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I made a bit of a mess of my fork's carbon steerer while cutting it:

enter image description here

I deliberately cut the steerer around 10 mm longer than I needed it. While cutting, the steerer was secured in a stem up to this 10 mm mark, with the clamping area on the other side from where the splintering has occurred, so that the fibres were well supported here. It looks like the splintering doesn't extend further down the steerer from where it was clamped in the stem.

enter image description here

How should I proceed? I can afford to take a further 10 mm off the steerer if necessary (basically down to the bit where the splintering seems to stop).

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  • I will think of how to answer, but meanwhile, are your cutting tools in good condition? For a hacksaw blade, I believe you want at least 32 tpi, and be aware that they won't last many cuts.
    – Weiwen Ng
    Apr 29 at 20:13
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    @WeiwenNg I'm using a tungsten carbide gritted blade. I've cut two other steerers with it in the past, so it's not particularly old.
    – Will Vousden
    Apr 29 at 20:18
  • @WillVousden I'm using a tungsten carbide gritted blade. I've been using bog standard (but relatively cheap) wood-cutting blades in a miter saw with good results. I'd be worried that the grit blade has to push against the carbon fibers pretty hard before it wears through them - that would explain the delamination at the bottom of the cut - the grit blade is pushing against the fibers and probably vibrating them like crazy and breaking them loose before finally getting through them.
    – Andrew Henle
    Apr 30 at 21:36
  • (cont) And FWIW, that cut surface looks rough, with a lot of gouges from the grit.
    – Andrew Henle
    Apr 30 at 21:37

3 Answers 3

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With carbon fiber structures, the designers are counting on every layer of carbon being there. They build in a safety margin, naturally, but I would guess they are counting on the structure not being compromised by something like a tear - technically, this should count as a delamination, which means the material has fractured into layers.

Visually, it does look like the tears extend more than 10mm below the top, but scale is hard to judge on a photo without some sort of indicator. If you cut the damaged part of the steerer off, though, you should be able to see if there's tearing further down the structure. If you do this, I would inspect it regularly. For example, after the first ride, maybe weekly or monthly for a while, then probably annually. Carbon is stronger than people think, and it is not likely to shatter into pieces if you knock the bike. However, it can fail in ways that not everyone might anticipate. While the OP took precautions, this is one of those ways. A more risk averse rider might sell the fork to someone who is going to cut it even shorter. I am leery of this because I am not a composites expert, and because the fork steerer is safety critical. If your seatstay gives way during a ride, even if it breaks all the way through for some reason. you can seriously just finish the ride and find a repair shop afterward. Seat stays are lightly loaded. Steerers are not lightly loaded, and a broken steerer means an instant loss of control and possible facial injuries.

For other users, I would generally recommend having a bike store do the cut. They have the tools. Their mechanics do this task repeatedly, whereas ordinary consumers will probably only cut their steerers once or twice with each bike, so on average a bike store mechanic will be further up the learning curve than almost all consumers. Plus if the store messes up, they can be liable (and at the very least they'd be able to purchase replacements at dealer rates).

For tool nerds, Dave Rome at the Escape Collective wrote a detailed article on cutting carbon steerers, including steps to take to avoid delaminations. This is why I pressed the OP about their tool use in a comment.

For the record, a worse delamination is illustrated in that article, photo reproduced below. Another worse one is in this user page on Bike Forums.

enter image description here

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    It may help to wrap electrical tape around the steer tube prior to cutting, centering the tape over the area to be cut. I would use a blade of 32 tpi or higher and labelled for use on carbon fiber composites.
    – Jeff
    May 1 at 0:40
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Next time you cut, cut fully around the circumference of the steerer to nip any splinters/delamination in the bud.

Tape helps only so much.

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In the end, I took a different approach here.

The manufacturer assured me that the extent of the damage is no big deal, and that keeping the damaged section of the steerer was preferable to removing so much of it that the stem extends beyond the top of it. I wanted to deal with the loose splinters somehow, to prevent propagation, but I figured that epoxy (the manufacturer's suggestion) would have a hard time penetrating well enough to fill all the gaps.

So I decided to file/sand away the damaged part of the steerer, since all but a couple of millimetres would be above the stem anyway, and hence not subject to any significant load. This part of the steerer was structurally compromised anyway, so I saw no reason not to just remove it.

This involved removing several layers of laminate on the frontal section of the steerer. To keep the delamination from propagating further down the steerer, I worked on about 3 mm of the steerer at a time, using a tightly secured strap to demarcate the work area and hold the splinters in place below. I kept going until the cracked area shrunk to nothing, then moved down another 3 mm and repeated, until there was no more visible cracking and the surface was smooth.

In the end, the volume of material removed was about 10 mm tall (from the steerer cut down to the bottom of the splinters), 7 mm wide (around the front of the steerer), and 0.5 mm deep. I found that rubbing a small amount of isopropyl alcohol onto the work area and letting it seep into the laminate (before drying up) showed up any remaining cracks that weren't immediately visible. From the photos below you can see contours indicating around 3-4 layers of laminate were removed in this area.

The worked area extends around 2 mm below the top of the stem, meaning there's a slight gap between the stem and the front of the steerer tube there. I plan to fill in the removed volume with J-B Weld epoxy resin when I have the time, to restore the cylindrical profile.

Below are some photos of the work and the result.

Work beginning. You can see the outline of the damage, with the help of some isopropyl alcohol that was applied beforehand. enter image description here

Filing the steerer down, with the help of a strap to support the delaminated splinters. enter image description here

The full width of the steerer, below the damaged area, is 28.65 mm. enter image description here

The width after removing the delaminated material is 28.17 mm. enter image description here

Here you can kind of see the contours of the layers that were removed. enter image description here

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