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Here's why cross bikes don't have disc brakes.
by The CycloCross Geek

Next to "why would you ride a road bike in the dirt" the most commonly-asked question in the sport of cylocross is, "why don't cyclocross bikes have disc brakes?"

It's a great question. Disc brakes are powerful, with superior modulation, and they're virtually unaffected in mud. I know this because every mountain bikers-turned-CX noob takes the time to tell me these things.

First off….THANKS but we know about disc brakes. Most cross riders/racers have mountain bikes of their very own, with disc brakes and everything, and we know how well they work. Still, most of us run cantilever brakes on our cross bikes, on purpose. The reason: The UCI, math, and road racing.

REASON NUMBER 1: The UCI.

If the politics of cycling were a neighborhood, the UCI would be the old man yelling, "You kids get off my lawn" at most things new or innovative relating to bike racing. Cycling's international governing body has kept bike frames looking traditional, placed tighter restrictions on aerodynamic developments for road and time trial gear, and relegated World Hour Record track bike requirements to 1950's technology. And they were quick to ban disc brakes for cyclocross, or at least semi-quick, as they waited until 2003.

The UCI's ban on disc brakes was flipped for a time, and then it went back in effect in 2008. Interestingly, in the years between bans, top racers could have gone all willy-nilly and raced whatever brakes they wanted, but that didn't happen.

Of course the UCI's curmudgeonly rules don't prevent the majority of US cross racers from running discs in any regional series, but they're banned in big international races, and this does a lot to prevent bike and component manufacturers from equipping (and perhaps even optimizing) disc brakes on cyclocross bikes.

REASON NUMBER 2: MATH.

Stopping power = braking force x available traction.

With cross tires, your contact patch is smaller than that of a mountain bike, and you don't have suspension. This means you have less grip, and therefore less braking traction. So theoretically, you could strap the brakes off a Porsche 911 GT2 to your Ridley, and you'll just lock up a tire and go skidding off the trail that much sooner. A properly adjusted cantilever brake system will provide enough stopping power and modulation for the amount of grip available to a cross bike, at least for racing. Trail riding is an entirely different matter.

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REASON NUMBER 2.5: MORE MATH

Then there's weight. Decent cantilever braking systems are lighter than the few cable-actuated disc systems on the market, and the hydraulic options won't work with STI/Ergo/Doubletap shifters. Rotors are heavy, and that's rotating weight. And all the lightest race wheels? Not disc compatible.

REASON NUMBER 3: ROADIES DON'T USE DISCS

If disc brakes make sense for cyclocross, then they might be an even bigger boon for road racing. The descents are faster (requiring heavier braking) and a disc's wet weather performance would be an even greater advantage in a rainy road race than it would a muddy cross race. Think about it…when you're racing in the mud, you slow down painfully quick if you just….stop pedaling. Discs would make a rainy alpine road descent safer, if not faster. But between the weight penalty and the UCI's negative stance on anything new, don't hold your breath.

Cyclocross technology is driven by road technology. Every technology we have was first developed and introduced for the road, and all the disc brake systems currently available were all designed with mountain bikes in mind.

There are already options aplenty for anyone wanting to run discs brakes on their cross bike, as long as they want to do so with mountain bike parts. Some riders do just that, but still more stick with cantilevers.

THE FUTURE: MORE POWERFUL, WITH BETTER MODULATION?

Even though discs are, without a doubt, the better mousetrap, most cross riders will continue to roll around on cantilever brakes, at least for now. But it wouldn't take much to turn things around.

IF ProTour teams were to show some interest and support for disc brake use in the pro peloton, and IF the UCI could find it in their hearts to allow such a safety and performance-boosting innovation to occur in road racing, we'd likely see a new breed of superlight disc brakes (and lighter, stronger, disc compatible road wheels, perhaps even hydraulic systems compatible with road drivetrains) for the road that would also be nearly perfect for cyclocross.

When or if all this happens, I suspect we'll all jump ship. But until then, cantilevers will have to do.

Do you have a question for The CX Geek? Email him at [email protected]