Bike engine torque

[The Ultimate M]

i want to understand ---- maximum output=14 PS/7500 rpm & maximum torque= 14 Nm / 6000 rpm (a bike engine's specs)--- thoroughly what it means and how it affects bike ride (performance,power,speed)

Thnx for ur time

[Schrödinger's hat]

Torque is a measure of how strong the rotation is. Roughly equivalent to force.

14Nm means that, if you were to put a 1 metre wheel on the crank shaft, you would have to put 14 Newtons of force on it to slow it down.

Power is force multiplied by velocity, or torque multiplied by angular velocity (rate of rotation).

 

If you had a perfect and infinite range gearbox the only thing that would matter would be peak power, you could just rev the engine at 7500rpm all the time and get maximum acceleration.

 

Unfortunately there are limitations to gearboxes, engineers have to trade off weight, life time, strength and other factors, so you have a limited (and usually discrete, although many scooters now have continuous) range. This means the amount of torque is important.

High torque also usually comes with a big flywheel, and massive pistons. This helps smooth out the power over the course of one stroke a bit. It means the engine won't lose revs quite as easily under load.

 

Now, on to torque and power curves.

The higher the torque, the higher the power. But where peak torque occurs is important too, along with how quickly it increases.

 

 

 

Here is an example of a highly tuned comparatively small engine. It produces a moderate amount of torque but revs quite high. This means it produces lot of power, but only at high revs. It will go just as fast as something with the same amount of power, but it will require changing gears more frequently, and will probably not accelerate as quickly at low speed (depending on what ratios are available). This doesn't really matter for an engine this powerful, because the motorbike is limited by the amount of force that will take the front wheel of the ground and the rider can do things like slip the clutch.

This kind of torque/power curve is known as being peaky (at least compared to my other examples). Small, highly tuned engines tend to act like this, in extreme cases (single cylinder racing two strokes) this behavior is known as a power band where both the power and torque increase dramatically for a short rev range and then drop off again. Note that the peak torque (the light line) and peak power (the heavy line) are quite close in revs, and the peak torque is not far from the maximum revs. These are ways that you can detect a highly tuned engine that will act in this way.

 

 

Here is an example of two engines, both of which are extremely powerful for a motorcycle. One of them has a moderate or somewhat flat torque curve, the other has a very flat (even decreasing) torque curve.

 

The Vmax is a balance between getting the largest possible amount of power out of an engine, and making that power available without changing gears/waiting for speed to build up, this will also make the bike accelerate more quickly in general compared to a peakier engine with similar power. By increasing the mass of the engine slightly, and detuning resonances so they have broader frequency ranges the torque profile can be flattened. Peak torque is somewhat below peak power.

 

The rocket 3 is an extreme example of peak torque at low revs. It is at half the revs of peak power, and about a third of the revs the engine is capable of. This behaviour leads to almost flat power throughout the entire rev range, changing gears on this motorcycle is only necessary when slow manoeuvring. This is known as a very broad power profile/power band, or a high torque engine. You will often hear the word grunt to describe this, depending on where you live.

This doesn't mean the motorcycle will go any faster than one with similar peak power, but it will accelerate at any speed, in any gear (providing the engine does not rev out) about as quickly as a more highly tuned engine will in optimal conditions.

 

 

 

The figures you quoted are much more modest, as befits a learner motorcycle. I'm guessing this is either a 250cc twin, 125-200cc single, or something of older design.

 

There is a reasonable range between peak torque and peak power, this indicates that you won't have to work constantly changing gears to find a spot where the engine has power.

The overall output of the engine is modest, but enough for around town, depending on aerodynamics and weight I'd say it has a top speed of 110-140km/h. Taking it into hilly areas with >100km/h speed limits may be taxing, but short trips on the motorway should be okay.

Without knowing how heavy it is and how many cylinders it has this is less certain, but I would imagine that this motorbike will accelerate about as well as most family cars up to about 60km/h.

