So I started researching other options. Of course - coil-over shocks came to mind - lots of tunability, proven performance, reasonably compact package, and easy to mount. The only problem is - they are quite an expensive option, especially once you add up the cost of the required springs for a double or triple rate setup.

It was right around this time that I came across Ron, driver for Team Purple, a professional rock crawling team, and owner of Off Road General Store. He had posted pics and comments about the "new" Fox Racing Shox 2.0 Airshox he was using on his wild new competition rock buggy. I place the word 'new' in quotation marks because air shocks are not a completely new concept, they have been in use for sometime in various vehicles from motorcycles to race trucks to monster trucks - but they are just recently beginning to make inroads with the off-road/rock crawling crowd.

After asking around amongst those with experience with them, doing some research, and talking directly with Fox, I surmised the following (note that, unless specifically noted, the comparison is with "traditional" suspension setups consisting of a spring (leaf or coil) and separate shock absorber. This is simply because this is currently my only frame of reference, having had no personal experience with coil-over suspensions):

Advantages:

• Weight - each unit, comprising "spring" and very high quality rebuildable shock weighs only six (6) lbs!!!
• Compact size - the entire unit has an OD of only 2.0" making them extremely easy to fit.
• Adjustability - by varying initial nitrogen pressure ("spring rate" and ride height) and the volume of oil in the shock (spring curve progression), along with adjustable valving - they offer a broad range of adjustability.
• Price - at a suggested retail price (as of this writing) of approximately $225 US they compare very favourably with a high quality spring and shock absorber setup and are quite a bit more economical than traditional dual or triple-rate coil-overs.
• Ease of mounting - Great flexibility in mounting options and configurations is afforded by the fact that each end of the unit mounts with a high quality Aurora brand spherical bearing and a single 1/2" bolt.

Disadvantages:

• Requirement for high pressure nitrogen source. - this is more of a neutral, as traditional springs obviously don't require one - but then again aren't adjustable either. Note that this requirement is true of both Airshox and coil-overs as coil-overs also need to be periodically charged with hp nitrogen.
• Price - compared to junkyard springs and shocks, these are obviously more expensive, but then we're hardly comparing apples to apples.
• Affect of Heat - It is theoretically possible to heat the shocks up to the point that there will be a detrimental effect on the volume/pressure of nitrogen with which they are charged, potentially altering the spring rate/ride height during use. Whether my practical experience bears this out you will have to read on to find out! Note that the magnitude of ambient temperature changes (for example, seasonal changes in outside temperature) have neither a theoretical nor practical effect on the shocks.
• Weight carrying capacity - maximum capacity for the shocks to support is 1000 lbs per corner of sprung weight. That equals about a maximum of 5000lbs GVW, depending on vehicle. They generally perform best on lighter vehicles.
• Natural roll resistance - similar to softly sprung coil-over setups, these shocks offer very little natural roll resistance. Not a problem for a trail-only rig but may be an issue for a daily driver. Easily fixed with a sway bar though.

After talking to Ron and to Fox I then sought the opinion of Dave at Poly Performance. I have been a satisfied customer of Dave's in the past, and I know he deals and services a large array of high quality aftermarket shocks, from coil-overs to Airshox and I trust and respect his opinions. In the end I decided that the only possible concern for me would be the weight of my buggy, as I'm right at about 950 lbs sprung weight per corner in front. Other than that, the Fox 2.0 Airshox seemed like they might be a perfect solution for me. So I decided to try them and what follows is what I found:

Part 2 - Description

When I ordered my Fox 2.0 Airshox, the longest available was a 14" travel unit, so that's what I ordered As I write this, I have just heard from Dave at Poly Performance that there is now a 16" travel version available and that he will have them in stock by the time you read this.

Part 3 - Function

So, the burning question everyone has is: how do they work? The answer is both complicated and simple. I shall endeavour to stick to the simple :-). Basically, the answer is "not like a spring, but like the piston in an engine"

The Airshox are both a high quality, rebuildable, re-valvable shock absorber and a means os suspending the vehicle in one package. That is, they are a "shock, and a "spring" in one. The shock portion works exactly the same as virtually any other rebuildable monotube shock absorber and so I will cover it only briefly before moving on to the "air spring" portion that makes the Airshox, Airshox!

