Took apart my cockpit to make some adjustments and decided to inspect my steerer. At first sight, it looks fine. No cracks, no bulging, no obvious signs of delamination, everything is smooth to the touch. The "coin test" produces a constant sound. Fork was inspected at the end of the season by LBS.

Buuuuut under a very very bright light, I'm seeing a few things I don't really understand, as I haven't seen raw carbon before. I don't think anything is broken, I just want to make sense of everything I see. Can some carbon expert chime in ?

So first picture : https://imgur.com/fpCk2zz


The horizontal arrow indicates a very small surface imprint where my stem met the tube. Nothing to worry about here.
The vertical arrow... well I don't get it. Nothing is cracked, but it has a crack like pattern. Nothing can be felt on the surface.
Red box... don't get it either. Poorly cured resin? Delam? Again, nothing can be felt on the surface.

Second picture : https://imgur.com/b1UbDFo

Blue arrow: all the steerers of this brand of bike have this line. Guess it's a demarcation between two sheets of carbon.
Red arrow: same look and feel as red arrow of first picture. It looks wayyy worse than it is in reality. Nothing ca be felt on the surface.

So I'm just trying to make sense of all this.

Your seeing the layup (i.e.how it's made), stick the stem and spacers back on, then you won't see it, and all will be well (i.e. your over analyzing it)

I agree that the lines look to be ply edges and not cracks. Note that the steerer looks to have an AL core. This is added insurance for compressive issues (stem clamping) and integrity.


Out of curiosity does the fork/bike instructions say anything about a compression plug need? Andy.

Yes they do. The plug has to be positioned at the lower stem bolt and no spacer shall be used on the top of the steerer.

Just out of safety, I replaced the plug with a Specialized compression plug, which has a greater contact surface than the stock one. Now, both the top and bottom bolt are well supported.

I'm not 100% sure it's an alu core. When viewed from the top, the grayish part does not have the same width on all circumference. Here's a picture :

But you probably know better than I do, Cannondale specs aren't very clear on the subject.

I sure don't see an Aluminum core. Looks 100% carbon to me. But it's strange to see the two resin shades. I'm guessing the layup was cured in 2 steps, probably to build up the inner diameter to achieve the right ID spec. I gotta say IMHO the vertical cracks at the Red and Blue arrows (second photo) look horrible to me, but if you say you can't feel anything with your fingernail on the outside, then I'll take your word for it that it's standard Cannondale layups.

The main stress-riser you have to look out for is from the base of the stem causing wear on the steerer, or over-tightening causing shear/crushing of the steerer. In both cases it should show up as marks/cracks around the stem rather than longitudinal.

The first picture looks like the resin failed to fully saturate the fabric, but doesn't look serious to me.

You can see an horizontal mark on the first picture, which was indeed caused by the bottom of the steerer. Inspection by LBS revealed it is within what's normal. Just to make sure, I have since changed my stem to one with more progressive edges and a greater contact area. Also installed a Specialized expander plug and nothing moved, so I guess all the vertical lines are normal (I know for sure the one with the blue arrow is, the one with the red arrow looked more weird because of the jagged pattern but it is really fine to the touch inside and out and doesn't produce a different sound when tapped)

I agree. Looks like layup and molding marks that might be slightly worn due to shifting spacers or stem.

I agree that it's not "likely" a crack, but it is a bit irregular for a production or mold mark of some kind. Given that, you probably want something better than internet assurances. So here's something that might give you the answer you want.

If it's a crack, then the likely cause is "hoop stress" form an overly tightened expansion plug. So, that's how you'll know. Inert the plug and tighten up to about what you normally do. Observe the crack for any hint of expansion. Even if it is a crack, it still might be OK because some of the resins makers use are very brittle and will crack before the structural fiber does, but I probably wouldn't ride it if I confirmed that it was a crack, except (possibly) on a city bike where I don't tress the bars much.

Your are talking about the vertical lines right?
If the resin was cracked, wouldn't I be able to feel it?
I did that test, no sign of expansion with or without the expander.

Yes, I was taking about the vertical "crack".

No you wouldn't feel cracked resin until or unless you open it up a bit. But I wasn't offering an opinion as much as giving you a way to know either way.

But if you've tightened the expander and the "crack" didn't expand, then you have your answer.

The advice is greatly appreciated. Tbh, I still find those jagged patterns super odd. I can't comprehend how a mold would have left this kind of imprint, and I really don't think it's a demarcation between two sheets as it would be straighter. Cracked resin is probably as bad as cracked carbon itself as it's part of the structural integrity.

I'm with you and from the photo it smells more like a crack than demarcation line.

But this is one of those things where we each have to make our own calls. On the bright side, that part of the tube is fairly sheltered, being sandwiched between the stem and expander. It's possible to tighten both so there's neutral stress on the tube walls.

My rule has always been to tighten the plug only as much as needed for it not to slip, and I use a bit of a traction aid so that doesn't need much force at all.

If you decide to continue riding it, and I probably would too, mark off the ends of the "crack" and set up an inspection routine to see whether it creeps further. If so, it's a crack for sure and I'd toss the fork.

It's already the full length of the steerer, down to the crown of the fork. I'll try to submit the pictures to Cannondale. What bothers me is that they will probably ask me to go through my LBS and I know my mechanic will play it off, while he's the one who overtorqued the expander. Uuuugh

Not an engineer but I've worked plenty with composites including carbon composites. Always, in my experience, a good laminate will be completely uniform in appearance. It is easy to end up with a crumby laminate and worst, it is difficult to tell just how bad it may be.