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SRT Calculator Certifiers and Users Course Course Outline (morning) >Regulating Size and Weight >Stability related Performance Measures >Derivation of.

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Presentation on theme: "SRT Calculator Certifiers and Users Course Course Outline (morning) >Regulating Size and Weight >Stability related Performance Measures >Derivation of."— Presentation transcript:

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2 SRT Calculator Certifiers and Users Course

3 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

4 Course Outline (afternoon) >SRT Calculator – Advanced Topics in Loading >SRT Calculator – Advanced Topics in Suspensions >Review > Advanced Users Test

5 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

6 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

7 Prescriptive Limits Pros >Simple to regulate >Easy to enforce >Relatively straightforward compliance >Relatively low cost >Usually unambiguous

8 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

9 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

10 Performance Based Standards Examples >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

11 Performance Based Standards Pros >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

12 Performance Based Standards Cons >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

13 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)

14 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

15 Fleet Distribution of SRT SRT Distribution of Fleet Static Roll Threshold (g) Percent

16 Crashed Vehicles Distribution of SRT SRT Distribution of Crashed Vehicles Static Roll Threshold (g) Percent

17 Relative Crash Rate as a Function of SRT Relative Crash Rate vs SRT Static Roll Threshold (g) Relative Crash Rate

18 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

19 Fleet Distribution of DLTR DLTR Distribution of the Fleet DLTR

20 Crashed Vehicles Distribution of DLTR DLTR Distribution of Crashed Vehicles DLTR

21 Relative Crash Rate as a Function of DLTR Relative crash rate vs DLTR

22 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

23 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

24 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

25 SRT Calculator Derivation 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.

26 Static Roll Threshold Determination >Experimentally through a tilt- table test >Analytically by computer simulation >SRT Calculator

27 Tilt-Table Test >Pros >No vehicle instrumentation reqd >No vehicle parameters reqd >Cons >Facility cost >Testing cost Accuracy depends on good test procedures

28 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

29 2D Model – Horizontal Forces

30 2D Model – Vertical Forces

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

32 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

33 Graphical Method (Winkler et al)

34 Graphical Method with Lash (Winkler et al)

35 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

36 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.

37 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

38 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

39 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

40 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

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

42 Validation results Tilt-table tests Tilt table test * Yaw Roll Computer simulation SRT Calculator Generic steel suspension SRT Calculator User-defined suspension ±

43 Validation results Generic Suspensions

44 Validation results User-Defined Suspensions

45 Rollover Example

46 SRT Requirements in Rule >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

47 SRT Requirements in Rule Continued >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

48 Using the SRT Calculator Basics Start the calculator either > On the internet at the LTSA site > Or for certifiers from the Start menu or the desktop icon – SRT Calculator SRT Calculator

49 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)

50 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

51 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)

52 Main Data Entry Page - Tyres >For each axle tyre size and configuration should be selected >Selection affects unsprung mass (standard wheel masses) value and Cg height >Selection determines track width >Calculator does not allow for the effects of low profile tyres as they are not significant

53 Main Data Entry Page – Axle Loads >For each axle group, gross mass and tare mass must be entered >Calculator automatically calculates payload mass and total mass as numbers are entered >Payloads and totals are not correct until all data have been entered

54 Main Data Entry Page – Axle Loads continued >Tare mass values should come from a weighbridge docket or from the manufacturer >The gross mass should be based on either the current RUC value or a higher value specified by the operator >Gross mass should not be the vehicle GVM unless requested by the operator >Distribution of gross mass between axle groups is normally in proportion to the axle group load limits

55 Main Data Entry Page – Load Categories >This is used to determine the payload Cg height >Mixed Freight – Assumes 70% of load mass is in bottom half of load space and 30% in the top half >Uniform Density – Assumes the payload Cg is at the vertical midpoint of the load space. Expects the load space to be symmetric about a horizontal axis. >Other – Requires the user to calculate the vertical position of the payload Cg. This option is not available to level 1 certifiers

56 Main Data Entry Page – Load Geometry >For load types Mixed and Uniform, the load bed and load height are used to calculate the payload Cg > Implicit assumption that the values are constant along the vehicle but > Sloping decks/roofs – use values at longitudinal midpoint (level 1 certifier) > Step decks – can use a weighted average of values based on load mass carried at each level (level 2 certifier) > Anything more complex use load category Other (level 2 certifier)

57 Main Data Entry Page – Load Geometry contd >For load type Other the payload Cg height must be calculated by the user and entered explicitly > A load height value must also be entered but this is only for inclusion on the certificate. It is not used in the calculations

58 Main Data Entry Page – Suspension Data >Suspension type selection > generic suspension data come from reported measurement results and are at the compliant end of the spectrum, i.e. resultant SRT will be lower > user defined requires the user to input suspension parameters. These data must be obtained from the supplier or by measurement and documentary support should be kept. > The user defined option is not available to level 1 certifiers

59 Main Data Entry Page – Suspension Data contd >Suspension track width and lash can be easily measured >Values can be entered for both generic and user defined suspension types >NB: Lash is the movement at the axle not at the spring hanger >Ensure correct units are used

60 Main Data Entry Page – Suspension Data contd >Generic displays the embedded suspension parameter values. These cannot be changed by the user >Two types of generic air suspension > Low roll stiffness type > High roll stiffness type >High roll stiffness type uses the axle as an anti-roll bar. This requires that: > Suspension has beam axle(s) > Trailing arms are rigidly connected to the beam axle(s) >If in doubt assume low roll stiffness type