Shock / Valving

On the end of the shock shaft is a piston with (8) holes. (4) compression and (4) for rebound. Layered on either side of the the piston are washers in different diameters and thickness. These washers make up the shim stack. The shim stack has to flex to allow the flow of oil in either direction of the shock. When the shock compresses, this is called compression, and when it extends this is called rebound.

The shim stacks come in ranges from 30 to 110. The higher the number, the more damping force the shock will have. When the shocks are built , Fox engraves the damping numbers (compression and rebound, separated by a slash (/) ) on the shaft eyelet just under the rubber bottom out bumper or O-ring. The first number relates to the compression shim stack, the second number to the rebound stack. If there is nothing engraved on the shock eyelet, then the shock has the standard 40/60 set up.

The valving can be adjusted by disassembling the shock and changing the shim stacks. I have not yet had occasion to do this, but if and when I do I shall update this article with pics and descriptions. In the meantime, here is a pdf file of Fox's instructions for rebuilding the Airshox:

Fox Racing Shox - Instructions for rebuilding 2.0 Airshox.

Air Spring Function

Instead of using a conventional spring to suspend the vehicle's weight and carry out suspension duties, the Airshox use high pressure nitrogen, contained inside the body of the shock itself. While the finite physics and chemistry involved can be quite complicated, the basic concept is very simple. The shock contains an emulsion of nitrogen and oil (emulsion just means that the nitrogen exists in suspension in the oil, a "mixture" of oil and nitrogen if you will). The emulsion exists in a container (the shock body or canister) of fixed volume. The nitrogen is compressible and the oil is not. The nitrogen is placed in the shock at some initial pressure (the charge pressure). As the shaft travels into the cylinder (shock shaft compresses) the cylinders effective volume decreases, and therefore the pressure of the nitrogen increases. Picture this exactly the same as a piston travelling up in a cylinder bore, compressing the fuel/air mixture. Since the oil is incompressible, the more oil present in the emulsion (i.e. the greater the oil volume in the shock) the less volume there is available for the nitrogen to be compressed into (using our engine/piston example, picture this as the more oil in the shock, the smaller the "combustion chamber"). It is this pressure inside the shock that provides the force able to suspend the weight of the vehicle. The pressure inside is measured in pounds per square inch (PSI) and the shaft of the shock shaft has a fixed and known area upon which this pressure acts, measured in square inches. Multiplying the pressure (PSI) by the area of the piston (SI) yields a result in pounds which is the weight of the vehicle that can be supported at that pressure. "That pressure" is affected by 3 variables - the initial pressure of the nitrogen in the cylinder (charge pressure), the volume of the cylinder (i.e. the amount of oil in the cylinder), and the "travel" of the shock (i.e. - how far the shock is compressed, or how far the shock shaft is into the body of the shock.)

If forces greater than the static weight of the vehicle act on the shock shaft (e.g. the vehicle coming down after going over a bump, or the vehicles weight being re-distributed to conform to uneven terrain (flexing the suspension) ) the shaft will travel into the shock body more, and the pressure will increase. Similarly, if less weight is on the shaft (again, because of shifting weight distribution as the vehicle crosses uneven terrain, or getting a wheel in the air or whatever) the shock shaft will extend as their is less force to act against the pressure inside.

Note that all these actions and reactions can easily be proven to yourself using a few simple free body diagrams, an understanding of newton's 3 laws of motion, and Boyle's, Charles's, and Dalton's laws - if you were so inclined! :-)

Alternatively, we can simply agree that the volume of oil as well as the charge pressure inside the shock determine what the shock travel is for any given weight being supported by the shock. These 2 user-adjustable factors also determine how the shock acts in use, as the vehicle is driven over rough terrain - i.e. the "ride" of the shock. Of course, the shocks valving also plays a role in how the shock "rides".

How we understand this is often attempted by modelling the action of the Airshox as if they were springs. The advantage to this is that it uses terms with which we are familiar, and can readily apply to our vehicle tuning and suspension design, terms like "spring force", "spring rate", and such. The disadvantage to this method is that, so far, nobody has developed an entirely accurate model, and given the complexities, I'm not entirely convinced that anyone ever will. So where does that leave us? The models in existence do a good job of explaining and helping us understand the concepts and inter-related variables, even if they do not give us precise, accurate results which we can use for exact design specification.

