The problem with mechanical disc brakes…

28 01 2011

Avid BB7's on my Surly Big Dummy...

I must say upfront that this post is a bit of a rip off from an article Jan Heine posted in Bicycle Quarterly.  However, it’s something I was thinking about before I read that article and I’ll add a bit to what Jan discussed.

All of my initial experiences with disc brakes were with Avid mechanical discs.  This was simply because they were popular and easy to work on at home since they used the same brake housing and cables I was used to.  That meant I could overhaul my bike at 3am without a visit to a LBS for help bleeding hydraulic brakes. To their credit these brakes work well. I find the stopping power of a set of quality v-brakes and Avid mechanical discs comparable.  In the dry I can flip my bike over with both and in the wet they both stop the bike well albeit not as well as when dry.

I won’t argue there is no difference at all between a good set of v-brakes and a good set of mechanical discs, but the difference has not been in the range that it matters to me one bit which I use.

One issue I’ve had with mechanical discs is that after the initial part of the lever travel that does stop the bike reasonably well there is a very mushy feel to the lever as you keep pulling that seems to have little additional braking effect.  It’s worse on the rear wheel although that wheel isn’t particularly important for stopping the bike.  Since they work fine it’s not something I’ve spent too much time worrying about.  Then one day Bicycle Quarterly published an article that explained why I was experiencing this.

If you imagine that a rim brake is really a disc brake with a very big rotor.  This gives it a lot of leverage to stop the bike.  So that generating the same braking force  on a disc brake bike takes more pressure on the brake pads than for a rim brake bike.  Just the same as if you used a long and a short pry bar to open a wooden crate you have to push harder on the short pry bar to generate the same force at the far end of the lever.  In some ways the higher pad pressure of a disc brake system is a good thing since this is what is supposed to give it better wet weather braking as the pads squeeze water off the disc rotor more effectively than the lighter pressure from the rim brake pads on the bigger rotor that is the rim.

The problem is that the housing used for the mechanical brake cable is only able to resist the compression forces of the brake lever to a certain point.  Before that point most of the power you put into the lever gets to the disc pad and squeezes the rotor resulting in good braking.  Beyond that point more and more of the extra force you put into the lever goes into compressing the brake housing.  This means as you double the force you only get a small increasing in brake effect at the rotor.  This explains why after some good initial braking the mechanical disc brake lever feels mushy and doesn’t seem to have much effect.  It also explains why you can brake effectively with rim brakes since they don’t require the same high forces. Rear disc brakes also tend to use a long full run of cable housing which exacerbates the problem.

So what can you do about it?

  • if you want to stick with mechanical discs use some high quality brake housing that resists compression better.
  • if you’ve got $$ to spend install compressionless metal cable housing like the one made by Nokon.
  • if you are buying brakes go with hydraulic disc brakes since they don’t have this problem
  • the cheapest solution is to understand the issue and use the braking your mechanical discs provide…when you get to the mushy part of the lever’s pull don’t bother squeezing harder since you know not much will happen.