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Why air springs ride the way they do
03-31-2017, 13:24 (This post was last modified: 03-31-2017 13:30 by davidbrady.)
Post: #1
Why air springs ride the way they do
Under the, "Oh, that's interesting David" heading, which is a warning that geek stuff follows. LOL!

Have you ever noticed how air-spring suspended buses ride soft and plush over the long undulating bumps but are somewhat stiff over cobblestone and the like?

If we take a look at the Prevost air spring graph I provided earlier there's a clue:

İmage

The area of interest is at the center of the plot, at nominal design height, where it says "300# Load Difference (Thermal & Material Hysteresis)". I highlighted the portion with a red circle here:

İmage

Take a look at how the jagged edged jounce and rebound lines don't overlap. If this were a perfect spring which returned all the energy put into it then the lines would overlap. The fact that they don't means energy is lost. In our air springs there are two losses, one is due to the nature of the rubber material which makes up the bellows and the other is due to the heat transfer from the work done in compressing air. Rubber is a viscoelastic material. Its elastic characteristic returns the energy put into it; whereas, its viscous characteristic dampens and absorbs a portion of the energy put in.

Here's a blowup view of short vertical cobblestone suspension inputs right around the design height of the air spring.

İmage

In blue you can see how the air springs oscillate between the jounce and rebound lines, but notice how the spring rate has changed. Now the air spring is compressing say around 1/4" over a load delta of 300# (as Prevost pointed out). If we do the spring rate calculation we get: 300/.25 = 1200 lbs/in. But wait, doesn't the plot say we normally have a much softer spring rate of 650 lbs/in? Yes it does, but that's for long timespan undulating bumps where the elasticity of the spring dominates. Over short timespan small disturbances the viscous nature of the spring dominates and we pay the price of a stiffer ride!
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