Question about springs installed backwards

[Silver Supra]

Originally Posted by TachyonBomb

..

On a side note I noticed that a lot of spring rates for springs are kind of missing. There may be a simple way to calculate the spring rates for all those springs.

The equation for the spring force is:

k*Distance=Spring force or force on spring
-(k) is the variable used in physics for the spring constant which I believe is interchangeable with how the automotive industry used the phrase "spring rate".

And the downward force of any mass due to earths gravity is:

Mass*9.8m/s=downward force

So if someone has one of those fancy 4 wheel alignment machines that displayed weight distribution and if someone would make a set of dummy shocks that lifted the car to the point where the springs are not compressed yet they are installed at the max uncompressed height. One could use this equation:

k*distance = Mass*9.8m/s

to make this equation:

k=(Mass*9.8m/s)/distance the spring compressed in meters

where the mass could be the original weight of the car plus some extremely fat friend you talked into hopping in the car while on the alignment/scale machine. You can measure the distance each spring compresses and enter the info in the above equation and that should give a rough idea of the spring rate for each spring.

This is what physics calls the static spring rate.

BUT!!! If spring rates and shock rates in the automotive industry are based on a time derivative (fancy way of saying how something changes over time) then that would require a differential equation and a measurement of how fast this spring or shock is recovering after being compressed (the measurement after you let go of a spring and how much time it takes for the spring to rebound to its max height.

I don't see why a spring would need this version of measurement but the shock will need to go through this measurement since it "damps".

damp = slows down the wave (of a bouncing/oscillating spring)

So you're measuring the change in force over time as the spring exerts from its max force (once let go) till no force is exerted (spring reaches max uncompressed height).

That equation looks a little funkier and needs a precise timer. So lets just forget about the shocks for now.

So ya if @Racecomp Engineering has one if these machines, a shock that provides no damping force and some free time to work some of this stuff out it is not impossible to find the spring rate of any spring on the market.

I'll be expecting my letter of recommendation before the November deadline thank you very much. End Tangent lol

Originally Posted by Silver Supra

... A unidirectional spring is a spring but may respond differently to high frequency movements....

Oh, you will be in academics. Trust me.

I am pretty sure the manufacturers test single units in a dynamic loading apparatus, not on a car.
The equation of motion for axial force and displacement is relatively simple:
Say axial displacement = X
velocity V = dX/dt
acceleration A = dV/dt
relevant mass = M
Damping = C
spring rate = K
equation is
F(t) = M*A + C*V + K*X
but the damping C and spring rate K (and even mass, M) may be nonlinear with X so it gets interesting.

It's all to say it may make some difference in response - or maybe not, but the manufacturer had a reason for the defined configuration.

I suspect the OP will think twice about starting another thread!!

[TachyonBomb]

Originally Posted by Silver Supra

Oh, you will be in academics. Trust me.

I am pretty sure the manufacturers test single units in a dynamic loading apparatus, not on a car.
The equation of motion for axial force and displacement is relatively simple:
Say axial displacement = X
velocity V = dX/dt
acceleration A = dV/dt
relevant mass = M
Damping = C
spring rate = K
equation is
F(t) = M*A + C*V + K*X
but the damping C and spring rate K (and even mass, M) may be nonlinear with X so it gets interesting.

It's all to say it may make some difference in response - or maybe not, but the manufacturer had a reason for the defined configuration.

I suspect the OP will think twice about starting another thread!!

We all physics high jacked this poor thread!!

I was trying to maybe get a simplified static (not dependent on time) spring rate value which would be the maximum spring rate for a set testing downward force. Meaning a junk 86 that will never be touched again will have to be used to keep things constant. Also like I stated to keep things simple the shock can't damp at all.

But you are completely correct. I didn't want to get into the equation of motion but that is exactly what is needed to find the spring constant(spring rate denoted k in both our equations) and damping rate/coefficient (denoted by C in your equation) due to a time dependent (also called dynamic) force.

Since we are in agreement to springy-ness being based on a spring constant/rate and that damping is based on a damping constant, I'm very curious as to how manufacture come up with a spring rate for a shock. It always throws me off in how to handle that definition and how it relates to the damping the shock does. Maybe racecomp or one of the other members more experienced with these numbers can chime in and define it for me.

Also in your given equation of motion:


F(t) = M*A + C*V + K*X

you mentioned that it doesn't need to be linear which is 100% true let me blow your mind!

Set the force to 0. The system is in equilibrum.

