define torque

[Baltobuell]

That was the most real and results way I could think of to describe the difference Torque is the grunt that gets it going, Hp is what gets it going faster once it's going. Sorry, I wasn't too clear either. At least you know I'm not an engineer.

[251am]

  Torque is the machine's ability to put the hp to the ground.
  Torque is a Powerstroke diesel. I do not disagree with Mr. Chin on the two, hp and torque, being inseperable, but the "feel" difference of torque exploding from the wheels of a TurboDiesel vs the HP of a Shelby is looking at kiwi and apples.
  Ahhhh, the sound of red-winged blackbirds in a Minnesota marsh is calling.....    

[tzracer]

Alright, finally something fun to talk about. (besides being a physicist I teach college physics for a living)

Torque can be best described as a twist. It is the rotational version of force. It is a force times a lever arm (actually a cross product for you vectorally inclined). It has the same units as energy, but it is not energy. In the metric system, N-m and J are the same (1 N-m = 1 J), but N-m is used for torque and J is used for energy and work.

Power is the rate at which work is done. Power is what is important when it comes to acceleration, the more power, the quicker your acceleration, all else being the same.

Power and torque are related as stated previously, change one and you change the other. It really doesn't make much sense to say you can feel one but not feel the other. People see acceleration and feel force. Force causes your head to snap back when someone steps on the gas. This force can be described from the point of view of torque or power. When I lower my gearing (such as larger rear sprocket), do I gain acceleration because I increased torque or because I am making more power for a given speed?

Equations are like a picture, they express much more than words ever can. Avoiding them can be a disservice to the audience. Equations are the key to truely understanding the subject of physics. Math is to physics, like a motorcycle is to a racer.

To answer the question, for an engine, torque is the twisting (rotational force) of the crankshaft at a given rpm. Power is the torque times the rate the crank is rotating. In some sense they are really 2 different ways of measuring the same thing.

[tcchin]

Whew, my dribble actually almost agreed with the Professor's answer! Why couldn't that have happened back in school when it counted?

And now for some trivia: How can two properties have the same units and not be the same? Why is torque not energy? Obviously, torque is work, otherwise it couldn't become power when multiplied by RPM and a constant, and work defines the increase or decrease in the energy state of an object... Please help a lowly layman fit the pieces together.

[chris_chops]

HP is work,
Torque is force.

The answer to WHY
is why not



Now everyone lets define Irony
:D X

irony- n. 1. a method of humorous or sarcastic expression in which the intended meaning of the words used is their direct opposite of their usual sense:  as, the speaker was using irony when he said that the stupid plan was "very clever."  2. an instance of this.  3.  a combination of circumstances or a result that is the opposite of what might be expected or considered appropriate: as, it was an irony of fate that the fireboat burned and sank  4. the feigning of ignorance in argument: more frequently Socratic irony (after Socrates, who uses this device in Plato's Dialogues).

[tzracer]

Whew, my dribble actually almost agreed with the Professor's answer! Why couldn't that have happened back in school when it counted?

Because then you would not have the humbling experience that is being a student.

And now for some trivia: How can two properties have the same units and not be the same? Why is torque not energy? Obviously, torque is work, otherwise it couldn't become power when multiplied by RPM and a constant, and work defines the increase or decrease in the energy state of an object... Please help a lowly layman fit the pieces together.

First, torque is not work. One way to tell is that torque is a vector quantity and work is a scalar quantity, so torque cannot be work.

Rotational work is the dot product of torque and angular displacement (tau times theta). Torque has units (N-m, ft-lb, etc) but angles do not have units. We measure angles in degrees, radians and on rare occations gradians. These are not units, they are labels. Don't like that? Look at the formula (see formulas do come in handy) for arc length. Arclength (s) = radius (r) * angle (theta, measured in radians). Rearrange the formula solving for the angle : angle = arclength/radius. Radius and arclength are both lengths, a length divided by a length will have no units, therefore the angle has no units.

So since angles have no units, torque, work and energy have the same units. Some in the engineering community will argue that the units are not the same. Engineers tend to use lb-ft for torque and ft-lb for work and energy, then tell you that these are not the same units. It would be much easier if everyone used the much better metric system in which torque is measured in N-m, and work and energy are J (Joules).

Power is work divided by time, so it is (torque * angle)/time, but due to commutivity and associativity in math this can be re-written as torque*(angle/time). Angle/time is rotational speed (omega), rpm is one of the units of rotational speed. The formulas show how it works out, multiplying torque times rpm converts the torque to work (the angle) and divides by time.

