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SRT Calculator

SRT Calculator

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SRT Calculator

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  1. SRT Calculator Certifiers’ and Users’ Course

  2. Course Outline (morning) • Regulating Size and Weight • Stability related Performance Measures • Derivation of SRT Calculator • Basic Use of SRT Calculator • Test on Basic Use of the Calculator

  3. Course Outline (afternoon) • SRT Calculator – Advanced Topics in Loading • SRT Calculator – Advanced Topics in Suspensions • Review • Advanced Users Test

  4. Dimensions and Mass Rules – Why? • To promote safety • Stability • Manouevrability • Fit on the road • To protect the infrastructure • Road damage • Bridge damage • Fit on the road

  5. Dimensions and Mass Rules – How? Prescriptive Limits • Maximum or minimum mass values • Maximum or minimum dimensions Specify what a vehicle must look like rather than what it needs to be able to do

  6. Prescriptive Limits Pros • Simple to regulate • Easy to enforce • Relatively straightforward compliance • Relatively low cost • Usually unambiguous

  7. Prescriptive Limits Cons • Not directly linked to the safety or infrastructure protection outcome that is intended • Less safe vehicles may still be legal • Cumbersome – lots of rules • Relatively inflexible • Inhibits innovation

  8. Performance Based Standards • Performance Standard = Performance Measure + Acceptance Level • Performance Measure - Some quantity that is measured (or calculated) during a specified set of test conditions. • Acceptance Level – Minimum or maximum level required to pass. This may vary with operating environment • Specify what a vehicle must be able to do rather than what it must look like

  9. Performance Based StandardsExamples • Basic concept is not new • Braking requirements – Stopping distance from 30km/h or a dry sealed surface shall be less than 7m • Turning circle requirements – a vehicle must be able to complete a 360° turn inside a 25m wall-to wall circle

  10. Performance Based StandardsPros • Directly related to the factors that are to be controlled • Allow for innovation and flexibility in vehicle design • Improve industry understanding of vehicle factors that contribute to safety

  11. Performance Based StandardsCons • More complicated and expensive to assess for compliance • More complex to regulate • Risk of reducing safety by encouraging vehicles to the minimum standard • Risk that the set of PBS is not complete

  12. Performance Measures for Stability and Safety • RTAC Study in 1980s to characterise the Canadian HV fleet • Range of measures relating to stability and safety • Static Roll Threshold (SRT) • Dynamic Load Transfer Ratio (DLTR) • Rearward Amplification (RA) • Yaw Damping Ratio (YDR) • High Speed Transient Offtracking (HSTO) • High Speed Steady Offtracking (HSO) • Low Speed Offtracking (LSO)

  13. Rollover Related PMs • SRT – steady speed cornering • Maximum lateral acceleration that a vehicle can withstand before wheel liftoff • DLTR – evasive manouevre stability • Load transfer from one side of the vehicle to the other during a high speed lane change

  14. Fleet Distribution of SRT SRT Distribution of Fleet 20 15 Percent 10 5 0 0.3 0.35 0.4 0.45 0.5 0.55 0.6 0.65 0.7 0.75 0.8 0.85 0.9 0.95 1 1.05 1.1 Static Roll Threshold (g)

  15. Crashed Vehicles Distribution of SRT SRT Distribution of Crashed Vehicles 35 30 25 20 Percent 15 10 5 0 0.3 0.35 0.4 0.45 0.5 0.55 0.6 0.65 0.7 0.75 0.8 0.85 0.9 0.95 1 1.05 1.1 Static Roll Threshold (g)

  16. Relative Crash Rate as a Function of SRT Relative Crash Rate vs SRT 5 4 3 Relative Crash Rate 2 1 0 0.3 0.35 0.4 0.45 0.5 0.55 0.6 0.65 0.7 0.75 0.8 0.85 0.9 Static Roll Threshold (g)

  17. SRT Conclusions • Fleet distribution bi-modal • 15% fleet have SRT < 0.35g • 40% crashed vehicles have SRT < 0.35g • Improving performance of the worst vehicles will have a significant impact on crash rates

  18. Fleet Distribution of DLTR DLTR Distribution of the Fleet 18 16 14 12 10 8 6 4 2 0 0.05 0.15 0.25 0.35 0.45 0.55 0.65 DLTR

  19. Crashed Vehicles Distribution of DLTR DLTR Distribution of Crashed Vehicles 30 25 20 15 10 5 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 DLTR

  20. Relative Crash Rate as a Function of DLTR Relative crash rate vs DLTR 3.5 3 2.5 2 1.5 1 0.5 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7

  21. DLTR Conclusions • Fleet distribution tri-modal • increase in crash rate for DLTR > 0.7 • limited evidence for significant effect of crash rate for lower DLTR • Note that DLTR and SRT are not independent

  22. Levels for PBS • SRT • From crash data 0.4g-0.45g is desirable • Internationally 0.35g minimum is widely suggested • Higher targets affect too many vehicles and have too big an effect on productivity • DLTR • Internationally 0.6 maximum has been suggested but some debate • From crash data 0.67 approximately equivalent effect to 035g SRT in New Zealand

  23. Potential Impact on Crash Rate • 15% of vehicles below 0.35g SRT involved in 40% of rollover crashes • Reducing their crash rate to the average could reduce rollover crashes by more than 25% • SRT and DLTR are related. Improving one will improve the other

  24. SRT CalculatorDerivation and Validation Static Roll Threshold (SRT) Maximum lateral acceleration that a vehicle can withstand during steady speed cornering before the wheels on one side lift off.

