Shock damping solves wheel restraint issues

To date, most wheel restraint systems have been relatively unsuccessful in keeping the wheels attached to the car in accidents. Dean Palmer talks to a private inventor that thinks he has solved the problem

UK private inventor Nigel McKrill has developed and patented a wheel restraint system for Formula One racing cars that is unique, safer and more effective than any other system to date. And, it can be adapted to almost any type of wheeled vehicle, not just racing cars.

McKrill says his invention relates to vehicles equipped with wheels, most typically motor cars, although the system may be applied to other types of wheeled vehicles that require their wheels to be retained in an accident. And just as important, the system also prevents the risk of injury to the driver from the restrained wheel or wheels rebounding back on their restraining straps and hitting the driver. As McKrill explained to Eureka, “A demonstration of the dangers of a restrained wheel held on a very long restraining strap flailing uncontrollably, were provided recently when the front suspension system on the Jordan [F1] car being driven by Ralph Firmin collapsed at high speed, resulting in the restrained wheel on the end of its long tether, smashing into the cockpit area many times, fortunately this time without injury to the driver.”

McKrill reckons the weight of his wheel restraint system can be engineered to come in at the same weighting already used on Grand Prix cars, and according to McKrill, the new Ferrari is, “allegedly carrying 100kg of such weighting.”

He added that, for many years now, typical racing car suspension systems have been prone to wheel loss in accidents, some incidents causing death or injury to spectators and drivers. And so the sport’s governing body has recently stipulated that wheel systems and their associated appendages must remain with the vehicle in an accident.

McKrill said that such legislation has brought about wheel retention systems that generally utilise a strap or straps, these being attached to each wheel via its hub and the car’s hull or body. So far, according to McKrill, such systems have not worked effectively, hence his invention.

“I wanted to provide a wheel restraint system that was capable of restraining the wheels so that they would remain with the vehicle even when detached from the chassis, and if required, may also pull the wheels back into the car after the initial shock of the accident has been overcome, to prevent the equipment from flailing about in an uncontrolled manner on the end of its restraining tether,” he added.

“Present wheel restraining systems attempt to restrain the wheels by the use of straps which are attached rigidly (to their strap attachment points on the vehicle chassis) with little or no provision being made for the high shock loads occurring in an accident, other than local reinforcement of the strap attachment points.

While McKrill conceded that designs such as these allow the minimum intrusion on the vehicle’s design, this method of installing retaining straps has invariable failed to prevent the separation and loss of wheels and/or suspension systems.

McKrill’s design solves these problems by restraining the wheels via a shock restraint system that incorporates shock load damping using extensible and retractable restraints. This permits a controlled and graduated extension of the restraining system to reduce the shock loadings to the restraint system attachment to the vehicle in an accident sufficient to separate the wheels from the chassis.

This allows the components to remain with the car even when the suspension attachment points are separated from the car. And, the extensible system may be arranged to retract the separated components, selectably or automatically, and for the system to vary its restraint pressure according to the severity of the accident.

The system may be actuated by rapidly changing loads ie negative force or positive force loadings, automatic sensing system or selectively, and may incorporate both compressible and/or incompressible means for extension and retraction.

On McKrill’s racing car mock-up design, the restraint system comprises four shock attenuator assemblies, installed back-to-back, each one being linked by supply lines to a pressure boost accumulator. The pressure boost accumulator is connected to each shock attenuator assembly via supply lines and is arranged to supply increased pressurised operating means to each shock attenuator.

Attached to each of the shock attenuator assemblies are restraint straps which pass through a strap guide to the an attachment point at each wheel. The shock attenuator assembly comprises a cylinder, a piston which is motivationally controlled by pressurised means, a shaft, a port body and fork strap attachment, to which is attached a retaining strap secured by a shear pin. Supply from the pressure boost accumulator is delivered via supply lines to the port.

The supply of increased pressurised operating means delivered by the supply lines is controlled by the port body this being opened by the piston. When this is in the maximum extended position, the abutment of the piston against the port body forces it to rotate on the thread to align the port with the supply port. This allows the flow of increased pressurised operating means from the pressure boost accumulator to be superimposed over the normal operating pressure of the shock attenuator, thereby increasing the pressure to the piston, to increase system retraction ability when the restraint strap extends the shaft sufficiently to activate the port body.

A shear pin is provided which shears at a pre-determined load to prevent breakout of the attenuator assembly in extreme and unforeseen operating circumstances.

Each attenuator may be provided with individual boost supply or may have two sources of boost supply, divided between the four attenuators, or may operate without boost supply or its associated supply mechanisms.

Tom Shelley

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