Twin Tube vs Monotube Suspension

Monotube vs. twin-tube vs. foam cell shocks: A beginner’s guide
A shock absorber is an incredibly simple and effective part of any suspension, and it has only one shortcoming: high temperatures. All the energy created by the springs has to go somewhere, and it ends up morphing into heat. Unfortunately, when the oil’s mercury starts to rise, its viscosity starts to thin out and absorbs less energy. If the oil gets too hot, it can start to bubble like a pot of boiling water, causing a dreaded case of shock fade.

Besides just dampening, a good shock must also be engineered to dissipate heat efficiently for consistent performance. The primary way to keep fade and foam in check is through maintaining high internal pressure. Ever wonder why a radiator doesn’t boil over unless you open the cap? It’s because your coolant is pressurized, and pressure greatly increases a fluid’s boiling point. Shock absorbers work the same way. In general, shock fade is kept at bay through gas charges and tube design. Let’s investigate the 3 most common designs:

Twin-Tube Shocks:
The twin-tube shock can trace its lineage back to the mid 20th century, and its ingenious design still stands the test of time. As its name implies, twin-tube shocks have 2 chambers: an inner and an outer tube. The inner tube is where most of the work takes place. Here, the piston plows up and down through the main supply of oil. The outer chamber holds an extra supply of oil and a low pressure gas charge at around 100–150 psi. The gas, usually nitrogen, provides the backstop against the flow of oil for mega dampening. However, as the oil heats up, it can mix with the gas and cause foam and fade.

Twin-tube shocks deliver comfortable, stable all-around dampening for cars, trucks and SUVs. In fact, twin-tubes are probably the most commonly used shocks in the automotive market today. Ideal for street driving, they feel smooth yet still reduce body roll and sway during cornering and dipping during hard stops and spirited acceleration. However, rugged off-roading can cause the twin-tubes to overheat, which leads to foam and fade in extreme conditions.

Foam Cell Shocks:
Foam cell shocks are nearly identical in design to twin-tubes. The key difference is that the gas is trapped inside of tiny little capsules rather than free-floating. By separating the gas charge from the oil, the risk of foaming is nearly eliminated while still providing the comfortable ride quality of a twin-tube shock. However, foam cell shocks are vulnerable to overheating. If the oil in these shocks gets too hot, the foam cells can erupt. After that, no matter how long they cool down, they’ll never regain the same level of performance.

Monotube Shocks:
One may be the loneliest number, but monotube shocks have no problem leading a solitary life. Rather than using two chambers, these burly shock absorbers do it all in a single tube. What’s more, they have twice the piston power as a twin-tube shock. The first piston is located right where you would expect it–at the end of the piston rod. This first piston pushes against a section of oil, which is forced downward into the second piston. This floating piston is sandwiched between a section of viscose oil and a pocket of highly charged gas (between 200–360 psi). By separating the oil from the gas, monotube shocks greatly reduce the risk of oil foaming and fade.

The dual piston design of monotube shocks provides stiffer dampening, which is preferred for sportier handling and grueling off-road conditions. Monotube shocks are also air cooled, so they do not retain heat the way twin-tubes do. Plus, they can be mounted upside down for extra weight support. However, monotube shocks are difficult to mount in place of stock twin-tube shocks on cars and stock-height trucks and SUVs because of the added length and range. Also, monotube shocks are slightly more vulnerable to dent damage because there is no outer buffer.

Ref: http://www.autoanything.com/suspension-systems/50A26A163A4.aspx