  25. Static Roll Threshold Determination • Experimentally through a tilt-table test • Analytically by computer simulation • SRT Calculator

  26. Tilt-Table Test • Pros • No vehicle instrumentation req’d • No vehicle parameters req’d • Cons • Facility cost • Testing cost Accuracy depends on good test procedures

  27. SRT by Computer Simulation • Pros • Cheaper than physical testing • No instrumentation or measurements required • Cons • Detailed vehicle parameters needed • Too costly for routine use • Skilled analysts required to ensure accuracy

  28. 2D Model – Horizontal Forces

  29. 2D Model – Vertical Forces

  30. Simple 2D Rollover Model Solving force and moment balance equations gives a simple equation for SRT

  31. 2D Model Complications • Roll angle, , is the result of all the compliances in the vehicle. It is not simple to determine • Two ends of the vehicle are not necessarily the same. Need to consider the interaction between them

  32. Graphical Method(Winkler et al)

  33. Graphical Method with Lash (Winkler et al)

  34. SRT Calculator Basic Assumptions • Applied to a single vehicle unit with no more than two axle groups • Two axle groups are connected by a rigid body i.e. chassis flex is not taken into account • Suspension stiffnesses are approximated as linear i.e. constant rate but suspension lash is taken into account

  35. SRT Calculator Basic Method • Develop equations for graphical method (see Schedule 1 in Dimensions and Mass Rule 41001) • Equations are piecewise linear. Solve for transition points, checking for validity. • SRT is maximum lateral acceleration for which a valid solution exists.

  36. Vehicle Parameters in Equations • Sprung mass by axle group and Cg height • Unsprung mass by axle group and Cg height • Tyre vertical stiffness • Tyre track width • Suspension vertical stiffness • Suspension roll stiffness • Suspension track width • Suspension roll centre height • Suspension lash

  37. SRT Calculator Software Specifications • User inputs known or easily obtained • Web-based software • Three versions • Public – on internet • Level 1 Certifier – generates compliance certificates for relatively standard vehicles • Level 2 Certifier – generates compliance certificates

  38. SRT Calculator Implementation • Aim to minimise user data input requirements but maintain enough flexibility to represent key vehicle parameters accurately enough • Assumptions on default parameter values are conservative so that actual SRT will be at least as high as calculator result

  39. Calculator Implementation -continued • Vehicle width is assumed to be 2.5m – tyre track width is back-calculated from tyre size and configuration • Generic tyre properties based on size and configuration are used • Standard axle and wheel masses for each vehicle type are assumed • Empty sprung mass Cg height is assumed based on vehicle type • Generic suspension parameters are embedded so that in many cases actual data are not needed

  40. Calculator Validation • Tilt table test on a 4-axle trailer • Comparison with results from Yaw-Roll simulations for a selection of vehicles

  41. Validation resultsTilt-table tests

  42. Validation resultsGeneric Suspensions

  43. Validation results User-Defined Suspensions

  44. Rollover Example

  45. SRT Requirements in Rule 41001 • Principle of Safety at Reasonable Cost • SRT level 0.35g • All heavy vehicles of Class NC and Class TD have to comply except for those on the exempt list

  46. SRT Requirements in Rule 41001Continued • Distinction between compliance and certification • All vehicles listed above must comply • Only vehicles of Class TD with a load height greater than 2.8m need to be certified

  47. Using the SRT CalculatorBasics Start the calculator either • On the internet at the LTSA site www.ltsa.govt.nz/srt-calculator • Or for certifiers from the Start menu or the desktop icon – SRT Calculator

  48. Vehicle Type Choice • Affects default no of axles and tyre configurations but these can be changed • Affects axle mass values and empty sprung mass Cg height which are embedded values • For a semi-trailer only the rear bogey is analysed and it is treated as if it were an independent vehicle (like a simple trailer)

  49. No of Axles • Choosing a vehicle type inserts a default number of front and rear axles. These should be changed if necessary • Some basic error checking is done. Eg a semi-trailer must have zero front axles

  50. Main Data Entry Page • Schematic showing vehicle type and axle configuration selected. If wrong go back. • Data entry boxes have pop-up help on labels (not functioning on Netscape 4)