Preform, hydroforming method, and hydroformed product

A hydroforming method comprising the steps of:

• disposing a preform inside forming dies having cavity surfaces that correspond to an outer shape of a hydroformed product, and
• applying a hydraulic pressure in an inside of said preform to form reinforcement ribs that divide a hollow cross section of said hydroformed product,
• said preform comprising first and second outer members composed of sheet materials having edges overlapped and jointed to each other for forming outer surfaces of the hydroformed product, and reinforcement members composed of a sheet material jointed to said first and second outer members to form reinforcement ribs that divide a hollow cross section of said outer surfaces, and said reinforcement members having dimensions capable of suppressing elongation in a tensile direction due to a tensile force that develops during hydroforming;
• and
• wherein said reinforcement members comprise a sheet material that maintains slackness to bend due to said dimensions during inflating deformation of said preform, and said reinforcement ribs are formed in bending shapes with slackness.