Air Suspension 101: Part 3

Now that we have explained the basics of the air spring, it is time to talk about the next important component – air. There are several methods for getting air into the air spring. You can have an out-board compressor, such as a shop compressor or you can have an on-board compressor. There are different types of on-board compressors. These can be engine driven or electric. We will discuss all the different ways to get air into your bags in this article.

Out-board air compressor refers to using a compressor that is not ran off the vehicles power. These can be shop or gas station air compressors. This is the cheapest way to get air to your springs. Some people will start with this and then add on other components when they can afford them.

There are two ways to use an out-board air compressor. One way to do this is run an air line to each bag with a “Schrader” valve on the end. A “Schrader” valve is the same style valve used on a tire.

The other option is to put air in an on-board air tank through a hose and “Schrader” valve. This will allow you to air up the air springs and have a reserve for adjustment. All the valves, air lines, gauges, etc. are in place as a normal air system, but the air source is external from the vehicle.

The most common option is to use an on-board air source. The most popular on-board air system is an electric compressor such as those from Firestone or Viair. These run off of the electrical system and can be used with or without the vehicle running. These run at different CFM rates, PSI rates, and duty cycles which will be explained later.

An engine driven compressor mounts to the front of the engine and is driven by a belt just like other engine accessories. These offer the fastest fill rates because they can move large volumes of air. The units look very similar to an air conditioner compressor. In fact, some people have used air conditioner compressors to do the same thing. These units require an oiler to keep the internal parts from burning up. The downside to this is finding a place to mount it. Some companies like Chassis Tech and Air Lift make brackets for these, but this is still a custom installation and will take more work to install than a normal electric compressor. The other downside is that the engine must be running to use this compressor.

The other key component needed with any on-board air compressor is a pressure switch. A pressure switch senses the pressure inside the air tank and shuts the compressor off when it reaches the set pressure. It also turns the compressor on when the pressure drops. Some pressure switches are adjustable allowing you to set the on/off pressure settings. Never try to use a pressure switch that is higher than what the compressor is rated for. This means if your compressor is rated at 150PSI do not use a 175PSI pressure switch. You will ruin the compressor.

Compressors have three ratings, CFM, PSI, and duty cycle. CFM stands for cubic feet per minute. CFM is the measurement of the volume of air that a compressor can produce. CFM ratings are usually rated at a given pressure. So a compressor that has a CFM rating of .99 @ 100 PSI means that it will produce .99 cubic feet per minute of air at 100 PSI. Of course at a lower PSI rating it should produce a higher CFM rate. The higher the CFM rate; the more air the compressor will produce.

PSI means pounds per square inch. This measurement is how much maximum pressure the compressor can produce. Most compressors will do at least 150 PSI which is fine for air suspension. Most air suspension systems only need 100 PSI to operate. Of course the higher the PSI, the faster the air bags will fill. You want to make sure that you match the PSI rating to the rest of the system. Air tanks, air line, valves, air bags, etc, all have PSI ratings. If you use a 175 PSI compressor with components that are only rated for 150 PSI, you could do serious damage to the other components.

Duty cycle refers to the amount of time the compressor can run before it needs to cool down. Duty cycle is measured at 100 PSI with ambient air temperature of 72 degrees. The duty cycle percentage is expressed as compressor on-time / (compressor on-time + compressor off-time). These means a 25% duty cycle compressor needs 30 minutes of cool down after running for 10 minutes. Here is how that is figured – 10 / ( 10 – 30) = 10 / 40 = .25 or 25%. You should never exceed the duty cycle rating of the compressor or you will damage the compressor. Compressors produce heat and need time to cool down. The only exception to this is a 100% duty cycle compressor which does not need to cool down. Remember duty cycle is measured with an ambient air temperature of 72 degrees. If the outside air temperature is higher, your compressor may need time to cool down even if it is 100% duty cycle rated. Also, the compressor should be mounted in a location that will allow adequate air flow and ventilation otherwise you will raise the ambient air temperature and require a cool down time for the compressor.

Electric compressors also come in different voltages and amp usage. Make sure you use 8 gauge or larger wire. If you wired the compressor directly to the battery, you may find that it will kick on anytime the pressure drops in the tank. The solution to this is to use a relay to only allow the compressor to kick on when the ignition switch is on. This will save you from getting stranded with a dead battery.

That’s it for this month, check back next month as we will discuss the valves and switches. As always, if you have any questions about this article, please drop us an email.

Vincent Wright
sales@kmwperformance.com
http://www.kmwperformance.com