I have just noticed this on the top of my saddle bar and where the seat post clamp is. Is it a crack probably caused by the saddle pressure? enter image description here

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    Take this same photo monthly, and compare it over time. If the line changes and grows in width or thickness then its a crack. If not, it isn't.
    – Criggie
    Commented Apr 6, 2019 at 23:28

1 Answer 1


Doesn’t look like a crack, looks more like a surface void that formed during the resin injection molding. These types of imperfections are not uncommon, especially on the inside of the frame where you can’t see them. Less common to see them on the outside as they are usually caught in quality control inspections. While they are not ideal, as areas with voids will be somewhat weaker than areas without voids (depending on the size and depth of the void), they are not critical like a crack.

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    Most importantly, the irregularity seen in the image has mostly smooth edges (except where worn/machined for the clamp). A crack will generally have sharp edges. However, that very thin line below the more obvious dent could be a crack. But it's more likely just a less severe indication of the mold seam that created the dents at the top. A closer examination would be needed to see if its edge are rounded or sharp. Commented Apr 6, 2019 at 22:02
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    Even a void is bad, as it serves as a stress concentrator and results in rapid fatigue cracking, possibly leading to eventual catastrophic failure.
    – Vikki
    Commented Apr 7, 2019 at 3:07
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    @Sean a void in the structural carbon is bad because in interrupts the grain structure, but typically we can’t see these types of void because they are within the carbon layers. Most frames have a cosmetic layer of carbon wrapping on the outside, a void here is less critical. The void we see here is likely superficial, affecting the cosmetic layer.
    – Rider_X
    Commented Apr 7, 2019 at 4:02

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