Saturday, March 23, 2013

CX Disc Brakes

A bunch of people have asked me about my experience with short pull cable actuated mechanical disc brakes, specifically for cyclocross and specifically in regards to pad wear during foul conditions. What I have told them is that if the disc brakes are properly setup, they are not in my experience a problem for cyclocross. They are also a big advantage in that they provide consistent, reliable braking performance regardless of the conditions.

Avid BB7 mechanical cable disc.
With that, let me go into a bit more detail on the point. First, let me start by saying that most of my 13 years of mechanical disc brake experience are with the Avid BB7 though I will try and speak in general. Some of the lower end models have limited adjustability in comparison and some of the new models that will hit the market will be more advanced. Proper setup of mechanical disc brakes is a little trickier than their hydraulic brothers.

At present, with most of the mechanical disc brakes out there now, you have control of the cable length and tension as well as the position of both the inboard and outboard brake pad. Through these individual adjustments you control how far away from the rotor the pads sit at rest. This is very different from hydraulic brakes, which are self centering in their pad position and distance from the rotor. They also self compensate for pad wear, a key aspect of the mechanical brake issues.

The only way to adjust for pad wear with a cable actuated disc brake is to readjust the pads. This unfortunately requires getting off the bike and screwing the inboard and outboard dials individually on an Avid BB7, a pain in the butt. You can, however, get half way to dynamic in-ride adjustment by installing an inline cable adjuster, which effectively lengthens the cable housing, shortening the slack on the cable. This has the effect of actuating the lever arm which sets the outboard pad inward, toward the rotor. The downside is that the inboard pad remains fixed still. The new TRP cable actuated mechanical calipers that are supposed to be out soon actuate both pads via a crossover arch and will solve this issue, making them manually adjustable on the fly. Of course, the TRP hybrid cable actuated hydraulic calipers will be really sweet when they show up.

Avid BB7 SL rear.
So that is the mechanical lowdown in a nutshell. Let me get to the proper setup of which I spoke. No big secrets here, it is all pretty straight forward. Because the road brake levers take up so little cable, the brake setup at the caliper is crucial. This is also why pad wear as felt at the lever is far amplified over the longer pull MTB models. Anything you can do to improve the feel and minimize flex in the system helps. To that end, good quality cables and housing are important. Compressionless brake cable housing makes a big difference. It is also crucial to have rotors that are straight. A small bend is going to produce an annoying and costly pad strike when you run the pads really tight to the rotors. That is the next key, getting the calipers setup level and even so that the pads make even contact with the rotor is important. If the caliper toes one way or the other it means the pad will contact unevenly, reduce power and probably squeal. Getting the pads in as tight to the rotor is also important. You want them on the verge of rubbing but not quite.

The last tip is one that I've discovered to be helpful over the years. What I do is to preload the actuator arm a little bit. This means that with the cable taught, there is some back tension on the cable from the arm wanting to return to full rest. I'm not sure if the arc that the actuator arm makes during the travel stroke is slightly non-linear (doesn't appear to be) but this seems to drastically improve the feel. Of course by preloading you start actuating the outboard pad but as long as it isn't rubbing, you are fine.

With this setup I have been able to race up to 1:08 in some very, very sloppy and gritty conditions (2012 Plymouth P/1/2/3 day 1) and 45 minutes in some insanely sloppy conditions while working the brakes over really, really hard on the steep downhills (2012 Downeast M45+ races), for the entire races on the same bike, without losing braking. I was also never required to adjust mid-race via the inline adjuster. I'm not sure why so many people were running out of brakes but I have to guess that it was due primarily to setup.

Noticed the bulge last night. All done.
Oh yea, and this is the biggest reason why disc brakes are a way better choice than rim brakes. You don't wear away a structural component to the bike by applying the brakes, namely the rims. This is the latest of many casualties I've had over the years, which I discovered this morning. It is often catastrophic, especially at road pressures. When the brake surface of the rim separates enough that it can no longer support the tire pressure, it explodes. That usually involves shrapnel and long shards of sharp aluminum. It also makes the wheel un-usable. Not fun.

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