Sudden Steerer Tube Failure on Canyon Endurace

I think you are right to question Canyon's response. The reason it sounds like nonsense BS is because it is. 

Please can I suggest that you get a second or third opinion from a qualified engineer and or repair specialist, and get it writing. Would be great if you come back to this forum to tell us what happened, please? I sincerely hope you are looked after by Canyon, in which case, do tell, but if the opposite happened, please do share. 

Lastly, well done for surviving a snapped steerer!

Firstly, sorry to hear about the crash, but at least you're okay.  Forum discussions regarding Canyon failures are all too common.  It may be a function of high volume sales and 'normal' failure distribution, or they suffer from poor design / quality control / materials.   Either way,  none of the above explains their crappy, 'f**k you' customer service which I read about time-and-time again.  

My 2003 Litespeed (winter bike) has done well over 120,000kms without failure - riding done safe in the knowledge that the frame carries a lifetime warranty.  

Sounds like a load of pony from Canyon, as above you'd have noticed an issue in how stiff steering had become or how loose the steerer was, it really would be noticeable even just taking a hold of the bars and moving them when off the bike just walking from your front door to the kerb.

In any case would you expect a headset bearing to be so shit that it does that much damage, at all never mind after that period of time. Christ my Specialized Globe which has an alu steerer on the carbon fork has tens of thousands of miles on it in all the worst conditions 7 years of UK commuting will bring. I'm a heavy lump too and I've never had to adjust the headset at all.

For a headset bearing to go pearshape like that to me says it was faulty from the off, the spacers shouldn't be cutting in like that anyway.

think you should get another assessment and the original bearing returned if they are flipping you off.

I agree that the expander bung seems the more robust design but on the other hand if play in the headset was causing steerer tube failures they would be going frequently because most if not all headsets develop some play as they wear. 

The expander bung does provide some support to the steerer if it's correctly positioned within the depth of the stem, which can counteract over tightening of the stem bolts to some degree. I'd still say the most likely explanation is over tightening of the stem or a steerer defect. If it's the latter then headset design is irrelevant. If not, it doesn't help but it's not the cause.

If you have a Canyon you could always add an expander if you were concerned, Exustar make them.

The OP could tell Canyon that either the spacers must be faulty to damage the steerer or the steerer must have had a fault to fail as it did and what are they going to do about it? 

Maybe email a few of the companies who repair carbon bikes. They may have seen a similar pattern.

I had a Canyon a while ago which used the same headset. In my opinion it is a flawed design (Campagnolo used it on one model as well).

I have built dozens of bikes and stripped similar amounts of headsets. Maybe I was unlucky but I always found with time that model would slip so thr headset became loose. If that was the cause I could not say.

My theory was the small grub scew that sets the tension must ever so slightly loosen with vibration etc. It does not need much movement to create play in the bearings.

The Canyon headset uses an expanding headset top cap rather than a compression bung I understand but I don't see that being particularly relevant here. Did the steerer fail within the depth of the stem or where the stem meets the spacers? I'm assuming the latter.

I can see a scenario where a seized upper bearing causes relative movement between the top cap and the stem with any spacers between them moving relative to both. Usually it's the lower headset bearing that goes first however and as its more heavily loaded it's noticeable in the steering whereas the upper bearing may not be.  Is the upper headset bearing seized? Did you hear any noise from the front end before it went? I imagine it would creak if the spacers were rubbing on either the stem or the headset.

What Canyon are saying I think is just about plausible but there are a couple of things I'd question. First the spacers are usually a close fit on the steerer and secondly shouldn't have any protruding edges to cut into the steerer. Bit of a manufacturing issue if they do I think. If they did or were a loose fit then yes scoring of the steerer could cause a stress riser and sudden failure of the steerer. 

The more likely explanation is either excessive stress caused by over tightening the stem bolts or a defect in the steerer but they have seen the bits and I've only seen pictures. 

Am I right in thinking that Canyon headset's are tensioned by a device in the headset cap, and not via a an expander bolt pushing down on the stem as a normal aheadset?

If so, I can think of a couple of challenges. First, with no need for an expander bolt, does the lack of this mean that there is less material to tolerate the forces put through the stem / fork interface?

Similarly, in a normal headest, does the pre-loading of the stem against spacers / headset top cap dissipate stress into a wider area? 

Looking at the pictures, the thickness of the steerer seems minimal, is there a question around the integrity of the steerer in the first place? 

I can't see the photo without signing up for a Live drive. Think I'll skip that.

I have never seen this before, but you can clearly see where grooves have been worn into the steerer at intervals that seem consistent with the headset spacers, but it doesn't make sense to me initially though.

Did the headset bearing and top cap not move at all? I can only see what they mean if turning the handlebars rotated the steerer, but that the seized headset stayed still and in some way managed to keep the headset spacers still with it. 

The spacers slide down over the steerer, so shouldn't be able to dig in and score the surface, although I have had bikes in where the expanding wedge has been seriously over-tightened and caused the steerer to bulge and jam the spacers in place.

I do struggle to see quite how a headset would cause this, I would have thought it would have to be so seized, and that you would have to put so much effort into turning the bars to steer that the bike would be unrideable.

I'd ask for an explanation that states what the worn bearing did to cause the marks, and say you will take it to an engineer for an independent report if they don't supply a reason that makes sense.

Edit to say that I did just think of a scenario where the headset adjustment was really loose, so the whole thing was slopping about and could cause the steerer to move back under braking etc, and with the extra space caused by the slop, the spacers could be forced into the steerer at a slight angle repeatedly.

What was the state of the bearings and adjustment of it all?