Disc rim - failure by (circumferential) cracking
 

(Title edited for accuracy)

Here's a new one for me: The rear rim on my 650b disc wheelset has developed two circumferential cracks. The cracks span about 3 spokes each and are in the same area - separated from each other by only a spoke or two. Both cracks are on the drive side of the rim.

The wheelset is 3 years old and has a couple thousand road miles. Wheels were built by me. This particular rim is somewhat notorious for cracking at the spokes holes, but I haven't had that issue.

Any thoughts about what might've caused this? Just asking out of curiosity - I already have replacement rims to rebuild the wheelset.

I have never seen that before.

I've only seen cracks like that on rim brake rims, when the brake tracks had been worn down so far that the sidewalls were no longer strong enough to hold the pneumatic pressure. Are the cracks inside or outside of the tire bed wall? If they are outside, then my best guess is that in order to wave weight on this disc rim, they removed the extra thickness of the brake track, but that they went too far and made it too thin. It's probably not a coincidence that the crack in the 2nd photo is around a point where the rim sidewall has been dented inward (as if by hitting a sharp edge pothole).

(By the way, those cracks run circumferentially, not radially.)

Excessive tire pressure pulling the rim flange outward?

Should I have been able to infer the rim model?

I'll have to see if the cracks are inside the rim bed as well. Tires were 42mm run at 45-50 PSI.

Rim is discontinued now, so make & model don't really matter. They served me well for 3 years. I'm just surprised by how it failed.

Assuming it's an extruded shape, that's either a thin point, a stress point or some sort of material inclusion that would do that.

aren't those circumferential cracks? radial cracks would be parallel to the spokes?

I think you're onto something....usually there's a smooth transition b/t the sections though those look like two distinct sections. I'm guessing there's a bond/seam failure along that line? either way, I would hope the replacement rims the OP mentions are from a different manufacturer with a stronger quality reputation.

+1

I was going to point out the same thing.

Radial = same as the direction of a radius
Circumferential = same as the circumference

Please excuse the PFPG (Paceline Forum Pedant Group).

Tubeless? That can have quite a lot of stress on sidewalls. I wonder if an extrusion 'line' is there...a burr or something on the extrusion that cause stress along the rim there. What's the manufacturer/distributor say?

Pressure acts equally within a vessel. Tubes or tubeless wouldn’t matter. It’s also on the external material- I’d say bad rim to begin with.

Disc rim - failure by (circumferential) cracking
 

Maybe it was using wider tires without lowering pressures to match the increased width?



Hoop stress
http://flocycling.blogspot.com/2014/...-tire.html?m=1
https://www.velonews.com/2017/04/bik...m-width_435447

Great demonstration
http://www.killasgarage.bike/uncateg...demonstration/

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Just for grins, I cut through the rim at the crack. Turns out it's between the rim bed and the spoke bed - not in the sidewall. That makes me think that it wasn't tire stresses that caused the rim to fail.

Thanks for sharing that photo, interesting to see!

I'd wager this area is the "weld area" on the extrusion which is caused by the material flowing around the die bridges as it is extruded. The material properties are usually a little worse in this area. Also, failed just above the radius so maybe they could have gone a little bigger on that.