There is one major thing that I see overlooked on most custom suspension systems that one would think to be the most important... airbag type and size.
Before we talk about how to choose a 'bag, let's explore how air is able to lift a 4,000 pound. vehicle with less than 100 psi. Air pressure is rated in pounds per square inch (psi). What that means is that a 1x1x1-inch cube with 100psi of air in it, will exert a 100 pounds of force on all sides. So knowing that, a rough calculation can be done to figure out the lift capability of a 'bag at a given pressure. Let's take a generic 'bag with a top and bottom mounting plate of 5 inches with only 50 psi. A five inch circle has a surface area of 19.625 square inches, now multiply that dimension by 50 psi and you have a weight carrying capacity of 981.25 pounds. That is a lot, considering that you will most likely have two of them to lift the front end of your vehicle.
Now where it gets a lot harder to explain is when you try to calculate the spring rate. The spring rate is the amount of weight that it takes to collapse a spring one inch. It is much easier to figure the spring rate on a steel spring (and it's consistent too) but the very nature of air, where both volume and pressure need to be taken into consideration, an accurate calculation becomes exponentially more difficult. A large 'bag will be able to lift more weight, but will have a lower spring rate. Where a smaller 'bag will have less weight capacity but a higher spring rate. The spring rate is almost directly related to the pressure, and the weight capacity is closely associated with the volume. Therefore, because a large 'bag has more volume, it takes less pressure to lift the vehicle and thus a softer spring rate and vice-versa for a smaller 'bag.
With that out of the way, I have seen 'bags that I would personally only use on a minitruck, being used on midsize or even fullsize vehicles. But why would it matter? If the part will lift the vehicle and it's even easier to install, why not use it? There are more things to consider than simply ease of installation. The weight of the vehicle, the leverage ratio that the part will be installed in, and the actual intended use of the vehicle are all more important than how easily the part will be to install. Of course, this is assuming that the owner of the vehicle cares how their vehicle is going to ride once it is 'bagged. There are some companies out there that sell products that are fairly easy to install but everything else is a compromise. So how do you pick the right 'bag for your application? That's not that easy explain, but then again, is anything that I write about? I would say that the first thing to consider when choosing an air spring is how much weight the 'bag will be lifting and at what leverage ratio. Most of the time, you will end up with two or more different sized 'bags that meet your weight carrying capacity needs. From there, you need to decide what the vehicle's intended use will be. If some handling performance is desired, a smaller 'bag will have a higher spring rate thus offering a more firm ride. This should be accompanied by some good quality shocks or it will just end up bouncy. If the plush "Cadillac" ride that everyone brags about having-but only a few truly do-is what you desire, then use a larger 'bag and set up with more suspension travel.
Many of the 'bags being made these days have a higher spring rate designed into them by using stiffer or thicker rubber, thus changing the style of the 'bag can lower the spring rate and still keep the weight capacity up. But once again there are far too many variables to fit into a one page column, luckily for most of you the homework has already been done for your vehicle and it is just a matter of whether or not to install the bigger 'bag or to tolerate the stiff ride for now...
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