Friday, September 4, 2015

Disc Brake Failure

I recently read this pretty scary story on BikeRadar:



Apparently, one of their test riders was testing a Specialized Tarmac Pro with Shimano R785 hydraulic disc brakes on a long ride into Rocky Mountain National Park, with some major climbs and descents -- with elevations between 5,300 and 12,000 feet. On a descent down Trail Ridge (reportedly the highest continuous highway in the U.S.) the rider felt his rear brake get spongy for a few seconds -- then nothing. The brake lever bottomed out at the handlebar, and the rear brake was completely dead. His front brake was not affected, so luckily he was able to stop without further incident. A cursory roadside check found oil on the chainstay below the caliper.

The cause of the brake failure was that the brake fluid leaked out of the caliper. According to the article, Shimano collected the brake caliper, hydraulic lines, and the rotor to investigate the incident. Their finding was a cracked ceramic piston inside the caliper, which then let the brake fluid escape. Under such circumstances, each pull on the brake lever would just pump the fluid out of the caliper until it was gone.

Shimano concluded that it was unlikely that heat buildup under braking caused the piston to crack, as they didn't see any other signs on the pads or rotors to indicate excessive heat. But then, it still leaves the question unanswered why did it crack? One also has to wonder if this was an isolated incident, or have there been others?

Shimano issued a response to BikeRadar, which can be read in full HERE. I enjoyed the following excerpts, though:

"We are very sorry for the oil leak and trouble it caused on your ride." Yeah - "trouble." That's an understatement.

After explaining about the cracked piston (without being able to explain why), it concludes:

"Again, we are sorry for the trouble you experienced with this brake caliper. This is a rare occurrence for us, we will continue to study for further refinement and improvement.

"We always recommend that you inspect your braking system prior to riding. Disc brake system trouble may start to appear as a spongy feeling at the brake lever. A visual check of pad wear, contact of the pad to the rotor, and motion of the piston are all recommended. If you experience any problems we recommend you seek professional service at a bike dealer."

Of course, when the spongy feeling presents itself suddenly mid-descent (followed by total braking loss), doing the visual checks and seeking professional service at a bike dealer are kind of precluded by first being able to stop without getting killed.

Such an incident shouldn't necessarily be seen as a wholesale indictment against disc brakes on bicycles, but to my view it does highlight a certain problem with the brakes, as well as a lot of the new technology we're seeing on today's wünderbikes. On traditional cable-operated rim brakes, everything is pretty well out in the open, easy to see, easy to understand -- just as with most traditional bicycle components. On a hydraulic disc brake, a lot of the critical componentry is hidden inside. One can't visually inspect the caliper for a cracked piston the way they can spot a frayed cable. A faulty master cylinder can look no different than a good one. If there's something wrong, it can be harder to detect before it fails -- and if failure does occur, the results can be dire. Not only that, but diagnosing or repairing the problem can be much more difficult.

Somebody is bound to point out that we use hydraulic disc brakes in our cars without incident, and we put complete faith in them. But it's also worth noting that all the brake components in a car are much larger, and much more robust than those on a bike. In order to bring the weight down to a level that a cyclist can tolerate, the discs, the calipers, master cylinders -- everything -- has to be pared down to tiny scale, making them more prone (it seems to me) for failure.

Not that I needed another reason to stay happy with my traditional rim brakes, but something like this should still keep people alert to the potential downsides of the latest tech.

13 comments:

  1. Additionally, with a car, there are other methods of coming to a stop should your normal brakes utterly fail: Emergency brakes. Shifting into neutral and slowing on the shoulder (if available) Throwing it into park/reverse (admittedly, this is the "OH S..T" option, as this will blow up your transmission, but still...). Also, you're surrounded by tons of protective steel and airbags.
    On a bike, you're road pizza. That test rider was very lucky.

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    1. The part that shocks me is that he continued on with the ride, even with those descents, with only a front brake.

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    2. Well, the front brake is the most effective and important one, but, yes, given how the rear one failed, why would he trust the front one?

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  2. Hmm. A manual, lever and cable operated emergency braking system for vehicles. This automotive innovation just might be useful on bicycles. Oh, wait a minute.....

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  3. This is best described as a hydraulic failure, not a disc brake failure. Cable-operated discs are nearly as easy to check and maintain as cable-operated rim brakes.

