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Torque

Torque

(OP)
I know what it means, but the motoring press and my Dad don't.  I'm about to spend a week with him telling me about his new car and how even the power's not the same as his old one, the torque's higher.  Etc.  Dad is a caravan (US: trailor) tower, so torque & power are things that car retailers quote at him.  And he always asks me for my advice.  Typical conversation:

Dad: "What does power mean?"

SG: "It's the rate at which work can be done.  In other words, it's the speed you can tow your caravan up a hill."

Dad: "What about torque, what does that mean?"

SG: "It's the amount of grunt/pull your engine has."

Dad: "But what if I change down to a lower gear?  Then I can pull whatever I like."

SG: "But you can't pull it very fast though, because it'll require a low gear."

Dad: "Fast! But wasn't speed to do with power?"

Does anyone have a layman's description of what torque does for a car?

RE: Torque

Torque will make a car able to pull a heavy load (or accelerate quickly).
Power will make the car able to do the same at higher speed.
For example a Ferrari and a truck both have 400hp (power).
However the truck has way more torque. That's why the Ferrari can go 250 km/h but only carries two people, while the truck goes slow, but carries 20 tons.

RE: Torque

I've struggled explaining the difference to non-techies many times and the best I could come up with is this:

Low torque, low horsepower:  A weak person on a bicycle in a low gear - he can't pull much and he can't go fast.

Low torque, high horsepower:  A weak person on a bicycle in high gear - he can't pull much, but he can go fast.

High torque, low horsepower:  A strong person on a bicycle in low gear - he can pull a bunch, but he can't go fast.

High torque, high horsepower:  A strong person on a bicycle in high gear - he can pull something fast.

It's simplistic and way off of the mark in engineering terms, but I've found it's something that the layperson with some practicality of mind can get his arms around.

RE: Torque

Power = torque x speed x appropriate unit conversion factor

Put your dad on a ten speed bicycle.  Explain to your dad that he is essentially a constant power supply.

Put the bike in a high gear and point him to a hill.

Put the bike in a low gear and point him to a flat.

Two other permutations.

RE: Torque

(OP)
Murec:

"Torque will make a car able to pull a heavy load (or accelerate quickly)."

Not true.  Low gear ratios can do that too (as my Dad consistently points out).


ctopher:

Interesting and useful links, thanks.  But acceleration can always be increased by lowering the gear ratio in which you are accelerating.

I guess in the (possible) future, CVTs will make torque irrelevant?


I do notice that the UK's most outspoken journo (Clarkson) always postfixes the word "torque" with "whatever that might be".

RE: Torque

yeah, when it comes to moving a vehicle down the road, the faster you can convert chemical energy to work, the faster you can go.  Having lots of torque helps to overcome the practical obstacle of power dropping with reduced engine speed.  An engine with "gobs of torque" simply has a greater power output at low rpm than one with the same rated power but "no torque".

RE: Torque

Funny thing this topic comes up now.  My 12-year old daughter moved from a 1-speed bike to a 15-speed bike 6 months ago.  She still uses only 5th speed.

When I was growing up in the hills of New York, I didn't know beans about power and torque but I learned to shift by necessity.  Low gear on the way up and high gear on the way down.

I don't have the uphill/downhill to help teach my daughter here in the flatlands of Texas.  I tried showing her how much easier 1st gear was on the grass and 10th gear when going fast level, but somehow it didn't quite take yet. Not knowing what else to say, I resorted to talking about torque, speed and power for about 15 minutes until she walked away shaking her head.  Afterwards I realized that I probably would have done a better job explaining it if my background were business or sociology instead of engineering.  That seems ironic but I think it's true. Has anyone else ever experienced that?

=====================================
Eng-tips forums: The best place on the web for engineering discussions.

RE: Torque

You are explaining torque to a 12 year old for 15 minutes?

Boy, you have a great daughter. Most 12 year olds would've walked after 10 seconds (and that is out of politeness).

smarty

"Do not worry about your problems with mathematics, I assure you mine are far greater."   
Albert Einstein
Have you read FAQ731-376 to make the best use of Eng-Tips Forums?

RE: Torque

Torque is like how much you can bench press at once.

Power is like how many times you can bench press 50kg in one minute.

RE: Torque

(OP)
Tomfh:

I hear what you're saying, but...

You can bench press as much as you like once, given the right pulley system.  The amount of force your actual body can provide (i.e. the "torque") is irrelevant.

But it doesn't matter how much mechanical equipment you have at your disposal, you'll never increase the amount you can press in one minute.  This is limited by your power output.


I guess the answer to my original question is that torque in itself is irrelevant so long as you have adequate gearing to allow the power available to do useful work.

And given that most vehicles have a finite (low) number of fixed ratios, we should really measure "flexibility" - the flatness and width of the torque curve.

RE: Torque

To address your issue with gearing:

The right pulley system will slow the movement of your weight down, i.e., you'll do the same amount of work in MORE time, thereby using LESS power.  Torque is NOT irrelevant.  If you had more torque (a stronger guy doing the bench press) you wouldn't have to use as elaborate a pulley system, thereby raising the weight faster, doing the same amount of work faster, hence more power.

