A twin-frame bicycle, comprising: a primary bicycle body, having a primary bicycle frame, a front wheel, a rear wheel and a pedal mechanism, wherein the front wheel and the rear wheel are connected pivotally to the primary bicycle frame and distanced from each other, and the pedal mechanism is used to drive the front wheel or the rear wheel; an auxiliary bicycle body, having an auxiliary bicycle frame and a first auxiliary wheel, wherein the first auxiliary wheel is connected pivotally to the auxiliary bicycle frame; at least one articulation mechanism, having a first end, a second end and at least two linking portions (53, 54), wherein the first end and the second end are connected to the primary bicycle frame and the auxiliary bicycle frame, respectively, the linking portions are substantially disposed in parallel between the first end and the second end, and the first end, the second end and the linking portions forming a movable quadrangle, wherein the first end comprises a first foundation and the second end comprises a second foundation; and a resilient tension mechanism, having an elastic element disposed between the primary bicycle body and the auxiliary bicycle body, wherein two ends of the elastic element are connected to the first end and the second end, respectively, and wherein the ends of the elastic element are connected to the first foundation and the second foundation, respectively, characterized in that the elastic element is substantially parallel to the linking portions (53, 54) and in that the resilient tension mechanism further comprises a first adjustable unit disposed at the first end of the elastic element and a second adjustable unit disposed at the second end of the elastic element and the first end of the elastic element is connected to the first foundation through the first adjustable unit and the second end of the elastic element is connected to the second foundation through the second adjustable unit. a primary bicycle body, having a primary bicycle frame, a front wheel, a rear wheel and a pedal mechanism, wherein the front wheel and the rear wheel are connected pivotally to the primary bicycle frame and distanced from each other, and the pedal mechanism is used to drive the front wheel or the rear wheel; an auxiliary bicycle body, having an auxiliary bicycle frame and a first auxiliary wheel, wherein the first auxiliary wheel is connected pivotally to the auxiliary bicycle frame; at least one articulation mechanism, having a first end, a second end and at least two linking portions (53, 54), wherein the first end and the second end are connected to the primary bicycle frame and the auxiliary bicycle frame, respectively, the linking portions are substantially disposed in parallel between the first end and the second end, and the first end, the second end and the linking portions forming a movable quadrangle, wherein the first end comprises a first foundation and the second end comprises a second foundation; and a resilient tension mechanism, having an elastic element disposed between the primary bicycle body and the auxiliary bicycle body, wherein two ends of the elastic element are connected to the first end and the second end, respectively, and wherein the ends of the elastic element are connected to the first foundation and the second foundation, respectively, characterized in that the elastic element is substantially parallel to the linking portions (53, 54) and in that the resilient tension mechanism further comprises a first adjustable unit disposed at the first end of the elastic element and a second adjustable unit disposed at the second end of the elastic element and the first end of the elastic element is connected to the first foundation through the first adjustable unit and the second end of the elastic element is connected to the second foundation through the second adjustable unit.