Standard braking system

The brake compensator valve is the most complicated part of the hydropneumatic suspension. This component is not only responsible for the operation of the brakes but forms a central part of the whole system.

With no pressure applied on the pedal, the brake valve connects both the front and the rear brake circuits to the operational return, ensuring that the brake pads retract from the disks. Note that both braking pistons (4 and 5) have internal conduits (drawn as Y-shaped lines) offering a passage for the brake pressure to escape into the return line:

At the same time, the pressure coming from the rear suspension exerts pressure on the right hand side of the piston 1. As soon as this pressure exceeds a given (albeit quite low) level, the piston will be forced to the left. The cup 2 will be forced to the left as well which, in turn, will compress the spring 3. As soon as the piston 1 starts to move, the lip on its the face will move away from the edge of the housing, effectively increasing the area the hydraulic liquid can exert its pressure upon. This setup introduces some hysteresis in order to prevent unwanted oscillations in the system: although the piston 1 and the cup 2 moves as soon as the pressure exceeds the threshold level, it stays in that position even if the pressure drops a little:

The reason for this is to lower the counter force on piston 4, forming the valve for the rear brakes. This makes the rear brakes extremely sensitive at the very beginning of braking, minimize their delay in order to allow the anti-dive behavior of the rear trailing arm suspension to take effect.

As soon as the driver starts to brake by pressing on the brake pedal, which in turn presses the front piston 5 behind the rubber boot at the end of the unit, the feed from the main accumulator becomes connected to the front brake circuit:

In addition, the high pressure liquid will also pass through the channel in the middle of the front piston 5 and enters the chamber between the front 5 and rear 4 pistons. The pressure of the LHM will push the rear piston 4 to the left, too, which in turn activates the rear brakes:

The presence of pressure in the rear suspension has kept the piston 1 and cup 2 pushed back to the left (against the force of the spring 3) until now. But the increasing pressure in the rear brake circuit, being present on the left side of the piston 1 as well, will exert a counter-force, forcing the piston 1 to the right. Via the passage in the rear piston 4, the pressurized LHM of the rear suspension enters the chamber between that and the piston 1 (all these pistons have lipped faces to prevent unwanted oscillations by introducing hysteresis action, as described above).

We have discussed piston 1 as a single component so far but in reality, it is made of two parts. We introduce part 6 here so that we can refer to its right side (forming a chamber with the left side of the rear piston 4). In the following formulae, piston 1 will only denote the part in direct touch with the pressure coming from the rear suspension. Thus, the force on the left side of the piston 1 comes from both the spring 3 and the braking pressure (blue) exerted on the left side of piston 1:

Fleft = F3 + pbrakeA1left

As we can see from the cross section of the valve, the combined areas of 6 and 1 right are equal to that of 1 left:

Fleft = F3 + pbrake (A1right + A6left)

Similarly, the force on the right comes from the suspension pressure (green) on the right side of piston 1 as well as the braking pressure (blue) on the right side of piston 6 (arriving via the channel in the rear piston 4):

Fright = prearA1right + pbrakeA6right

Brake force regulation is performed by the movement of pistons 1 and 6 until an equilibrium of forces is obtained on their two sides, and the consequent influence on pistons 4 and 5. Limiting of rear brake pressure with respect to rear suspension pressure is insured by the force generated from the pressure in the rear brake circuit being assisted by the spring 3. Because of this spring, that pressure is always smaller by an amount proportional to the spring tension:

Fleft = Fright

F3 + pbrake (A1right + A6left) = prearA1right + pbrakeA6right

pbrakeA1right = prearA1right – F3

pbrake = prear – (F3 / A1right)

The balance of forces depend on the spring, as well as the suspension and brake pressures. Limiting occurs whenever the rise in rear brake pressure causes the pistons 1 and 6 move to the right:

As the LHM trapped between the rear 4 and front 5 pistons is not compressible, the second one will also move to the right, providing a counter-force to the pressure of the driver’s foot. As this movement will partially close the passage between the main pressure feed and the front brakes, the pressure in the front circuit is also forced to reach an equilibrium between the force applied to the brake pedal and that of pushing the piston 5 to the right.

The springs between the pistons 6, 4 and 5 are not taking part in this process. They are small, soft springs, aiming only to center the rear piston 4 between the other two but their effect is otherwise negligible.

From this description, we can draw a few conclusions. First of all, the maximum rear brake pressure can never exceed that of the rear suspension. Consequently, if there is no rear suspension pressure, the rear wheels do not brake.

When the driver starts to brake, the rear brakes will bite immediately as the suspension pressure overpowers the spring 3,  pushing the pistons 1 and 6 to the left and allowing the rear piston 4 to open quickly and completely. This piston will also balance the forces in the front and rear brake circuit, so, during this initial braking period, the rear brakes operate just like the front ones. As the rear brake pressure builds up, it will always be dynamically limited with respect to the rear suspension pressure, as described by the formula above.

Stop breaking, please…

The brake compensator valve used on early CXs was slightly different from the one described above, withouth the built-in brake force limiting effect of the piston 1 and the cup 2. These CX Break models had a separate rear brake force limiter to provide this functionality.

Brake force limiter

When there is no pressure in the rear suspension (for instance, the suspension is set to low), the force of the spring 4 keeps the piston in the neutral position, completely closing the feed to the rear brakes from the brake compensator valve.

When the suspension is under normal pressure, the force 1 supplied by the rear suspension fluid exceeds the counter force 2 provided by the spring. The piston stays in the open position, letting the fluid pass to the rear brakes. As soon as the driver starts braking, the force 2 increases by the additional pressure coming from the front brakes, via the LHM entering through the ball valve 3.

As soon as the incoming front brake pressure exceeds the rear suspension pressure by more than 28 bar (in other words, the combined force of front pressure and that of the calibrated spring 4 becomes larger than the rear suspension pressure), the piston moves again to the left, cutting out the additional pressure to the rear brakes, which will then continue to brake with this constant pressure.

To avoid a sudden cut-off of pressure, the ball valve 3 is combined with a damping element 5 to smoothen the changes. This element is a coil made of hydraulic piping, closed at the far end. The elasticity of the pipe and the coil acts as a small hydraulic spring capable of lowering the resonance of the chamber, hindering unwanted oscillations.