Abstract
ABSTRACT Servo hydraulic two-poster is an accelerated structural durability rig for motorcycles like a fourposter in cars. It simulates the vertical wheel loads and evaluates the life in terms of actual kilometres covered on road. However, motorcycles pose a greater challenge for simulation as they are not self-balancing and require an elaborate restraint system to hold them on rig. Further, significant payload (normally more than vehicle weight for small motorcycles) and rider/ pillion movement affect ride (system) dynamics. This causes more complexity in simulation. As a result, although vertical loads are simulated accurately, matching local strains and thereby life is very difficult.
Two/ four-poster rigs are popular as vertical loads dominate fatigue life and are economical as compared to multi-axial rigs. To improve the accuracy of simulation and have a better match of local strains, it is important to identify and separate out the effect of longitudinal and lateral loads. In this paper, a new approach to separate out the vertical wheel loads from the road loads is described.
Simulation was done with the present dead weight restraint system and compared with new target. Fatigue life prediction was used to analyse the simulation results. A minimum correlation to be achieved to ensure good simulation on the rig was arrived at. It was decided that the life with the restraint system should be between 0.75 to 1.25 times that of the target to ensure good simulation. With dead weights, the life was found to be only ~0.25 times the target. Also, the correlation co-efficient was a low 0.3. Even with a “Rescue Randy” (mannequin) as restraint system, it was not possible to meet the criteria for good simulation. Therefore, a new and optimised dummy-rider restraint system based on fatigue life prediction was developed. With the new system, not only was the life on the rig found to be almost matching with that of the target, but also the correlation co-efficient was a high 0.95.
The benefits of this exercise include: 1) With the revised target, local strains match well. 2) No abnormal failures on the rig. 3) Better road to rig correlation.
Keywords: Fatigue life prediction, fatigue testing, road to lab correlation, servo-hydraulic road load simulation, vehicle restraint system