Please?

Power = (rate at which work is being done) = torque x revs

where torque = (turning force) = (force on pedal) x (length of crank)

Torque at the wheel can be different to torque at the crank, depending on the gear ratio you're using, but power is constant throughout the drivetrain (except frictional losses).

I've always thought of it as Power compares to top speed, Torque is how quick you'll get there.

Power determines how fast you hit the wall, torque determines how far through the wall you travel.

Power determines how fast you hit the wall, torque determines how far through the wall you travel.

wrong on both counts!
power is how quickly you're accelerating (or how steep you're climbing)
torque is how hard you're pushing the pedals

A useful quote about driving
Under steer = Hitting the fence with the front of the car
Over steer = Hitting the fence with the back of the car
Speed = How fast you hit the fence
Torque = How far the fence and the car travel after the impact.

Torque = How far the fence and the car travel after the impact.

No! No! No!

Momentum = How far the fence and the car travel after the impact.

Well that's clear then 🙄

Well that's clear then

the first answer was clear and correct 🙂

elliptic got it spot on. In real terms, torque is the FORCE that you create at the cranks, and power is the WORK that's done (i.e. the total energy used/transfered to the wheels). Take a little example….

If you get into the big chainring and try and go up a big hill, then that's a lot of Torque because your pushing the pedals really hard, but chances are you'll go really slowly and topple off, so there's not a lot of climbing going on and therefore not a lot of energy (and therefore not much Power) going into climbing the hill.

Another example…take Mark Cavendish sprinting to the line in the TdF, he's cranking a huge gear, but also pushing a cadence of 120rpm. He's creating a massive amount of force at the pedals (which relates to massive torque at the BB), and turning them quickly, so creating lots and lots and lots of Power.

Another-other example…take Mr Contador climbing a tasty 25km alpine pass. He'll be in the 34t front ring, a 25 on the back and spinning merrily. So he'll not be pushing the pedals all that hard, and therefore not creating a lot of torque at the BB, but he's going uphill Fast and therefore pushing out a lot of power.

Torque is the same as moment of force. It's just how hard something is being twisted or turned.

Good practical examples from petesam.

torque = a rotational force

work = energy transfered

energy transfered = force * distance

power = enrgy transfered / time

so…………

power = force * distance / time = force * speed

so imagine two engines, both with identical gearboxes and both in identical cars.

One is tuned to give 250 lb/ft torque at 2500rpm, the other 250 lb/ft torque at 4000 rpm, both with similar torque curves, just shifted by 1500rpm. The torque curve builds upto the max then falls away rapidly.

Car A produces 250*2500/5252 = 119bhp

Car B produces 250*4000/5252 = 190bhp

1/5252 is the constant if using imperial (lb/ft and bhp) units

So both cars have the same feeling of acceleration (both push you allong with 250lb/ft / drive ratio) but car B will be faster as its capable of puting out more power.

Take F1 as an example, the engines use the best available tech, producing 700bhp from 2.4 ltr engines at 19000 rpm (aprox). In actual fact its no more torquey than a normal engine, see………..

700*5252/19000 = 195 lb/ft torque.

One of the reasons (that and the high gear ratios) why F1 cars dont accelerate as fast as you'd imagine.

Power = Torque x RPM

That's it!

Power = Torque x RPM

actually power = force * velocity
torque = force * radius
so power = torque * ( rotational velocity in radians/sec )

retaining dimensional integrity

1 RPM = 2.pi / 60 radians/sec

And contador would be on 39×23 or 25 not 34 unless it was the Angliru which isn't alpine

Ah, boo to the technicalities of which gear contador would be using…I thought the pros were into compacts these days? Or are they just a solution for weak plebians? Never used one myself I have to say.

Torque is Force

Power is Transfer

Torque is Force

Power is Transfer

wrong & wrong :o) In fact the 2nd one is so wrong I don't know what it means.

