Publication

Connected to the road with the Direction Sensitive Locking Differential (DSLD).

The topic of this project is the bigger picture of vehicle dynamics and handling characteristics of cars and more specifically about highlighting a hidden potential to significantly improve the driving safety of cars to decrease the number of serious road accidents.

The main outcome is the verification of the substantial improvements to the directional stability of passenger cars made possible by the preemptive use of the semi-active differential DSLD. The proving ground results show that the stability improvement significantly surpasses the improvement provided by the brake based Electronic Stability Control system (ESC).

Stemming from the obvious need to enable low speed maneuvering, the open differential was developed already at the dawn of the automotive era and it has ever since, maintained a position as the default solution for all driving situations. However, since then highways have emerged, and the travelling speeds have increased substantially which means, although the automotive industry has not yet realized this, that the routinely made assumption that the open differential is the best solution for virtually all driving situations in not really valid. There is however, a general understanding that there is a fundamental compromise between high speed stability and low speed maneuverability. Despite failing to realize this mentioned misconception, the automotive industry has made great progress in making cars with better and safer road manners, partly due to more advanced suspension systems etc. but in recent decades even more so by introducing safety systems like ESC that have been facilitated thanks to the development of modern sensors and computing power. One of the main reasons for the commercial success of ESC-systems, next to the fact that it has proven to save lives, is that it utilizes already existing hardware such as the braking system, including the ABS hardware albeit supplemented by a few extra sensors and components. This means that the add-on cost has been kept low enough to be justified when compared to the proven safety benefits.

Another way of accomplishing similar and even more effective stability gains would be to properly utilize active differentials. Within the industry the electronically controlled limited slip differential (eLSD) has been developed. However, due to the increased hardware cost the big commercial breakthrough for eLSDs has not happened yet. Also, the way in which eLSD have been controlled up until now means that their role has been more or less limited to improve other aspects of vehicle dynamics. ESC systems have also been regarded as more safety critical which means that the ESC system have been mandated to take precedence in critical driving maneuvers such as evasive maneuvers at speed.

However, as proven in this project, a more preemptive use of the stabilizing properties of locking a differential such as eLSD or DSLD, significantly postpones the need for ESC interventions and in most situations even render the help from a brake-based stability control system unnecessary. The results even show that the stability improvements substantially surpass those achieved with the ESC system.

Finally, all of this can be achieved in spite of and thanks to less complicated and faster reacting feed forward control strategies. In addition, the direction sensitiveness of the DSLD gives it unique properties making the control task even simpler than for eLSD systems. This means that the added cost of the DSLD, both regarding its control needs and its hardware, is significantly lower compared to current eLSD systems. The use of the DSLD and the suggested more preemptive feed forward control will transform the medium to high speed behavior of road cars, making them extremely stable and at the same time allow for tuning the cars for better low speed maneuverability. Virtually giving two cars in one.

For the automotive industry this is a game changing opportunity to circumvent the abovementioned fundamental compromise in vehicle design. Provided that the industry takes full benefit of this opportunity, future drivers and passengers will enjoy cars that are fun to drive and at the same time will have greatly improved medium to high speed stability characteristics meaning that road-users will run significantly lower risks of being involved in serious accidents.