Remember, power is intimately related to torque - you cannot seperate the two.  Torque is the only way to measure what's going on.  You can't measure horsepower in a car engine;  you measure torque and calculate the power from the speed the engine is turning.

Part of the problem of understanding this when talking about IC engines, electric motors, and bicycling is that the torque generated by them VARIES WITH RPM.  It's tough to compare apples to apples when the torque is varying.

Let's look at a bike that is driven by someone or something that can create constant torque:

In a high gear, that constant torque will create a constant force at the tire and the bike will accelerate at a certain rate.

In a low gear, that constant torque will create a greater constant force at the tire and the bike will accelerate faster.

The key is, the constant torque applied in the lower gear has to be applied FASTER than the one in the high gear (the rpm at the crank is higher).  This is where your higher power number comes from with the same amount of torque.

Now lets look at a bike that is driven by someone or something that can provide constant power:

In a high gear, that constant power will create a high torque at the slow speed of the crank, creating a certain force at the tire and the bike will accelerate.  However, as speed increases, the torque will decrease (constant power) and he will level off at a certain speed.

In a low gear, that constant power will create a low torque at the high speed of the crank, creating THE SAME FORCE at the tire as in the above example.  The result?  The bike will increase in speed, and level off in speed, at the same levels as in the previous example!

If you stand back and look at it, these examples make sense - in the first two scenarios, the person that can maintain torque will create more power in a higher gearing, whereas in the second two scenarios, if you have a given power, you will accelerate at a given rate and top out at the same rate - power is work over time and you're moving the same mass over the same distance in the same amount of time.

RE: Torque

(OP)
Yes, I know all this.  It's high school physics.  Anyone who doesn't know it isn't qualified to be posting here.

My problem is explaining it in a sensible way to the average layman.  And in that regard, I think 25362's second link has by far the best explanation.

RE: Torque

I have seen a, normally, perfectly sensible, forum, riven by this.

Either, you believe the maths. Or, you settle down to a long religous war.

Incidentally, clever engineers, make damn sure you know  where the number 5252 comes from. 5252 is not exact.

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

RE: Torque

If you used a unit conversion table, 1 hp is around  33,000 ft*lbf/minute. Then there's those two pies.

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Eng-tips forums: The best place on the web for engineering discussions.

RE: Torque

Torque measures how "hard" the crankshaft can turn, like the difference between Peewee Herman trying to take off a lug nut and Ahhnold taking off a lug nut.  

Power measures how fast that the motor can do work.  Gearing down doesn't change how much torque is applied.  It increases the distance over which force is applied so that a smaller force can do a given amount of work.  It's still the same amount of work.  Maybe you're trying to pull 1000 pounds up 10 feet with a rope.  Directly pulling requires 1000 ft-lbs of work.  With a pulley system, maybe now it only requires 500 pounds, or 250 lbs, of force, but now you need to pull for 20 feet or 40 feet, respectively.  Same work, different way of doing it.  

Same thing with a car's gears.  A lower torque motor can pull a lot of things if you gear down, but it will have to do that work over a greater distance, or more turns of the crank.  Same work, different way of doing it.

That's my shot at it, lol.  Make any sense?

RE: Torque

(OP)
To my original post, I think my tactic will be to talk about diesel-electric trains when the conversation moves to this topic.  They effectively have almost perfect CVTs, so they can always run at the optimum condition (peak efficiency or peak power).  Torque isn't an issue then, except in the sense that if it's too high, things start to break.

RE: Torque

Sompting,

Or (if torque is too high) the wheels slip.

RE: Torque

(OP)
"Or (if torque is too high) the wheels slip"

Nope, if tractive effort is too high the wheels slip.

RE: Torque

//Or (if torque is too high) the wheels slip.//
You mean "NATO"?
No Action, Torque Only?

RE: Torque

   An acquaintance of mine bought himself a Z28 Camero.  He proudly told me that he never takes the engine above 3000rpm.  I asked him why he didn't just buy a mini-van instead, and he insisted that it is torque that makes you accelerate.  Power is unimportant.  I suppose he did not understand what the transmission does.

   My biggest failure at this dicussion occured one winter when I helped a woman get her car started out of the deep snow.  Eventually, I got into the car, put it into second gear and eased it gently out onto the road.  She asked me how such a counter-intuitive procedure could work.  Another car was happening along at just that moment and I think I quickly tried to say something to the effect that tall gears mean low torque means you don't spin the tires on the ice.  I think the only convincing thing I did was get her car unstuck.

                        JHG

RE: Torque

Here's the one I use.

Think of you having to remove a large rusty bolt with a wrench.
Torque is how hard you pull on the wrench handle.
Power is how fast you can unscrew the bolt depending on where you grab the wrench.

