* For those interested in a little more detail on the issue of gas charging to minimize aeration and foaming of the hydraulic fluid, it works like this: When the bumpstop cycles, the motion of the piston through the oil creates an area of high pressure ahead of the piston and an area of low pressure behind it. If the pressure of a fluid is reduced below its vapour pressure, the fluid will spontaneously change state from a liquid to a gas. This means that tiny air bubbles can form in the low-pressure area directly behind the piston. This process is called cavitation. Left unchecked, the tiny air bubbles that form will mix with the oil  - a process called aeration. The resultant mixture of oil and air inhibits the functioning of the bumpstop because the piston and valving are designed to produce the required damping by operating in a column of incompressible oil, not in a column of frothy air and oil. Once the oil is mixed with air, or aerated, it is no longer incompressible. The faster the piston pumps up and down, the more rapidly cavitation aerates the oil on both sides of the piston, and eventually the oil will be churned into foam. The resulting foam offers little resistance and causes extreme fade of the bumpstop’s damping ability.

The addition of pressurized nitrogen gas to the shock helps to prevent the low-pressure zone behind the moving piston from falling below the vapour pressure of the oil, reducing the chance of cavitation, aeration, foaming, and eventual fade.

Customizing Travel

One of the things you may or will want to do when installing air bumps is to adjust the travel. The 2.0 series are sold in 3" or 4" travel versions, but as we shall see shortly - they are exactly the same except for one internal component - the internal spacer on the shaft. Also, depending on your rig, type of driving, suspension setup, etc., it's possible that 3" may be too long or 4" too short. The nice thing is, you can simply and easily convert either of them to anything from 0 - 5-3/8" travel.

But, in order to do that, first we have to take it apart.

What Length?

OK, so now we know how to customize a bumpstop, and how it works, the question is - how long should it be (i.e. - how much travel should I shoot for?).

Well - the easy answer (easy for me to explain, that is) is to try by trial and error. By having a few different length internal spacers on hand you can easily try 2", 3" and 4" and see what the difference is. The problem is - that's an awful lot of work to pull out, disassemble, modify, reassemble, re-charge, and re-install up to 4 bumpstops every time. And on top of that - yo get the most valid results from this method of "test and tune" you have to run the rig over the same terrain/course at the same speed under the same conditions (like load/weight) every time - otherwise you don't know what the cause of any difference noticed is. Most of us just don't have the time or patience for this - although it works well if you do.

Instead, it's nice to have some guidelines or rules of thumb to start with.

ASSUMING your suspension is reasonably well tuned beforehand (e.g. assuming it's not to soft and you are trying to use the bumpstop to add some stiffness or roll resistance) and assuming you have a "normal" amount of up-travel or (say up to about 8" or so), then 2-3" of air bump travel seems to be a good range to shoot for.

In my experience, no more than about 25% of your shock's up-travel works well as a target. I run about 8" of up-travel front and rear, and initially tried 4" bump in front and a 3" in the rear and the results were TERRIBLE! After some tweaking, I settled on 2" bumpstop travel front and rear.

Basically, the length or travel of the bumpstop relates to how soon in the up-travel, and for how long, you need to start slowing down the axle before it reaches the hard stop.

Of course, the pressure you charge them too will have a fairly dramatic effect too, and generally the shorter the travel the higher the pressure. Stock pressure is in the 180-200 psi range.

The Fox Airshox owner's manual has the following information:

"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)"

And since the air bump is very much like a short airshock (albeit with different valving and piston, and one end left unattached) I assume that a similar range is possible. However, in practical terms, I would question the need to go less than 50psi or higher than about 400psi.

One final note - if you are using coilover shocks with dual-rate springs, you almost certainly want the bumpstop length adjusted so that the bumpstop only comes in AFTER the coilovers have transitioned to the final rate (the rate of the main, or lower, spring).

Remember - a bumpstop, even a fancy air bump, is just a bumpstop and is their to help achieve a controlled deceleration of the up-travel and eventual full stop of your suspension up-travel before other suspension or chassis components are damaged when the suspension bottoms out.

That said - obviously an air bump is a very tunable device - valving, length, pressure, and theoretically even fluid volume and fluid viscosity - so if you can or want to experiment with all these adjustments to get what you want, more power to you, and don't let anyone tell you you're doing it wrong!

