Tube Bed Crack — Velocity Chukker

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Velocity Chukker rim, 26". Used with 2.0" tires at 4 bar (60 PSI). Tube bed ("web") cracked between spoke holes. Note the "Chukker" is a renamed/reintroduced "Deep-V MTB".

From http://journal.goingslowly.com/2009/12/velocity-rims-part-2.html and http://journal.goingslowly.com/2010/01/velocity-rims-part-3.html as of 2010/02:

Rim use approx. 3,500 km. Rider plus bike plus gear weight about 110 kgf. Used in hilly riding.

The force spreading the tire beads is proportional to the circumference of the air chamber made by the tire and rim; and proportional to the tire pressure. Riders with 2.0" tires often run lower pressure for comfort and traction riding off-road, and road riders often use smaller-section tires. However, the combination of section and tire used here is still common.

The direction of the spreading force is also related to the weight being carried: more weight causes the tire casing to deflect further, causing the spreading load to be more directly between the rim beads and less in line with the tire casing. The 110 kgf load carried here is higher than typical, but again common, and riders carrying more weight often also use higher tire pressures.

Braking pinches the rim, so can reduce the tension in web, thus "working" it on every wheel revolution.

The crack does not show darkening typical of a fatigue failure.