Ok, so here's the setup of the problem. A motorcyclist tries to jump over some cars by using a ramp. Right after he leaves the take-off ramp, he notes that his motorcycle is angled slightly upward and has zero angular velocity (meaning that if he did nothing and stayed in the same position, the bike would remain in the same orientation with respect to the ground). If this tilt is maintained, then a problem will arise when he hits the landing ramp as he should be tilted downwards to ensure a smooth landing. The question is what should the biker do in order to make his bike tilt forward? Also, ignore any energy loss/air resistance.
According to my professor, the biker should hit the brakes. The reasoning is as follows.
The wheels are spinning quickly in the forward direction at takeoff, so they have substantial angular momentum. If the brakes are applied, then some of this angular momentum is transferred to the main body of the bike, because the total angular momentum of the system is conserved. The bike will therefore rotate forward somewhat, as desired.
I understand that the motorcycle-biker system must have its angular momentum conserved (only gravity acts on the system, but since it acts on the center-of-mass, it provides no torque) with respect to its center-of-mass. What does the statement "the wheels...have substantial angular momentum" really mean? With respect to what? Intuitively, it feels as there should be angular momentum, as the object is spinning. But, doesn't angular momentum have to be measured with respect to a fixed point/axis or the center-of-mass of the motorcycle-biker system? If so, what is the fixed point/axis in this situation?
I want to understand the solution, but I cannot understand what point the angular momentum is measured about. If someone could either explain the reasoning behind the solution or tell me what axis/point the wheels would have angular momentum, I would greatly appreciate it.