Air versus coil - missing a fundamental

[Frankd3000]

So, with the Cloud Nine's being exercised to their fullest extent i'm left wondering about something which is "troubling me to no end".

The old Noleen NR-2 has a NB2-350 spring on it. Okay, so that means the spring has a rate of 350 pounds/inch on it. That's fine - I get it. FWIW, I already know that this spring is too stiff for my scrawny, 150 pound butt. Maximum travel I ever noticed with it was about 1".

The C9, on the other hand, is set to ..... 70 pounds! I achieve my best sag/travel numbers with ..... 70 pounds! Okay, NOW i'm missing something.

I COMPLETELY realize that I am comparing something adjustable to something that is not adjustable. I also completely realize that i'm not necessarily making a "fair" comparison with respects to air and coil springs.

So, my question is this - how is it that there is such a BIg difference between the two? If I were to put a 70 pound spring on the Noleen we all know i'd be sitting on the bottom out bumper of the shock, and if I put 350 psi in the C9 it would explode. So, what gives? What basic fundamental am I missing here? Is this simply a case of the different compression characteristics between the two?

Also, is there any kind of formula to convert one to the other?

As always, PATIA!

[jinder]

Ummm, yes...  good question...  anyone?

[Matno]

It's a question of air volume. Every air shock has a slightly different volume, and that corresponds to a different compression rate. I think a smaller air volume will compress more easily and thus require a higher pressure. That's why most air shocks now have higher volumes than say 5 years ago (including Risse) and also run at much lower pressures (it used to be that you needed a LOT of air pressure for any air shock, which led to poor seal reliability/durability).

My Stratos, which is a coil with a small air assist, uses almost 200 lbs of pressure for my 145lb weight. On the other hand, I have a 400lb spring on it. Go figure.

[Simon]

Put your shock pump on your shock,
now compress the shock and you will see the air pressure reading rise,thats why 70psi doesn't bottom out because as it move through its stroke the pressure increases,if you set the pressure to its max and compressed it you'll be suprised as to just how high a pressure can be obtained,
simply the air shock has a variable spring rate,this is good,but its susceptable to changes in temperature altering the pressure (spring rate) this is bad,
Simon.

[will]

Pneumatics... a lovely science!

Air cylinders and air motors were my absolute favorite tools for making things work when I was building machinery for a factory. They can take a lickin' and keep on tickin'

Now that 70psi thing; the spring strength is the product of the area of the piston and the psi. So, if the inside diameter of your air shock cylinder is, say 1" that means the area acted on by the 70psi = .5 *.5*3.141593=0.785 square inches.

Then if you have 0.785 square inches and 70 pounds per square inch, your spring at rest exerts 55 pounds of force.

As Simon mentioned, that force will skyrocket as you compress the shock. (Boyles' law:  P=VT)

You can calculate that spring rate on an air shock using Boyles' Law. But that's only theoretical...

Try starting with 70 psi and crank it down (in a vise?) exactly 1" with the shock pump/gage attached. When you get that pressure, you can then calculate the force. That number will give you  something you can compare to a coil spring. Whatever the number is would be the same as a spring label here in the states.

At our local spring manufacturer, they put springs in a big load cell gizmo and squeeze 'em 1". They use the spring force at 1" of compression to label the spring.

Sorry to ramble... quiet day at the office.

[proflex252]

Will is right!

But I like to ad one thing.

Frank what you are comparing is the effect of a force on a spring meassured in lbs and its effect of a length reduction of that spring with  the air pressure in the cylinder meassured in pounds per square inch.
You can see already that the formula is not the same. On the one side it is one dimensional and on the other side it is squared.

And the coilspring behaves linear because how ever it is compressed it is still in your case 350 lbs to contract it by one inch while the airspring behaves progressive, due to the reduction of the volumina in the cylinder and its result of the increasing of the pressure.

That means you can't compare those two things.

[Frankd3000]

Well crap - now I see exactly what i'm missing!!!!!!!

A COIL spring at rest isn't compressing anything. An AIR spring needs to be pumped up to achieve a "starting point", so it isn't "flat". A coil spring doesn't need this because of the differing medium and it's inherent physical properties.

Thanks guys! (meant in an honest "thanks for the info", not "thanks for making me feel dumb" way.  [smiley=laughing.gif] )

Geez, do I ever feel stupid for missing something like that. [smiley=laughing.gif]

[rapiddescent]

I thought it was the air pressure is measured in lbs/square inch and the coil spring is measured in lbs/inch travel - two very different measurements. The noleen springs have fairly linear compression rates whereas most air shocks have a non-lenear rate.

my new COR springs for proflex 856 are non-linear rate coil springs

also a top tip: never believe the lbs/inch measurement on the noleen springs.  

callum
rds