# Review: A Survey on Configurations and Performance of Flow-Mode MR Valves
## 4 Analysis
### Base Functionality: Benchmark Valve, Bypass  
Next, let us analyze the valve in Figure 7d. The configuration incorporates a thru-hole in the structure. The bypass (usually located in the valve and rarely on the outside of it) is a feature to control pressure in the MR actuator by diverting a portion of the fluid flow into a secondary flow path. It is indispensable in suspension dampers in particular. There are several reasons for it. One characteristic feature of any flow-mode MR damper without the mechanism is the presence of high field-dependent intercept forces at the velocity of 0 m/s (of the force-velocity characteristics), which is a result of energizing the fluid in the control gap volume. Only upon exceeding the breakaway pressure can the flow through the main gap be initiated. This is typical of any controlled friction damper and unwanted in controlled chassis applications in passenger vehicles, for instance, due to stick-and-slip and severe harshness effects [61]. In the conventional shim-stack valves used in passive suspension dampers, the behavior can be achieved by preloading the shims in valving stacks. The bypass function in such valves is realized by thru-holes in pistons, orifices, or slots manufactured into the shims. The bypass is a simple means for altering the force-velocity characteristics in a passive damper or a semi-active MR damper so that the high intercept forces can be effectively degraded and shaped for optimal ride quality. It is theoretically possible to realize the control function with a non-bypass MR controllable damper, however, it would require the exact knowledge of the damper's relative velocity and ultra-fast controls. MR bypasses are most often manufactured in the form of thru-holes (pure bypass) in the core assembly as in Figure 7d or cutouts on the inside/outside diameter of the sleeve (forming the primary path for the MR fluid flow) or the outer diameter of the core assembly (flux bypass) [59; 60; 78; 79]. While the former resembles standard hydraulic bypasses as those used in any hydraulic suspension damper, for example, the latter takes advantage of the magnetic flux local distribution in the area of the cutout. As a result of the lower flux density distribution in the cutout region the yield stress is made non-uniform and degraded which effectively allows the fluid to pass through the gap at a lower pressure drop than in the other volume of the gap. The flux in the pure bypass is zero or near zero. As a result, the fluid's rheology within the bypass flow regime is virtually unaffected by magnetic flux variations in the electromagnet. The results in the envelope are simply illustrated in Figure 9b. For comparison, the flux bypass is located directly in the flux main path, and any changes in the flux level in the region translate into the breakaway pressure changes as in Figure 9c. Both features can be combined in one MR valve assembly to develop a dual-slope bypass for even more effective tuning of the damping force envelope at low velocities (flow rates) [79]--see Figure 9d. The functionality of conventional bypasses is limited, though, as the slope of the curve (damping coefficient) below the breakaway force cannot be altered or it can be altered only in combination with the flux bypass. In simplified terms, they are characterized by a single and high damping coefficient. In general, the damping coefficient in the bypass region is independent of the magnetic field. For comparison, a variable slope bypass would allow altering the damping coefficient at low velocities. This would require shaping either the magnetic poles or the gaps(s) and/or designing the orifices in such a way to be controlled by the flux [9]. However, so far, the invention of the pinch mode has not yielded any viable variable slope contributions in the area despite the initial developments and claims by the inventors [9]. At the time of writing this review, no other experimental evidence with adequate performance had been developed to support the inventor's claim [45].