Torque - lbs/ft, lbs.ft, ft.lbs or Nm

otispunkmeyer

otispunkmeyer

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Nm is the one i usually work in, and looking round google, its always quoted as Nm. it makes sense, torque is like a bending moment, its a force x a distance from a center of rotation for example

newtons are a force (weight x gravity) and meters are the distance.

but the imperial version, i have seen this quoted as

lbs.ft pounds feet (seems the same as Newton meter though pounds is a weight and not a force)

then theres ft.lbs foot pounds which doesnt make a great deal of sense to me

and then i have seen lbs/ft which is pounds per foot which again doesnt make sense to me because thats a weight divided by a distance

can some one explain the lbs.ft thing to me?

also car makers quote engine power in HP, when engineers will typically use kW instead. HP is the bigger number, so basically that gets to go into the marketing gash. but torque measured in lbs.ft or what ever is a lower number than torque in Nm. so why do they keep quoting it like that? tradition?
 
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otispunkmeyer said:
newtons are a force (weight x gravity)
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Now that's just rubbish. I see, you remember it from the f = mg equation, but that's just one particular case. It's like saying color is always red because you've seen a red dress one day. It's just force, which is no more than a force, but sometimes it equals to something or other.

Regarding the lbs x ft (i believe multiplication is the proper form of spelling it), i think it's lbs being substitute for the force compared to the n x m form. Because weight (as opposed to mass) is actually a force, at least that's what i've been taught quite some years ago :D
 
Newton is the force an object of 0.1 kg exerts on it's supporting object in the earth's gravitational field (called weight, which as you night notice is something else than mass). Newton is not constant. It varies depending on where on the earth you are as the earth's gravitational field varies. So yes, Newton is a force. In the imperial system both weight and mass are expressed in pounds.
 
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otispunkmeyer said:
Nm is the one i usually work in, and looking round google, its always quoted as Nm. it makes sense, torque is like a bending moment, its a force x a distance from a center of rotation for example

newtons are a force (weight x gravity) and meters are the distance.

but the imperial version, i have seen this quoted as

lbs.ft pounds feet (seems the same as Newton meter though pounds is a weight and not a force)

then theres ft.lbs foot pounds which doesnt make a great deal of sense to me

and then i have seen lbs/ft which is pounds per foot which again doesnt make sense to me because thats a weight divided by a distance

can some one explain the lbs.ft thing to me?

also car makers quote engine power in HP, when engineers will typically use kW instead. HP is the bigger number, so basically that gets to go into the marketing gash. but torque measured in lbs.ft or what ever is a lower number than torque in Nm. so why do they keep quoting it like that? tradition?
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I'm used to Newton metres, being brought up using the metric system and all. I can't really judge how torquey a motor is by looking at how many foot pounds it produces.

I'm pretty sure that ft.lbs and lbs.ft are the same thing. You're right, though, lbs/ft isn't a torque, it's a stiffness.
 
lbs/ft, lb-ft, ft-lbs...while they may not be semantically correct, people say them as if they mean exactly the same thing.
 
1 pound force feet = 1.355 newton meters, more or less. Like epp_b said, lb.ft.=ft.lb. etc etc. The only reason we stick with it is because it would be a huge pain in the ass to change over to the metric system lol. They've been teaching the metric system in schools for like thirty-plus years, and it's slowly being used more and more.
 
Measuring tourqe with weight is quite interesting, as it would mean that an engine outputs different numbers depending on it's location :p A Koenigsegg CCX would have about 4094 lbft of torque on the moon. And you might gain something from doing dyno-runs as close to the equator as possible, as the gravity is slightly lower there.
 
ahhh yanks and their crazy imperial system

granted I don't think in metric and have been brought up in a metric world to think in miles, bhp, lbft and all that bollocks.

ftr weight is a force, so lbs is kinda acceptable. I think a jet engine developing 56K lbs of thrust and you get 249.1KN.

now looking at bugatti.... 922 lb ft = 1250 Nm.... (I'm using a rough diagram off of bugatti's site for this info) will produce a tractive effort of 30kN...

don't ask me why I spent 15 minutes calculating that out....
 
a ft-lbs and a lbs-ft are the same thing, it's just both values multiplied by eachother.

