4WD Starion on RallySport mag
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SpidersWeb
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Ask even a well educated engineer to describe how a car’s differential works in words you or I can understand and he will be lucky to succeed. So, when I try to tell you the systems of controlling differentials on rally cars, please be sympathetic - I will do my best!
Mitsubishi's Starion 4WD Turbo was the first rally car to use a controlled centre differential. Lampi drove it in the Lucky Strike Rally in Malaysia in 1983.Four wheel drive rallying brought a huge new world into motor sport technology, and the subsequent graduation into hydraulically controlled active transmission control one galaxy beyond. Maybe it is easiest to simply say that four wheel drive cars have three differentials.
The most important is the central differential which controls and divides how much of the engine torque is passed to the front and rear axles, and when. The front differential controls the side to side division of the torque which passes to each of the front wheels, and the rear differential the same function at the other end of the car.
The moment that Audi stepped into the competition arena with their Quattro cars in 1981, the work of rally car differentials changed for ever. Life started off at a very basic level, indeed for a long while Audi forsook even a limited slip differential at the front, because the extra weight would only unbalance the already front-heavy car further.
By 1985 Audi were using torque sensitive with Torsen centre differentials, but these wore out badly and so Audi experimented with viscous units instead. In the eighties Group B rally cars were arriving in a variety of different layouts, and it was widely believed that the Peugeot 205T16 had the best inherent layout, and this factor enabled the four wheel drive system to work the best.
The Group B days were times when teams were still trying to discover the directions in which they should be aiming in their work. For example, suspension changes could easily override perceived inadequacies in the transmission systems. Certainly the differentials could not lock themselves up under power as much as the increased torque outputs from engines demanded.
Controlled differential systems came onto the scene with the Mitsubishi Starion four-wheel drive cars in the middle 80's. These cars were never homologated but were seen in various free-formula rallies in 1984. Former driver and current Mitsubishi technical adviser Lasse Lampi remembered it well.
Lasse Lampi and Pentti Kuukkala finished third on the 1983 Swedish Rally in the revolutionary Audi Quattro - the first 4WD rally car to make it to the top of world rallying.“This car used hydraulic systems to control the work of the central differential and were coupled with a sprag clutch system attached to the rear axle. It was a big step forward, as it shifted the torque in overrun situations from the rear to the front. The problem at that time was that the available hydraulic controls were not powerful enough.
“1987 was the time when the VR-4 came along. The VR-4 started with a solid central transmission, as had been used by Audi, and this was then replaced by viscous coupling central differentials. In 1991 we started to use viscous front and centre differentials as well.
“With the Lancer, which arrived in 1993, there was a lot of confusion and a lot of handling issues ensued. As the work of hydraulics were developed, so the need for the sprag type clutch faded away in significance, so that when the Lancer WRC series of cars was designed, this system was abandoned and full permanent four-wheel drive systems are used in the now orthodox way.”
There are three ways in which differentials can be controlled. Firstly by the way mechanical devices are set up; secondly by controls which the drivers can operate from inside the cockpit; and thirdly by automatic ‘closed-loop’ systems, which means actively altering the set ups according to the information from sensors which note the behaviour of the conditions and the car at every moment.
Subaru transmissions sealed by the FIA scrutineers in Mexico. This prevents on-event specification changes.It is this last mentioned system of operation where the state of the engineering art lies. This is the area of high electronics technology, and which the FIA wish to curtail in their quest for reducing the levels of costs in running rally cars in future.
Although originally there were hybrid designs in which both mechanical mechanisms were combined with hydraulic systems, hydraulics are now the sole means by which the differential mechanisms are commanded.
The strangest thing about active transmission systems is that they do not, of themselves, create a great improvement in absolute performance. David Lapworth, technical director at Prodrive, explained.
“Active transmissions offer a lot of advantages but not so much in sheer performance. They help you discover the ideal set-up much quicker, and substantially reduce testing time. You can make radical changes quickly, which is invaluable when you are completely lost in trying to find the correct technical direction in which the setting up process should proceed.
“The problem with passive systems is that you are forced to have the same set-up for all a rally. Testing work with passive systems takes a lot longer. I can see, however, that the lack of need to engage in extensive technical testing of electronic systems could make the idea of entering the sport more attractive to outsiders. But in other respects, the justification of saving costs generally is not appropriate.”
