SRT Calculator User Guide M A I N R O A D S W E S T E R N A U S T R A L I A SRT
SRT Calculator User Guide M A I N R O A D S W E S T E R N A U S T R A L I A SRT Calculator User Guide TERNZ Ltd PO Box 11029, Ellerslie Auckland 1542, New Zealand Phone +64 9.579 2328 SRT Calculator User Guide T able of Contents T able of Contents ........................................................................................................................... i Background ................................................................................................................................. 2 Performance Measures and Standards ........................................................................................ 3 Static Roll Threshold (SRT) ........................................................................................................ 6 Using the SRT Calculator ........................................................................................................... 9 Examples .................................................................................................................................... 19 SRT Calculator User Guide 2 Background Why prescriptive dimensions and mass regulations are not always enough. Heavy vehicle size and weight regulations exist to try to achieve a number of objectives including: • Ensuring a fit between the vehicle and the infrastructure it will operate on • Protecting the infrastructure from excessive wear or damage • Ensuring the vehicle is safe. The traditional approach to regulating size and weight has been through prescriptive requirements. These have the advantage that they are relatively easy to understand and thus to comply with. Equally, enforcement is straightforward as only simple apparatus such as a tape measure and weigh scales are needed to check compliance. For some aspects of the objectives, there is a clear direct link between a prescriptive requirement and the desired outcome. For example, the maximum height limit of 4.3m means that road controlling authorities can design overhead structures on roads with a clearance of 4.3m plus some tolerance value and be confident that all legal vehicles will fit underneath it. However, for other aspects of the objectives the link between the prescriptive requirements and the desired outcome is less clear cut. For example, the various vehicle length restrictions on the units making up a combination vehicle influence the off-tracking and hence the lane width requirement on curves and corners, but it is not a simple matter to convert from one to the other. One of the major safety issues for heavy vehicles is stability and in particular how easy it is to control the vehicle and to avoid rollover. Although prescriptive dimensions and mass requirements can be used to promote improved stability, there are many factors other than size and weight that influence stability. It is not practical to use prescriptive size and weight limits to ensure stability. For example, the steady speed rollover stability of a vehicle depends on the centre of gravity height, the track width and the compliance in the tyres, suspensions and other parts of the vehicle structure. If the size and weight limits were set so that all vehicles (including those with very compliant suspensions) would always have adequate rollover stability, this would unnecessarily reduce the load carrying capacity of the better vehicles and significantly degrade the efficiency of the road transport system. The solution, therefore, is to have some measure or measures which quantify the vehicle’s stability and impose a requirement that vehicles meet some minimum standard for these measures. This is the basis of the performance-based standards approach. SRT Calculator User Guide 3 Performance Measures and Standards What are performance measures and why are they useful. A performance measure is a defined quantity that is measured during some standard prescribed manoeuvre or test condition. Generally the manoeuvre or test condition reflects some typical in-service operating situation. A performance standard is a performance measure together with some level or levels of what constitutes acceptable performance. Although the use of performance measures and standards for controlling heavy vehicle stability is a relatively recent development with most of the measures being developed since the 1980s, the use of performance standards for some aspects of vehicle regulation is well established. For example, Table 1 in ADR 35 on commercial vehicle brake systems specifies test speeds and deceleration levels that must be achieved for various braking tests. Similarly there are regulatory requirements for turning circle based on performance. Compared to prescriptive requirements, performance standards are generally more complex to check both for initial compliance and for enforcement. However, the link between the performance measure and the in-service vehicle requirements is much more direct than is the usually the case with prescriptive requirements. For example, although the prescriptive length restrictions