In simple terms, modeling the Airshox as if they were springs helps us understand how to make adjustments, and gives us a good starting point from which to begin our tuning. But we must be aware that we can't count on the model for 100% accurate results, and a certain amount of trial and error is required.

The best model I am aware of was originally developed by Poly Performance, and subsequently tweaked by yours truly!

The following is a screen shot of the program, which is available as an Excel spreadsheet by clicking on the image or clicking HERE.

Let's concentrate on just the first 4 columns initially. Column x represents the shock travel, with 0 being fully extended and 14 being fully compressed. (note that when discussing suspension, we always define travel as being the amount the shock shaft is IN the shock body - i.e. how compressed it is, so that full travel - fully compressed.) The next column labelled"FORCE" is the modeled spring force for the given travel. The third column, "RATE", is the modeled spring rate at that travel.

To populate the chart with data, the user simply enters 2 variables - the charge pressure in PSI (cell H7) and the Volume of oil in the shock in cc's (cell H9). The user could change the diameter of the shaft also, but this is of no practical value. Note that the "stock" values of these variables are 200 PSI nitrogen charge and 325 cc oil volume.

By varying the volume of oil and the pressure, one can now observe from the chart the effect on "spring force" and "spring rate".

The program also graphs the data from the table, the following is an example:

Note the highly exponential nature of the curves. In practical reality, this means the the spring force and spring rate remain constant and quite low (soft) for the first 2/3 of the shocks travel, and then increase rapidly and dramatically from there - meaning they get quite stiff quite fast. This graph is of the settings for my front shocks, which are 365 PSI and 370 cc of oil - remember that I am right at the max. weight per corner in front (940 lbs) for these shocks, and that's why my pressure and oil volume figures are reasonably high, so that I can achieve the ride height I want (about 8") while supporting the weight.

Note that the pressure and volume values entered have a dramatic effect on the shape of the curves, and it is this visualization that is the real utility of the program. The following graph is that of the "stock" values, and clearly illustrates the principle, as well as illustrates why the Airshox are best suited to "lighter" vehicles.

Remember I said that the model doesn't produce precise results? Go back and have a look at the screen shot above, and observe the area titled "Actual suspension scratchpad". Here, you can see that the model predicts that at the given pressure and volume, and at my sprung weight, my ride height should be between 8" and 8.5" (to determine this, look in the chart of travel and spring force for a value of spring force = sprung weight). However, actual measurement shows that my ride height was closer to 7.75" Of course, inaccuracies of measuring both the volume of oil in the shock, the pressure, the weight on that corner, and even the actual ride height could easily account for this difference. More telling are the enormous force figures in the chart as the travel approaches its maximum value. By measuring shock travel during operation with a zip tie around the shock shaft, I know that I have compressed the front shocks to within 1.5" of maximum, that is, to at least 12.5" travel. According to the chart, that means I would have had to have had a force or weight on that corner at that time of 2328lbs, which is well over twice the static weight of 940 lbs. Given that I hadn't jumped the vehicle I feel that this is unlikely - however, I can't say for sure without testing with an accelerometer (which would be getting silly - even for me!), and we do know that Force = Mass x Acceleration, so it is possible - just unlikely in my opinion.

Another reason the model will not give precise, accurate results is that it does not account for the negative spring. The negative spring is inside the body of the shock at the bottom end (if you depressurize the shock and then fully extend it by hand you can feel the spring inside in the last 2" of stroke as you extend the shaft) The negative spring is active for the first 2.0" of shock travel. It has a 70# spring rate. At full shock extension, it provides 140# of force to help you compress the shock. (If you run less than 115 psi, the shock will not fully extend)

The point is, the model is an excellent tool for understanding the operation of the shocks, their limitations (high weight vehicles) and as a starting point for tuning oil volumes and nitrogen pressures, as well as modelling and understanding how those 2 variables interrelate. It should not, however be counted on for so precise a ride height prediction that you'd want to weld in a complex mounting system based solely on it's prediction. In other words - some trial and error testing and tuning will be required.

