Is it torque or horsepower that raises the front tire of a motorcycle during acceleration?

Question

Years ago my dad and I saw a motorcycle popping a wheelie, and he said it was the high torque of the motor that did it. This made sense to me: if you had a bike that was stationary (with the back wheel fixed to the ground) then torque would spin the bike-front upward.

But if you simply look at an accelerating object (motorcycle, car, any back-wheel driven object) then acceleration will cause the front to raise up.

So I can't decide which is right. Note: I realize there is a relationship between torque and HP, but you can have high torque and low HP, as well as vice versa.

Answer 1

These are all related concepts, but usually we colloquially talk about forces, and by extension torques, as the "cause" of changes in motion. That change in motion is called "acceleration", and the rate at which the force causes a change in energy is called "power". Really the dynamics can be described by all of these things at once. e.g. if there is a net torque then there will be an acceleration as well as a rate of energy change (power).

Answer 2

It's the torque reaction that tends to lift the front wheel, as follows.

The motorcyclist first revs the engine up to high RPM to prepare to pop a wheelie. This puts the engine up onto the rising part of its torque curve. Then s(he) begins gradually slipping the clutch into engagement while holding the RPM high, which also holds the torque output high.

Now note that power is the ability to perform work quickly and is numerically equal to the product of torque times RPM. All that power is then on tap and ready to go, and can be dumped into the load (the motorcycle mass) through the transmission, which divides down the high RPM at the rear wheel while magnifying the torque. The rear tire digs in as the clutch approaches full engagement and the pavement applies a reaction force to the rear wheel which gets translated into a reaction torque acting on the motorcycle frame.

If the engine power is big enough and the transmission gear ratio low enough, then the resultant torque will be sufficient to lift the front wheel off the ground.