Reassembly

Reassembly is the reverse of the removal process, with the following notes:

• Make sure the piston and valve shims go back in the right orientation (on the right side of the piston, facing the right way).
• Tighten the locknut to 30 ft./lbs.
• Make SURE the snap ring that holds the bearing / shaft guide in place is FULLY seated all the way around.
• Make sure you fill the body with the correct volume (125 cc) of the correct shock oil (Fox 5wt or equivalent).
• Make sure you install the valve core back in the Schrader valve BEFORE you attempt to fill the body with the correct volume of oil (Don't ask!)
• Unlike a coilover with an internal floating piston separating the oil from the nitrogen, there is no bleeding procedure for an air bump.
• Pressurize the bumpstop with 180-200 psi - nitrogen ONLY.

Mounting

Mounting air bumps is fairly easy and straightforward, especially if you use the handy air bump mounting cans sold by Polyperformance and other good vendors. However, there are a few things to keep in mind:

• They take quite a bit of room, especially vertically.
• You have to allow room to get a nitrogen chuck on the Schrader valve, while mounted, so pressure adjustments are easy and convenient (Don't ask!)
• If you're planning on grabbing some air, especially if your rig is on the heavy side, make sure your mounting has adequate bracing so the bumpstop doesn't just bend the chassis where it is mounted.
• In most case, you will have to fabricate and install on the axle a contact area for the bumpstop to contact.
• The contact pad on the bumpstop is fairly small, but there will be a wide variation in the position of the axle's contact area as the axle travels up in either "ride travel" (axle remains parallel to the ground as it travels vertically) or "flex travel" (axle articulates up on one side and down on the other). So you will have to make the axle's contact area large enough to accommodate all possibilities.
• Air bumps cannot be mounted upside-down because the pressure would hold the shaft extended up, but gravity would leave the fluid collected at the bottom.

Function

So how do they actually function?

Well, their purpose is two-fold - first they slow the motion of the axle as it travels upwards, and secondly they provide a "safe" hard-stop to serve as a limit to the uppermost position of the axle relative to the chassis.

Advantages & Disadvantages

There are both advantages and disadvantages to running air bumps, some of which are:

Disadvantages:

• Cost
• Packaging - as mentioned, they take quite a bit of room vertically, and are generally more work to install than a simple rubber or poly bump.
• Requirement for high-pressure nitrogen
• Maintenance - more moving parts means more seals, O-rings, fluid, and wear bands to occasionally replace.
• Size of axle contact area - because of their travel, they most often are engaged (are contacted by the axle and start to work) earlier than would be a simple lump of rubber. This means, the range of axle positions when contact initially occurs is quite large and therefore the area of the contact pads on the axle must be fairly large.

Advantages

• Damping - Their ability to progressively damp the upward motion of the axle before a hard stop is unequalled by other styles of bumpstop.
• Adjustability - Easily adjusted for different lengths of travel, as described above.
• Tunability - With the ability to tune valving, travel, nitrogen pressure, and to a limited degree even fluid volume and fluid viscosity, their function and manners can be tuned in a way that a lump of rubber or polyurethane cannot. If you use a rubber or poly bump, its rate or stiffness is what it is and if it's too harsh or too soft - there's not much you can do about it.
• Ruggedness - Because they damp the axle by heating the fluid (kinetic energy transformed to heat energy), they can do so reliably and consistently for many thousands of cycles and with little wear and tear. Conversely, a traditional rubber or poly bumpstop relies on plastic deformation to damp the axle, as it works its structure is continuously deformed and this results in wear and tear - eventually they chunk or tear and fall apart.
• Bling - If it's your thing, there's no denying the curb appeal of high-tech machined aluminum air bumps compared to a chunk of rubber.

If you're racing (many hundreds or thousands of bumps in a single event), and / or needing to progressively damp and cushion the up-travel of long-travel suspension to a full stop then air bumps are almost a must. If, on the other hand, you spend all day slowly crawling around trails and rocks, then they probably won't do that much for you and simple rubber bumpstop may suffice. If your use falls somewhere in between (e/g/ you like to occasionally rip around over the rough stuff) then you will have to make your own decision based on needs and budget.