Damping and Oils
Riders can be tempted to experiment with different damping oils and levels in front suspension. However, you can't just put any oil into your suspension and expect it to work correctly.
Here are the reasons why...
A change in oil level will affect the air compression; it does not affect damping. However, too large an air gap in forks may cause forks to dive too fast. Conversely, if the oil level is too high with too small an air gap, it can have a significant effect on the last bit of fork travel.
Viscosity refers to the thickness of the oil. Damping oils are at the thin end of the scale for viscosity and generally in the range of 2.5 to 7.5. Changes in oil viscosity will affect both compression and rebound damping and while thick oils increase damping, they are not suitable as they are more likely to cavitate. Metal parts move against each other creating friction and wear. The higher the oil viscosity, the more it will protect moving parts. Suspension damping, by the nature of its design, requires oils of low viscosity and, together with the additional demands made on it, should be changed more regularly than other oils to protect parts from wear.
Oil Viscosity Index
The oil viscosity index tells you how well the oil viscosity is maintained in high temperature environments. Most damping oils have a viscosity index above 140 and will work well in forks because they are air-cooled. However, shocks that work hard with a short stroke and hidden from the air-stream behind the engine can be more prone to fade due to reduced viscosity with the increase in temperature.
Cavitation refers to the foaming of the oil or creation of air bubbles. When these air bubbles collapse, they cause a small implosion in the oil producing heat, light and sound. Cavitation produces fade as will oils with a low viscosity index. Anti-foaming agents are added to suspension oils to reduce cavitation and thick oils are more likely to cavitate than thin oils.
The oil's lubrication qualities are required to keep suspension parts protected and their action consistent. Metal parts move against each other creating friction and wear. The higher the oil viscosity, the more it will protect moving parts. Suspension damping, by the nature of its hydraulic design, requires oils of low viscosity and with the demands made on it, should be changed more regularly than other thicker oils.
The basic requirements of damping oils are as follows:
Viscosity must be suitable
Suitable anti-foaming agents are used
The oil does not harden seals or cause them to swell
Lubricating properties are appropriate
Oil chemistry has advanced to adding chemical agents which control viscosity over a range of temperatures.
It is important that these specially designed damper oils should be used with the desired viscosity, viscosity index, appropriate lubrication properties, compatible with all the materials used in forks (e.g. bearing and sealing materials) and resistance to foaming.
Damping and Oils