Tech blog: Stuff breaks sometimes

Why is everything so unnecessarily complicated. I have a set of thm carbon forks ,weigh 218 grms . They have a built in expander . Just put on you top cap screw it in " make sure your bars are straight " done . Simple as that. . If all forks were made like that , no issues whatsoever.  Never comes loose.  Simplicity . 
I had some UMS stiletto forks  that were the same . 

Similar to my experience but my steerer was aluminium. The break was about 22mm below the bottom spacer, just about inside the head tube. Fortunately I  wasn't racing or speeding, there was this funny rubbery wonky feeling holding the bars before I safely stopped. Never found out why, even my LBS was confounded never having seen my problem before. 

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?                          

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?

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.

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)?

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. 

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?

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.

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

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?

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.

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?