Of course you're technically right sb, but I think the OP wanted simple 🙂
Radians are quite obscure to most people.

Thisisnotaspoon has the correct explanation, albeit one to which I guess most cyclists will not be able to associate.

Torque is the ability of something (an engine, a human, whatever) to move a weight at a distance. Usually referred to in Newton Metres (Nm) or pounds feet (lb/ft). That is to say something that applies a force of 1Nm, means that it can lift a weight of 1 Newton (approx 100grammes on Earth) at a 1 metre distance. Similarly, 1 lb/ft of torque is the ability to lift a pound in weight at 1 foot of distance.

What may be easier to understand, in the question of what torques is versus power, is to compare the cyclist on a bike (as the bikes engine) Vs a car engine…

The situation as follows… A cyclist weighing 200lbs is capable of pedalling at up to 120rpm, and of applying a pressure at the pedals equal to that of exactly half of his own weight. He is running 175mm cranks)

To work out the torque applied:

Torque = 100lbs / (175/(12×25.4))

Torque = 57.4 lb/ft of torque

So a fairly strong bloke, riding a bike, is capable of creating as much torque as a small car engine!

But to work out how many brake horsepower he is creating, we observe the following…

Power = 57.4 x (120/5252)

Power = 1.3 BHP

So not much power at all!

If we compare this to a 1 litre car engine, which for the sake of arguments produces exactly the same 57.4 lb/ft of torque (a totally realistic figure for a small car engine), but it produces this torque at 6000rpm, we get the following for its power…

Power = 57.4 x (6000/5252)

Power = 65.6 BHP (There or therabouts what a 1 litre Toyota Yaris engine, or similar, creates)

So, we have two different engines (a human on a bike, a 1 litre petrol engine in a small car), but because they deliver their torque in totally different ways, we get vastly different Power figures. This is also why the guy on the bike is unlikely to be able to top 35mph on a flat level road, whereas the car would most likely top 100mph on the same road given the space. Yet they are producing the same peak torque! 😉

It's funny to think that an F1 car engine only produces approximately only 3 times as much torque as a strong cyclist does. But because the engine in that can spin up to 18,000rpm or more (in other words more than 150 times as fast as the cyclists legs), it will produce more than 450 (150×3) times the outright power!

Crazy stuff this physics… 😉

voltage x current = …………..

Excellent stuff!

I've learned a lot from reading this thread (except for SFB's contributions).

Cheers guys.

"Torque is cheap…."

Cracking phrase used by a mate in an XC race when the Torque powder guys 'dissed him', then he whooped their collective asses…

From what I understand, and another phrase used by Mr Shelby:
"Horsepower sells cars, torque wins races".

Look at it this way – my little RC car has an engine that revs to 44000rpm (yup – fourty four!) and produces 2.1Hp. This is a lot of power to weight/displacement. However – I could grab the tyres and easily stop the engine as it has pi$$ all torque. It's all revs based power.
Now, an engine producing similar power (say a few Hp) but revving at 10RPM would be pretty hard to stop by hand wouldn't it, if you were to grab hold of the driveshaft that is. That's the torqe that is. Same power, more torque.

However, it depends on what application you're using it for.
Using the example above, you could have the 10rpm engine connected to horrendously tall gearing in order to turn the drive wheels at the same speed as the 44000rpm engine, but it'd be a pig to get going (accelerate basically)…. and my little RC can accelerate quicker than my type R (well, to 40mph at least)

DrP

(except for SFB's contributions).

well, mine were the most literally accurate :o)
But mechanics wasn't meant to be easy…

I think I'm getting it. 😀

But mechanics wasn't meant to be easy…

As a teacher I value good, clear descriptions where the audience isn't patronised or treated in a smug, condescending manner.

Stick to the day job Simon. Sorry.

clear descriptions where the audience isn't patronised or treated in a smug, condescending manner.