Lasse LampiHaving said that, the advantages of active systems are real. The work of the central differential concerns the dynamic balance of the car, particular to overcome the difficulties at low speeds. This means that actions such as turn-in are much improved.
Second priority is the front differential. This has more relevance than the rear differential because it has also to handle the demands of the steering. Control of the rear differential adds more subtle opportunities to control the wheel torque side to side, especially when the car lifts a wheel.
Transmission development has been one of the most fundamental tasks in recent rally work. Specialists Xtrac and Hewland have led the field in producing the necessary hardware equipment, but the software work is carried out by individual teams. This work has helped change the whole engineering of rallying.
Lapworth: “Twenty years ago everything was subjective and empirical. Try it and see. Nowadays engineering is completely analytical. In the old days a driver came along in testing and suggested how we might try to change the set-up of the car. Now they arrive and simply explain to the engineers the exact behaviour of the car, and the engineers know exactly how to change to counteract the unwanted characteristic.
“The capacity of the datalog systems today is fantastic. It is absolutely unbelievable exactly how much information can now be collected from even the shortest test.”
Things, however, are going to be made to change, by regulation. The 2006 World Championship regulations, which will introduce a lot of new technical limitations in the name of cost savings, are now being finalised.
Lasse Lampi and Tommi Makinen at the Monte Carlo Rally in 1999.The front and rear differentials must be mechanical, so the only control allowed is by torque sensing ramps and plates, and then only before an event. The main effect of all this will be an almost total lack of control, other than by severely increasing the pre-loading when the equipment is originally prepared, for when wheels lift off the ground.
Currently this action can be controlled either by active controls or by the use of viscous differentials, both of which are banned in 2006.
The central differentials can be controlled in a limited number of ways. No longer will sensors collect data by monitoring engine behaviour, speeds of each wheel, acceleration (longitudinal and lateral), though certain sensors, for instance to measure driveshaft torque, will still be permitted.
It has been information in the past from each of these systems which are fed into an ECU, which then dictates how the transmission is to react and produce the desired performance. When it was realised that without handbrake mechanisms being available through the central differential, driving round tight corners on asphalt roads would present insuperable difficulties, the FIA allowed controlled systems to the extent that the handbrake for turning the car, but again viscous couplings are banned.
The controls therefore will be far less comprehensive, and very little use will now be made of hydraulics. The first reaction to the effect of the changes is that there will now be far more search for mechanical grip.
Mitsubishi's Starion 4WD Turbo was the first rally car to use a controlled centre differential. Lampi drove it in the Lucky Strike Rally in Malaysia in 1983.Four wheel drive rallying brought a huge new world into motor sport technology, and the subsequent graduation into hydraulically controlled active transmission control one galaxy beyond. Maybe it is easiest to simply say that four wheel drive cars have three differentials.
The most important is the central differential which controls and divides how much of the engine torque is passed to the front and rear axles, and when. The front differential controls the side to side division of the torque which passes to each of the front wheels, and the rear differential the same function at the other end of the car.
The moment that Audi stepped into the competition arena with their Quattro cars in 1981, the work of rally car differentials changed for ever. Life started off at a very basic level, indeed for a long while Audi forsook even a limited slip differential at the front, because the extra weight would only unbalance the already front-heavy car further.
By 1985 Audi were using torque sensitive with Torsen centre differentials, but these wore out badly and so Audi experimented with viscous units instead. In the eighties Group B rally cars were arriving in a variety of different layouts, and it was widely believed that the Peugeot 205T16 had the best inherent layout, and this factor enabled the four wheel drive system to work the best.
The Group B days were times when teams were still trying to discover the directions in which they should be aiming in their work. For example, suspension changes could easily override perceived inadequacies in the transmission systems. Certainly the differentials could not lock themselves up under power as much as the increased torque outputs from engines demanded.
Controlled differential systems came onto the scene with the Mitsubishi Starion four-wheel drive cars in the middle 80's. These cars were never homologated but were seen in various free-formula rallies in 1984. Former driver and current Mitsubishi technical adviser Lasse Lampi remembered it well.
Lasse Lampi and Pentti Kuukkala finished third on the 1983 Swedish Rally in the revolutionary Audi Quattro - the first 4WD rally car to make it to the top of world rallying.“This car used hydraulic systems to control the work of the central differential and were coupled with a sprag clutch system attached to the rear axle. It was a big step forward, as it shifted the torque in overrun situations from the rear to the front. The problem at that time was that the available hydraulic controls were not powerful enough.