For example, if you grab the wrench handle at the farthest end, you have to exert the least amount of force on that handle. But it will take you longest to unscrew a bolt that way because your hands have more distance to travel. If you grab the head of the bolt directly with your fingers you can unscrew it the fastest, but you had better have some VERY strong fingers!

http:/www.jraef.com
Eng-Tips: Help for your job, not for your homework  Read FAQ731-376 pirate

RE: Torque

Yet curiously, the torque exerted in both those cases is identical. Neat way of confusing people!

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

RE: Torque

Torque -- kind of screw invented by a Monkee?

GregLocock: "5252 is not exact."
-- True, need to also specify the temper: 5252-O, 5252-H25, 5252-H38...

RE: Torque

As a lowly chemical engineer, I have to say I'm even more confused now than I was when we discussed torque in high school!

RE: Torque

Better than being a Torquemada!?

RE: Torque

Sompting Guy try this website,

http://trucks.about.com/od/autobuying/a/torque.htm

I think it gives a fairly simple answer to your torque or hp question.

Better than you explaining it to him... hand him a print out.

Reading it the same thing a few time might help him grasp the concept well enough that he need not discuss it with you ever again.

I hope this and all the other suggestions bring you some peace.

BSK

RE: Torque

What's confusing is the fact that torque and horsepower 'should' go hand-in-hand: if an engine produces a given HP, you should be able to calculate the torque for any given rpm.  This works pretty well for a DC electric motor that produces its peak torque at 0 rpm.  But it's different for an engine.

"Torque", when referred to in an automobile, usually means "low-end torque", or the torque that the engine can produce at low rpm.  Internal combustion engines don't usually do very well at low rpm...in fact they stall if you go too low.  Most engines produce their peak power at a relatively high rpm (like 5,000 or more).

So if a vehicle is said to have "good low-end torque", then it can burn rubber (or pull up a stump) with relative ease, though it might not reach as much peak power at high rpm as another car with lower torque.  My Honda S-2000, for example, has terrible low-end torque, but the V-Tech engine screams by the time you get up to 3000 rpm or so.

It all has to do with the design of the fuel injection/carburetor, the heads, the ignition system, the gearing, etc.

Don
Kansas City

RE: Torque

If we go back from physics to driving, then ctopher's first link explpains things quite well:
Torque and power are related by rpm.
What is important is not just max torque and hp, but the whole curves - at what rpm each of them is achieved. The engine will work most economically around the max torque rpm. It will accelerate up to the max power rpm (at the price of increased gas consumption), beyond that it will only produce more noise up to the red line.
Changing to a lower gear will make the engine go faster and consume more gas to do the same work. Also there will be no power reserve left for accelerating.

RE: Torque

SomptingGuy,
just print this thread out and give it to your dad.
Either he'll understand the answer(s) or he'll know not to ask again.

JMW
www.ViscoAnalyser.com

RE: Torque

“Relationship between torque and power
If a force is allowed to act through a distance, it is doing mechanical work. If torque is allowed to act through a rotational distance, it is doing work. Power is the work per unit time. However, time and rotational distance are related by the angular speed where each revolution results in the circumference of the circle being travelled by the force that is generating the torque. This means that torque that is causing the angular speed to increase is doing work and the generated power may be calculated as:
 
Power = torque×angular speed

Mathematically, the equation may be rearranged to compute torque for a given power output. However in practice there is no direct way to measure power whereas torque and angular speed can be measured directly.
Consistent units must be used. For metric SI units power is watts, torque is Newton-metres and angular speed is radians per second (not rpm and not even revolutions per second).


Some people (e.g. American automotive engineers) use horsepower (imperial mechanical) for power, foot-pounds (lbf·ft) for torque and rpm's (revolutions per minute) for angular speed. This results in the formula changing to:
Power (hp) = torque (lbf) × angular speed (rpm) / 5252
 This conversion factor is approximate because the transcendental number n appears in it; a more precise value is 5252.113 122 032 55... It also changes with the definition of the horsepower, of course; for example, using the metric horsepower, it becomes ~5180.”

For the same power the lower the torque the higher the angular speed
And the higher the torque the lower the angular speed
Low torque high rotational motors = Formula one cars

High torque low rotational motors = Trucks.

luis

auto

RE: Torque

The more you try to "simplify" an already perfectly simple subject (like torque) the worse it gets. I think large parts of this thread illustrates that.

Gunnar Englund
www.gke.org

RE: Torque

'nuff said, let's find a new subject to torque about.

RE: Torque

Just one last question:

SomptingGuy, how did it go with your dad?

RE: Torque

(OP)
You know, for once it never came up.  Still I have the ammo for next time.

RE: Torque

To beat the dead horse:

http://en.wikipedia.org/wiki/Torque

For a layman examples of a "torque" wrench or water hand pump (if one had seen it) or leverage-fulcrum will best explain torque. Its the angular force.

Power is at the rate you use the torque.

Using the hand pump example: You need more power to pump more water using the same pump and handle. That is constant toruque, speed varies.

Or
Torque= Strength in arm
Power=Stamina. How fast you use that strength or for how long.

Two boxers may have same strength (torque, how hard can they hit), however the guy with more power can deliver punches in quicker succession than the other. Speed.









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