 

If you told me how many cylinders it has and what it revs out to I could help more (or just the model number/name).

[lemur]

i want to understand ---- maximum output=14 PS/7500 rpm & maximum torque= 14 Nm / 6000 rpm (a bike engine's specs)--- thoroughly what it means and how it affects bike ride (performance,power,speed)

Thnx for ur time

Without looking up the specifics of what torque and HP refer to exactly, I'll just go ahead and post how I think of these things intuitively as a bicycle-rider. Torque I think of as the amount of force put into pushing the pedals. E.g. higher gear and going uphill without downshifting requires pushing harder on the pedals. When shifting into a lower gear that requires faster pedaling, or just when going at top speed in a gear where you're "spinning" the pedals, this is where I think of HP as being high despite lower relative torque. I know that I'm generating torque at every pedaling speed, but when it's slow cranking with a lot of effort I think of this as torque. I hope this isn't incorrect in some way. I like have intuitive experiences that involve "feeling" forces in different situations because I think it gives you a good basis for understanding the physics and interpolating diverse mechanical situations as a result.

[Danijel Gorupec]

You told us a lot about your engine, but you said nothing about your gearbox (gear ratio and gearbox type) and diameter of your wheels. If you understood above answers, then you also understand now that gear-ratio and wheel-diameter is as important to your bike (how it behaves in driving) as engine specifications.

 

Do you have any kind of gear-ratio-adjustable gearbox on your bike at all?

[Schrödinger's hat]

You told us a lot about your engine, but you said nothing about your gearbox (gear ratio and gearbox type) and diameter of your wheels. If you understood above answers, then you also understand now that gear-ratio and wheel-diameter is as important to your bike (how it behaves in driving) as engine specifications.

 

Do you have any kind of gear-ratio-adjustable gearbox on your bike at all?

 

One can fairly safely assume that the gearbox will be optimized for speeds between 60 and 110km/h with those power figures -- unless the bike is very old. Five-six speed gearboxes are normal and even four speeds is enough to get a decent range of ratios.

Even cruisers and scooters behave fairly similarly at slow speeds (up to 100km/h) with modern suspension on good roads, learning to control them is a bit different, but the performance is much the same.

[Danijel Gorupec]

One can fairly safely assume that the gearbox will be optimized for speeds between 60 and 110km/h with those power figures -- unless the bike is very old. Five-six speed gearboxes are normal and even four speeds is enough to get a decent range of ratios.

Even cruisers and scooters behave fairly similarly at slow speeds (up to 100km/h) with modern suspension on good roads, learning to control them is a bit different, but the performance is much the same.

 

Yes, I agree... But from question I assumed that The Ultimate M needs explanation of more basic stuff (power, torque...). That's why I want to be sure he/she understands the gearbox is important. If The Ultimate M already knows this stuff, then I apologize for underestimating his/her knowledge.

[The Ultimate M]

Torque is a measure of how strong the rotation is. Roughly equivalent to force...........................................I could help more (or just the model number/name).

 

Thnx Schrodinger's hat , for ur time. u guys r quicker than i was expecting. u explained on my question very well. But i want to clear this thing with a little more simplification.

the torque & HP value i mentioned are about yamaha FZ motorcycle (loaded with single cylinder, 2 valve, HOTC).

 

Q. suppose i put a lot of weight (quite much like 100-500 Kg) on the engine fitted in a vehicle (not particularly this bike, but in general, any engine however designed) then which component (torque or Horse Power) will b responsible in engine by which it will rotate the wheel. & similarly in an other case which component (torque or Horse Power) will b responsible in engine by which it can pull heavily loaded cart with great speed (like in case of railway train). If my question is clear to u , then please clearify it, & if not , then tell me. I'll try to make it more clear.

