The KYB monotube design offers additional performance and can have a more aggressive ride.
The monotube design has a single cylinder: The cylinder is divided into sections: A fluid area and a gas chamber. The piston and shaft move in the fluid portion. It uses a single fluid valve assembly in the piston. The diameter of the single working cylinder and piston valve is larger than in a twin tube even though the outside dimensions of each may the same. There is no need for an air or gas in the fluid area so the valve can operate more responsively and without any aeration or performance fade. The high pressure gas chamber is separated from the fluid area by a floating piston & seal. That provides an expansion area for the excess fluid movement during the compression stroke. On more aggressive movement the floating piston is pushed further into the gas chamber which increases gas pressure quickly and provides additional damping force. Because of its higher performance capabilities, the monotube design is used as original equipment on some vehicles and offered as an upgrade on vehicles that came equipped with the twin tube design.