Disc brake failure question
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  1. #1
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    Disc brake failure question

    Hello, my first post here but trying to find out why Shimano disc brake pistons crack and shatter.
    My son and I have both run R685 levers on RS805 on our bikes, recently his bike has had rubbing issues, first call was try cleaning the pistons to clear any grit if any, pads out and cotton bud with ipa, and gently push the pistons back in with a tyre lever, one side would go back no problem but one side would only retract a little and leave itself protruding by 2mm or so, no real worries as a quick loosening of the caliper bolts would have the disc in the centre of the gap, but after another ride his bike came back dragging again, repeat but pushing the piston back felt gritty, and noticing the piston would be extending a extra mm further out. Something amiss. Checking the rear brake also showed the same asymmetric piston arrangement. I tried bleeding the system thinking maybe the seal had popped or wasn't sitting right. This did nothing even opening the system the piston would not retract. As my son needed his bike for a local run, I thought I would quickly swap out the calipers from my bike to his as I have been injured and haven't ridden for a while, but my RS805 also suffer from asymmetrical pistons on both front and rear also pushing them back also felt crunchy. So a new pair of RS8050 were bought and put on his bike.
    Meanwhile I took apart his old calipers. Waiting to take apart mine too.
    What would cause the pistons to crack inside? I am pretty sure heat would and should cause them to crack within fluid, the hottest part would be the pads on the outside, and why would this afflict two different sets of RS805s?
    Thanks for reading.
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    Welcome to the forum. Are these pistons ceramic? It appears that others had similar issue, and Shimano has responded:

    https://www.bikeradar.com/news/shima...liper-failure/

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    Thanks Nova, I've done quite a lot of searching but there hasn't been a definitive or real life answer as to why it happens, I don't have warranty on the bikes, but thats fine, much more expect the ceramic to crack on the outside than within the caliper.

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    Yikes! I'm sure glad these didn't fail while riding. Makes me wonder about my own disc brakes on my gravel bike which are Shimano RS505's. Not sure what the calipers on those are made of.
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  5. #5
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    Question about the day of failure. Were you dragging you brakes a lot?
    That can make for a lot of heat problems, more so than hitting them hard & release as long as possible. Also you could alternate front.... rear....front to acheive the same effect.
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  6. #6
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    Quote Originally Posted by Lombard View Post
    Yikes! I'm sure glad these didn't fail while riding. Makes me wonder about my own disc brakes on my gravel bike which are Shimano RS505's. Not sure what the calipers on those are made of.
    All of Shimano's calipers are aluminum. The pistons on the other hand might be ceramic. All of the higher end Shimano pistons are ceramic for the one reason they are supposed to deal w/ heat better than aluminum or phenolic pistons.
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    I believe the failure is progressive, as I am sure it isn't helped by pushing the pistons back in and feeling them crunch a tad. Where we live the descents are not that demanding, but in saying that if the ceramic pistons can't take any sort of normal abuse then there is a big problem with Shimanos design and manufacturing, lucky for me is that the initial problem was purely a non returning piston that dragged and finally seal leakage most likely from the ceramic particles between the seal and piston, after a bleed and bike was on the stand. So no nightmares on the road.
    I am waiting for parts to arrive, then I will also take apart my other set of RS 805 which I have a very sneaky feeling the pistons have also cracked, and that particular bike hasn't done very much mileage.

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    hmm could this be happening because ceramic are brittle material to be using in an application where vibration could be a bit too much???
    Jesus christ why would Shimano deviate from the tried-n-true aluminum piston? I mean, forgosake the aluminum piston has been used for oh only the last 3 decades in mtb with great success. I say this is one of the rare time that Shimano engineering has disappointed me. I guess I'll stick with mah rim brakes for now ;)

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    Quote Originally Posted by aclinjury View Post
    hmm could this be happening because ceramic are brittle material to be using in an application where vibration could be a bit too much???
    Jesus christ why would Shimano deviate from the tried-n-true aluminum piston? I mean, forgosake the aluminum piston has been used for oh only the last 3 decades in mtb with great success. I say this is one of the rare time that Shimano engineering has disappointed me. I guess I'll stick with mah rim brakes for now ;)
    I've seen someone crack a ceramic piston when they were GENTLY pushing in the piston with a composite tire lever. Still not a good reason to stick with yo rim brakes.

