All torque AND action
A power transmission technology developed for performance cars is now being applied to military and civilian off-road vehicles. Tom Shelley steers his way through its intricacies
Add two clutches and a double epicyclic gear train to a standard differential and what do you have? The means to control the amount of torque being delivered to two output axles. And these can be axles to wheels on the two sides of the vehicle or propeller shafts driving front and rear axles.
The patented technology, developed by Ricardo and called ‘Torque Vectoring’, has been shown to deliver additional cornering power to a performance car being driven under extreme conditions. Now the developers are seeking to apply it to improve handling and manoeuvrability of special purpose vehicles. Applied to a multi axle military or civilian truck or armoured personnel carrier, it is anticipated that it will enable improved steering and, at low speeds, a marked reduction in turning circle by imposing a controlled amount of skid steering on top of that available from the conventional axle steering. It could also empower the vehicle to make a skid steer spin on the spot.
According to Ben Reynolds, Ricardo’s chief programme engineer, military applications driveline and transmission systems: “We have been involved in torque vectoring since 2003. R&D came up with the idea of an epicyclic gear train that would modulate the output speeds of a differential.”
The differential provides a way of inducing and controlling driving torque and traction force differences between wheels.
“This applies a yaw moment to the vehicle, which can be managed by a control system to increase response to steering inputs, or stabilise the vehicle back on track if it starts to oversteer,” he says.
The system was first applied to the centre differential of a four-wheel drive BMW X5. It was therefore not used to apply a yaw moment directly, but indirectly by managing the relative amounts of torque applied to the front and rear axles, so as to control the balance between understeer and oversteer. “It’s effect was extremely noticeable, if you pushed the vehicle hard so that the tyres were heavily loaded sideways,” adds Reynolds. This is not the same as the electronically controlled systems offered on less upmarket four-wheel drives and SUVs, he comments, which effectively have front wheel drive, with clutches to bring in the rear wheel drive as well when required.
In the Ricardo design, there is a conventional differential, plus two nested, hydraulic, wet, multi plate clutch packs and a double epicyclic gear train on one side. One clutch vectors torque to the left hand side, the other to the right hand side. Other designers and companies have come up with systems designed to achieve similar goals, but, says Reynolds: “One of the best aspects of our system is the unconnected pinions, which float freely and share the load nicely”.
There are many variations in the way the system could be applied in practice. For example, actuation could be provided by: an existing external hydraulic supply, a local externally mounted hydraulic power pack, a self-contained hydraulic system with smart actuator functionality or an electromagnetic system.
The system, which can be built into transaxles and rear differential modules, in place of limited slip devices, is currently on the rear axle of an Audi A6 as a demonstration vehicle. Ricardo is also involved in a programme with two sports car trans axles for an unnamed client and is looking at licensing the technology for road cars.
But for off-road civilian and military system, the company says it is still at an exploratory stage where it wants to “talk to customers and come up with best solutions”.
There is no doubt a need for something that could continuously adjust the amount of torque being applied to the wheels on the two sides of a vehicle to help keep it on a straight line over rough and slippery ground (especially when the wheels get into ruts), improve steering control and reduce the risk of rollovers, always a problem with vehicles moving over irregular ground at speed, with high centres of gravity.
There is also a need for a solution that would allow wheeled vehicles to make sharp turns or spin round on the spot. This is particularly the case for military vehicles, where threats can come suddenly from unexpected directions - and at a time when designers are coming up with armoured 6x6, 8x8 and even 10x10 vehicles that are simply unable to get round tight corners, even with all but one axle being steered.
In the civilian sector, there are a few vehicles, mostly small loaders with front and rear wheels very close together, that have no conventional steering at all, but rely on varying wheel speeds so as to effect what is termed ‘skid steering’.
Agricultural tractors generally have the means to apply brakes separately to each rear wheel, both to suppress wheel spin and to help them get round corners. Tracked construction equipment often has separate hydrostatic drives for each track, which allows the tracks to be run in opposite directions when required. Hybrid technologies, too, potentially offer the means for easily reversing wheel or tracks by switching motor current direction on one side. However, current thinking is that this is not a very cost or weight effective solution, and it is better to have one set of central driving motor (or motors) and gear trains and/or steering motors to direct torque to one side or the other.
Ricardo has a number of solutions to this problem, it say, with torque vectoring itself providing the central means of delivering substantial benefits. Using a torque vectoring differential, for example, a driver could spin the vehicle round by selecting a ‘pivot’ command, which would open an axle clutch and close an axle brake. He would then only have to select first gear to turn the vehicle clockwise or reverse gear to turn it anti-clockwise. On switching to ‘normal’, the axle brake would open and the axle clutch close.
Some of the other possible solutions require separate propeller shafts, while others demand special gear trains that would only be needed for the spin-on-the-spot manoeuvre. While the torque vectoring differential is moderately complicated, the other solutions look to be even more so - and expensive.
* System adds on to a conventional differential
* It has two wet clutches and a double epicycle gear train on one side
* Its function is to control the amounts of torque being delivered to two output axles
* These can be wheel axles on the two sides of the vehicle or sections of propeller shaft delivering torque to front and rear axles
* The technology has been demonstrated as effective on high-performance road cars, and is now being applied to off-road and military vehicles
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