thermite

My seat post collar has a label - Max Torque 70KGF-CM
Could anyone explain this please?

ryder47

70 kilograms force per centimeter

70 kgf = approx 154 lbf
1 cm = approx .3937"

This would be 154lbs force per .3937", however this sounds awfully high. Converted to foot lbs this would be approx 4694 foot lbs. Even if the 70 was a 7 that's 469 ft lbs. Still real high. Any chance it's a missprint and should read 70kgf/m? At that point it approx 48 ft lbs.

This refers to the max torque applied to the clamp bolt other wise you risk distorting the seat tube and or post.

eubi

The general formula for torque is T = r X f, or the radius crossed with the applied force. The operator "X" is the cross product (a vector operation); it is usually, but not necessarily "times". We will presume the applied force vector is normal to the moment arm vector, so it will be "times". Enough of my pedantic engineering ramblings...

The torque you gave above is in kgf * cm. Not kgf/cm.

Assuming the above conversion is correct, your torque would be:

154lbf * .3937in = 60 in*lbf. Reasonable for a seat post collar bolt.

Dusk

Formulas for converting other torque designations into inch pounds (in-lb.).

in-lb. = ft-lb. × 12
in-lb. = Nm × 8.851
in-lb. = kgf-cm / 1.15

70 Kgf-cm / 1.15 = 60.87 in-lbs.

60.87 in-lbs /12 = 5.07 ft-lbs.

Cheers,
Dusk

thermite

Thanks guys, still a bit confusing for me but the label does say exactly that -
"70KGF-CM" What would it be in lb-ft ??

ryder47

Oh s***. I feel like Homer Simpson, DOH!.

INstinct told me I made a wrong turn but I still crashed.

To: thermite. As other were quick to discover my error the correct answer is 5 ft lbs

CdCf

And the proper unit for torque is Nm, not kgf-anything...

thermite

So that's 1lb at the end of a 5 foot radius right?
Or 5 lbs at one foot?
Still can't get my head around all this.

ryder47

Yes, they are one in the same. Think of the radius as being a simple lever. 5lbs at one foot is "5 ft lbs" of torque. Now extend the lever to 5 feet and you have a mechanical advantage of 5:1. Now 1 lb at the 5 foot point becomes "5 ft lbs" of torque. This is the amount of torque applied to the center of rotation.

This is why cheater bars work on wrenchs. By extending the length of a wrench (a lever), the mechanical advantage gained mulitplies the amount of force you can apply to the center of rotation, or that stuck and frozen bolt.

thermite

Got it!

eubi

I'm glad you are all properly torqued now.

CdCf, you are right about kgf. Good catch. Kilogram force? What the heck is that? kg measures mass, not force. Force is measured in Newtons (N).

In the US, force is measured in lbs (I always write lbf - pounds force), and mass in slugs. Now who ever heard of that besides in a US engineering class? We also use lbm (pounds mass). Ugh. I always hate working with lbm. I let MathCAD do the conversions...

rmfnla

Great thread! One detail; though, kg measures weight, not mass. Remove gravity from the picture and you still have mass w/ no kgs.

I am impressed by the slug reference, though.

CdCf

kg is the unit for mass!

juicemouse

Impressed by the slug reference? Oh jeez...

The kilogram (kg) is indeed the SI unit of mass. The SI unit of force is the Newton (N). But when you bring kgm, kgf, lbm, and lbf (not to mention the beloved slug) into the picture, it gets real confusing real quick. First of all, the weight of something is how much force it exerts on whatever it's sitting on, so weight is a force. It is defined as the mass of an object multiplied by the force of gravity on the object. Mass is a constant, but since the force of gravity can change from planet to planet (and even slightly at different elevations on Earth), weight is not a constant.

Now, what's the difference between kg and kgm and kgf and N, or between lb and lbm and lbf and slug? And why to Europeans commonly measure their weight in kilos (short for kilograms) instead of Newtons? Well, for one thing, since most people only travel into space a few times a year or so, we tend to use an "Earth-centric" system of measurements that differs somewhat from the "standard" SI and English systems. What happens is that we tend to ignore the fact that the force of gravity is not a constant, because it is so close to being a constant as long as we stay on Earth. Believe it or not, it makes things easier to understand, because it takes the variable force of gravity out of the equation. But again, the requirements are that you have to be dealing with objects on the surface of the Earth, and it's a good idea to stay within one unit system without converting back and forth (it can be done but it's a bit more of a PITA looking up obscure conversion factors between kgf and N and lbf, for example). Here's how it works:

1 kgm is equal to the SI standard 1 kg. It is a unit of mass. We stick the "m" on the end so we don't get it confused with the kgf when we're dealing with both.

