On Sunday, September 24, 2017 at 8:04:15 PM UTC-7, Tim McNamara wrote:
My concern with buying a disc brake bike now and planning to keep it for
possibly decades is that the technology is not mature yet. Rim brake
parts, as noted, will be available for pretty much the rest of my life.
But disc brakes may be still evolving to get rid of the weight penalty,
optimize rotor size, pad design, etc. Pretty much the only replacemetn
parts that rim brakes need are cables and pads.

Disc brakes are evolving, but the real incompatibilities result from different through axle standards and proprietary hub/dropout spacing (Specialized SCS). Disc caliper evolution is a problem only if mounts change -- which they have, but there are so many adapters out there that it doesn't matter. I'm running BB7 cable discs on my warranty replacement CAADX frame which has rear flat-mount and front post-mount for 140mm rotors (I use 160mm front). That meant getting two adapters which were actually quite cheap -- about $10 apiece. My cable discs only require a cable and run off ordinary levers.

Another concern, which may be unfounded, is that disc brakes stress
frames and wheels in much different ways than rim brakes do. I am not
sure that tecnhlogy has caught up with this yet. For example, on the
front wheel disc brakes can create a steering input during hard braking.
Braking forces are higher than acceleration forces, so this may affect
spoke and rim life.

Discs apply forces differently to the fork, and one hopes that the forks have been engineered to withstand those forces. I've been riding wheels I built for my commuter probably ten or more years ago with no problems -- plain-old 32 spoke, cheap Shimano M525 hubs (rear hub shot now) and Velocity Touring Disc rims, which is a discontinued 450g rim. The great part is that the rims are pristine apart from whatever beating they've taken from pot holes, etc. I would have worn them out in two years of commuting in the rain with a rim brake.



In terms of performance, I have only ridden a handful of cable actuated
disc brakes. I found them abrupt and grabby and I didn't like them.
But I may not have ridden best examples of the technology and things
have probably come along since then (several years ago). My brazed on
centerpulls, cantilevers and single pivot brakes have consistently an
safely stopped me for decades and I have no complaint about them, but
maybe someone will design the rim brake that converts me. Those brazed
on Mafacs are wonderful, BTW. What a great brake, but few people have
had the opportunity to try them.

Cable discs can be better or worse than rim brakes in terms of stopping power and modulation. If you don't watch your pad adjustment and wear, you can find yourself with really bad braking -- like "oh, crap, man!" That happened to me on my way home from work, flying down a super-steep hill. I stopped (after a while) and adjusted the pads, and everything was AOK. Hydraulics adjust automatically. You can blow through pads on both and have to watch pad wear. My front hydraulics on the Roubaix were stopping me with NO pads -- I was stopping with the pad carriers. Ooops. Not so great on the rotors. Other downsides are complexity (bleeding hydraulic), dragging, pad break-in, adjustment for different wheels if any change in disc location. Wheel changes are a little more complicated, but then again, if you had fat tires and were trying to get them through calipers, that would slow you down more. If you have through axles, that can mean rack and work-stand incompatibility. There is a weight and aerodynamic penalty, which is meaningless for most people.


Like I said, discs modulate well and have excellent stopping power -- far better in wet condition and on CF rims. I think they should be standard on rain bikes. They are standard on my rain bikes. They are totally unnecessary on a dry weather bike with metal rims. Other pluses are tire and fender clearance.

-- Jay Beattie.