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complete rotor destruction!

5K views 37 replies 20 participants last post by  jeffscott 
#1 ·
I searched and didn't find any results on rotor cracks or rotor failures. This happened to me 2 days ago and it is an absolute mystery what exactly could have caused this amount of damage.

I just started to descend, did not hit anything, did not crash, and all of a sudden there's this incredibly loud noise and I think my rear wheel locked up at this point. When I stopped I thought maybe I picked up a stick that was lodged in the wheel or something like that. But then I see the rotor all mangled. Very weird. No idea about what could have happened. Did the rotor crack and part of it hit the caliber as it spun around and then with the force of the bike descending and the wheel spinning it just went into complete pretzel mode?
 

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#5 ·
Just as well that wasn't the front disc, or that could have caused some pain for you! I don't think I've ever even heard of anyone breaking an actual rotor before.
Maybe a rock of something did get caught up in the rotor, causing it to jam/break, and consequently end up looking like that. I'd make sure the the rear wheel is still good to go and hasn't sustained any damage either.
 
#6 ·
Front failure would have been a lot of pain! As it was, I still had a good 275m of steep descending and the front wheel was difficult to keep from locking up and skidding. I almost went of the bars a couple times.

I think you may be right about the cause. I will take a picture of the brake pads later and add it here. I remember seeing a deep scratch on one of them. Something must have went inbetween and jammed the rotor. Funny, looking at the rotor again, the outer ring is not broken, just folded on itself.
 
#7 ·
just a guess... but from the look of the one bolt missing and the torn threads only on the very outer most "enter/start" section, and it looks like the inner threads are fine...? I'm guessing you had at least one loose or damaged bolt...

if the bolt WAS completely torn from the hub... the hub is toast.. DO NOT try and use it by re-threading and using a diff bolt... Aluminum massively weakens under high stress and this could easily happen again if the material has been stretched/stressed...:nono:
 
#29 ·
just a guess... but from the look of the one bolt missing and the torn threads only on the very outer most "enter/start" section, and it looks like the inner threads are fine...? I'm guessing you had at least one loose or damaged bolt...
From what I can see, I would agree with this. It could have been a loose bolt that came out and the rotor gave up the ghost to the braking forces.
 
#8 ·
You can see by the look of the rotor you have got something caught in it for sure.
Theres no other explination i can think of to create that sort of carnage, ive never seen anything like it.
Id say a stick started it then your bike might have done the rest of the damage getting a bit of the rotor caught on it
Tine for a new hub n rotor
 
#9 ·
just to add to that theres no way its from a rock getting stuck between the rotor and pads, theres only 1mm at most on each side of the pads, its just impossible for that to cause the destruction there, and if something did get caught in there theres no way that it would have the strength at 1mm or less to do that damage, just not possible.
youve got something caught in the rotor, to shred your rotor and dethread a bolt like that it would take some massive force to do that
 
#10 ·
actually there is.. the vent holes on those are large enough that something "could" get caught in there and hit the outer part of the caliper... but there would be noticeable damage to the entrance of the caliper,... and I agree it's very, very unlikely...:skep:

also the bolt/hub interface is about the strongest part of the whole thing, to actually tear the bolt out of the hub is not likely with the failure being out by the caliper.. the arms of the rotor would snap first (like all the rest did) most likely something happened to/with that bolt... :skep:
 
#18 ·
If the metal had a defect, extreme thermal stress could allow the defect to become a crack. If that crack then turned into a broken arm, chaos might ensue.

Meterial defects in stainless parts combined with corrosion and stress have brought down airliners, and one could assume there is quite a bit more quality control in aircraft parts than mtb parts (I believe it was a 747 that dropped an engine which ripped out the second engine on that wing, causing a disaster...I can probably look this up if needed - it's covered in a Modern Marvels/Engineering Disasters episode).

I don't think anything short of expensive failure analysis via a metalurgy lab could pinpoint what went wrong here, so speculation is all we've got unless there is other evidence available.

-Pete
 
#16 ·
Take a very close look at each of the arms connecting the bolts to the disc contact area. If one of the arms deviated far enough to catch on the caliper, there should be an obvious dent on both the arm and the caliper where they made the initial contact. If that's what happened, then it's on to see what caused it, that missing bolt looks too suspicious.
 
#19 ·
^^ fair enough Pete, good point, i was gonna say that thermal stress could only result in cracking the rotor, i suppose it might have cracked n made it easier for the break, but you can see the massive bending of the rotor, im not sure its cause of that, but you have a valid point mate, cheers
 
#20 ·
Aloha Aizu1,

I keep looking at the picture and that one rotor mounting hole looks clean. The bolt did not get pulled out so it must have been loose and eventually fell out. I'm guess with this but thinking the rotor eventually twisted enough till it bent around and broke. I've never seen anything like this so you'll probably be OK just replacing the rotor and new bolts. Definitely use lock-tite on the bolts. I was lucky a while back in that I used Ti bolts but didn't use lock-tite. The bolts were all getting loose, I heard some weird rattling. Caught it in time to get the bolts re-done with the lock-tite. Haven't had any issues with loose rotor bolts since then.
 
#22 ·
Rotor was not mounted backwards.
I have had issue with bolts loosening in the past. Usually I check them from time to time. However, I've been doing a lot skiing and hardly any biking lately. In this case I just topped off the air in the tires and took off.

Regardíng the bolt hole, it's fine as this bolt was missing. So it was not "ripped" out of the hub as someone suggested.

Appreciate all the theories!
 
#26 ·
Im not sure if thats secondary to the broken rotor ripping it out and across the pad, i dont think that could have caused the rotor to turn out how it did, im not convinced, but its an interesting thread OP thats for sure.
 
#27 ·
I think that's more a consequence of the damage, not the cause. I know those pad retention springs are tough, but that much? Still think the stick/stone in the rotor (which may have had previously unseen damage) is more likely.
 
#30 ·
The missing screw is the cause of the catastrophic damage. Something else may have jammed the disc/caliper, but the missing bolt is why the rotor folded. Formula rotors have no center ring and rely on the hub to support the center. Some hub manufacturers even warn that their hubs aren't designed strong enough to use Formula rotors.
 
#32 ·
Also, I'm guessing you're running something other than full length hard steel rotor bolts? If so, that's why they're loosening up. You can get away with weight weenie bolts on a rotor with a center ring, but the extra movement you get at the bolt interface with the Formula design requires full torque on a hard bolt with at least 2x the bolt diameter in length.
 
#34 ·
Nicely done! I second the disc screw(s) being the culprit. Back in 05 when working out west a manufacturer was spec'ing their own mount hardware. The downside is we saw a bunch of them coming back loose after some heavy rides. Turns out the culprit was the threads- they were over cut, meaning the thread diameter was smaller than they should've been. This resulted in loose rotors aplenty. Switched to the brake manufacturer's rotor screws and voila, problem was gone. If one (or a couple) of those screws loosened enough to allow the rotor to bend/float side to side a bit more than usual, kablammo. If you have access to another co's rotor screws, unthread one of the remaining ones and thread the other one in and feel the difference in thread engagement. If the stock ones feel looser when both screws are over half way in, that'd explain why you've had to tighten them frequently. Of course, if you've been running three screws instead of all six, then you pretty much did it to yourself. That's a lot of torque and vibration for just three screws to handle.
 
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