Brake Pad Cotter/Tube Failure

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Braking caused "chuff-chuff" sound. On inspection, tire sidewall was torn at bead/rim edge. Tube poking through hole was chafing on brake pad retaining cotter pin, which had moved up from its standard position.

Original cause of failure unclear. Usual causes are brake pad rubbing on casing or grit in bead abrading casing. Neither was obviously the case here, but also not obvious that elevated cotter could contact the sidewall until already torn and bulging out. (Note: brake is Shimano XTR; parallelogram has some vertical slop.) 26" x 2.1" nominal tire size, mounted on narrow rim (WTB Power Beam). Tire sidewall marked for up to 65 PSI; inflated to 60 PSI.

Field fix included patching the tube where chafed; putting a "boot" inside the tire casing; remove cotter and check others on bike; adjust brake pad away from tire casing; check others; inflate tube to 30-35 PSI; avoid front braking, especially causing high rim temperatures and elevated tire pressure. Did not obviously get worse during about 4 hours further riding.