I think they use BHP and FT-LBS more often than kW and Nm because they're usually similar numbers on a car that revs to 6000 - 7000rpm, and because of that it's easy to compare the two.

Take the Z06 for instance, it makes 505hp and 480ft-lbs while a regular C6 makes 400hp and 400ft-lbs of torque. Now we can assume that the LS7 is tuned to produce peak torque near the higher 7000rpm redline. Since the C6 revs lower (6500rpm) the torque is tuned to it's max (4400rpm). You can compare the peak horsepower rpms compared to torque to see where the car's powerband is. Using units like Nm and kW make it hard to compare the two values and picture the dyno chart in your head.
 
Vitamin J said:
a ft-lbs and a lbs-ft are the same thing, it's just both values multiplied by eachother.

I think they use BHP and FT-LBS more often than kW and Nm because they're usually similar numbers on a car that revs to 6000 - 7000rpm, and because of that it's easy to compare the two.
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Aren't the peak power and torque at the same rpm whatever the unit is? 1kW = 1.36hp and 1Nm = 0.74 lb-ft etc. The dyno chart is the same whatever the units. It's easier to imagine it in your head if you use units that are familiar to you of course.
Familiarity is just the reason why marketing uses hp and lb-ft etc.
 
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Aren't the peak power and torque at the same rpm whatever the unit is?

neg. power = torque * engine speed[rads^-1] (in SI units - imperial you need to use conversion factors)

and we'll take some examples to prove it.

Landrover freelander.. just cos it's the only one I have to hand

peak torque = 295lb ft @ 2000 rpm = 400Nm - power at 2000 rpm = 83.733 kW

peak power = 158bhp @ 4000 rpm = 118 kW - torque at 4000 rpm = 281 Nm

I would draw a graph based on that, but I can't be arsed.

ok, I had a script all ready for such a moment... (It's for a different vehicle I did for a uni project but it shows enough all the same)

powervstorque.jpg
 
I think what geeman said is that the curve is the same (although I haven't seen anyone claiming the oppisite ;))
It's linear conversion from one unit to another. You can use whatever unit you're feel more comfortable with to judge the magnitude, but obviously this doesn't affect the curve shape. Peak is still at same place.
 
I think what geeman said is that the curve is the same

it's not because the factor you multiply it by (engine speed) changes , therefore it's not a linear relationship
 
Engine speed? What are you talking about, rpm is rpm, it doesn't change.

1 ftlb ~= 1.356 Nm

1 hp ~= 0.746 kW

It's just a scaling factor. I.e. you change the scale but not the shape, because revolution per minute is always revolution per minute SI units or not SI units..
 
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Blythy said:
I think what geeman said is that the curve is the same
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Yes. I guess the wording I used could be misunderstood easily. What I meant was that if for example the peak power is at 4000 rpm when the units are kW, the peak is at 4000 rpm too if the units are bhp.
 
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MXM said:
Engine speed? What are you talking about, rpm is rpm, it doesn't change.

1 ftlb ~= 1.356 Nm

1 hp ~= 0.746 kW

It's just a scaling factor. I.e. you change the scale but not the shape, because revolution per minute is always revolution per minute SI units or not SI units..
Click to expand...

i dunno

we always always use any rotational speed as radians per second rather than revolutions per second. so i guess thats the engine speed he means.
 
geeman said:
Yes. I guess the wording I used could be misunderstood easily. What I meant was that if for example the peak power is at 4000 rpm when the units are Nm, the peak is at 4000 rpm too if the units are bhp.
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Maybe I'm still misunderstanding your wording. Power isn't measured in Nm, torque is. Power and torque are two different things.

Peak torque (Nm, lb ft etc) will almost always be at a different engine speed than the peak power (kw, bhp, ps etc)

Peak Nm rpm = peak lb ft rpm

Peak Kw rpm = peak bhp rpm

Peak Kw (or bhp) rpm != peak Nm (or lb ft) rpm

Power (Kw or bhp) = Torque * rpm. So while your torque figure may drop off as revs increase, that increase in revs is more than enough to offset the fall in torque, so your power still rises.

Peak Nm / lb ft will most likely occur at a different rpm than peak Kw / hp as they're measuring different things.
 
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