A gas charged monotube shock absorber is a single tubular structure holding the oil, a shaft with piston/valve, a free floating separator piston, and a gas chamber. The piston rides up and down the tube in the oil and further down tube there is free floating piston suspended as a separator. The job of this piston is to separate the monotube body into two halves - one containing the oil and piston and the other containing high pressure nitrogen gas. The monotube suspension was pioneered by Bilstein.
The function of the high pressure nitrogen gas is to apply pressure on the gas by means of the floating piston to avoid cavitation. If the oil is aerated the damping properties of the shock absorbers change as the rate with which the piston pushes the now aerated oil through the valve body. Applying a constant pressure on the fluid mitigates cavitation and the shock provides a consistent damping.
The other type of shock absorber is a twin tube and each have their own pros and cons. Twin tubes usually have linear valving and the gas pressures, if gas charged, are very very low.
If monotubes were pioneered by Bilstein why does a Dobinson IMS Monotube ride much better than an equivalent Bilstein Monotube? Two reasons - type of valving in mass market Bilstein monotubes and gas pressure.
Valving
All Bilstein shocks listed above have digressive damping, stiff at lower (piston) speeds and they ease up at higher speeds. But a sudden large impact on less than perfect road will jolt the vehicle and have experienced this many times on the roads here. Good for track and circuit.
Dobinson IMS shocks have a progressive damping curve, not as stiff at slower speeds and gets progressively tighter as speed increases. Good at low vehicle speeds and sudden impacts as one would expect on un sealed roads.
Gas Pressure
Both Bilstein and Dobinson IMS are monotube shock absorbers with internal reservoirs for oil and high pressure gas separated by a free floating piston. The Dobinsons IMS has a recommended fill pressure of 150PSI of Nitrogen. Bilstein holds a higher pressure N2 charge in their internal reservoir monotubes with recommended fill pressure between 180PSI and 200PSI and with a factory fill pressure at 200PSI. The Bilstein is at a minimum 30PSI to a maximum 50PSI higher pressure compared to the Dobinson IMS. This higher internal gas pressure in a Bilstein acts on the piston shaft giving it a gas spring effect and a static preload. Lower the gas pressure less the static preload.
A question asked frequently: Why does my car sit higher after I put a set of Bilstein monotube shocks (on stock springs)? Ans: High gas pressure.
Especially true if the vehicle was previously on a set of any low gas pressure or hydraulic shocks which were replaced with a set of Bilstein monotubes. The high gas pressure adds higher static preload to the initial weight needed to get the shaft moving. This combined with the spring rate adds to the total suspension preload at rest and at lower speeds. The effect of gas spring preload is more at low piston speeds and weans off at higher piston speed.
I have used Bilstein monotubes in two of my own vehicles, an Octavia RS Mk1 with stock and VW OE sport springs and in an Innova Crysta 2.8 Diesel with stock and Eibach lowering springs. Also had a friend's VW Vento 1.6TDI with me for many months (when he left the country) which had a B12 coil over kit. Although the B12 coilover was supplied with springs as a kit, it rode very similar to the other two. Not pleasant on our less than perfect roads, except for the Crysta.
Run off the mill Bilstein monotubes including the B12 coilover rode harsh except for the one I put in the Crysta. It was well behaved because the set was made for the previous Innova and the Innova Crysta I put it in weighed a good 200Kg more than the old Innova. This additional weight had a calming effect on the Bilstein's gas spring at low piston speeds.
Rebuildable?
Dobinson IMS is rebuildable by design and they have a rebuild kit available for the monotubes, they have a gas port to let out gas pressure and charge it post rebuild.
Bilstein supply all the minor and major internal parts as spare parts for their shock absorbers. The rebuild process of the Bilstein is a bit more involved because they have not included a gas port in their shock body. So First a hole needs to be precision drilled at a specific location to let out gas pressure and then use a high pressure Schrader valve (supplied as spare by Bilstein) installed in the shock body to charge the shock.
Both can be revalved during the process of rebuild.
Is there a better way that I could have put this?
I wish, watch the video this is what I wanted to convey in this blog entry about gas pressure. Also read the comments.
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