Dennis (and Jeff) -
You're right - the coils are loaded in torsion in a coil spring. But, the
spring rate is still normally expressed as lb/in (or, force/distance using
whatever units you want.)
It neither makes any sense- or is useful- to describe a coil spring in
terms of "ft-lbs/inch" any more than it makes sense to describe it in terms
of "psi/inch".
It's kind of like saying " my car uses 1000 watts/mile".
What you really want to know about a coil spring is how stiff it is - that
is - how many lbs (not ft-lbs) it takes to compress it each inch. Think of
how you would measure the stiffness of a coil spring. You would load it in
compression with a force ( for example, 1000 lbs) and measure its deflection
( for example, 2 inches) . Then you could calculate its stiffness as
1000lbs / 2 inches = 500 lb/inch. You would not load the spring with
torque ( and, neither does your car).
The only other useful thing to know about a coil spring is how much load (
in lbs, not ft-lbs) it can take before it fails ( by either breaking or
permanently deforming). That maximum load is expressed in force ( lbs) not
ft-lbs.
And, even for a torsion bar, the units " ft-lbs/in" would not apply. The
units of stiffness for a torsion bar are torque/angle ( for example : ft-lbs
/ degree or ft-lbs/radian) .
In general, if you want to check your units, you can reduce them to see what
you get. In this case, with "ft-lbs/in" you can cancel the units of
length on the top and bottom, and you come out with units of lbs ( weight).
That just doesn't make any sense for the stiffness of a spring. The only
thing that has units of lbs is a weight or a force.
Phil Marino
On Mon, Oct 26, 2009 at 9:49 AM, Dennis Miles <
[hidden email]> wrote:
> On Mon, Oct 26, 2009 at 12:15 AM, Phil Marino <
[hidden email]> wrote:
>
> > On Sun, Oct 25, 2009 at 6:50 PM, Jeff Shanab <
[hidden email]>
> > wrote:
> >
> > I think you mean "spring rate" which is force per deflection ( or, pounds
> > per inch, for example)
> > The deflection per inch is pretty much the same for all springs - 1 inch
> > per
> > inch. : )
> >
> > >
> > > ie 600 ft-lbs compressed 1" vs 200 ft-lbs compressed 3" is the same
> > > ride height but that first bump is more noticable on the 600 ft-lb/inch
> > > spring.
> > >
> >
> > The units here should just be lbs ( not ft-lbs), right? Ft-lbs is
> torque,
> > and you're not talking about torsion bars, or are you?
> >
> >
> > Phil Marino
> >
> > ============ A torsion bar is a straightened coil spring
> =================
> >
> Hi, Phil, Yes, Jeff has it right, it is "FT-LBS," and not "LBS." Because
> a coil spring works as it is compressed or extended in length not by
> flexing
> as a flat leaf spring seems to do but, by twisting the wire or rod stock of
> which the coil spring is formed.
> And as you said, that is what a "Torsion bar" does but it simply is using a
> different mounting technique. (I mention this only to clarify it for the
> many readers who haven't had the Physics or Engineering classes to
> have analyzed the "Simple" spring!)
>
> Regards,
> Dennis Miles <Director>
> E. V. Technical Institute
> Lakeland, Fl.
> Ph. [863] 289-0690
> Training the Tech/mechs
> to build,repair, and service
> Today's and Tomorrow's Electric Cars !
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