Clamping mechanism, for a tyre testing device, enabling compensation of a tyre sidewall collapse during testing, which comprises: - A supporting bracket provided with a vertical guide, - A trolley deposited on the vertical guide, provided with a first centring member, - A vertically arranged spindle formed by a lower fixed spindle and an upper moveable spindle both arranged on one rotational axis A, where o the lower fixed spindle as well as the upper moveable spindle are provided with rims (14, 13) facing each other, for clamping a tested tyre, o the lower fixed spindle is adapted for rotational movement, o the upper moveable spindle is positioned under the trolley and freely suspended in the supporting bracket in order to enable rotational movement as well as movement in the direction of the rotational axis A, o the lower fixed spindle as well as the upper moveable spindle are provided with a bayonet lock (10a, 10) for their mutual locking, o the upper moveable spindle is provided with the second centring member adapted to latch into the first centring member of the trolley, o the upper moveable spindle is further provided with a compensation chamber that enables insertion of the lower fixed spindle for compensation of the tyre sidewall collapse, - a pneumatic cylinder for controlling the trolley movement in the supporting bracket along the vertical guide in the direction of the rotational axis A for centring the upper moveable spindle relative to the rotational axis A through the first and second centring members (2 a 3), - a servo drive for controlling the movement of the supporting bracket in the direction of the rotational axis A between an open position enabling insertion of the tested tyre between the rim of the upper moveable spindle and the rim of the lower fixed spindle, and a locked position, where the tested tyre is clamped between the rims (13, 14). - A supporting bracket provided with a vertical guide, - A trolley deposited on the vertical guide, provided with a first centring member, - A vertically arranged spindle formed by a lower fixed spindle and an upper moveable spindle both arranged on one rotational axis A, where o the lower fixed spindle as well as the upper moveable spindle are provided with rims (14, 13) facing each other, for clamping a tested tyre, o the lower fixed spindle is adapted for rotational movement, o the upper moveable spindle is positioned under the trolley and freely suspended in the supporting bracket in order to enable rotational movement as well as movement in the direction of the rotational axis A, o the lower fixed spindle as well as the upper moveable spindle are provided with a bayonet lock (10a, 10) for their mutual locking, o the upper moveable spindle is provided with the second centring member adapted to latch into the first centring member of the trolley, o the upper moveable spindle is further provided with a compensation chamber that enables insertion of the lower fixed spindle for compensation of the tyre sidewall collapse, o the lower fixed spindle as well as the upper moveable spindle are provided with rims (14, 13) facing each other, for clamping a tested tyre, o the lower fixed spindle is adapted for rotational movement, o the upper moveable spindle is positioned under the trolley and freely suspended in the supporting bracket in order to enable rotational movement as well as movement in the direction of the rotational axis A, o the lower fixed spindle as well as the upper moveable spindle are provided with a bayonet lock (10a, 10) for their mutual locking, o the upper moveable spindle is provided with the second centring member adapted to latch into the first centring member of the trolley, o the upper moveable spindle is further provided with a compensation chamber that enables insertion of the lower fixed spindle for compensation of the tyre sidewall collapse, - a pneumatic cylinder for controlling the trolley movement in the supporting bracket along the vertical guide in the direction of the rotational axis A for centring the upper moveable spindle relative to the rotational axis A through the first and second centring members (2 a 3), - a servo drive for controlling the movement of the supporting bracket in the direction of the rotational axis A between an open position enabling insertion of the tested tyre between the rim of the upper moveable spindle and the rim of the lower fixed spindle, and a locked position, where the tested tyre is clamped between the rims (13, 14).