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Bikes and components occasionally fail - especially in races - here's some thoughts on why that happens and some advice on how to minimise the risks of it happening to you

The bike failure that caused Simon Pellaud’s dramatic crash in the final sprint  is a reminder that stuff breaks sometimes. Despite the sudden failure of this bike, he still managed to cross the line in fifth place. 

What are we to make of this bike failure? Probably nothing, other than it serves as a reminder that bikes and components can fail from time to time. Just like a car can break down for no clear reason or the office printer refuses to work. 

When a bike or component breaks human error is usually to blame - where in the chain that leads  from the designer,  through the builder, then mechanic, to finally the rider determine how much other riders of that particular bike or component have to worry. Usually any human error involved is much closer to the end of the chain than the beginning. When the error is at the front of the chain recalls and lawsuits are the consequence – and bike companies are very keen to avoid those so have all sorts of systems and mitigations in place to guard against things going wrong in the process of designing and building a bike. Of course sometimes, thankfully rarely, they still do.  

We have no details about the incident involving Pellaud's Bianchi, what led up to the failure and what happened in the aftermath. The internet can and will speculate, lay the blame at the bike brand or the choice of material, carbon fibre in this case. 

From watching the video a few dozen times, we can determine it’s a Bianchi Specialissima and the failure has happened where the stem clamps the steerer tube. It doesn’t appear the rider was sprinting before the sudden failure, but it does fail suddenly and without warning, as carbon is prone to.

This area of the steerer tube is under a lot of stress. If we are to speculate, it could be a result of stem bolts done up above the recommended torque settings. That would lay the blame at the rider or mechanic, rather than the bike brand. 

Many years ago Trek issued a reminder specific to carbon steerer tubes following a number of steerer tube failures with its Trek Madone in 2010. It stated that a torque wrench should be used, only the recommended stem fitted, and spacers should be added above and below the stem. It specified a 5mm spacer above the stem to prevent the stem from creating a stress riser on the steerer tube. We all know that racers love to “slam” their stems and spacers usually get chucked in the bin for aesthetic reasons. 

Bianchi also issued the same advice a couple of years ago about a 5mm spacer being used above the stem [interesting because we'd assume it would make more difference below the stem]. On its website Bianchi says: “Bianchi has recently learned that stem assembly’s on some bicycles with a full carbon steerer tube may have been incorrectly assembled with spacers above the stem totaling [sic] more than 5 mm.  Bianchi is requiring all consumers that are using bicycle stem assembly’s with this configuration on a full carbon fork to immediately stop using your bicycle and inspect your fork for damage and proper installation.”

Here are the Bianchi carbon fork installation instructions in case you’re interested https://www.bianchi.com/wp-content/uploads/2019/09/Fork_Installation_ins...

This could support our hypothesis that incorrect installation led to the steerer tube snapping. If not, then we’re looking at a manufacturing problem. But the Bianchi bike in question has been around since 2015 and surely any problems would have arisen by now? https://road.cc/content/review/170534-bianchi-specialissima

A Trek bike was famously involved in the 2006 crash of George Hincapie at Paris-Roubaix, when his Trek 5200 failed in very much the same way at this more recent incident. That bike was modified with a cantilever-brake-compatible fork from a Trek hybrid bike, that had an aluminium steerer tube, and Hincapie had crashed earlier in the race and kept going.

We’ve contacted Bianchi for a statement and are awaiting a reply, and it’s no surprise the picture of the broken bike posted by Simon Pellaud (who is okay by the way) has vanished from social media. Hopefully Bianchi can launch an investigation into the cause of the failure and share its findings, we’d love to know the exact reason and put to bed this speculation. 

Should you be concerned about a sudden fork failure? It’s obviously imperative that your bike is assembled following the manufacturers instructions and using the correct torque settings. But other than regular checks how are we likely to be able to detect a failure before it can lead to a crash? You’d need a scanning machine to detect early signs of fatigue in a carbon steerer tube. 

