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 can call the torque-curve “flat” but it just means the graph curve is flatter than average. There is more torque at the higher RPM’s
How do you know the bike is accelerating faster at 9000 RPM? Did you actually measure it or is it just a feeling? Because it the laws of physics are pretty clear on this subject 😉
So an analogy ..
torque is like how strong you can unscrew a lid and
Horsepower is how many lids you can unscrew in a minute …
The bike starts to pull at high rpm’s because it’s twisting ~3000 times more each minute..
So replace lids with back wheel and That many more turns a minute makes ur accelerate and you can feel it
Here is a very simple answer. The more torque *at the wheels* the faster you accelerate.
[Here is a graph of torque at the wheels on a CB500F](https://motostatz.com/wp-content/uploads/2021/07/CB500F-in-gear-acceleration-e1660277766742-1024×588.jpg)
Note how torque at the wheels in first gear is higher across the *entire rev range* than peak torque in second.
So if you for example accelerate through 3nd gear, then no, it won’t pull harder at higher revs, it’ll pull hardest at peak torque.
But you have gears, so you wouldn’t accelerate in 3rd, you’d drop to 2nd (and higher revs) and use a lower gear to turn that power into more torque at the wheels than anywhere in 3rd.
So no, you don’t accelerate harder with more revs, you accelerate harder in lower gears, and lower gears require more revs to do the same speed 😛
There are some not so good answers here. If the bike makes the same torque at 3000 and 9000rpm, then it will accelerate about the same at 3000 and 9000rpm in the same gear. The acceleration will actually be lower at 9000rpm since the bike is moving faster, resulting in higher aerodynamic drag.
I think the likely answers are 1. Your perception of acceleration is not accurate, and is being influenced by the motorcycle making a lot more noise at higher revs. You’re also starting to focus a lot more on needing to shift as you get higher in the rev range. 2. The torque curve isn’t actually flat, a quick look shows it increasing a decent amount from 3-4 k, and then continuing to increase to 7k. What looks like a small increase on a graph may feel like a larger increase while riding.
Ever bicycle uphill in a higher gear where you have to basically stand on the pedal to get it to move? Versus in a lower gear where you can pedal normally and putter uphill at a steady pace. Standing on the pedal is high torque low power, while pedaling at a steady pace is low-medium torque and higher power.
And when you pedal super duper fast, most of your energy is making the pedals spin and not really making the bicycle go faster, well it turns out your torque is dropping off to near zero at that point.
But just before that point, in-between getting the bicycle to start moving versus being maxed out, that’s your powerband, you can still put some weight on the pedals to push you ahead, but you are moving the pedals fast enough where each rotation gets you some more speed.
Mathematically, Power = torque x RPM. So given constant torque, you have more power at higher rpms.
You can’t feel torque from an engine, you can only feel power. And power is what makes an object accelerate. So max power is where you pull harder at high revs.
However in all engines, torque eventually drops off, the pistons can’t effectively rotate faster, and the power will decrease. So you shift gears!
This idea that torque determines acceleration isn’t a very helpful way of understanding what’s going on. It’s not wrong, exactly. It’s easy to see that the forward force at the drive wheel(s) is directly proportional to torque, and acceleration is directly proportional to the forward force. But that static view masks a lot of important details.
A better way to understand engine output and acceleration is that the average acceleration between two speeds is proportional to the area under the power curve between the corresponding two RPMs. Forget torque, it’s just a secondary indicator of the shape of the power curve. The shape of the power curve is what matters.
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.
Torque at the wheels determines acceleration. The figures you see quoted are for the engine. We use the gearbox to convert the power og the engine to basically whatever torque we want at the wheels.
So it’s the power an engine makes through an rpm range that determines acceleration.
The real answer? It doesn’t. You’re probably feeling the peak of your torque curve (yes, even the flattest torque curves have a rise, peak, and fall) motorcycles tend to have their peak torque pretty high up.
But, let’s do some mental exercise. Let’s assume your engine makes a constant 50 torques. That torque is applied at every rotation the engine makes. So at 3,000 rpm (rotations per minute) you can apply that 50 torque 3,000 times within one minute. At 9,000 rpm you can apply that same 50 torque 9,000 times within one minute. The more times you can apply the torque within a time span, the faster you can move an object.
Of course, this is all simplified and hypothetical, and engines do not make constant torque, and there are hundreds of factors that affect their operation and power/torque output.
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