A few weeks ago we were visiting the North American facilities of Ohlins Advanced Suspension Technology, Ohlins USA for a special look inside Ohlins’ operations and to be able to get a close look at some of their technology. We are sure that most of you MotoIQ readers are very familiar with the Ohlins name and associate it with high-end professional motorsports from IMSA and Indy cars to Moto GP and Supercross, but Ohlins also makes many consumer affordable suspension systems for high-performance street and dual-purpose street/track cars.
Their main offering in this market segment, the Ohlins DFV line of coilovers and MacPherson struts, impressed us with the quality of manufacturing and some very unique technical features that set the product line apart in the market. The main technical feature of this lineup is the DFV or Dual Flow valve. The Dual Flow Valve is Ohlins’ answer to make a conventional deflected disc valve system more sensitive and faster reacting to high-frequency ripple and square-edged bumps. This type of bump often causes tire shock, a loss of traction and ride discomfort. The DFV valve is good at absorbing these kinds of road shocks without affecting low-speed control. Low-speed control helps keep roll, squat, dive at bay and helps sharpen transient response which is critical for a performance car. Many times, a shock that is good at controlling platform movement does poorly at providing comfort and traction finding suppleness over small bumps. The DFV valve is designed to be the best of both worlds, an absorbent supple shock that controls body movement well. How does the DFV valve work? Let’s take a look.
In this cutaway picture of the DFV valve, figure 1 is the compression stroke. Under low-speed conditions which is 0-1.5 inches per second of shaft velocity, fluid mostly flows through the hollow shock shaft through a metering orifice whose flow volume is determined by an adjusting needle valve. After passing through the needle valve the low-speed circuit fluid flows out of an orifice in the side of the shock shaft to the other side of the shock piston. You can see the low-speed flow indicated by the dotted line going through the shaft. At higher shaft velocities the fluid flow volume increases and to keep up with the flow demand, the fluid backing up behind the piston travels through holes in the piston where the pressure must overcome and bend the stack of stainless steel shims blocking the holes. This is your high-speed compression circuit. The high-speed circuit is activated when you hit a bump or strike an FIA curb and helps your suspension react to larger bumps. How the high-speed circuit reacts is controlled by size, shape, diameter, and number of holes in the piston and by the diameter, thickness and stacking sequence of the shims blocking the holes. Typically these can be tuned really well to a car’s weight, tire type, and suspension motion ratio.
Now there is a zone in the mid to mid high speed of shaft velocities and higher frequencies where a high-speed circuit that is calibrated to prevent a car from blowing through its travel isn’t so sensitive, a lot of this is due to the amount of fluid flow needed to bend the high-speed shim stack. This causes some lag in the high-speed circuit’s response. This is where the DFV comes into play. In the DFV shock, there is another fluid flow path, hence dual flow. Fluid flowing through the shock shaft can flow through another orifice in the shaft, before the needle valve, leading to a second valve and another set of shims. This second set of shims is set up to be softer and quicker responding and gives the DSV shock good compression responsiveness.
On the rebound side of things, as shown in the dotted lines in figure 2, at low shafts speeds, the fluid flows through holes in the shaft and past the needle valve to the other side of the piston. At higher speeds, the shims blocking holes in the piston deflect and allow fluid to flow through the piston just like on the compression side. At mid speeds, or during the piston valve’s lag time, fluid can flow through the shaft before the needle valve to the softly shimmed DFV valve.
Here is a cutaway DFV strut. It is a strong inverted shaft design. We wrote about how an inverted shaft strut works last week, so be sure to read that article. The DFV strut is of monotube construction with a gas chamber at one end where the gas is separated from the fluid by a floating piston.
The DFV valve is the one that the finger is pointing at, behind the main piston. You can see how the shims on the DFV piston are thin for quick response compared to the shim stack on the main piston. You can see the flow passages painted yellow. You can see the damping adjustment needle valve in the center of the shaft. The damping adjustment affects the low-speed rebound mostly although it is a global adjustment.