The gas in every shock is necessary to accommodate the change in internal volume that results from the shock shaft. The internal volume of of the fluid reservoir is reduced by the volume of the shock shaft as it compresses. Without a gas chamber all shocks would basically act as a fixed length linkage and would not move at all, or they would rupture.

The goal then is to use a the gas to allow for volume changes while not mixing with the oil. Gas and oil foam doesn't create the same resistance to flow through the valves and the dampening goes out the window. So there's a few different ways to keep the gas and oil separate. One way is to use a floating piston in between the gas and oil (how it's done in monotubes - either inside the main body as an "IFP" or in a reservoir - same thing, different location. The only real function of the reservoir is to provide more volume of gas for less change in internal pressure through the stroke of the piston) and the other most way to separate the gas and oil is a rubber bladder or bag of gas (how it's done in most twin tube shocks).

The foam cell is basically the same thing as a bag of gas - just a lot of little bags of gas. It provides the same function as a floating piston and air chamber or the rubber bladder. The unknown is how long the foam lasts, how many cycles it can handle before breaking down, etc.

The reason you pressurize a monotube gas chamber is because you need to maintain positive gas pressure through the entire stroke of the shock. Even at full extension you want plenty of gas pressure inside to make sure that the floating piston is being pushed against the oil on the back side. That keeps the oil on the oil side where you want it. The seals are not designed to handle a scenario where there is a vacuum created on the air chamber side - gas will be pulled through the seals to the oil side. The pressure also helps to keep the oil from foaming or boiling at higher temps - but that's pretty rare if not impossible in a 4Runner. All oils will have some suspended gasses in them that would tend to off-gas at higher temps. More pressure helps prevent that off-gassing, but it's a pretty minor issue in a 4Runner and generally not a big issue in most applications anyway. There's basically no way you can push one hard enough to overheat most shocks. The self contained air bladders in typical twin tube shocks don't have that same issue. Nor does it appear the foam cells do. They can expand and contract without as much initial gas pressure required.

If the foam lasts a long time and doesn't break down or rupture the cells, it should be a fine way of making a shock. And it could be used just as easily in monotube by using a block of foam instead of the gas and piston design. The most obvious downside I see is that the foam probably has some limits on expansion and contraction volume. So the shock shaft will be limited in diameter as a result. That's why they use a pretty small piston shaft. An IFP design can accommodate a much larger piston shaft's volume displacement. That's why you'll see much larger heavier shaft and pistons in a race type shock. - Does that matter in a 4Runner that has 8 or 9" of suspension travel? Probably not at all. I've never seen a broken shaft that wasn't the result of a spacer lift causing a bottom out.


The shock shaft diameter beomes a lot more important on a 40" long extended shock (it has to do with the thin column issue - the longer it gets the less force it takes before an out of plane failure - bending under compression or collapse- is expected). That just isn't really an issue with the short travel suspensions we have on our 4Runners.