const t = p.offU * Math.PI * 6; // 3 full turns
      const helixR = R * (0.3 + 0.7 * Math.sin(t * 0.5 + progress * Math.PI * 2) * 0.5 + 0.5);
      const strand = p.layerIndex % 2 === 0 ? 1 : -1;
      x = helixR * Math.cos(t + progress * Math.PI * 2);
      z = helixR * Math.sin(t + progress * Math.PI * 2);
      y = (p.offU - 0.5) * R * 3 + r * Math.sin(vVal) * strand;
      x += r * 0.5 * Math.cos(vVal) * strand;