i started riding a CB500F, it has a flat torque curve, pretty much the same torque at 3k and 9k rpm, then why does the bike (in the same gear) accelate much faster at higher than lower revs?
In: 88
You have discovered the phenomenon that torque and power output are both important for maximum acceleration, despite what some people say or believe.
Your maximum torque determines the maximum amount of force that your vehicle can exert at the wheels. If you exert almost no force at the wheels, you can’t really accelerate very much. But in order to accelerate, you don’t just need to provide a force. You also need to increase the kinetic energy of your vehicle (as well as overcome the energy that you are losing to drag and other sources of friction). If you can apply a tremendous amount of torque but very little horsepower, what that means is that you can tow a million pounds, but at 0.0000001 miles per hour. Horsepower (power) is the amount of energy you can add to the vehicle per unit time. You need a lot of horsepower if you want to accelerate quickly, because you need to add a lot of energy to your vehicle.
The reason your motorcycle can accelerate more quickly at higher RPM given that it has a flat torque curve is that, at a higher RPM, the engine is putting out more power. More fuel is being injected and burned in the same amount of time. Since the amount of torque it’s providing is the same, power output is what determines exactly how fast you accelerate.
E: to be clear, of horsepower and torque, the fundamental parameter of the engine is horsepower. Output torque can be changed to any arbitrary value you want (within reason) through gearing. An engine that outputs 1,000 horsepower but only provides one foot pound of torque at the crankshaft is much preferable to one that outputs 100 horsepower but with 10,000 ft lb of torque at the crankshaft if you want to use the engine to make something move quickly. You can design a gear train to provide the torque you need at the wheels, but no gear train allows you to output more power.
E2: it has been pointed out to me, and I agree, that depending on how you interpret your original wording, what I said is wrong.
If what you mean is that, with the motorcycle in the same gear, and at 5,000 RPM versus 9,000 RPM, that you observe more rapid acceleration at 9,000 RPM, then what I said is wrong. If that’s the case, what must be true is that, if you’re already operating at 9,000 RPM, pushing the pedal down a little bit more must be increasing both torque and power output more than torque and power output increase if you are at 5,000 RPM.
The way I interpreted what you said is that you were asking why, if you really push the pedal down and go from 5,000 RPM to 9,000 RPM, you end up accelerating a lot. That’s because you have to, because power output is higher. The fact of the matter is that torque curves are maximum torque curves, not actual torque curves. That is, your engine can produce far less torque at a particular RPM then the torque curve would suggest, because the torque curve is generated by putting the engine at wide open throttle and seeing how hard you have to brake it before it slows down.
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