So... now that road disc is "here"... where will they go in future?
Essentially, all that has been done to date is to take a MTB disc brake, which has evolved over the last couple of decades to suit the needs of MTB, and stuck it on a road bike. Now I expect we'll start seeing some divergence, with road discs slowly (?) evolving to fit the needs of road.
The two critical factors for road bikes, obviously, are weight and aero. Weight is also a consideration for MTB, but not to the same extent, and aero is really an afterthought.
One thing that amazes me about disc brakes is just how heavy the disc itself is. A 160mm rotor can weigh up to 200g (for a rotor with minimal slotting, which is where I think they should be heading [1]). That's within coo-ee of the weight of a light road rim.
The rubbing track is where all the weight is; the rest of the rotor is mostly holes... and doesn't even need to be steel [2]. But there's no reason why the rubbing track of a disc brake needs to be as wide as the current standard design (with pads ~20mm square). If the rubbing track was narrower, we'd strip a heap of weight out of the rotor.
Say we go from a 160mm rotor with a 20mm wide rubbing track to a 160mm rotor with a 10mm wide track: you've almost halved the mass of the rubbing section of the rotor. And does it even need to be 10mm wide? Could we brake on a 5mm wide brake track?
When calculating brake effectiveness, we usually idealise a disc brake caliper as acting at a single point on the rotor - at the effective radius. Apply a pressure there, and a given friction coefficient will produce a friction force at that point, resulting in a brake torque about the axle. So, this narrow-track brake rotor would just have all the brake force concentrated on the effective radius.
Now, there's nothing wrong with a narrow brake track and a narrow pad to fit. But to get a reasonable contact area and pad volume, you'll need to make it longer. Home mechanics might be familiar with the difference between the pads that fit Falcadores (banana-shaped friction material) compared to a more typical Euro car (a squarer block of friction material). For a variety of reasons Euros tend to use big single piston calipers, Australians tend to use longer twin-piston calipers. And that's what I imagine happening to road bike disc brakes. Longer multi-piston calipers on longer banana-shaped pads.
The biggest aero nightmare I see on MTB-style disc brakes is the hub. The standard 6-bolt disc mounting flange is a disgrace. That will have to change. It's big and lumpy and the front one especially is right out in the free-stream airflow, not even sheltering in the wake of the tyre.I haven't had much to do with the Shimano centre-lock system, but at least that's a bit more aerodynamically compact. I thing we'll see a few iterations here, as the Big S and Small S develop incrementally more aerodynamic disc mount standards.
The caliper itself, I think is aerodynamically better than a rim brake caliper. It's already tucked in behind the leg of the fork. The fork leg creates an aerodynamic wake, and anything that hides in that wake essentially gets a free ride. The caliper just needs a bit of smoothing out - right up to the arbitrary limit of fairing that the UCI will allow - and it may even be able to recover a bit of pressure, reduce the size of the wake behind the fork leg, and improve the aerodynamics of the bike. By a poofteenth, but that's enough to market the hell out of it
Cables or hydro lines - internal, or clipped on to the back of the fork leg. Hydro will give the benefit that the brake cable no longer needs to travel in a graceful sweeping arc; sharp elbows work just fine, so we could end up with a bit plumbing used to keep the brake lines close to the bike, out of the wind.
Could we see frames and forks with integrated brake lines? For the fork especially - there could be a hollow tube moulded in to the fork steerer and leg to be filled with brake fluid. Connect a brake line from the lever to the stem, and another from the fork tip to the caliper, and the fork contains the fluid in integral internal hydraulic tube in between. Likewise the frame: plug a flexible line from the lever into somewhere at the head tube, and the internal fluid ways will deliver pressure to a short line from the rear dropout area to the caliper.
What other incremental developments do people imagine coming in the future of road bike disc brakes?
tim
[1] My day job is testing automotive disc brakes. I am yet to see slotted or drilled car rotors improve anything other than bling factor. All they do is cause pad wear, as the edges of the slots shave the surface off the pads. We saw in a recently discussed article, disc brakes in CX failing in mud due to excessive pad wear. I reckon this was largely due to abrasive debris (ie mud) getting into the rotor slots, and getting into the brake contact area that way. I'd like to get rid of the slots. On principle, I
bought a set of Avid "solid sweep" un-slotted rotors, but haven't fitted them yet... damn they're heavy with all that steel where the slots used to be!
[2] I'm leaving aside the carbon-ceramic friction couple systems for now, although I have no doubt they'll be significant... in time, at the bleeding edge. But metal rotors will always be a big part of the scene.
Still bloody expensive
By then all the issues and hub spacing a should be standardized.