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    Quote Originally Posted by aclinjury View Post
    hmm could this be happening because ceramic are brittle material to be using in an application where vibration could be a bit too much???
    Jesus christ why would Shimano deviate from the tried-n-true aluminum piston? I mean, forgosake the aluminum piston has been used for oh only the last 3 decades in mtb with great success. I say this is one of the rare time that Shimano engineering has disappointed me. I guess I'll stick with mah rim brakes for now ;)
    Interesting with the vibration, but my thinking would be surely the pad rattling and vibrating against the ceramic would chip and crack easier than inside where the brake fluid is only in contact with the piston pulsating under hydraulic pressure? Rims brakes are hell of a lot easier!;)

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    Quote Originally Posted by Seven1816 View Post
    Interesting with the vibration, but my thinking would be surely the pad rattling and vibrating against the ceramic would chip and crack easier than inside where the brake fluid is only in contact with the piston pulsating under hydraulic pressure? Rims brakes are hell of a lot easier!;)
    i'm not a materials expert so who knows. But ceramic is hard and brittle, which reminds me of diamond. If you strike a diamond at the right point, then the crack could develop on other side of the rock rather than at the point of impact.

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    Quote Originally Posted by cxwrench View Post
    All of Shimano's calipers are aluminum. The pistons on the other hand might be ceramic. All of the higher end Shimano pistons are ceramic for the one reason they are supposed to deal w/ heat better than aluminum or phenolic pistons.
    Sorry CX, I meant pistons! Do you know what the pistons on the RS505's are made of?

    Quote Originally Posted by aclinjury View Post
    I mean, forgosake the aluminum piston has been used for oh only the last 3 decades in mtb with great success. I say this is one of the rare time that Shimano engineering has disappointed me.
    I've had a few reservations with Shimano engineering lately, like rear shifters that digest cables every 1500-2000 miles. It's only a matter of time before I become a SRAM guy.
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  13. #13
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    Quote Originally Posted by Lombard View Post
    Sorry CX, I meant pistons! Do you know what the pistons on the RS505's are made of?

    Ceramic.
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    Quote Originally Posted by Lombard View Post
    Sorry CX, I meant pistons! Do you know what the pistons on the RS505's are made of?



    I've had a few reservations with Shimano engineering lately, like rear shifters that digest cables every 1500-2000 miles. It's only a matter of time before I become a SRAM guy.
    i'm still giving Shim the benefit because in terms of price, performance, quality, reliability, they're still king, for now. They did correct the cable chewing issue with in the 9100/8000/7000 series. I never used their 9000/6800/5800 series.

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    Quote Originally Posted by Seven1816 View Post
    Interesting with the vibration, but my thinking would be surely the pad rattling and vibrating against the ceramic would chip and crack easier than inside where the brake fluid is only in contact with the piston pulsating under hydraulic pressure? Rims brakes are hell of a lot easier!;)
    After giving my vibration theory some more thought, I think this could be the reason why the pistons are breaking at the back end (where it come in contact with the brake fluid). Here's what I think why:

    As you squeeze the brake lever, the piston is pushed outward; i.e, a portion of the piston is protruding out from the caliper block. Well, because the piston is protruding out, now you have a "lever" (with a fulcrum) for the vibrating brake pad to act on. And in such senario, the weak-link is at the fulcrum where it'll break

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    Quote Originally Posted by aclinjury View Post
    hmm could this be happening because ceramic are brittle material to be using in an application where vibration could be a bit too much???
    Jesus christ why would Shimano deviate from the tried-n-true aluminum piston? I mean, forgosake the aluminum piston has been used for oh only the last 3 decades in mtb with great success. I say this is one of the rare time that Shimano engineering has disappointed me. I guess I'll stick with mah rim brakes for now ;)
    Given that the piston effectively acts as a direct heatsink from the pads, it would get much hotter than the caliper body as they are not in direct contact to each other. If both the caliper body and the piston are aluminum, the thermal expansion differential between the two will likely seize the piston to the caliper. Ceramic is much more thermally stable than aluminum, which is likely why it was chosen by Shimano.

    My theory on the root cause of this failure is bad caliper to rotor alignment. If the caliper/piston are not parallel in all axis to the rotor, it can cause off-axis loads on the piston skirt against the cylinder/caliper body that would eventually lead to a failure. More often than not, I see caliper mounts that need to be faced and/or badly aligned calipers.

    OP did mention of a rubbing issue, but I suspect the damage was already done by the time he noticed it. I also suspect most folks use the pads rather than the pistons as a reference for alignment. While this may be fine with brand new pads, it should not be done using worn pads as if they have uneven wear, the caliper can be misaligned, further exasperating the situation.

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    Quote Originally Posted by nova_rider View Post
    Given that the piston effectively acts as a direct heatsink from the pads, it would get much hotter than the caliper body as they are not in direct contact to each other. If both the caliper body and the piston are aluminum, the thermal expansion differential between the two will likely seize the piston to the caliper. Ceramic is much more thermally stable than aluminum, which is likely why it was chosen by Shimano.