1 kgf is defined as the force exerted on Earth on an object that has a mass of 1 kgm.

1 kgf absolutely does not equal 1 Newton. 1 Newton is defined as 1 kgm times the force of gravity on earth (9.81 meters per second squared).

So how much does a 1 kgm object weigh on Earth? Either 1 kgf, or 9.81 N. Both answers are correct.

The kilo that Europeans commonly use as a measure of weight is actually the kgf. Are we having fun yet?

1 lbf is equal to the English standard 1 lb. It is a unit of force. Again, we stick the "f" on the end so we don't get it confused with the lbm when we're dealing with both.

1 lbm is defined as the mass of an object that weighs 1 lbf on Earth.

1 lbm absolutely does not equal 1 slug (the English standard unit of mass). 1 lbf is defined in the English system as 1 slug times the force of gravity on earth (32.2 feet per second squared).

So what is the mass of an object that weighs 1 lbf on Earth? Either 1 lbm, or 0.031 slugs (1/32.2 = 0.031). Both answers are correct.

That felt way too much like freshman year.

rmfnla

I never would have exerted that much effort (measured in ergs?) but I am impressed.

I haven't thought about that stuff in years.

dedhed

Above is what the Engineer wrote, we brought in the tech who tells the customer, "tighten it up so the seat won't slip"
DED HED (the tech who constantly has to interpret the plans, specs, and engineers intentions to the contractor in the real world)
I remember freshman year too, even though it was long ago.

rmfnla

It's also why the engineering students never got laid...

juicemouse

What happens when the seatpost isn't slipping anymore, but it gets crushed because some ham-fisted mech torqued it too much? Just wanted to fill in the gaps for the few people who seemed somewhat confused so they'd be able to know what the terms really meant, and that they weren't technically being used improperly. And while I am studying engineering, I am not your stereotypical "head buried in the sand" engineer with no real world experience and understanding of how stuff works. Don't assume I am. I don't assume that all mechs have no theoretical understanding of the principles involved in what they work with, even though that's another stereotype.

moxfyre

kg is a unit of mass, not force.

Weight is a kind of force, namely the gravitational force exerted by Earth on a massive object.

kgf isn't a canonical SI unit, but an engineering unit. It is the weight of an object whose mass is 1 kg. 1 kgf = 9.8 N = 2.2 lbf (approximately)

ryder47

If we think about "weight" isn't it really just a value associated with a measurement of the "force" my "mass" applies to the mechanism in the scale as the gravitational field on earth applies to me. As long as were on earth. Any where else my weight and force change depending on the strength of the gravitational field present on my mass.

BTW, great thread. Forcing me to use the grey matter.

juicemouse

Yup, that's the right way to be thinking about it.

dedhed

That's the trouble with the net you can't see the tongue in my cheek. After 20+ years of sparring with both the engineers and contractors we all agree it's the architects and bean counters who screwed up the design. the other thing reinforced daily is that the most important sign in any engineering equation is the dollar sign $. It's the only one all parties understand.

What happens is it doesn't slip and the customer is happy...until some day he wants to remove it and has a hard time getting it out. Then the shop gets to sell a new one.

I don't think I could get an allen wrench to deform the clamp and crush the seat post before the flats in the head or the threads in the clamp stripped. (All based on assumtions and hearsay, not actual inspection of clamp and post in question, for the stereotypical lawyers)

I just hope my seatpost is of higher quality than the crushed one or my 6 year old is gonna be in for one heck of a ride when the tag a long lets loose.

CdCf

This is the only part of it all I don't agree with.

We measure our bodies' masses, since the gravity can assumed to be a constant, and so the body weight divided by the gravitational "constant" equals the body mass.
So, we really measure the body mass in the end, and the unit is correct.

The variation of the gravitational "constant" is around 0.5%, depending on where you are, and that's probably less than the error of most body weight scales. And also much less than the daily weight (or rather mass... ) fluctuations caused by varying levels of water retention.

53-11_alltheway

40N/m on my SRAM cassette lockring. If I used a SAE torque wrench how may foot lbs would that be.

Hemlock

Post #4 has the conversions listed...