山地车干土滚胎循环

Seamless 6-second loop, mountain-bike tyre rolling steadily on packed dry dirt trail: a smooth continuous mid-range roll around 400–1200 Hz, fine even grain, no low rumble, no sub bass, no high-frequency hiss, no sandpaper sizzle. Slow natural variation, no repeating crunch pattern, no distinct stones, no music, close perspective, dry.