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
I’ll try to first explain where you went wrong, then answer what I think you’re really meaning to ask. TL;DR at bottom.
Let’s say your engine has a perfectly flat torque curve; 100 Nm at any engine rpm. If you’re in 2nd gear, which lets say has a 2:1 ratio, then the wheel feels twice the torque that the engine puts out (just simple gearing). So in 2nd gear, no matter what speed you go, the wheel will always feel 200 Nm of torque. Ignoring resistance (which actually slows you down more at higher speed), you’ll always feel the same acceleration since the wheel torque is the same, even though the engine rpm is changing. So your assumption is wrong, if you don’t change gears your acceleration is actually slightly worse at higher rpm.
Now instead, let’s say you’re keeping a constant speed and comparing acceleration in different gears. Let’s say 4th gear is a 1:1 ratio. So with a wheel speed of 2000 rpm, 2nd puts your engine at 4000 rpm, and 4th puts your engine at 2000 rpm. But if they make the same torque, what’s the difference? The difference is, in 2nd gear you multiply the engine torque by 2 to get the wheel torque, so 200 Nm. In 4th gear, you multiply the engine torque by 1, so 100 Nm. 2nd gear gets you better acceleration because of the gearing. With a flat torque curve, you always want to be in the lowest gear possible so you can multiply your engine torque by a bigger number.
It seems like it would get complicated to compare engines with different torque values at different rpms, because you would be in different gears with each. But we found a way to make it super duper simple, just compare horsepower and it’s taken care of for you. If you are making more horsepower, and just use the proper gearing, you get more wheel torque and accelerate faster. So just pick the engine with the most horsepower, and put the vehicle in whatever gear allows the engine to make the most horsepower. It’s almost as simple as that.
The reasons to consider more than just horsepower pretty much all boil down to: sometimes you care about the shape of the torque curve.
1. You don’t always have the proper gearing; when you start from zero, you usually can’t just decide to put the engine at the rpm where it makes peak power, so you need it to make torque at low rpm too.
2. A driver expects a flat torque curve so it feels the same whenever they press the pedal.
3. In some cases (tractors) you want an engine to have a torque hump in the middle, so if it encounters a high load it just gets stronger as it slows down, and overcomes the load rather than stalling.
+others I didn’t think of
TL;DR (which is kinda long on its own):
Flat torque curve? If you don’t change gears, your acceleration will be the same regardless of speed (minus resistance). If you do change gears, put it in the lowest gear you can so you multiply your torque by a higher number.
Choosing which gear you should be in (regardless of engine)? Always the one that lets your engine make the most power.
Choosing an engine? Pick the one with the highest power output. Except you might also care about other things, like towing and “feel” and lugging and more.
Yep, I was pretty bored, that’s what motivated me to type this whole thing out.
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