Chapter 8
Conclusions
In our research we analyzed the effects of the drive torque redistribution between the front and the rear axle on the vehicle motion, in order to study the feasibility of a new Active Yaw Control. Moreover we compared it with a traditional ESP and we explored the possibility to make this systems cooperate.
It was concluded that adopting a redistribution of torque strategy we manage to create a contra-cornering yaw moment in a rear-wheel drive car and a pro-cornering yaw moment in a front-wheel drive car. This is possible as the transferred longitudinal and lateral forces influence each other on account of the natural tyre characteristic (ellipse of grip). Then by means of a central coupling opportunely controlled it is possible to correct the vehicle behavior, even if not with the same efficacy in a rear and in a front-wheel drive vehicle.
The results obtained simulating the rear-wheel drive car motion manifest that providing a vehicle with an “Haldex” coupling we are able to govern the car motion such a way to avoid dangerous manoeuvres even if the speed and the steer angle imposed by the driver are very extreme. Nevertheless this innovative stability control system is not as powerful as an ESP. In fact a system acting on the brake system can create much greater pro-cornering and contra-cornering moments. If the car instability is not due just to the excessive speed but it is caused by an external disturbance (an ice spot, a very strong lateral wind, etc.) the redistribution of torque can be not efficient enough. In this cases the ESP is still necessary, but making it cooperate with our system we manage to improve its performance, pushing ahead its limits. Moreover we improve the car performance and the vehicle comfort, limiting the accelerations the passengers are liable to.
With regard to the front-wheel drive car we observed a more stable behavior, also without adopting any Active Yaw Control, with respect to the rear-wheel drive vehicle. To supply the vehicle with the central coupling improves the handling, limiting the natural
understeer this kind of car is subjected to, even if the effects of the redistribution of torque are not as good as in case of rear-wheel drive car. This is due to the tyre behavior. Indeed side forces are less influenced by longitudinal forces if the slip angles are small, rather than if they are large (in an understeering manoeuvre they are inclined to decrease, and the opposite occurs in case of oversteer). The performance and the comfort are improved also in this case.
The results obtained simulating a car equipped with tyres which size is 205/55-R15 are good although the Active Yaw Control set-up is developed for tyres 195/65-R15. In particular the redistribution of torque system is not negatively affected by the different tyre response, and also the ESP exhibits anyhow a good potential. Consequently we can think about the possibility to supply a normal car with our system, indeed if the customer decides to change the tyre type, the stability control system does not became inefficacious or dangerous. Furthermore the fact to be allowed to keep a constant set-up can be an advantage in the competitions as well.
For the reasons mentioned above we think that in particular the results obtained con-cerning the rear-wheel drive vehicle are very interesting. In fact an excessive oversteer is usually considered more dangerous than an understeering behavior and our system can prevent the negative consequences of the high speed. Moreover a rear-wheel drive car is mostly an high performance vehicle and also the effects regarding the increased cornering speed suggest to test GT cars and race vehicles provided with our Active Yaw Control. It could results in a lowered lap time or in any case in an improved handling which makes the vehicle easier to drive. A normal car maybe does not need our system since for a complete stability control it needs in any case an ESP. A central coupling cannot correct in the same vehicle both understeer and oversteer, and that is already a very efficacious system. In addition to supply a vehicle with a central coupling makes the price increase, and it could be acceptable just for a luxury vehicle.
We recommend to make an experimental campaign to check the results obtained by means of the simulations. In particular it is necessary to compare our results with experi-mental acquisitions, to take into account the driver reactions. In fact the driver inputs are governed by a closed loop (man-external environment) but to model the human reactions is very complicated, and it can be confusing.
