Following our yesterday’s blog where we revealed the basic concept behind an anti-dive / anti-squat system (possibly similar in scope and design to what Mercedes AMG are rumoured to be running), my friend who is behind this idea was kind enough to provide a few insights into the concept, as well as some figures, which will help everybody understand its application a little bit better.
The first picture is a logical extension of yesterday’s sketch, with a gas (N2) cylinder accounting for the increase in system volume due to thermal expansion. For the benefit of our presentation we will assume that the system operates with mercury (and all calculations have been based on that assumption) but other heavy fluids can be used as well, such as gallium and its alloys (gallium-indium-tin), cesium formate, barium sulfide, sodium metatungstate, etc etc. Less heavy fluids will require pistons & cylinders of increased diameters to reach the required force (due to the inevitably reduced ΔP), but it’s do-able.
The mercury circuit is set at an approximate fixed volume of 500 cc, with a pipe line of 8 mm (inner diameter) connecting the front and the rear heave cylinders at a distance of approximate 3200 mm (i.e. the wheelbase of the car). As you understand, the volume of heavy liquid required is quite minimal.
The oil circuit and the mercury circuit are set at a constant pressure of 200 bar. The accumulator decides the “system pressure”, since lower pressure values can also be used without any primary effect on the system (10 bar would do as well). The benefit of this system, in comparison to the Lotus one that was recently banned by the FIA, is that the car is not “rested” on it, so any changes on its stiffness will not have dramatic effects. The beauty of this system is that it’s not “in-line” with the main suspension load, but “in-parallel”, so it doesn’t carry it all the time.
During the braking phase, a -5g deceleration would produce a delta-P (differential pressure between front and rear) of around 22 bar (similarly, a 1.5g acceleration during the accelerating phase would produce a delta-P of around 6.6 bar), counter-acted by higher spring load at the front and lower at the rear, thereby creating a legal high density fluid anti-dive / anti-squat suspension system.
These 22 bar of ΔP when put into work against a piston of 65mm diameter will produce a force of around 7200N. Assuming that the front downforce is at around 40.6% of the overall, then it’s easy to calculate (taking into consideration the aerodynamic force, the mass of the car, the driver and 1/2 fuel tank) that the force which the system will need to overcome during the maximum (5g) deceleration phase is in the order of 6200N, approximately. All very straightforward and easy, even accounting for a 5% approximation error.
A sealed crossover (utilizing a floating piston) between oil and mercury circuits is provided in order to allow movement in the heave mode – front and rear axles move simultaneously. The high-pressure system is set in order for the deceleration-induced pressure at the crossover not to cause too much of a circuit volume change, and also to secure a low percentage of system-pressure differential from both deceleration and thermal expansion taken up by the accumulator, as well as to increase the “stiffness” of the oil and avoid possible localized cavitation effects in the circuit during operation.
The gas cylinder, as we explained, allows for the thermal expansion. This presents another advantage against the banned Lotus RRH system. The Lotus system would operate in working temperatures of around 130C for the oil (the pistons and fluid lines situated next to the 850C red-hot ceramic brakes). However, the system that we assume Mercedes is running is completely inboard, which means that the working temperatures are not expected to be higher than 70 – 85 degrees Celsius, therefore things like the change of the oil bulk modulus due to temperature and pressure do not come much under consideration.
Another, purely mechanical advantage, is that in order to raise the pushrod (as in Lotus’ system) you will need a higher force to counter the “bad” motion ratio of the inclined pushrod, whereas in this system you can have a 1:1 ratio in relation to force Vs wheel movement.
The second figure below shows an iteration of the above system, that utilizes a position-sensitive valve which controls the maximum amount of lift of the nose. This arrangement is implemented to prevent the nose from “overshooting” the desired position, lifting all the way to the end of the suspension travel. Please do not assume that this can be termed as “fully active” because it’s not – the valve can be triggered mechanically. This closing valve can be arranged just in front of the front and rear heave cylinders and can be mechanically activated by the suspension positioning in order to ensure optimal ride height during braking / accelerating as well as preventing the overshoot.
It is fair to assume that Mercedes are probably using two separate systems, cross-linking the left rear to the right front, and the right rear to the left front. The underlying principle and function remain the same but such a system can also account for combined condition (longitudinal + transverse acceleration), helping in conditions where braking is combined with turn-in (i.e. majority of the cases). By fighting the roll as well as the dive, the overall aero platform is more stable and predictable to drive.