I receive many questions about road disc brakes and the pros and cons of such. This FAQ to Lennard Zinn of Velonews caught my eye and discusses some of the specific problems associated with discs on a road bike setup. As always, Lennard is thorough in his research and offers some insight into this subject. you can find the original article via the Velonews website here:
Editor’s Note: Lennard Zinn’s regular column is devoted to addressing readers’ technical questions about bikes, their care and how we as riders can use them as comfortably and efficiently as possible. Readers can send brief technical questions directly to Zinn.
Do you have tips for setting up disc brakes on road bikes? I’m using Avid BB-7 road calipers with XX 160F, 140R rotors with stock pads and standard cables/housing with Ultegra Di2 brake levers. They seem to be short on stopping power; the MTB versions I’ve tried have been much stronger. I’m planning to replace the housing with something compressionless. I’m wondering if the longer cable pull of the new Shimano levers doesn’t match the calipers well — maybe the standard BB-7 calipers would be better? I could also try different pads or a larger rear rotor. I’ve heard from others the burn-in period was really long on their road bikes, so maybe they will continue to improve.
You’ve knocked the lid off the reasons that cable-actuated disc brakes generally don’t stop as well as hydraulic ones — the cable and what pulls on it. You’ve also glimpsed why the braking performance we’re used to on road bikes with high-performance rim brakes will be hard to come by using discs without adding considerable weight and aerodynamic drag.
First, liquids are essentially incompressible, so a hydraulic brake will push essentially equally hard, no matter how long the hose is. This is not true with cables. Also, there is very little friction in liquid moving in a hose compared to a cable sliding in a metal tube, so your cables and housings must be low-friction and your cable routing needs smooth bends to minimize friction.
Unlike shift-cable housing, which is coaxial (its steel strands are longitudinal), brake-cable housing is spiral-wound to preclude its own compression. All cable housing, shift and brake, was spiral-wound before the days of indexed shifting — which, as you are aware, can compress. Compressionless coaxial cable housing was perhaps the essential innovation Shimano came up with when it introduced the world’s first truly functional indexed shifting system. Without it, shifting was vague and unpredictable; with it, shifting could be adjusted to hit each cog precisely every time.
But why didn’t brake cable housings become coaxial, too? To ensure the rider’s safety, that’s why. Coaxial shift-cable housing can split under high cable-pull pressure. It doesn’t happen often, because there is not as much leverage with shift levers as with brake levers. Nor is there reason to pull them as hard. And if the housing were to split, the worst that would happen is that the shifting wouldn’t work.
But brake cables get pulled much harder than shift cables (that’s why they’re thicker). And if the housing were to split, the brakes wouldn’t work. So brake housings are spiral-wound and do compress, which takes away from braking performance.
And the longer the cable run, the greater the issue, since the compression of the housing is additive over its length. Also, the cables themselves stretch, and that, too, is more of an issue the longer the cable run.
I believe you can find brake-cable housing that combines coaxial strands to prevent compression of its length with spiral winding to prevent bursting.
But back to your question: You clearly understand that brake levers that pull more cable have less leverage than ones that pull less cable. In order to pull more cable for the same amount of lever movement, it should be obvious that the cable hook (where the head of the cable attaches to the lever) must be further from the lever pivot. I’m sure you’ve played on enough teeter-totters to know how this lower-cable-pull brake lever would improve your situation, proved you don’t have so much housing compression and cable stretch that the lever comes back to the handlebar before the brake is applied as hard as you need it.
The second issue also involves mechanical advantage, and has to do with the diameter of the braking surface and the design of the caliper. Obviously, it takes a lot less force to stop a bike wheel by pinching brake pads down on a brake rotor attached to its hub that is 630mm in diameter than it does if they pinch down on a 140mm-diameter one. Well, guess what? The outer diameter of the braking surface on a 700C wheel is about 630mm. So the answer to part of your question is, yes, if you get a bigger rotor, your braking power will increase.
Caliper design is also an issue. The pivoting arms of rim brakes both pull against the rim, and longer brake arms and improved pivot orientation increase leverage and hence braking power. There is very little friction in the caliper — just a small amount at each pivot.
A cable-actuated disc-brake caliper, however, generally has a spiral track on an internal inclined plane upon which a number of ball bearings ride (hence the “BB” in the Avid brake’s name, I imagine). Pulling (rotating) the brake arm drives the balls around the track and up the incline, which cannot move outward, thus forcing the pad inward. Only the outboard pad moves; the inner pad is fixed in position and serves as a stationary anvil to the extent that the caliper and its mount are stiff enough to hold it stationary.
Obviously, there is more friction inside this caliper than there is in a simple pivoting rim-brake caliper. And a focus on reducing metal thickness to reduce weight can result in a brake arm that flexes and twists as well as a caliper that flexes open under load, thus reducing the force pushing inward on the pads.
Road disc brakes will have to be nearly as light and aerodynamic as rim brakes to compete against them in the marketplace. The rotor sticking out there is always going to add weight and air drag, so it has to be small. The caliper will have to be light, and so will the mounts and the fork and rear-stay tubes to which they are attached, even though those will be taking much more load at their extremities. And the wheels will need more and longer spokes crossing each other at angles to counteract the twist on the hub from the rotor attached to it.
All of this adds weight and aerodynamic drag, so the impetus for manufacturers to reduce the size and lighten their materials will of course be strong.
And it’s not just cable-actuated disc brakes that have huge hurdles to overcome. Many of these issues apply equally to hydraulic discs, even if component manufacturers start putting hydraulic cylinders in their brake levers, thus eliminating the complexity and weight of satellite master cylinders.
The distance from the lever to the caliper will become a non-issue, since hydraulic fluid will not compress like brake-cable housing or suffer cable stretch. However, in the interest of reducing weight and air drag, the calipers, pads, master cylinders and rotors will be small, and the rotors may even be made of a less dense material than steel to further drop weight.
Have you begun to see the problem? It’s heat dissipation. The less the mass in the system, the less the system can get rid of heat. And if the fluid boils, the brakes don’t work, because what was liquid is now gas, and gases, unlike liquids, do compress.
I know I’m throwing ice water on the enthusiasm I see among cyclists licking their chops at the advent of road disc brakes, but I’m not sure that enthusiasm isn’t clouding some important issues.
Obviously, disc brakes are a huge advantage on a mountain bike, where weight and aerodynamic drag are smaller issues and mud, debris and water on the rims are bigger issues than on road bikes. The same can pretty much be said for cyclocross (with perhaps the exception of the weight issue). And I further contend that the braking power on a road bike has to be higher than on either of those bikes, because road speeds are higher and must be reduced substantially for a switchback, an obstacle or a crash.
So brake heating, I think, is more of an issue on road bikes, not less. And this indicates that calipers, master cylinders, rotors and fluid volume all ought to be greater to counteract the problem, rather than whittled away to cut weight and drag.
Thus, John, to get better performance from your disc brakes, get thicker cables and housings that won’t compress as much (or a hydraulic system), plus bigger rotors, bigger pads and stiffer calipers. You’ll have to suck up extra weight and aerodynamic drag as the costs of good braking.