 

Thnx

 

Also Thnx to Daniljel Gorupec & lemur or their precious time to this post

 

I also want a thorough detail on gearbox & gear ratio, but after understanding torque & HP completely

[ewmon]

Torque is force acting on a "lever arm", such as someone pushing on a wrench, or the piston pushing down on a crankshaft, or the teeth of one gear pushing on the teeth of another gear, or a wheel pushing against the road. Power is the torque multiplied times the speed at the point the torque is acting.

 

When power is sent through gear ratios, the rotating speed changes down or up by the ratio, and the torque changes oppositely – up or down – by the ratio. Except for losses due to friction, power is always conserved. This means that, in a frictionless transmission, the power of the engine is applied by the tire on the road; however, the torque will be different, as will the force that propels the vehicle, depending on the transmission gear ratio.

 

In “low” gears, you get slow tire/vehicle speeds and high torque/force.

In “high” gears, you get high tire/vehicle speeds and low torque/force.

 

The forces acting on a vehicle come from a few sources:

#1 – gravity (when moving up/down an incline),

#2 – aerodynamic/rolling drag, and

#3 – the forces from the powered wheel(s).

 

Let’s simplify the situation by saying that the road is flat and, so, gravity has no effect. So, we have only the drag (which is mostly aerodynamic) and the torque at the wheel pushing against the road.

 

When the pushing by the tire is greater than the drag at that particular speed, the vehicle will accelerate. When it’s the same, the vehicle will maintain speed. And when it’s less, the vehicle will decelerate. The acceleration and deceleration follow the F=ma rule where, when re-written, a (acceleration) equals F/m (force from the wheel’s torque divided by the vehicle’s mass). Adding mass to the vehicle will increase its mass, m, causing a slower acceleration/deceleration, a.

[Schrödinger's hat]

The Ultimate M, for the level of detail you seem to be after it might be a bit more efficient to go and find a book. I could write out all this stuff, but I would only be reproducing work which has been done by others, and my explanations would probably not be as good as someone who has spent years writing a book specifically on the subject.

Here's here's a link which may be useful (External links on wiki can be a great place to look for these things)

http://craig.backfire.ca/pages/autos/horsepower

 

Read that and come back with any questions you still have. If it's not detailed enough for you I'll try and find some book titles.

[Danijel Gorupec]

I once wrote a small article about torque vs power and their influence to vehicle acceleration. I re-edited it now and posted down (I planed to put it on my web site for long time but never finished it, so thanks). Maybe you will find it useful. Sorry, it is somewhat long. Also note that it only explain basic stuff. If you need more details (engineer-level knowledge), then, yes, you should turn to books.

...

 

The thing that moves your motorbike around is the force that tire exerts on pavement. I will call this force ‘the thrust force’. If the thrust force is greater than drag forces (mostly wind resistance) then your motorbike accelerates. If the thrust force is exactly equal to the drag, then your motorbike keeps running at constant speed.

 

There is a very simple formula that tells how much your motorbike will accelerate if we know the net force acting on it: a=(Ft-Fd)/m (where ‘a’ is the acceleration, ‘Ft’ is thrust force, ‘Fd’ is drag force and ‘m’ is the mass of the motorbike, driver and cargo). To make your motorbike accelerate faster you must either decrease its mass or increase thrust force (or you can even decrease drag force, but effect will be small on low speeds).

 

So, how large is the thrust force? It changes depending on how much throttle you give and what gear you use…. please, read further.

 

At any moment, the amount of the thrust force depends on the amount of torque exerted on the driving (rear) wheel of your motorbike. Larger the torque, larger the force. There is a simple formula: Ft=Mw/r (where ‘Ft’ is thrust force, ‘Mw’ is torque on the wheel and ‘r’ is wheel radius). You can see that the thrust force also depends on wheel radius – smaller the wheel radius, larger the force, greater the acceleration (theoretically). But you can rarely change the radius of motorbike wheels.

 

One important side-note: The thrust force is also very limited by friction between tire and pavement, and cannot pass over this limit. That is why often the best way to achieve better acceleration is by installing stickier tires.

