In this situation, the force from the back tires (Fb) also contributes to the torque. This is a frictional force between the tires and the ground, and it pushes forward as the truck accelerates into the jump. (There would also be a frictional force on the front tires, but I left it off since they will soon lose contact with the ramp.) The diagram also shows the downward arrow of the gravitational force. However, since it’s at the center of mass, it doesn’t contribute any torque.
After the truck launches, there are no external torques to change its rotational motion. But since it started the counterclockwise rotation, it continues to rotate in the air. The only question is whether it will make it back to a wheels-down position before landing. If not—oh well, these trucks are built for punishment. That’s why they’re monsters.
How to Pull a Wheelie
Before we get to the front flip, there’s another stunt you need to master first—the wheelie, where you drive with the front wheels off the ground. How does it work? Let’s start with another force diagram:
Illustration: Rhett Allain
In this position, the truck is stable—there is no rotational motion. As before, the gravitational force acts at the center of mass. The force from the ground is not applied at the center of mass, but notice that if you extend a line from that arrow, it would pass right through the center of mass. That means this force also has a zero torque arm, so it creates no torque.
But wait. Why does the ground force point that way? It’s actually a combination of two forces: First, there is the upward-pushing force from the ground. This is an interaction between the tire and the ground that prevents the truck from falling through the dirt. (It’s the same force that pushes up on your coffee cup to keep it from passing through the table.) This is called the “normal” force, because it’s perpendicular to the surface.
Second, there is the frictional force from the back tires. This force pushes forward, parallel to the ground. Combine those two and the net force points diagonally up to the right. This is the whole key to pulling a wheelie: You need to hit the perfect frictional force to aim that arrow right at the center of gravity. That’s all driver skill.