Measuring unit for measuring the forwarding of a workpiece while the latter is being bent in a given bending plane by means of a bending machine, the measuring unit comprising a first carriage which is arranged to be connected to the bending machine and comprises: a main body, a pair of support bodies (30, 32) carried by the main body, a pair of measure rollers (34, 36) mounted each on a respective support body (30, 32) so as to be idly rotatable each about a respective axis of rotation (z1, z2), the axes of rotation (z1, z2) of the measure rollers (34, 36) being parallel to each other and perpendicular to the bending plane, first measuring means (50, 52) arranged to provide a signal indicative of the angular position of each of the measure rollers (34, 36) about the respective axis of rotation (z1, z2), guide means mounted on the main body so as to guide the support bodies (30, 32) along a straight direction (y) perpendicular to, and passing through, the axes of rotation (z1, z2) of the measure rollers (34, 36); elastic means arranged to apply on the support bodies (30, 32) an elastic force tending to urge the support bodies (30, 32) towards each other, and therefore to urge each measure roller (34, 36) against the workpiece under bending when the latter is moved forward between the measure rollers (34, 36), so as to cause each measure roller (34, 36) to roll on a respective intrados surface (Si) or extrados surface (S e) of the workpiece while the latter is moved forward through the first carriage, and positioning means (66, 68, 74, 76) arranged to define the positioning of the main body, and hence of the guide means, relative to the workpiece under bending, in such a manner that during bending with a constant bending radius (Ri) the tangent (t) to the longitudinal axis (x) of the workpiece in the point of intersection between this axis (x) and the straight direction (y) of the guide means is perpendicular to said straight direction (y), characterized in that said elastic means comprise at least one spring which is connected at one end thereof to one of the two support bodies (30, 32) and at its opposite end to the other support body, whereby the resulting normal force applied by said elastic means on the workpiece under bending is zero, and in that said positioning means (66, 68, 74, 76) comprise a first pair of guide rollers (66, 68) and a second pair of guide rollers (74, 76) which are arranged upstream and downstream of the measure rollers (34, 36), respectively, and are supported by the main body so as to be idly rotatable about respective axes of rotation parallel to the axes of rotation (z1, z2) of the measure rollers (34, 36). a main body, a pair of support bodies (30, 32) carried by the main body, a pair of measure rollers (34, 36) mounted each on a respective support body (30, 32) so as to be idly rotatable each about a respective axis of rotation (z1, z2), the axes of rotation (z1, z2) of the measure rollers (34, 36) being parallel to each other and perpendicular to the bending plane, first measuring means (50, 52) arranged to provide a signal indicative of the angular position of each of the measure rollers (34, 36) about the respective axis of rotation (z1, z2), guide means mounted on the main body so as to guide the support bodies (30, 32) along a straight direction (y) perpendicular to, and passing through, the axes of rotation (z1, z2) of the measure rollers (34, 36); elastic means arranged to apply on the support bodies (30, 32) an elastic force tending to urge the support bodies (30, 32) towards each other, and therefore to urge each measure roller (34, 36) against the workpiece under bending when the latter is moved forward between the measure rollers (34, 36), so as to cause each measure roller (34, 36) to roll on a respective intrados surface (Si) or extrados surface (S e) of the workpiece while the latter is moved forward through the first carriage, and positioning means (66, 68, 74, 76) arranged to define the positioning of the main body, and hence of the guide means, relative to the workpiece under bending, in such a manner that during bending with a constant bending radius (Ri) the tangent (t) to the longitudinal axis (x) of the workpiece in the point of intersection between this axis (x) and the straight direction (y) of the guide means is perpendicular to said straight direction (y), characterized in that said elastic means comprise at least one spring which is connected at one end thereof to one of the two support bodies (30, 32) and at its opposite end to the other support body, whereby the resulting normal force applied by said elastic means on the workpiece under bending is zero, and in that said positioning means (66, 68, 74, 76) comprise a first pair of guide rollers (66, 68) and a second pair of guide rollers (74, 76) which are arranged upstream and downstream of the measure rollers (34, 36), respectively, and are supported by the main body so as to be idly rotatable about respective axes of rotation parallel to the axes of rotation (z1, z2) of the measure rollers (34, 36).