61 High roll stiffness type air suspension

62 Main Data Entry Page – Suspension Data for User defined >User defined requires suspension parameters to be entered. >Care is required to ensure: > Correct units > Roll stiffness is per axle > Spring stiffness is per spring assuming two springs per axle > Roll centre height is measured from the axle centre with +ve upwards

63 Main Data Entry Page – Calculate SRT >Some error checking is done on data entry but most is done when calculation is initiated >Masses are limited to a maximum Vehicle Axle Index of 1.1. >All input data is checked against upper and lower limits >Equation solver assumes small roll angles (<20°) and this is checked >If SRT is less than 0.35g, the calculator determines the reduced load height or reduced mass needed to achieve 0.35g

64 SRT Results >Calculated SRT is shown >If below 0.35g reduced mass and reduced height to pass is shown >Can use back button to return, modify inputs and recalculate or >Certifiers can login to generate certificate

65 SRT Greater than 0.35g

66 SRT Less than 0.35g

67 Certificate Pages >After login certificate data page >Info required for certificate – has no effect on calculations >Certifier details embedded in personalized copy of software >Generate Certificate button creates a certificate in a format suitable for A4 printing >Certificate includes all input data and hence can be used to replicate results >Attach SRT Cert to LT 400

68 Advanced topics in loading Removable bodies >Eg stock-crates > Option 1: Consider body as part of payload > Option 2: Consider body as part of tare mass >With load category Other option 1 is best >Otherwise need to consider overall effect. Empty sprung mass Cg is assumed to be 0.56m above axle centre for a truck and 1.25m above the axle centre for a trailer. Which option is more realistic?

69 Advanced topics in loading Sloping Load Beds > Determine longitudinal position of Cg > Measure (or calculate) load bed height and load height at this location

70 Advanced topics in loading Variable height decks > Load bed height = Weighted average of the different heights using the proportion of payload mass carried as the weighting > Alternatively can use load category Other and calculate the Cg of the payload explicitly

71 Advanced topics in loading No horizontal axis of symmetry > Use load category Other and calculate payload Cg height

72 Advanced topics in loading Unit Loads > Use load category Other and calculate payload Cg height > Use worst case typical load > Possible approaches include: > Obtain Cg heights from equipment suppliers > Obtain maximum cross-slope capabilityfrom suppliers and calculate Cg height

73 Advanced topics in suspensions Generic suspensions > Parameter values derived from UMTRI factbook and based on measurements but do not represent any actual suspension > Parameters selected to be at the more compliant end of the spectrum and thus give conservative estimates of SRT > Provision for users to enter measured values for suspension track width and axle lash

74 Generic Total Roll Stiffness > Generic steer axle Nm/radian > Generic steel Nm/radian > Generic air (high stiffness) NM/radian > Generic air (low stiffness) NM/radian

75 Generic Suspension Vertical Stiffness > Generic steer axle N/m > Generic steel N/m > Generic air N/m

76 Generic Roll Centre Heights > These are from the ground > Standard wheel approx 0.5m radius > Generic steer axle 0.48m > Generic steel 0.7m > Generic air 0.7m

77 Advanced topics in suspensions User defined suspensions > Must enter suspension make and model for traceability > Three key parameters needed > Composite roll stiffness > Spring vertical stiffness > Roll centre height > To determine these requires sophisticated measurement techniques and analysis > Thus the key data must be provided by the suspension supplier who must take responsibility for its accuracy and validity

78 User defined suspensions Conversions >Composite roll stiffness = auxiliary roll stiffness + roll stiffness from springs >Any two of the above (with spring track width) can be used to calculate the third

79 User defined suspensions Conversions continued > For steel suspensions (with no anti-roll bar) auxiliary roll stiffness is usually relatively small (5-10% of total) > For low roll stiffness air suspensions (trailing arms bushed on axle or no beam axle), the auxiliary roll stiffness is also relatively small > For high roll stiffness air suspensions (trailing arms rigidly clamped or welded to the axle), the auxiliary roll stiffness is high (80% or more of the total roll stiffness)

80 Composite Roll Stiffness > Input value is per axle assuming all axles in the group of equal stiffness > Manufacturer value may be per axle group. If this is the case, halve the value for a tandem and one-third it for a tridem. > Roll stiffness is required in Nm/radian. It may be supplied in in-lb/degree. To convert multiply by 6.47 > Input value is per radian. Supplied data may be per degree. Make sure and convert if necessary.

81 Spring Stiffness > Input value is per spring assuming two springs/axle and all springs of equal stiffness > For one spring/axle suspensions (eg camelback type) halve the spring stiffness values > For unequal stiffness springs, average the spring stiffness. If unequal load share, use load share weightings to calculate weighted average > Vertical stiffness is required in N/m. It may be provided in lb/in. To convert multiply by

82 Roll Centre Height > Input value measured from axle centre not the ground, i.e. independent of tyre size. > Influenced by all linkages in suspension > Determination by measurement is quite complex

83 Advanced topics in suspensions Effects of Parameter Changes > Increased roll stiffness improves SRT > If roll stiffness (relative to load) differs between ends of vehicle, increasing the stiffness of the softer one has more effect > Large axle lash values have a negative impact on SRT > Higher roll centres lead to a better SRT > Improvements of the order of 10-15% are possible with suspension improvements


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