From Figure 9, the change in the dry-state friction coefficient of the flocking sample without spray coating at a normal load of 2 N is approximately 1.61 ± 0.3. According to Equation (11), the angle θ of vibration due to the sprag slip mechanism is 27.6–37.4. If the angle of the inner belt weatherstrip is within this range, vibration noise owing to sprag slip may occur. Even if the coating solution is attached to the flocking, with time, the coating solution on the surface will be removed by abrasion. Therefore, a design guide to avoid this angle range is required. Although the sprag slip is more advantageous at smaller angles, an angle of 0 is impossible owing to the structure of the belt. Therefore, optimization is necessary. Unless a glass surface is contaminated, it is difficult to have a coefficient of friction of higher than 3. Therefore, if the angle is designed to be less than 18.4° or the friction coefficient of lower than 3 can guarantee performance and sufficiently prevent sprag slip noise (Figure 13).