Airsprings differ from conventional Leaf or Coils in that their behavior is not linear. This gives a significantly different ride to a conventional spring with great resistance to bottoming out. I wont get into better or worse, but will settle for different.
If you are running PolyAirs or similar helper airbags in your vehicle, it’s probably a reasonable idea to know what effect the pressure you are running has compared to the normal spring.
Attached is a spreadsheet that should help you work it out.
For some reason spring loads are commonly still in lb/inch, hence the combination of imperial and metric.
My Landcruiser springs (aftermarket Kings) are approx 250lb/inch. This means for every inch compression, it means another 250lb * 0.45 (112Kg) has been added to each spring.
With full travel of 240mm (10 inches) and a normal lightly loaded ride height of 4 inches compressed = about 1000lb per spring (2000lb total) in the rear of the car. Every extra inch of compression will take an additional 112KG
Polyairs’ rate depends on several factors. The base area of the bag X pressure = the current load. Where it gets interesting is as you compress an airbag it’s rate increases exponentially. The attached spreadsheet shows this effect compared to a normal spring. What is interesting is that the polyairs in the Landcruiser only compress by some 40% of their total length. This reduces the non-linearity significantly. In fact they increase in effective load bearing capacity at a slower rate than the factory spring. This is an unusual outcome for a normally exponential device. If your car has large locators inside the springs, or compresses them near to binding, then the behavior would be significantly different.
Applying the same formula to sleeve type airbags as used on trucks and Discovery III or Range Rovers is a little more difficult, as their internal volume also consists of some spare space down the sides of the bag. This tends to reduce the effect of the rising rate somewhat. Further the piston in the sleeve type bags is often conical instead of cylindrical. This again changes the rate based on height.
It is interesting to note that the factory Range Rover airbags make the piston smaller near the base, presumably to offset the rising rate effect and be more linear. Many aftermarket bags Rover are larger at the base, making the rising rate effect more pronounced.
Flat bag type bags on the other hand would be highly exponential in their behavior.
If you play with the spreadsheet somewhat it quickly becomes evident that airbags are quite a complex suspension mechanism, and one that holds significant potential.
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