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  4. My understanding of the hand operated cable brake in automotive application is that it is a "parking" brake used to keep the vehicle in a static status when operator is not present and not to use it for dynamic(i.e. moving) situations.

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    1. I believe that to be correct, though there was a time when that cable-operated brake, whether hand operated, or with a foot pedal as in a lot of older cars, was routinely called the "emergency" brake. In the earlier days of hydraulic brakes on cars, was there more liklihood of failure, or was it a matter of people mistrusting something new and unfamiliar? I never hear people call it that anymore -- always "parking brake." However, does that make it unsuitable in the unlikely event that the hydraulic system should fail?

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  5. Early automobiles used cable operated rear wheel brakes, no fronts. Increased speeds and vehicle weights led to the adoption of 4 wheel Bowden cable operated brakes in the '20s followed by hydraulic 4 wheel brakes in the 30s. The rear "emergency brake/hand brake" is used for parking but is also intended for last resort braking in the advent of hydraulic system failure (unlikely in these days of split/redundant brake lines).It is also used as a driving technique to induce oversteer in rally cars to aid cornering.

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  6. Of course cars have two independent brake circuits in a diagonal configuration (one front, one rear) and bicycles have independent front and rear brakes - although I have single shared "master cylinders" on some bikes which I suppose could result in sudden failure of both brakes.

    The "emergency brake" or hand brake (as it's usually called in Europe is mostly for holding a stopped vehicle and not for deceleration. I suspect it would have minimal effect at speed (try it and report back). In the UK, drivers are taught to select neutral at each traffic light and set the hand brake while waiting. Then, when the lights change, it's select 1st and release the hand brake. I've even read of the "danger" of remaining at a stop with your foot on the brake as it may warp the brake rotors! This might have been relevant about 75 years ago, and before automatic transmissions became the norm, but now it's just another hoop on the driving test - like US parallel parking (surely not an essential skill).

    Shimano's response was obviously crafted by lawyers and would fall apart under mild scrutiny - such as "How should I inspect the piston for imminent failure?" Personally, I'm pretty happy with cable-operated disc brakes, although I feel it inelegant that most actuate only one pad and warp the rotor when clamping. Even simple automotive disc brakes have sliding calipers so that the force is applied equally on each side of the rotor. Then again, most cars have quite solid cast iron rotors. I'm sure better designs will come along for bicycles. In the meantime, don't put too much faith in bicycle brakes

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  7. Having worked for some of the major corporations over the past 30 years as a engineer I can tell you that engineers no longer decide what to engineer and what to develop. MBA's, Wall Street and marketing now dictate what to develop. And these decisions are totally based on sale forecasts. Its basically a version of planned obsolescence. Disc brakes are the next phase of the corporate scheme to market a new technology in order to increase or just maintain their market share in the never ending pursuit for growth in a finite world. Even though the technology as in the case of disc brakes make no sense. Never in my 30 plus years of being a biking nut have I ever said "gee, I wish I had better brakes". Never heard anyone else for that matter. I've said and heard things like "I wish I can go faster", and "I wish I had lighter bike", but never said I wish I could stop quicker. "Compact geometry" is another case in point. This design is to save 50 cents on material. Eliminating the rake saved 50 cents on labor. It takes extra labor to bend those fork tubes, so let's not do it anymore and tell them it's better this way. Plus the robots building the bikes now days are easier to program with fewer steps. It's all about profits over the pure joy of riding, or the human experience for that mater. But this will all change soon. Frame building will soon be back in vogue. The robots in China are about to be shutdown as Trek, Specialize and all the other corporate bike manufactures can no longer operate in a globally depressed economy.

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  8. I should point out that we use hydraulic disc brakes on our mtb's for the last 20 years without any incidents. I really don't understand why they don't just take mtb brakes which are proven to be reliable under much higher demands of mtb riding and put them on damn gravel bikes or whatever.
    I guess they are not light enough :)

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    1. Actually, there have been virtually identical problems with pistons cracking on some of the MTB brake models, too. And from what I've read from a number of sources, disc brakes on the road are subjected to very high demands, probably higher than the demands on MTBs. Transferring MTB brakes directly to road bikes has lead to failures from overheating, and the way I understand it, that isn't recommended.

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