there are a number of potential reasons for tube blowouts on descents. another problem i've seen - although not directly causing a crash - is poorly aligned brake pads which bite into the tyre wall. what can happen is that as the pads wear, you tighten up the caliper (cable), which brings the caliper into more of an upward arc and raises the contact point of the pads - above the rim.
i'm a reckless descender at times - the thought of a blowout terrifies me. i'm pretty careful with looking after my bike.
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