Chair assembly with a backrest-adjusting device

A chair assembly comprising:

• a base frame having two mounting bars and each mounting bar having a front end and a rear end;
• a pivot frame mounted pivotally on the rear ends of the mounting bars of the base frame and having two pivoting bars mounted pivotally and respectively on the rear ends of the mounting bars;
• a backrest frame mounted securely' on the pivot bars of the pivot frame;
• a seat frame mounted on the base frame and the pivot frame;
• and
• a backrest-adjusting device mounted between the base frame and the pivot frame and having
• a mounting bracket mounted securely on the mounting bars of the base frame;
• a stopper mounted securely on pivoting bars of the pivot frame;
• an adjusting bolt having
• a connecting end mounted rotatably on the mounting bracket;
• and
• an adjusting end defined opposite to the connecting end;
• an adjusting nut screwed movably around the adjusting bolt;
• and
• a torsion spring mounted on the pivot bars of the pivot frame, biasing the pivot frame and the backrest frame to pivot forward and having
• at least one abutment end tightly abutting the stopper;
• and
• a regulating end defined opposite to the abutment end and presses against the adjusting nut;
• wherein rotating the adjusting bolt moves the adjusting nut along the adjusting bolt and changes a torque of the torsion spring.