 

The torque is a bit foggy term for those that never liked physics. In the world of wheels and axles, the torque is a value that is analogue to the force of the linear world. The torque tells with how much ‘strength’ something is twisted (while the force tells with how much ‘strength’ something is pushed/pulled). With the already mentioned (M=F*r) formula you can convert torque to force and vice versa… force can cause torque (like when you use a wrench), and torque can cause force.

 

So how large is the torque on your motorbike driving wheel? Well it depends on two things: on the torque generated at engine shaft, and on the gearbox ratio. The formula is simple: Mw=Me*N (where ‘Mw’ is torque on the wheel, ‘Me’ is the torque on the engine shaft, and ‘N’ is gearbox ratio – like 1:N).

 

As you can see, the gearbox ratio is very important. This is not surprising because the function of the gearbox is to adjust the torque level at your driving wheel. Larger the gear ratio, more torque on the wheel, more thrust force and therefore greater the acceleration…. again, in theory only. It is important to note that there is no sense to put 1:1000 gear into your motorbike – you may achieve great acceleration with this, but only for very short time because you will hit on maximum engine revs while still driving very slowly… and you would have to have some impossibly sticky tires.

 

Still, for greater acceleration, one must keep gears low as long as possible (late shifting).

 

And how much torque is there on engine shaft? Well this depends on your engine capabilities and how much throttle you give. Engine capabilities are described with torque curve. Torque curve tells what is the maximum torque (that is, ‘on full throttle’) the engine can produce at any given engine revs.

 

Still, absolute value of your engine torque is not that important for your motorbike acceleration. Even if your engine can generate only a small torque, you can easily enlarge it by using higher gear ratio in your gear box. Yes yes, we said that high gear ratio reduces maximum speed of your motorbike – but what if your low-torque engine is the one that can run into very high revs? This way, even with high gear ratio you could still obtain some respectable speed, and have acceptable acceleration at the same time.

 

Strange, isn’t it? You can have high-torque-low-revs engine or you can have low-torque-high-revs engine, and both can achieve similar results. You only need lower gear ratio to the first one and higher gear ratio for the second one... It really is that way.

 

Okay, if torque doesn’t matter, what matters then? What matters is product of the torque and revs. You must multiply the torque that is your engine capable with max revs that your engine can achieve – the product will be a good estimation of acceleration capacity of your engine… Do engine manufacturers already give this number in their engine specification datasheets? Yes, of course, this is the first number you see – the power!

 

The power is the product of torque and rotation speed.

 

Because gearbox ratios are always near-optimal in real-world motrobikes, you can quickly estimate their acceleration abilities only by looking at the engine power and motorbike overall weight (mass).

 

For more precise acceleration estimation, you must consult either power curve or torque curve. Their shapes are important. For example some engines will not want to accelerate your bike at low revs, but will go mad at high revs, while other engines may provide more constant acceleration over full range.

 

Obviously, from the torque curve we can produce the power curve of an engine and vice versa. Reasonably, the power curve will have its maximum at higher revs than the torque curve. But either of these two curves is enough to predict the acceleration behavior of your bike (sure, you must also know: gear ratios, rear wheel diameter, tire/pavement friction coefficient, aerodynamic drag and rolling resistance, mass of vehicle+driver+petrol+anything). This should be enough for ‘a regular person’.

 

As for gearboxes, it is better if they have greater number of ratios because with large number of gear ratios you can hold your engine at narrow rev band. Of course that you will choose the band that is around peek-torque on the torque curve… Actually, if you had continuous-variable-transmission, you could keep your engine always exactly at the peek-torque gaining maximal acceleration. There are however technological reasons why normal motorbikes don’t use transmissions with 30 gear ratios nor CVTs.

 

 

[The Ultimate M]

Thnx everybody. All of u've provided me good knowledge. Thnx for ur valuable time......;-)

 

Hope to talk again soon.....................................

 

Any updation/ further knowledge, if somebody want to publish on this topic, is welcomed.........;-)