A study was conducted to investigate issues surrounding the fitment of Roll-Over Protective Structures (ROPS) to small (300-600kg operating mass) vehicles currently available on the UK market, primarily for use in amenity, turf care and estate maintenance applications. Of particular interest was the potentially detrimental effect of ROPS installation upon small vehicle lateral stability, and the suitability (or otherwise) of ROPS performance test criteria intended for larger (>800kg) vehicles, for application to smaller vehicles in the 300-600kg mass range.
Computer-based dynamic simulation modelling techniques, validated by practical roll-over trials, were employed to investigate small vehicle lateral roll-over behaviour, with particular respect to the onset of vehicle lateral instability and the energy levels likely to be absorbed by the vehicle Roll-Over Protective Structure (ROPS) during the overturn event.
Vehicle mass was found to have no effect upon lateral stability: primary determining factors being vehicle wheel track (plus tyre) width and Centre of Gravity height. Consideration of operator mass can significantly reduce lighter vehicle lateral stability (side-slope angle at onset of instability) by up to 20%, whereas the addition of an (estimated) ROPS frame was found to reduce lateral stability by only 5.4-8%, depending upon vehicle configuration. The proportion of total roll-over kinetic energy absorbed by the ROPS frame varied between 43 and 61%, and was largely independent of vehicle mass. The remaining energy generated during the roll-over was absorbed by the vehicle wheels/tyres, sliding friction between the vehicle and roll-over/ ground surfaces, and ground surface deformation. Increasing ROPS stiffness reduced the amount of roll-over energy absorbed by the structure and the resultant structural deflection, but increased peak force levels.
Whilst application of the currently-accepted lateral roll-over ROPS performance test criteria (ROPS-Absorbed Energy = 1.75 x vehicle mass) may result in a 15-17% energy over-estimate for the lowest-mass vehicles considered (~300 kg), this criteria appears to become progressively more appropriate as vehicle mass increases towards 600kg. In conclusion, there is insufficient evidence to recommend the application of an alternative energy - mass relationship, even at the lower end of the vehicle mass range considered and, on balance, significantly more evidence to support application of the currently-accepted (larger vehicle) relationship to off-road vehicles in the 300-600kg mass range. Appropriately-engineered ‘conventional’ ROPS solutions would therefore appear to be suitable for application to these smaller vehicles, where a roll-over risk is deemed to exist.
This report and the work it describes were co-funded by the Health and Safety Executive (HSE). Its contents, including any opinions and/or conclusions expressed, are those of the authors alone and do not necessarily reflect HSE policy.
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