on vehicles do try to ensure that the vehicles have adequate manoeuvrability, satisfactory performance is not guaranteed. However, the turning circle regulation that requires a vehicle to be able to complete a 360º turn within a 25m diameter wall-to-wall circle much more directly ensures a level of capability. In the area of rollover stability there are two key performance measures that have been developed. These are Static Roll Threshold (SRT) and Dynamic Load Transfer Ratio (DLTR)1. SRT reflects the stability of the vehicle during steady speed cornering and is the lateral acceleration required to cause all the wheels on one side of the vehicle to lift off the ground. At this point the vehicle is on the verge of rollover. DLTR reflects the stability of the vehicle during an evasive manoeuvre. To determine this measure the vehicle is required to execute a specified lane change manoeuvre at highway speed and the load transfer from one side of the vehicle to the other as a proportion of the total load is determined. The maximum value of this ratio is the DLTR. The value of DLTR is always between 0 and 1 where 0 is no load transfer and 1 is complete load transfer onto one side of the vehicle i.e. rollover. There are some variations in the specification of the lane change manoeuvre that is used to assess DLTR and the value of DLTR is quite sensitive to which manoeuvre is used and to how closely the path is followed. These two measures between them reflect the most common situations where rollovers crashes occur. It should be noted that they are not independent. When a vehicle executes a lane change manoeuvre, there is a whipping effect so that the rear unit of the combination experiences more lateral acceleration that the front unit. How close the rear unit gets to rollover is related to its SRT. Thus the DLTR is related to both the magnitude of the whipping effect and the SRT. A study undertaken in New Zealand in 19992 showed a clear relationship between rollover crash risk and stability as characterised by the performance measures. Figure 1 shows the relationship between rollover 1 Some variations on these names have been used by other authors. Common alternatives are Static Roll Stability (SRS) and Load Transfer Ratio (LTR). There are also some variations in the definitions of the measures. 2 Mueller, T.H., de Pont, J.J. and Baas, P.H. Heavy Vehicle Stability versus Crash Rates. TERNZ Research Report prepared for LTSA, July 1999 available on-line at http://www.ltsa.govt.nz/publications/docs/Stability.pdf SRT Calculator User Guide 4 crash risk and stability performance as characterised by SRT. It can be seen that the poorest performing vehicles (those with an SRT < 0.3g) have four times the average rollover crash rate. Fifteen percent (15 %) of heavy combination vehicles in New Zealand did not meet the 0.35g SRT target. These vehicles represented 40% of the single vehicle loss-of control and rollover crashes. Clearly improving the performance of the poorest performing vehicles in the fleet should generate a substantial reduction in rollover crashes. Fleet Rollover Rate 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 <= 0.3 0.3 < <= 0.35 0.35 < <= 0.4 0.4 < <= 0.45 0.45 < <= 0.5 0.5 < <= 0.55 0.55 < <= 0.6 0.6 < <= 0.65 0.65 < <= 0.7 0.7 < <= 0.75 0.75 < <= 0.8 0.8 < <= 0.85 Static Roll Threshold (g) Relative Crash Rate Figure 1. Relationship between rollover crash risk and SRT. A similar relationship exists between DLTR and crash rate although in this case a higher DLTR reflects an increase in crash risk. This is not unexpected because of the inter-relationship between SRT and DLTR. SRT and DLTR can be determined by instrumenting vehicles and taking measurements during a physical test. However, this is relatively expensive (especially for DLTR where the instrumentation requirements are extensive) and thus is impractical for general application to all vehicles in the fleet. An alternative approach, which has been used in Australia for Performance Based Standards (PBS) assessments, is to use computer simulation modelling to evaluate the performance measures. Computer simulation is generally requires a substantial amount of component data which may be difficult to obtain. It also requires an analyst with a degree of expertise and experience to ensure accurate results. Although generally cheaper than physical testing, computer simulation is still too expensive to be a requirement for all vehicles. However, a good estimate of SRT can be obtained with the SRT calculator. The input quantities that are required are easily obtained. For vehicle parameters that are more difficult to obtain default generic values can be used. These are generally at the poorer end of the performance spectrum (in terms of the resulting SRT) and thus will result in a conservative estimate uploads/Management/ srt-calculator-user-guide.pdf
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Licence et utilisation
Gratuit pour un usage personnel Attribution requise- Détails
- Publié le Aoû 21, 2021
- Catégorie Management
- Langue French
- Taille du fichier 0.4167MB