We often get folks trying to compare the Ironman Foam Cell Pro to Dobinsons Monotubes, or want to know the difference between monotubes and twintubes. We want to make sure you get the most for your money, but also know what you are buying and why you are buying it. Lets go over that:
You need to look at the differences between monotubes and twin tubes and what makes the monotube a better shock. A high oil capacity, which the FCP offers, isn’t everything. Take total piston flow working area for example, an ironman foam cell pro has around 220 sqmm total and The Dobinsons 56mm monotubes have around 390 sq mm total, and the 66mm monotubes around 660 sq mm total. In comparison you are doing the same “work” or damping with 75% or more working area meaning the actual heat generated is considerably less. You also need to look at how heat is dispersed, twin tubes rely on the shaft displacement to move oil into the outer tube, like a monotube into the reservoir, where it can be transferred to the body and cooled. Monotubes on the other hand are working directly on the exterior body so any heat generated is transferred straight into outerbody and the cooling process begins earlier, rather than heating the inner tube first and then transferred to the outer tube. Also looking at oil capacity you will find that one you take out the foam cell, the inner tube and the additional piston the capacities are not that different. Heat build up increases substantially with speed offroad- and while oil capacity does play a large roll, a twin tube will never resist cavitation (see below) nor cool as well as a monotube shock.
Monotubes also react faster to road inputs than twin tubes, Due to the higher and more balanced working pressures and single piston design. Twin tubes are slower to respond and in more aggressive high frequency inputs in off-road terrain will be slow to react to the bump or dip and therefore cannot offer the best damping and control. This is why we ask our customers- how fast and/or aggressive they drive. A Monotube is almost always a better choice for someone with a lead foot 
The biggest issue most twin tubes have is in their piston and valve design, lack of port flow and lack of pressure to overcome cavitation – where the shock absorber pulls a vacuum inside the oil and is a result of having such large pressure variations on either side of the piston and the sudden drop in pressure can create cavitation much easier. Cavitation basically instantly “shears” the oil and damages it. Also it creates an air void behind the piston and your piston therefore passes back through this air void with no resistance and no shock valving and then slams into the oil which creates an uncontrolled and harsh ride. I think most people would also be surprised at the speeds that shocks will actually reach on your average 4wd when you go off-road. Although you wont see the same travel as race trucks, you can easily see speed in excess of 2 metres/second. The lack available piston flow area in a twin tubes mean that they simply cannot get the oil through the piston quick enough and will either cause cavitation or massively ramp up the compression damping and wont allow the suspension to soak up the bump. Dobinsons Monotubes have an internal floating piston, and are not emulsion shocks- which are more prone to cavitation.
Twin tubes are great and they do the job for most 4×4’s but If your really looking for performance out of the shocks you need to look at a monotube shock, there’s a reason even the basic production class race vehicles use them.
Generally speaking a twin tube (Like the Dobinsons Nitro Shock) is ideal for people who say drive both on road and off, but who partake in non high speed offroad activities such as mountainous, heavily wooded, rocky trails.
Monotubes are ideal for the person who is looking for performance, likes to drive faster and more aggressive both on road and off, especially those who have access to wide open desert trails and or wide open areas where they can go fast.