0=M*A+C*V+K*X

you stated:
axial displacement = X
velocity V = dX/dt
acceleration A = dV/dt

Now lets change what you said to this:
axial displacement = X
velocity V = dX/dt = X' (X first derivative)
acceleration A = dV/dt = (dX/dt)/dt = X'' (2nd derivative of X)

so 0=X''+X'+X

Doesn't that look alot like a non-linear equation since the leading coefficient is pretty much (X') squared.

So now lets take the damping term to the left side:

-X'= X''+X

X is the initial condition so time (t)= 0. So this is can be treated as a hidden 2nd derivative of X.

-X'=X''+X'' ~ X^2+Y^2 = the equation of a circle that is based on the damping term (X') on the left side.

To further add the spring coefficient K and damping coefficient C can 100% be based on changes in time. So K(t) and C(t) can be found, this can be further noticed when you follow the units.

Force = kg*m/(sec*sec)
K*X = (kg/(sec*sec))*meters
C*V= (kg/sec)(meter/sec)

So to conclude.... I suppose we should include time to be more accurate lol

Originally Posted by strat61caster

Ok, you're back into grad school, nobody wants to hire someone who would go through all that trouble.

I just used a drill press, ruler, and a scale rated to a few hundred or ideally thousand pounds, but you do you. Race teams will get into how each spring acts dynamically but the rest of us can't afford a sample size to actually do anything about it (other than put the slightly stiffer spring on the heavier side of the car), maybe something useful there for measuring lifespan of a spring (I'd presume elasticity degrades over time but who knows) but the deltas are probably too small on quality springs to be measured with amateurs equipment.

You hit the nail on the head if you want to include dynamics (changes as time goes on) well that will cost a pretty penny. This holds true for most fields of physics research.

You also touched on one of my favorite subjects and newest research fields in physics called soft matter physics! When it comes to measuring the lifetime of a spring if you used time according to physics all things will break down to a state of equilibrium as time goes to infinity. The 2nd law of thermodynamics literally can be used to troll and shut down most physics questions lol. So given enough time the springs internal elastic energy will be lost as the molecules of the material break down :-P You were very correct to say that the delta would be small over time.

So even though we are talking about metal/steel/some hard material we can still treat it like it is silly putty or a memory foam bed. The graph you will want to look at will be a stress over strain.

Stess = Force/Area
Strain = The distance you stretch something out to divided by the objects original length.

So as you stretch or compress a spring/silly putty/a foam bed it will tell you the force needed to accommodate this much stretch in a material.

So if you keep stretching a spring you are going to see a lot of force needed when you get started but after a certain point you will notice the force needed to continue the stretching will go down as the spring looses elasticity.

Remember that springs work that same whether stretching or compressing so this will give you a value of how much force your spring can take before it starts to lose elasticity. Since force = mass*acceleration you can calculate how much load or how fast your spring can be compressed and efficiently recover from before it starts to lose performance.

To get these values is were someone needs those expensive machines that can squish and stretch any material. :-( I actually got to play with some of these in college.

[OND]

Originally Posted by TachyonBomb

so 0=X''+X'+X

Doesn't that look alot like a non-linear equation since the leading coefficient is pretty much (X') squared.

If that looks like a nonlinear diff eqn to you, you better spend more time studying and less time on car forums before you start grad school.

Good luck with the applications.

[strat61caster]

Originally Posted by OND

If that looks like a nonlinear diff eqn to you, you better spend more time studying and less time on car forums before you start grad school.

Good luck with the applications.

Physicists don't like using numbers or unnecessary constants, that'd fly just fine in a theoretical class as a differential equation as actually finding a solution is "trivial" and it looks just like how I was lectured in my basic physics class in college.

Engineering grad school he'd get a thump for not elaborating.

[TachyonBomb]

Originally Posted by OND

If that looks like a nonlinear diff eqn to you, you better spend more time studying and less time on car forums before you start grad school.

Good luck with the applications.

Meant to write 2nd order linear. This could be deduced when one continues to read and places everything in context. But this should be taken as an excellent example of why the first draft of research/journal articles are never submitted and to further point out the significance of peer reviews being conducted on those submissions.

You are correct differential equations and ones ability to classify each type is an important aspect in itself not just with respect to grad school. I'm am mostly using my forum responses to hone in my communication abilities. One of the biggest issues I noticed in my undergrad experience was that a large portion of both students and professors had poor communication skills when it came to conveying ideas and concepts to others in a digestible way toward a lay audience. After presenting my research to an audience twice and having to write a these paper on that research as a graduation requirement I became aware of the importance to practice this sort of thing.

So by all means if you want to knit pick and correct anything I place out there please do. Better to get these kinks out of the way now than later down the line when it comes to scrutinizing my dissertation or I'm being paid to conduct and present research with an attached teaching requirement.

