218function beamAngle(t) {   // 弹簧阻尼，确定性逐步积分
219  let th = 0, v = 0; const dt = 1 / 240;
220  for (let s = 49.6; s < t; s += dt) {
221    const [L, R] = weightsAt(s);
222    const tgt = L + R > .01
            ? -.22 * Math.tanh(3 * (L - R) / (L + R))
            : 0;223    const a = -26 * (th - tgt) - 5.2 * v;
224    v += a * dt; th += v * dt;
225  }
226  return th;
227}

.22.50 最大倾角约 13°