I've just read my posts and they're strictly accurate while assuming people can understand simple sums. That doesn't correspond to my idea of condescension.

Jesus amateur engineers night is it??

I've just read my posts and they're strictly accurate while assuming people can understand simple sums.

simonfbarnes – Member

Power determines how fast you hit the wall, torque determines how far through the wall you travel.

wrong on both counts!
power is how quickly you're accelerating (or how steep you're climbing)
torque is how hard you're pushing the pedals

well, here;s why your explanation is wrong using simple maths,

"Power is how quickly you accelerating"

acceleration = dU/dt (change in velocity per change in time)

power = dw/dt (change in energy pe change in time)

If you folow those through to their logical conclusion you'll find that TORQUE is proportional to ACCELERATION, not power.

If you draw a power curve against velocity for a vehicle in a given gear and then superimpose the drag against velocity curve the point at which the two itersect is the top speed in that gear (in a car the engie cant supply air/fuel quick enough, on a bike you leg cant spin fast enough). The difference between the two lines at any point upto this is the rate of change of kinetic energy of the vehicle.

kinetic energy = mass * velocity * velocity

power transfered to kinetic energy = mass * velocity * velocity / dt

torque = mass * velocity / dt

or in its more usual form

force = mass * acceleration

Thus Mr SFB, you are in fact wrong on your point that power is related to acceleration, well technicaly it is, but only becasue power is related to torque which is related to acceleration, but seeing as this whole discusio is about the difference between power and torque i'd ay thats a very crucial point.

Jesus amateur engineers night is it??

or profesional in some cases :p Although car engines are more of a hobby, my degree and work is in proces engineering with my masters thesis on development of the wildfire constant lubrication system in ski's and telmetry for ice hockey sticks.

thisisnotaspoon ..you may have a degree but your spelling is atrocious!

Torque is what they do on world's strongest man when they arm wrestle. Power is when they have to drag along a lorry on a rope.
I only watch the programme to check out the competition mind.

You're all forgetting one crucial fact when defining torque (and my old biomechanics lecturer will thank me for this):

Torque = force x perpendicular distance

That is to say, the force value used to calculate torque on a lever must act at right angles to an imaginary line drawn through the centre of rotation of the lever and the point at which the applied force acts upon the lever. If the actual force is not perpendicular to this line, simple trigonometry can be used to determine the perpendicular component of the applied force.

For example, when starting off on your bike you set your cranks at 15 degrees to the horizontal (front foot a bit above the BB), and push down on the pedal vertically with a force of, say, 250 Newtons (no idea what that force might actually be). Part of that 250N is pushing the crank round the BB, part is trying to push the pedal towards the BB; only the rotational part of the force results in torque.

So, the vertical component of that force is: 250N / cos 15 degrees = 241.5N. If you have 175mm cranks and you push down exactly over the centre of the pedal, you create a torque of: 241.5N x 0.175m = 42.3Nm

Torque is not simply 250N x 0.175m = 43.75Nm. The force must act perpendicularly. There. Obsessive German biomechanics lecturer safely quelled.

For the record I'm ignoring wind resistance and Pi does not equal the square root of ten (nor does it equal 3.14, but that's beside the point. Isn't maths fun kids?).

Given some of the crap on this thread I feel qualified to answer

Power = how hard you push the spanner.
Tourque = how long the spanner is.

So a weak bloke with a big spanner can match a big bloke with a small spanner because even though he has less power he has more tourque.

who cares,get out and ride

well, here;s why your explanation is wrong using simple maths,

yes, you're right, I wasn't being rigorous in those statements – I was only thinking that on the level, applying power means you'll accelerate (ignoring wind resistance), but that if you're climbing at a steady speed power is required. The point being that torque only involves power if the point of application is changing. You can be stationary with the brakes on and apply a large torque to the crank without any power being developed.

Power = how hard you push the spanner.

more like how much you sweat turning the nut…

Power…whoosh
Torque…grunt
Simples.