Originally Posted by strat61caster

Physicists don't like using numbers or unnecessary constants, that'd fly just fine in a theoretical class as a differential equation as actually finding a solution is "trivial" and it looks just like how I was lectured in my basic physics class in college.

Engineering grad school he'd get a thump for not elaborating.

That is the truth!! We are highly encouraged to cut as many corners as possible. Me and my fellow classmates would joke around how engineers chop off as many numbers past the decimal point as they can get away with and the engineers would say at least the attempted to plug in numbers and get an answer. :-P

So are all of you guys engineers (OND. Strat61caster, silver supra)? Every job I come across here in the bay area seems to want an engineering degree. Right now my goal is a Ph.D. in cosmology, theoretical gravity or something similar. But I have a couple of friends that just got their masters degree in software engineering a few months ago from san jose state... lets just say their salaries can pay off my student loans, credit card debt and brz loan in less than a year with spare money to play. It honestly has me thinking about a software engineering masters program and just calling it quits.

[strat61caster]

Originally Posted by TachyonBomb

It honestly has me thinking about a software engineering masters program and just calling it quits.

If you can do the work and want an "easy" button to a comfortable life in the Bay Area software is the way to go.

In my experience Masters an PHD does not mean the person is more competent or capable of doing a job unless they studied and researched a particular field and continue to work in that field. The higher starting pay can be quickly closed in upon by someone who is good at negotiating and job hopping with a "lesser" degree.

And there are plenty in software around here who make great money without a degree, reputation can carry you a long way if you can network well.

[Silver Supra]

Originally Posted by strat61caster

....

Engineering grad school he'd get a thump for not elaborating.

Agree. Been there, done that.

Originally Posted by TachyonBomb

Meant to write 2nd order linear. .
That is the truth!! We are highly encouraged to cut as many corners as possible. Me and my fellow classmates would joke around how engineers chop off as many numbers past the decimal point as they can get away with and the engineers would say at least the attempted to plug in numbers and get an answer. :-P

So are all of you guys engineers (OND. Strat61caster, silver supra)? Every job I come across here in the bay area seems to want an engineering degree. Right now my goal is a Ph.D. in cosmology, theoretical gravity or something similar. But I have a couple of friends that just got their masters degree in software engineering a few months ago from san jose state... lets just say their salaries can pay off my student loans, credit card debt and brz loan in less than a year with spare money to play. It honestly has me thinking about a software engineering masters program and just calling it quits.

Who encourages you to cut corners? Teachers, fellow students, ?? Don't do it. Engineer or not, anyone working with numbers needs to understand significant figures - and how to determine how many numbers in an answer are significant (known with accuracy). Just because a calculator or computer can spit out 14 decimal places in a number, it does not mean they are significant!!

Read up on the definitions for terms like linear and nonlinear. They sound obvious, but have specific meanings.
We tend to think of springs as linear - i.e., a simple spring rate defining the ratio of force in the spring to displacement. However, many springs are not linear, the rate changes with various properties including load rate (dynamics), material properties, or geometric influences. Even the supported mass could be nonlinear - that 300-lb fat guy in your car may leave his seat, no longer be supported by that spring, after you hit a big bump. When he hits the seat again, his mass returns, but with added incremental acceleration (you know, that "theoretical gravity"!).
I wrote the simple equation of motion for s single degree of freedom (SDOF) model above. Same equation applies, in general, for a multi-degree-of-freedom (MDOF) model (where the terms are 1- and 2-dimensional arrays). I run a lot of dynamic, nonlinear analyses of MDOF models of assemblages and buildings as well as doing back-of-the-envelope SDOF calcs in a pinch.

Yes, I am an engineer with MS and PhD and offer this advice. Get the degree you want for you - not for some near-term job or even occupation. I did that because I wanted to learn more in my field and, incidentally, wanted to meet future requirements to teach in a major University if I decided to.
I also incidentally learned to program computers long, long, long ago and have also applied that throughout my career. I have worked in many different areas using my background knowledge - don't get pigeon-holed!

Anyway, we have jacked this thread beyond belief , so I need to get off my soapbox!

[TachyonBomb]

Originally Posted by strat61caster

If you can do the work and want an "easy" button to a comfortable life in the Bay Area software is the way to go.

In my experience Masters an PHD does not mean the person is more competent or capable of doing a job unless they studied and researched a particular field and continue to work in that field. The higher starting pay can be quickly closed in upon by someone who is good at negotiating and job hopping with a "lesser" degree.

And there are plenty in software around here who make great money without a degree, reputation can carry you a long way if you can network well.

Thanks for the input! That definitely gives me something to think about over the next few months.

Originally Posted by Silver Supra

Agree. Been there, done that.



