Adjustment device for chairs for adjusting resistance for a backrest part of a chair, the adjustment device comprising: a chair base; a backrest part pivotally arranged to the chair base around a pivot shaft; at least one spring element (31, 32) having a first end (31 a, 32a) arranged to the chair base and a second end (31 b, 32b) arranged to the backrest part, whereby the spring element (31, 32) has a spring force having a force direction parallel to the direction of the spring element from its first end to its second end, and that the spring element is arranged such that when the backrest part is loaded such that the backrest part is pivoted in relation to the chair base, the spring element will be loaded; a setting mechanism (50, 51) for setting the force of the spring element, whereby the setting mechanism (50, 51) is arranged to move the spring element (31, 32) such that the force direction of the spring element is changed and whereby the setting mechanism (50, 51) is arranged to move the first end (31 a, 32a) of the spring element substantially perpendicular to the force direction of the spring element, characterized in that the first end (31 a, 32a) of the spring element is arranged to the chair base via a spring shaft, and that the second end (31 b, 32b) of the spring element is arranged to the backrest part via an attachment shaft, whereby the adjustment device further comprises a spring frame having a first end part arranged to the spring shaft and a second end part and a spring frame shaft, whereby the second end part is arranged to the spring frame shaft, whereby the spring shaft is pivotally attached to the chair base via the spring frame shaft such that the spring shaft is movable parallel to its own extension, and whereby the spring frame shaft is arranged to the chair base in the area where the attachment shaft is arranged to the backrest part, when the backrest part is unloaded, such that when the backrest part is unloaded the spring element (31, 32) has an extension from its first end (31 a, 32a) towards its second end (31 b, 32b) that is substantially parallel to an extension of the spring frame from its first end towards its second end. a chair base; a backrest part pivotally arranged to the chair base around a pivot shaft; at least one spring element (31, 32) having a first end (31 a, 32a) arranged to the chair base and a second end (31 b, 32b) arranged to the backrest part, whereby the spring element (31, 32) has a spring force having a force direction parallel to the direction of the spring element from its first end to its second end, and that the spring element is arranged such that when the backrest part is loaded such that the backrest part is pivoted in relation to the chair base, the spring element will be loaded; a setting mechanism (50, 51) for setting the force of the spring element, whereby the setting mechanism (50, 51) is arranged to move the spring element (31, 32) such that the force direction of the spring element is changed and whereby the setting mechanism (50, 51) is arranged to move the first end (31 a, 32a) of the spring element substantially perpendicular to the force direction of the spring element, characterized in that the first end (31 a, 32a) of the spring element is arranged to the chair base via a spring shaft, and that the second end (31 b, 32b) of the spring element is arranged to the backrest part via an attachment shaft, whereby the adjustment device further comprises a spring frame having a first end part arranged to the spring shaft and a second end part and a spring frame shaft, whereby the second end part is arranged to the spring frame shaft, whereby the spring shaft is pivotally attached to the chair base via the spring frame shaft such that the spring shaft is movable parallel to its own extension, and whereby the spring frame shaft is arranged to the chair base in the area where the attachment shaft is arranged to the backrest part, when the backrest part is unloaded, such that when the backrest part is unloaded the spring element (31, 32) has an extension from its first end (31 a, 32a) towards its second end (31 b, 32b) that is substantially parallel to an extension of the spring frame from its first end towards its second end.