Drug delivery device and method to modify a drug delivery device

An arrangement comprising:

• a first drug delivery device comprising:
• a housing,
• a first drive mechanism,
• a first cartridge containing a drug, the first cartridge being retained in a defined position relative to the housing, wherein the first drive mechanism is configured to dispense the drug from the first cartridge, and
• a first cartridge bias system, which comprises at least one resilient member and at least one rigid force transfer body,
• wherein the first cartridge bias system is arranged and configured to exert a force on the first cartridge to maintain the first cartridge in the defined position, wherein the force is provided by the at least one resilient member, wherein the at least one rigid force transfer body is operatively connected to the at least one resilient member such that the force is reacted by the at least one rigid force transfer body, wherein the first cartridge is arranged in a first cartridge holder which is connected to the housing, wherein the first cartridge bias system extends into the first cartridge holder, and wherein the at least one rigid force transfer body is movable within the housing and is configured to be located at a position within the housing according to a length of the first cartridge, and
• wherein a piston rod of the first drive mechanism is arranged to travel through the at least one rigid force transfer body, wherein the housing comprises a piston rod guide section which is provided with a piston rod guide feature which cooperates with the piston rod to guide movement of the piston rod relative to the housing, wherein the piston rod guide section is radially offset from an inner surface of the housing, wherein at least a section of the at least one rigid force transfer body is arranged between an outer surface of the piston rod guide section and the inner surface of the housing, and wherein at least an inner surface of the at least one the section of the at least one rigid force transfer body faces towards the outer surface of the piston rod guide section;
• and
• a second drug delivery device comprising a second cartridge, a second cartridge bias system, and a second drive mechanism,
• wherein the first cartridge and the second cartridge have different lengths and/or diameters, wherein the at least one rigid force transfer body of the first cartridge bias system compensates or is suitable to compensate differences in the different lengths and/or diameters of the first cartridge and the second cartridge.