Who encourages you to cut corners? Teachers, fellow students, ?? Don't do it. Engineer or not, anyone working with numbers needs to understand significant figures - and how to determine how many numbers in an answer are significant (known with accuracy). Just because a calculator or computer can spit out 14 decimal places in a number, it does not mean they are significant!!

Read up on the definitions for terms like linear and nonlinear. They sound obvious, but have specific meanings.
We tend to think of springs as linear - i.e., a simple spring rate defining the ratio of force in the spring to displacement. However, many springs are not linear, the rate changes with various properties including load rate (dynamics), material properties, or geometric influences. Even the supported mass could be nonlinear - that 300-lb fat guy in your car may leave his seat, no longer be supported by that spring, after you hit a big bump. When he hits the seat again, his mass returns, but with added incremental acceleration (you know, that "theoretical gravity"!).
I wrote the simple equation of motion for s single degree of freedom (SDOF) model above. Same equation applies, in general, for a multi-degree-of-freedom (MDOF) model (where the terms are 1- and 2-dimensional arrays). I run a lot of dynamic, nonlinear analyses of MDOF models of assemblages and buildings as well as doing back-of-the-envelope SDOF calcs in a pinch.

Yes, I am an engineer with MS and PhD and offer this advice. Get the degree you want for you - not for some near-term job or even occupation. I did that because I wanted to learn more in my field and, incidentally, wanted to meet future requirements to teach in a major University if I decided to.
I also incidentally learned to program computers long, long, long ago and have also applied that throughout my career. I have worked in many different areas using my background knowledge - don't get pigeon-holed!

Anyway, we have jacked this thread beyond belief , so I need to get off my soapbox!

An interesting fact about most physics undergrad programs is that we are in no way formally introduced or taught non-linear spring systems. If memory serves I believe a dedicated non-linear differential algebra class was only a requirement for math majors so we got no deep experience with it. For the most part as an physics undergraduate exposed to spring systems as linear in the context of a particle/radiation waves, voltage oscillations, kinetic/potential energy transformations, maxwells equations etc. Nearly everything we are taught in an undergrad level is encouraged to be defined as the most simplest pendulum system as possible, this is what I meant when I said they (the professors/curriculum) push us to cut corners and wave off a lot of "less significant" details.

So that is why a more dynamic spring system with more degrees of freedom where a fat guy gets lifted off his seat for a split second didn't cross my mind haha.

I pretty much got introduced to non-linear systems when I decided to do my undergrad research on quadrupolar gravitational radiation and detection. So my limited experience with non-linear concepts is in the context of mass densities/gravitons being self interacting and tensor field comparisons to their scalar field Maxwell counterparts. The whole thing was a 6 month crash course in self teaching myself Einstein tensor notation and general relativity... I learned a hell of a lot but lord knows I only scratched the surface on the subject haha.

Also thanks for the wise advise as well

We should really give this guy his thread back

To sum up for the OP
-springs in this context function the same installed upside down
-spring rate and damper rate should be determined in a dynamic system

[venturaII]

All that brainpower and yet nothing about the obvious increase in unsprung weight at the motion end of the spring when upside down? There are ~twice as many coils on the tighter wound end; that end of the spring is twice as heavy, and springs aren't light to begin with. While it doesn't affect the spring rate or how the spring works in a sanitary, theoretical world, in real life you've doubled the spring's contribution to unsprung weight. The spring itself functions the same, the car's suspension does not. Take the free performance improvement and flip the spring.

[Gunman]

Originally Posted by venturaII

All that brainpower and yet nothing about the obvious increase in unsprung weight at the motion end of the spring when upside down? There are ~twice as many coils on the tighter wound end; that end of the spring is twice as heavy, and springs aren't light to begin with. While it doesn't affect the spring rate or how the spring works in a sanitary, theoretical world, in real life you've doubled the spring's contribution to unsprung weight. The spring itself functions the same, the car's suspension does not. Take the free performance improvement and flip the spring.

The value of unsprung items like spring and dampers, is debatable. Generally, half the mass is used, and not ratio'd out based on the coil design...at least not in my experience.

[venturaII]

Originally Posted by Gunman

The value of unsprung items like spring and dampers, is debatable. Generally, half the mass is used, and not ratio'd out based on the coil design...at least not in my experience.

Granted, the location of the coil in relationship to the suspension's pivot point has a different impact than static weight lost further outboard, like lighter brake calipers or control arms or whatever. But considering this reduction is a freebee, and no serious car guy looking for performance improvement would ever question the value of reducing unsprung weight, it'd be kinda stupid not to take advantage of the benefit in simply flipping them. I suppose if you were hard parking it, then there's no benefit to the OP...