But since that famous Hincapie crash, the UCI has stepped in with testing protocols based on safety, and there are international standards that bikes must meet to ensure they are as safe as possible. But it’s a reminder that there is always a risk involved and no easy solution.

14 comments

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LastBoyScout [664 posts] 2 months ago
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I'm interested in the details of specifying a 5mm spacer ABOVE the stem on a carbon steerer, but no mention, or hint of any problem, with the number, or lack of, spacers below the stem.

I'm aware you shouldn't clamp the stem at the top, due to the hoop strength being reduced at that point and so a spacer is required, but I've never before heard of any issues with too much steerer/too many spacers, which are surely redundant above that point?

My Sunday best bike has a carbon steerer and I've been running that with considerably more than 5mm of spacers both above and below the stem - terrified of cutting off too much steerer, as you can't stick it back on!

After the initial cut, I've lowered the stem a bit and cut a bit more off, but there's still a good 20mm above and about 10mm below - I might want to raise the stem in the future.

Am I running a potential death trap?

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srchar [1664 posts] 2 months ago
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No. Have a look at pretty much any bike being tested on these pages - plenty of demo bikes are ridden in anger by testers while wearing 20mm of spacers above the stem, so that the next reviewer can change the fit.

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tom_w [239 posts] 2 months ago
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LastBoyScout wrote:

I'm interested in the details of specifying a 5mm spacer ABOVE the stem on a carbon steerer, but no mention, or hint of any problem, with the number, or lack of, spacers below the stem.

I'm aware you shouldn't clamp the stem at the top, due to the hoop strength being reduced at that point and so a spacer is required, but I've never before heard of any issues with too much steerer/too many spacers, which are surely redundant above that point?

My Sunday best bike has a carbon steerer and I've been running that with considerably more than 5mm of spacers both above and below the stem - terrified of cutting off too much steerer, as you can't stick it back on!

After the initial cut, I've lowered the stem a bit and cut a bit more off, but there's still a good 20mm above and about 10mm below - I might want to raise the stem in the future.

Am I running a potential death trap?

 

If Bianchi have gone to the trouble of putting together a whole special page devoted to saying that having more than 5mm of spacers above the stem is bad, then if your bike is a Bianchi it seems likely it might be.

Their installation instructions show they use expanding plugs down the steerer tube for the top cap, so my speculation would be that the fork is designed with the assumption that the stem will be there to resist the expansion of the plug.  I'm guessing that if the expander ends up above the stem it could potentially split the steerer tube? 

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Sriracha [348 posts] 2 months ago
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I'm missing something. On the one hand they say to install up to 35mm of spacers below the stem on order to suit the position of the rider. So far so good, use spacers below to set the stem at the right height for the rider.

Then it says, "WARNING! Do not install more than 1 spacer of 5.0mm thickness above the stem. Application of more than 1 spacer of 5.0mm thickness may lead to a catastrophic fork failure, resulting in serious personal injuries. "

So, in fact you can not determin the height of the stem - it must be within 5mm (just less) of the top of the steerer. And you then have no choice about how many spacers to use below.

So clearly I have missed something.

Edit - is it that you must not use more than one 5mm spacer above, but you could add a second spacer of something different, like 10mm?

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John Stevenson [464 posts] 2 months ago
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I think tom_w has it right. 

Bianchi is saying that you have to use a 5mm spacer between the top of the stem and the top cap so the stem clamps the outside of the steerer to reinforce the section where the expander wedge presses from the inside.

I have to say I think this is bonkers.

For starters, designing your fork steerer so that it's sufficiently fragile it can be damaged by an expander wedge pressing on the inside of it seems … irresponsible. 

Murphy's Law as a design principle surely applies: whatever can go wrong, will go wrong, so if you design something so that it can be assembled wrongly, it will be assembled wrongly.

That said, I'm rather surprised that the expander puts that great a load on the steerer. It only has to resist a light load from the stem bolt to preload the headset bearing. In the early days of threadless headets top caps were made of plastic for this reason. People routinely over-tightened and broke them so now they're aluminium. Murphy's Law in action.