“1987 was the time when the VR-4 came along. The VR-4 started with a solid central transmission, as had been used by Audi, and this was then replaced by viscous coupling central differentials. In 1991 we started to use viscous front and centre differentials as well.
“With the Lancer, which arrived in 1993, there was a lot of confusion and a lot of handling issues ensued. As the work of hydraulics were developed, so the need for the sprag type clutch faded away in significance, so that when the Lancer WRC series of cars was designed, this system was abandoned and full permanent four-wheel drive systems are used in the now orthodox way.”
There are three ways in which differentials can be controlled. Firstly by the way mechanical devices are set up; secondly by controls which the drivers can operate from inside the cockpit; and thirdly by automatic ‘closed-loop’ systems, which means actively altering the set ups according to the information from sensors which note the behaviour of the conditions and the car at every moment.
Subaru transmissions sealed by the FIA scrutineers in Mexico. This prevents on-event specification changes.It is this last mentioned system of operation where the state of the engineering art lies. This is the area of high electronics technology, and which the FIA wish to curtail in their quest for reducing the levels of costs in running rally cars in future.
Although originally there were hybrid designs in which both mechanical mechanisms were combined with hydraulic systems, hydraulics are now the sole means by which the differential mechanisms are commanded.
The strangest thing about active transmission systems is that they do not, of themselves, create a great improvement in absolute performance. David Lapworth, technical director at Prodrive, explained.
“Active transmissions offer a lot of advantages but not so much in sheer performance. They help you discover the ideal set-up much quicker, and substantially reduce testing time. You can make radical changes quickly, which is invaluable when you are completely lost in trying to find the correct technical direction in which the setting up process should proceed.
“The problem with passive systems is that you are forced to have the same set-up for all a rally. Testing work with passive systems takes a lot longer. I can see, however, that the lack of need to engage in extensive technical testing of electronic systems could make the idea of entering the sport more attractive to outsiders. But in other respects, the justification of saving costs generally is not appropriate.”
Lasse LampiHaving said that, the advantages of active systems are real. The work of the central differential concerns the dynamic balance of the car, particular to overcome the difficulties at low speeds. This means that actions such as turn-in are much improved.
Second priority is the front differential. This has more relevance than the rear differential because it has also to handle the demands of the steering. Control of the rear differential adds more subtle opportunities to control the wheel torque side to side, especially when the car lifts a wheel.
Transmission development has been one of the most fundamental tasks in recent rally work. Specialists Xtrac and Hewland have led the field in producing the necessary hardware equipment, but the software work is carried out by individual teams. This work has helped change the whole engineering of rallying.
Lapworth: “Twenty years ago everything was subjective and empirical. Try it and see. Nowadays engineering is completely analytical. In the old days a driver came along in testing and suggested how we might try to change the set-up of the car. Now they arrive and simply explain to the engineers the exact behaviour of the car, and the engineers know exactly how to change to counteract the unwanted characteristic.
“The capacity of the datalog systems today is fantastic. It is absolutely unbelievable exactly how much information can now be collected from even the shortest test.”
Things, however, are going to be made to change, by regulation. The 2006 World Championship regulations, which will introduce a lot of new technical limitations in the name of cost savings, are now being finalised.
Lasse Lampi and Tommi Makinen at the Monte Carlo Rally in 1999.The front and rear differentials must be mechanical, so the only control allowed is by torque sensing ramps and plates, and then only before an event. The main effect of all this will be an almost total lack of control, other than by severely increasing the pre-loading when the equipment is originally prepared, for when wheels lift off the ground.
Currently this action can be controlled either by active controls or by the use of viscous differentials, both of which are banned in 2006.
The central differentials can be controlled in a limited number of ways. No longer will sensors collect data by monitoring engine behaviour, speeds of each wheel, acceleration (longitudinal and lateral), though certain sensors, for instance to measure driveshaft torque, will still be permitted.
It has been information in the past from each of these systems which are fed into an ECU, which then dictates how the transmission is to react and produce the desired performance. When it was realised that without handbrake mechanisms being available through the central differential, driving round tight corners on asphalt roads would present insuperable difficulties, the FIA allowed controlled systems to the extent that the handbrake for turning the car, but again viscous couplings are banned.
The controls therefore will be far less comprehensive, and very little use will now be made of hydraulics. The first reaction to the effect of the changes is that there will now be far more search for mechanical grip.
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