W = T * theta
P = T * omega
omega = theta/t
P = w/t

where
W = work
P = power
theta = angle
omega = angular velocity
t = time
        

[tzracer]

Why do people consider low rpm engines torquey and high rpm motors to not be torquey?

Power (hp) = [torque (ft-lbs) * RPM]/5252

Low rpm engines, such as a diesel or a HD twin, "feel" torquey. These engines also make more power at low rpm. If you have 2 engines at the same rpm, the one with more torque is making more power. So I could just say that the "feel" of an engine depends upon how the power is delivered. More power at low rpm will make the vehicle feel more lively. This is what most people call a torquey engine.

So can you "feel" torque and not "feel" power?

One of the problems with refering to the torque, depends upon where you measure the torque. Torque can be changed by gearing (up or down). The amount of engine torque, the useful rpm range and the gearing all contribute to the "feel" of the engine. An engine with low rpm torque may be easier to tow with, but it doesn't have to be able to tow more. Towing capacities have more to do with gearing and frame strength than engine type. I have driven plenty of 40,000 lb straight trucks with gasoline engines that worked fine.

[251am]

  I guess I figure more HP to generally = more wheelspin. More torque to = more forward movement. So, I guess I can "feel" torque in a forward rush of movement. I hear hp in spinning wheels, but then we enter the realm of inertial forces, clutches, and my brain's constipation. Anyway, let's put this in perspective we all understand a little better.
   Two bikes that each have 80 hp. Both machines are four strokes with engine size that is equivalent.(Oooh man, this can get messy.) One has a torque rating of 75 ft/lbs and the other is rated at 67 ft/lbs. Using the same rider for speed and lap times which machine has faster lap and 1/4 times?
   Mr. Mc Laughlin, are you the "Science Guy" Rick refers to? If so thanks for your time at the track. Now, back to the Minnesota marshes/Montana/church of quality book. Merry Christmas everyone. ;)        

[tzracer]

ÊI guess I figure more HP to generally = more wheelspin. More torque to = more forward movement. So, I guess I can "feel" torque in a forward rush of movement. I hear hp in spinning wheels, but then we enter the realm of inertial forces, clutches, and my brain's constipation.

If you have more power then you also have more torque. Wheelspin can explained interms of torque or power. How the power is delivered is as important as the amount of power. Things like heavier cranks help to control wheelspin.

Anyway, let's put this in perspective we all understand a little better.
 Ê Two bikes that each have 80 hp. Both machines are four strokes with engine size that is equivalent.(Oooh man, this can get messy.) One has a torque rating of 75 ft/lbs and the other is rated at 67 ft/lbs. Using the same rider for speed and lap times which machine has faster lap and 1/4 times?

Really can't say. Too many undefined variables. The engine with higher torque will probably have the power peak and torque peak at lower rpm. The shape of the curves are what is most important. The shape tells how power and torque are delivered, the width of the power band. Generally all else being equal, a more rider friendly powerband will make for an easier to ride bike (although not necessarily faster, at least under ideal conditions), especially when tires are worn or in the rain. I don't like to see big dips or steep increses when I look at a power/torque graph. For an engine power and torque are not seperable. Any description in terms of torque can be made in terms of power.

Wheelspin - too much torque, abrupt power increase.

Ê Mr. Mc Laughlin, are you the "Science Guy" Rick refers to? If so thanks for your time at the track. Now, back to the Minnesota marshes/Montana/church of quality book. Merry Christmas everyone. ;) Ê Ê Ê Ê

Yes I am. You are welcome. Remember we are here to help.

[Lowe119]

I had this discussion with an engineer a few years ago. It took awhile to understand it, especially when you look at how one horsepower is measured - in relation to a actual horse doing work. It was confusing, at first, to look at a 1,000 lb horse as having only 1 hp. Then I realized the torque behind that horse and how slow it is compared to a 10,000 rpm motor.  You can probably gear the horse and get a ton more hp out of one horse with all that torque.

[tzracer]

Only one problem, power is unaffected by gearing. Power output cannot exceed power input for any machine. A gearbox is a form of machine. Torque can be increased by a gearbox, but at the expense of rotational speed (output shaft slower than input shaft). Double torque, cut rotational speed in half. Cut torque in half, double rotational speed.

[K3 Chris Onwiler]

 You can probably gear the horse and get a ton more hp out of one horse with all that torque.

:D ;D :D ;D :D