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Pictures: https://imgur.com/a/0cWtcsS

scratches

My nail gets caught in the center and it feels sort of rough. I had my stem tightened at 4NM and the headset seemed to be properly adjusted. I have once reinstalled the stem; this time, tighenting the top cap a bit more (slightly more than needed to remove play) and setting the stem bolts torque to 5NM.

Any opinions on what this scratch could be from and how can I prevent it?

Really appreciate it, thanks!

Update:

I inspected again and it seems there is another mark from the spacer below. I think the marks may have actually been from the spacers rather than the stem. I thought I had tightened the top cap enough. Here are some pictures. https://imgur.com/a/6nkznf2. It’s very faint but there’s a line below.

  • On my Ritchey stem it recommends 6Nm of torque for the bolts clamping the steerer tube. I doubt it will make a difference but you certainly don’t want the stem to come loose. – Michael Sep 11 '18 at 10:44
  • I'd consider adding or removing a small 2~5 mm spacer to move the stem up/down a bit, and next time the stem comes off see if you have two scratch lines. If yes, then replace the fork and stem. Could apply some kind of top-coat to help seal the CF. – Criggie Sep 11 '18 at 20:17
  • Would you say the surface scratches are enough to warrant replacing the step/fork? – Dev Sep 11 '18 at 22:11
  • I inspected again and it seems there is another mark from the spacer below. I think the mark may have actually been from the spacers rather than the stem. I thought I had tightened the top cap enough. Thanks for all the help, here are some pictures. imgur.com/a/6nkznf2. It’s very faint but there’s a line below. It may be that the compression plug slips slightly? I feel that the lines are very superficial, but I’ll get half spacers tomorrow and install the compression plug at 8 Nm (range is 6-9) – Dev Sep 11 '18 at 22:28
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Looks like a surface scratch, so no need to worry.

Obvious source of a circumferential scratch on the steerer is the stem being rotated around the it, although in this case I suspect the stem moving up and down slightly from the normal varying forces on the handlebars during riding. Just check for a rough edge inside the stem.

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    In any case I'd put very fine sanding paper around a finger and pass it on the inner edge of the stem, just to be sure. – Carel Sep 11 '18 at 7:10
  • I inspected again and it seems there another mark from the spacer below. I think the mark may have actually been from the spacer rather than the step. I thought I had tightened the top cap enough. Thanks for all the help, here are some pictures. imgur.com/a/6nkznf2. It’s very faint but there’s a line below. – Dev Sep 11 '18 at 22:25
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Scratches in carbon fibre composite are a problem when any of the fibers are exposed or damaged. The plastic surrounding the fibers (in contrast to the plastic between individual densely packed fibers) has no (negligibly little) structural function, it just protects them from wear.

(the plastic between the fibers also fixes them in their form and bonds them together and the same plastic might be used to glue parts together)

On the picture it looks like probably the scratch is only in the plastic, but I'm less convinced than the other answerer, you should check again considering what I described above.

  • "The plastic "matrix" surrounding them has no structural function" - Sorry, but this is nonsense. The carbon fibre acts as a reinforcement for the plastic matrix, so yes, the plastic matrix provides a significant structural function. Without it there would be no solid material just a flexible sheet of woven fibres. – T_Bacon Sep 19 '18 at 13:25
  • @T_Bacon Sorry, but this is nonsense. The only structural function of the plastic is bonding together the fibers. There are high end carbon fibre parts which do away with the rest of the plastic and consist of just the impregnated fibers without the smooth plastic layer usually surrounding them. This is not like reinforced concrete, where the concrete still has a significant structural function. The less plastic, the better the strength/weight relation. – Nobody Sep 19 '18 at 14:21
  • And yes I know what I'm talking about, I've researched this quite a lot while I was preparing to hand laminate my first carbon fibre parts (which I have since done, and they work fine). – Nobody Sep 19 '18 at 14:23
  • Also I don't mean to be offensive with my first sentence, quoting it back to you is just supposed to show you that you might not want people to talk that way. – Nobody Sep 19 '18 at 14:24
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    @T_Bacon I think then we agree on the facts. The phrasing is debatable. I highlighted the word "surrounding" because that's the plastic I meant and removed the word matrix because I guess that implies the other plastic which I didn't mean. I thought about using the word impregnated, but that's kind of shady because the plastic is not in the individual fibers, it's only between them - impregnating is only really right with respect to the whole carbon fibre cloth/braid/band, and I wasn't going to explain about those. – Nobody Sep 19 '18 at 14:59

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