唔係好明點解向下尋。if the engine has more power than the brake provides, the car should just go or at least not move, as opposed to back off a little bit. What is the mechanism behind?
Hi, this phenomenon is commonly found in front-wheel drive (FWD) cars and it must not be confused with car body sqauting due to weight transfer under acceleration. On a FWD car, the engine drives the front wheels and the torque is transmitted to the wheels. The friction between the tyres and road surface allows this torque to be converted into a longitudinal force, driving the car forward. However, this also induces a moment (or "rotational force") that acts in the opposite direction of the rotation of the wheels. This moment is multplied by the wheelbase of the car (moment arm) and this results in a vertical force that "pushes" the rear of the car into the ground, causing the suspension springs to compress. (The magnitude of this reaction force (N) is moment (Nm) x wheelbase (m). With the handbrake on, the rear wheels are unable to rotate and this increases the drag of the vehicle. To overcome (or to counter) this additional drag, the driver applies more trottle, thus increasing the wheel torque and therefore the moment. The fact that the car is unable to move foward implies that moment that couters the wheel torque is larger. This is why the "squating" effect is more apparent when the rear wheels are locked. :)