    My theory on the root cause of this failure is bad caliper to rotor alignment. If the caliper/piston are not parallel in all axis to the rotor, it can cause off-axis loads on the piston skirt against the cylinder/caliper body that would eventually lead to a failure. More often than not, I see caliper mounts that need to be faced and/or badly aligned calipers.

    OP did mention of a rubbing issue, but I suspect the damage was already done by the time he noticed it. I also suspect most folks use the pads rather than the pistons as a reference for alignment. While this may be fine with brand new pads, it should not be done using worn pads as if they have uneven wear, the caliper can be misaligned, further exasperating the situation.
    I see your logic about Shim using ceramic, but is that really the case though? Before ceramic, manufacturers have been using aluminum pistons too.

    anyway, if heat is an issue, why don't they just use beefier pistons, calipers, and bigger rotors?? Oh wait, roadies don't are adverse to additional weight and aero drag, amirite!

  18. #18
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    Quote Originally Posted by aclinjury View Post
    i'm still giving Shim the benefit because in terms of price, performance, quality, reliability, they're still king, for now. They did correct the cable chewing issue with in the 9100/8000/7000 series. I never used their 9000/6800/5800 series.
    That's news to me. I'm still pulling frayed/broken cables out of the newest shifters. Well, the newest Shimano shifters...not a problem w/ SRAM. I will say Shimano has made it a lot easier to get the frayed/broken cable out of the shifter w/ the newer ones.
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    Quote Originally Posted by cxwrench View Post
    That's news to me. I'm still pulling frayed/broken cables out of the newest shifters. Well, the newest Shimano shifters...not a problem w/ SRAM. I will say Shimano has made it a lot easier to get the frayed/broken cable out of the shifter w/ the newer ones.
    really?? welp guess I'll have to make a mental note of this and keep an eye out for shifting this groupset then! Damn my DA 7900 shifters have been running the same cables for like 3 years now!

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    Quote Originally Posted by aclinjury View Post
    i'm still giving Shim the benefit because in terms of price, performance, quality, reliability, they're still king, for now. They did correct the cable chewing issue with in the 9100/8000/7000 series. I never used their 9000/6800/5800 series.
    Seriously??

    Quote Originally Posted by cxwrench View Post
    That's news to me. I'm still pulling frayed/broken cables out of the newest shifters.
    That's what I thought.

    Quote Originally Posted by cxwrench View Post
    I will say Shimano has made it a lot easier to get the frayed/broken cable out of the shifter w/ the newer ones.
    Well damn! If you're not going to correct the problem, then the least you can do is make it easier to repair when it breaks!

    Quote Originally Posted by aclinjury View Post
    Damn my DA 7900 shifters have been running the same cables for like 3 years now!
    Years mean nothing. How many miles is that? The 5700/6700/7900 was the first generation to have the cable eating problem.
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  21. #21
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    Maybe you are better to push the pistons back with the brake pads in place; i.e., load over a wider area less likely to crack the piston.

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    Quote Originally Posted by mfdemicco View Post
    Maybe you are better to push the pistons back with the brake pads in place; i.e., load over a wider area less likely to crack the piston.
    ^This^ I ALWAYS use the old pads to push pistons back when replacing them. Excellent suggestion.
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    Quote Originally Posted by cxwrench View Post
    ^This^ I ALWAYS use the old pads to push pistons back when replacing them. Excellent suggestion.
    This sounds like a given. I wouldn't think doing otherwise would make any sense especially with any sharp instruments.
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    Quote Originally Posted by mfdemicco View Post
    Maybe you are better to push the pistons back with the brake pads in place; i.e., load over a wider area less likely to crack the piston.
    from OP's description, the cracks occurred not at the end of the piston facing the pad, but at the end facing the oil, i.e, the end that's inside the caliper.

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    Disc brake failure question

    Quote Originally Posted by aclinjury View Post
    from OP's description, the cracks occurred not at the end of the piston facing the pad, but at the end facing the oil, i.e, the end that's inside the caliper.
    I was thinking in general, when dealing with those pistons, which are obviously brittle and fragile, that it would be a better practice to push them back in this manner. Anyway, maybe the piston got cocked when pushing them in and the one side bottomed in the bore and cracked the piston in this manner. I would think the piston could be pushed in more squarely, if they were pushed in by the pads. I don't know what happened; just postulating a cause.

    Anyway, the manual says this

    "Use a flat-shaped tool to push the pistons straight back in as far as they will go, while being careful not to twist the pistons.
    Do not push the pistons with a sharp tool.
    The pistons may be damaged."
    Last edited by mfdemicco; 01-14-2020 at 07:30 PM.

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