All seems like a really odd mix of over-design and under-design.

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matthewn5 [1432 posts] 2 months ago
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Colnago has always insisted on using a much longer internal expander than is usually fitted... perhaps this is to resist both the compressive force of the stem and the bending forces at the top of the steerer?

https://www.chainreactioncycles.com/colnago-fork-expander/rp-prod70234

//www.westbrookcycles.co.uk/images/colnago-fork-expander-for-carbon-forks-p279267-377309_image.jpg)

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Miller [316 posts] 2 months ago
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I do think the internal expander is key here. It doesn't just provide the anchor for the top cap preload bolt, it also reinforces the carbon steerer tube against the compressive force of the stem clamp. The point about one 5mm spacer above the stem is likely to ensure that the supplied expander is aligned with the stem clamp and not displaced above. Bear in mind that the stem transfers considerable loads into the steerer tube. Force that would be experienced during a sprint, for example.

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LastBoyScout [664 posts] 2 months ago
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I had thought about the expander plug - I should have mentioned that in my OP.

However, I also thought that you only need to do that up "just" tight enough for it to grip enough to allow the pre-load through the top cap, as John said. I'll accept that excessive tightening "could" split a steerer if it's not inside the stem clamp, but what if you put that in and tighten it up "before" fitting the stem - same problem.

The steerer shouldn't be relying on the expander to reinforce it, surely? If it does, you're over-tightening the stem bolts.

As it happens, the bike in question IS a Colnago, but I also have a Kona with a carbon steerer and 20mm on top of the stem - when I get home tonight, I'll pull both expanders out and compare them. I'll also check the owner manuals.

I'm not aware of any other method of pre-loading a carbon steerer than using an expanding plug?

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Miller [316 posts] 2 months ago
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I agree that you only need to do up the expander plug tight enough so that it doesn't slip upwards when you adjust the preload bolt on the top cap. No issue with that. The expander being a rigid thing filling the inside of the steerer tube (oo er missus, etc) is what provides reinforcement. As we're talking about 30-40mm of carbon tubing weighing just a few grams, which is expected to cope with hefty bending and twisting loads applied by a rider through the stem, I personally am happy with it being internally reinforced. 

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mdavidford [123 posts] 2 months ago
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Never mind all that - the important question is why is there a '[sic]' after "totaling" (acceptable US spelling), but not after "assembly’s" (just wrong wherever you are)?

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WeLoveHills [21 posts] 2 months ago
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Miller wrote:

I do think the internal expander is key here.

That's right. I have a Bianchi. The expander is very short. If you have more than 5mm above the stem, the expander will be too high up. However, what I don't understand is why, according to Bianchi, the problem is fixed in 2018 models and later ones. Mine is 2018 and the expander is short.

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srchar [1664 posts] 2 months ago
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Cervelo don't use an expander. A metal insert is bonded into the inside of the steerer and a star nut tapped in. The metal insert adds strength and rigidity. To be honest, I don't see the need for it, but Cervelo does, and I can't see an expander doing the same job.

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Cyclewalkbob [3 posts] 2 months ago
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I have just picked up my new Bianchi Impulso E-Road and the dealer has built it with 2 x 1cm spacers above the stem, their rationale was try the bike, be comfertable  and get the right stack height and we will then cut the fork to your chosen height.

 

Based on the comments above should I be worried?

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ktache [2343 posts] 2 months ago
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I am probably wrong but I thought that the carbon steerer needs to come up a bit above the stem so that the stem is clamped in the "middle" of the steerer rather than at the "end".  I have steel steerers on both of my aheadset bicycles (technically NoThreadset), on my good bike I never got around to getting it cut down so it protrudes a bit above the stem,  my new bike, I got cut down and the fine mechanic cut it down to about 1/4 inch below the top of the stem, big thick steel it is.

That 1/4 inch of carbon above the stem might make a lot of difference, the cut can never be perfect, but the weave should be better.  Is it better to have the pressure on the weave and not the slightly rough cut?