A final note - at the beginning of the article I mentioned that a possible drawback to the Airshox was the effect of temperature - we can now examine why this would be. Charles' law states that for a gas (in this case, the nitrogen that acts as our "spring") as the temperature increases, the volume increases. We know the volume inside the shock cylinder cannot change (for any given shock travel, that is), so we refer to Boyle's law that tells us that pressure and volume are inversely proportional (as one goes up, the other goes down) and we arrive at the fact that, as the shock heats up - the pressure of the nitrogen at any given amount of shock travel will have increased, altering the "spring force" and "spring rate" values, and therefore possibly the ride height and performance of the shock/spring. To convince yourself of this, go back to the Excel spreadsheet and play with just the pressure value and observe the effects. How much of temperature change is required for any noticeable effect I cannot say, as I have neither attempted to model it, not have I experienced it. I believe the magnitude of the temperature change required, i.e. how much the shock has to heat up, is quite high and is likely only to happen of the shocks are run for prolonged periods at high speeds over very rough terrain - like a desert race truck may do, which is something I don't do. I can tell you that ambient air temperature changes from +15°c to -15°C make no measurable differences in ride height, as that I have measured.

If all that makes your head hurt, don't worry. Fox actually supply far simpler instructions, and you can simply follow them and use the methods I will outline below to trial and error your way to satisfactory results:

On the Fox "owners manual" (actually a sheet of paper) it says:

"air shocks can be filled with Nitrogen from a range of 10 to 500 psi. To calculate the spring rate, multiply the psi by 1.105 (example 200 psi x 1.105 = 221 lbs/in spring force) 3:1 is the standard compression ratio for our shocks - you can modify this by adding or subtracting oil "

When I asked for clarification, they told me:

Initial Pressure:
With suspension extended, charge shocks to 200 psi.
Set vehicle on flat ground and check ride height.
Add or subtract pressure to get the correct ride height.
(Usually 35-50% shock compression)

Oil Level:
If vehicle bottoms out to easily, but rides well, add 15cc's of oil through shrader valve.
(Release pressure and remove valve core first)
Repeat until vehicle stops bottoming.

Part 4 - Installation

Part 5 - Tuning

For adjusting oil volume you need:

• A source of high quality 5w shock oil
• A syringe that will fit in the shrader valve with the core removed
• A shrader valve core remover
• A graduated cylinder of some kind, preferably graduated in cc's

For adjusting nitrogen pressure, you will need:

• A source of high pressure (at least 600 psi) nitrogen (i.e. a cylinder or tank of nitrogen)
• A nitrogen regulator capable of delivering from 10 to 500 psi
• A high pressure hose that fits the regulator
• A shrader valve fitting (air chuck) for the hose

ote that when you are adjusting pressures, a peculiarity of Airshox will become apparent. You MUST set the pressure in all 4 shocks, rock the vehicle side/side and front/back and then set is on perfectly level ground with equal (or at least real-world, in-use) weight distribution and tire pressure all around before evaluating the results. This is the result of two things. First, the first few inches of the Airshox travel is at a very low "spring rate" so that they are very sensitive in this part of their travel to any changes in terrain evenness or weight distribution (the exact magnitude of how sensitive they are will of course depend on your oil volume, nitrogen pressure settings, vehicle weight, and ride height). The other factor is what is known as the high level of "stiction" that the Airshox have. Because they have large diameter seals and high pressure grabbing the shaft, after an adjustment is made, or loading of the shock changes, the shock initially appears to "stick" where it was left (for example, if you were to jack all the weight of one corner, then make no adjustment but just gently set the vehicle weight back down on the shock on that corner, 9 times out of 10, the shock will not settle back to exactly where it was initially, but rather will likely "stick" at a lesser travel (more shaft sticking out). This phenomenon is at it's worst when the shock sits static for some time. (All of the oil gets squeezed out of the seals). As the shock is used, the "stiction" is greatly reduced. Also, this phenomenon does not adversely affect the shocks performance, it simply takes getting used to.

The following 3 pictures show the Wolf all set up for it's first run on the Airshox

One final note on adjustment. When you first begin to adjust and use the Airshox, the nitrogen is absorbed by the oil. This means, for the first 3 or 4 times you will need to re-charge the shock after each run. At some point the oil becomes saturated and the pressure loss diminishes. Keep in mind though, there is no foaming inside the shock due to the pressure. The nitrogen bubbles are microscopic.

Part 6 - Testing

So now we cut to the chase. What are they really like? How are they at speed? Are they rough? Do they overheat easily? Do you like them? etc. etc. I get all these questions, and more, frequently. Here's what I have to say about them: