be tucked in under the tire. Cantilevers, because of their longer lever arm (relative to the pivot point), can put more stopping power to the rim which makes them more powerful than single pivot side pulls. Dual pivot sidepulls increase the mechanical advantage and result in much more powerful brakes...close to, if not equal, to cantis.

You can tour with sidepulls...even single pivot. I've done it, as has my wife, long ago. That includes mountain touring with long fast downhills. If the bike has the proper geometry for touring and only takes sidepulls, that shouldn't be a barrier.

There is quite a variety of interesting brakes here:

https://sheldonbrown.com/harris/brake...ers.html#55-73

I was wondering in particular whether the BMX type would have enough girth to get around 32-37MM tires.