I recently discovered this horizontal line on my double walled aluminium rim (Triban road bike), pics attached, and was wondering if this was a crack or just a manufacturing artifact. Could anyone confirm if they've seen something like this? To me this looks too perfect a straight line to be a crack. Any ideas/opinions would be appreciated.

Many thanks JJ

left side - enter image description here

right side - enter image description here

  • 1
    Just for info, pinning is one way that the ends of the rim extrusion are joined. I think that typically, higher end alloy rims are welded, but it's also possible to make very high quality pinned rims with joints that aren't visible. First part of the linked answer talks about construction methods. Your rim is probably a more budget oriented rim, and it's possible you might feel a slight pulse as your brakes cross the joint. Otherwise, as already pointed out, there are no structural issues. bicycles.stackexchange.com/a/64832/38270
    – Weiwen Ng
    Nov 11 '20 at 21:56

That is the seam where the two edges of the rim were jointed. Cracks, as you have noted, do not form in such a perfectly uniform way.

As long as the edges remain closely butted together as they currently are, you are good to go.

  • 1
    If that is true, there should be something similar on the other rim.
    – Ralf
    Nov 10 '20 at 13:39
  • 1
    Assuming that the other rim is of the same construction method, then yes, I agree with you.
    – jwh20
    Nov 10 '20 at 13:47
  • 3
    It’s usually exactly opposite of the valve hole and exactly between two spoke holes.
    – Michael
    Nov 10 '20 at 16:09
  • 4
    All alloy rims have this. With black anodized ones it may be more visible.
    – Carel
    Nov 10 '20 at 18:07
  • 1
    @Carel Some are polished out and are very hard to find. H+Son TB14s for instance. Nov 11 '20 at 4:15

The other answers and comments are excellent.
Adding a video on how rims are commonly made and a picture of the seam.

A double wall aluminum rim starts life as a straight piece of extruded aluminum.(Factory Tour: Velocity USA Bicycle Wheels)
The bar is bent to shape (about 3:51 in the video)and cut to length (4:13). The ends are joined together with a sleeve and epoxy (5:20)
Sleeved Joint Connection

enter image description here As Ralf points out since this seam is part of the manufacturing process both rims will have the seam.

If the rim maker does a good job the seam will be difficult to find. A variety of factors can make the seam more visible on one rim than another including manufacturing quality control and the type of stresses a rim experiences while riding. Knowing where to look makes finding the seam easier. As Michael says the seam is usually opposite the valve hole exactly between two spokes.

  • Thank you very much - this is quite informative...
    – Jyo Jena
    Nov 11 '20 at 6:46

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