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Hydraulic disc brakes: A cautionary tale (Alternate title: OW!)

Wednesday, April 25, 2012 by  
Filed under Daily Blog

April
25
2012
The Little Dipper is much faster and safer with the addition of the clay!

The Little Dipper is much faster and safer with the addition of the clay!

Yesterday morning the weather was absolutely amazing: clear, cool and sunny. We had some rain a few days ago, and over the weekend many improvements were made to the trails at Mt. Dora. Where to ride was a no-brainer.

As expected, the trails were running super fast! My average moving speed on the first lap was more than 1 MPH faster than my personal best ever. Five new bermed corners had been added, and several sketchy areas had been improved with the addition of clay. For example, check out the re-worked “Little Dipper” shown in the picture to the right. Before the clay was added this fun little dip was very sandy, and its shape needed a little tweaking. Check out the “before” picture and the differences are pretty obvious. Yesterday I was able to hit this feature much faster than ever before, catching some air on the backside.

So I was having an absolute blast, but I was aware of an issue with my brakes that I stupidly ignored. That brings me to the main subject of today’s blog.

The first section of trail at Mt. Dora is a twisty downhill with lots of tight turns and switchbacks. As I made my way down the trail (called “Gravity Destroyer”), I noticed that my brake lever travel (both front and rear) was much greater than usual.

My bike has Avid Elixir hydraulic disc brakes, which are extremely powerful: a fairly light pull with just my index finger is all it takes to bring the brakes up to full power. In fact, when I first got this bike I almost went over the bars several times because I was not used to how effective hydraulic brakes are. Learning to modulate the powerful hydraulic brakes took some practice, but once I got used to them I felt they were light years beyond my old mechanical disc brakes.

I digress…

So I was aware of the brake issue, but I decided to ride anyway. I shouldn’t have. I was able to stop the bike, but the amount of finger force required to do it was significantly higher than what I’d become used to. When I had to brake the response was always delayed, and the braking force was less than what I expected: muscle memory from hundreds of miles of riding with properly functioning brakes was working against me. I had to use two fingers to bring the brakes to full power, and the levers were traveling so far that they were smashing into my gloves.

So here’s where things went sideways (literally). I was hauling ass into a bermed corner, and I came in a little too hot and my line was too high. Normally a little tap of the brakes would have allowed me to correct my mistake, and that’s what I tried to do. Of course the brakes didn’t respond to my tap and my bike and I went airborne and sideways. I crashed down onto the hard clay with my bike on top of me.

The first thing I noticed was I could not breathe: the crash had knocked the wind knocked out of me. While I was trying to get my breath back I remember thinking, “Holy crap, clay is really hard!”

As I began to recover my breath I realized that my left leg was hurt. The pain was pretty bad, and I was worried that I might have broken a bone. My shin was cut where the pedal hit, and was already swelling up. There was a small cut on my ankle as well. But none of that stuff hurt at all compared to my left hip and calf. Have you ever had a calf cramp? That’s exactly how my calf felt.

I scraped myself off the ground and did some poking and prodding. I was relived that nothing seemed to be broken. After a few minutes I got back on my bike and rode on. I finished that lap, and even did a fifth lap. Hey, I was having fun and didn’t want to stop. 🙂

On the ride back home my calf kept cramping up while I drove. I have no idea what impacted it, but this morning there is a silver-dollar shaped bruise on my calf and I literally can not walk using that leg. My hip hurts, but the real problem is the calf. It’s clearly soft tissue damage and I’ll be fine, but for now I can hardly walk.

Once I got home, showered and ate, I decided to go out to my my bike shop and investigate the brake issue. When I pulled the brake pads I knew instantly what was wrong.

Now you’re probably thinking (and I don’t blame you), “What an idiot! How come you don’t check your brake pads?!”

Ignorance is what led to yesterday’s crash. This is the first bike I’ve owned with hydraulic brakes, and I was ignorant of a key difference between hydraulic brakes and mechanical brakes…

With mechanical disc brakes, one notices brake pad wear gradually; increased lever pull would happen slowly as the pads wore down, and it was easy to tell when they needed replacing.

One of the advantages to hydraulic brakes is that they are self-adjusting. What I mean by that is hydraulic brakes automatically adjust the piston as the pads wear down so that lever pull remains consistent. This is a fantastic feature, but I simply did not know about it until yesterday.

The brake pad backing is 2.20mm thick, and so this pad is worn to 0.96mm.

The brake pad backing is 2.20mm thick, and so this pad is worn to 0.96mm.

So I think what happened is the hydraulic brakes automatically adjusted as the pads wore down, but they simply could not compensate for any more pad wear. I suspect this is what resulted in a fairly sudden increase in lever travel.

Avid recommends that brake pads be replaced when the pad+backing have a total 3mm or less remaining. As you can see in the picture to the left, when I used my digital caliper to measure the pad and pad backing the result was 3.16mm, so these pads were right at needing to be replaced.

As an aside, the pistons must be reset when the pads are changed. A comprehensive guide to brake repair is beyond this scope of this article, so pick up a copy of Zinn & the Art of Mountain Bike Maintenance if you want to learn more, or have a qualified bike mechanic do the work.

Checking and replacing disc brake pads is very quick and easy. I would have happily checked the pads on a regular basis had I known about this key difference between mechanical and hydraulic brakes. Thankfully I learned this important lesson yesterday with nothing more than a few cuts and bruises (and a pretty bad limp) as a penalty. It could have been much, much worse.

So, check those brake pads regularly! I’ll be checking mine weekly from now on, and I’ll always have a spare set on-hand.

Also–and I’m clearly guilty of this–don’t ride if you know something is wrong with your bike. It’s a major drag to be on the trail with a ride-altering mechanical or safety issue, but mountain biking is dangerous enough without adding a poorly functioning bike to the mix. You don’t have to be happy about it, but it’s always best to pack it in and ride safe another day.

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