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30/09/2002
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Tom Shelley uncovers what could be the answer to one of the long-running quests of automotive design
A new differential automatically locks and unlocks output half shafts to maintain maximum traction on the straight while eliminating tyre scuffing on cornering. Derived from a very successful design proven in last year's Formula Three motorsport season, it has the potential to improve greatly on available limited slip designs for road cars, off-road vehicles, trucks and agricultural tractors.
Further development is needed, but this new differential looks to be cheaper to make than existing conventional differentials, let alone limited slip products, requires no electronics and solves an apparently impossible problem in a way that should be of interest to mechanical engineers in many disciplines.
The basic principle of the old and new designs, both developed by motorsport specialist Speed Dynamics and the latter the brainchild of Jeffrey Lucking, one of the partners in the business, is that wheel hubs should normally be locked together. However, whenever the car takes a bend, the outer wheel should be allowed complete freedom to rotate faster, but them come back into lock on the straight.
Almost all racing and rally cars have a system that does this to some degree. Formula One cars, for example, use plate differentials, which press plates together under hydraulic force to limit slip between the two hubs. The products available are often extremely sophisticated, applying more force to press the friction plates more tightly together in response to greater detected torque. And different ramp angles to activate the gripping action and different numbers of friction plates may be selected to suit the conditions expected around different racing circuits.
Lucking describes the setting-up of such systems as "now down to a fine art compromise between wheel spin and understeer resulting from the two hubs being partially locked together".
In traditional designs of agricultural tractors it is, of course, the operator who decides how much force to exert with his or her boot to brake the wheel that is beginning to slip. But more recent and upmarket designs often come with very sophisticated electronically activated traction control systems in which differential locking forms a part. American Army vehicles, including the HMMVV or 'Hummer' use the Torsen Limited Slip Differential. However, not only is this a very intricate device full of worms and wheels but if one set of wheels loses traction completely, the differential is unable to supply any torque to the other set of wheels.
The Speed Dynamics designs, old and now, rise above these drawbacks in a very clever way. However, we are unable to describe the successful F3 design in detail because it remains a commercial product, even though a rule change has banned it from F3 this season. (Was it considered an 'unfair' advantage we wonder?) An indication of how seriously the secrets of its functioning are guarded is that the users do not service them themselves but return them to Speed Dynamics. The policy is underpinned by a warning that if opened by any unauthorised person, the units is ‘unlikely to function for very long’ – an implied threat that Eureka can confirm is far from being an idle one.
Despite the secrecy about the details of the design, some idea of how it works can be deduced from the new concept, which employs no gears but two hubs and sets of spring biased pawls on each hub that interact with each other.
The casing, driven by the engine, has sets of notches on its interior. The pawls on the hubs can either engage on the notches on the casing or be withdrawn clear. Linkages between the two hubs ensure that when one pair of pawls is pushed down, the pawls facing in the opposite direction on the opposing hub are pushed out and engage. Conversely, when one pair of pawls is engaged, the pawls facing in the opposite direction on the opposing hub are disengaged.
The way the pawls interact with each other through a continuously rotating joint is particularly clever. Each pawl has a bell crank that incorporates a ramp, which acts on a push rod. The push rods from each pair of pawls act on one of two concentric slip rings. The rings then act on push rods acting on the opposing direction pawls in the other hub.
This arrangement allows the hubs to be locked together in normal running. But when the wheel on the outside of the corner wants to speed up, torque reverses in such a way as to push the relevant pawls down. This pushes the opposing side pawls out at the same time so that they are in a position to engage with the notches when required.
A simpler device would achieve the same goals were it not for the need to be able to use the engine as a brake during driving, so the differential has to be able to lock the hubs with torque acting in either direction. Lucking says the successful F3 design is based on "exactly the same idea except that, in this case, it is done with rollers".
The new design exists only as a demonstration prototype in aluminium but the intention of making it with pawls and notches as opposed to rollers and friction is to allow the transmission of greater amounts of torque, as might be experienced in a road car. Since it is not based on gears, it looks as if it should be inexpensive to manufacture. And although the concept is protected by patent, the reason it is being shown to people such as us is that the developers are looking for sufficient funds and technical support to turn it into a commercial reality.
Pointers
Locking differential works automatically but requires neither gears nor electronics
It locks instead of applying friction and allows completely free movement when unlocked
Demonstration prototype is untested but is based on principles successfully applied by the same company in F3 motorsport
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Author
Tom Shelley
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Source
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