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Technical Bulletin

TB 2020-2

Rigid Drawbar Tandem Axle Converter Dollies

Specification Guidelines

Introduction: In May 2018, the Australian Trucking Association’s (ATA) Industry Technical Council embarked on an exciting engineering ‘proof of concept’ project, exploring further development of a rigid drawbar convertor dolly.

The project was the result of member discussions regarding the dynamic issues with hinged drawbar convertor dollies, especially brake re-activity and tyre wear and other experiences with air suspension dollies. Consequently, there has been some trending by operators to mechanical suspensions where braking re-activity seems to be less obvious but does not eliminate nor the solve the issues.

Development: As a result of ITC membership partnering, a “proof of concept” converter dolly was built and displayed at the 2019 Brisbane Truck Show.

Following the Brisbane Truck Show the converter dolly was trialled by ATA members and customers of the supply partners with very positive feedback on performance and handling.

Suspension: A suspension with neutral roll steer at nominal ride height is preferred (ie: the trailing arm eye bolt and the axle centre are aa near as possible to the same height) with one (1) height control valve (HCV).

A rigid drawbar MUST only be fitted with a single height control valve, for access to valve the rear axle is likely the preferred. Coupling: V-rating design requirement increases with GTM (gross trailer mass) and decreases with drawbar length and must be calculated in accordance with AS 2213.1 Section 7. V=5xGTM/L.

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The graph (below) is based on a GTM of 17tonnes. For drawbar length in the range of 3000mm to 5500mm.

NOTE: Applicable to TANDEM AXLE CONVERTER DOLLY - ONLY

The coupling type will be typically be limited to those displayed below.

Flanged drawbar eye

Special Flanged drawbar eye

Known suitable 50mm towing eyes are available from: BPW Transpec (Ringfeder); JOST (Rockinger); and SAF-HOLLAND (V.Orlandi) Drawbar Length: This will largely be determined by combination/application. ADR 63/00 limits converter dolly drawbar length (for use in a road train combination) to ≥ 3000mm and ≤ 5000mm; however, there is no dimensional restriction on a converter dolly approved as part of a PBS combination, the graph has been extended to 5500mm to assist PBS considerations. Note: The 35m Modular A-double proposal recommends a drawbar length within the range of ≥ 3.95metres and ≤ 5.0metres.

A Rigid Drawbar Converter Dolly in a 30metre PBS A-double Combination

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16

18

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30

3000 3500 4000 4500 5000 5500

Coup

ling

Requ

ired

V-ra

ting

(kN

)

Theoretical Drawbar Length (mm)

AS 2213 V-rating

GTM 17

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Axle group spacing: To limit static vertical loading at the tow coupling of a tandem axle converter dolly (when crossing an obstacle such that one axle may be clear of the ground contact) the axle spacing range needs to be reduced as the drawbar length reduces.

Based on a reference design parameter of a maximum static coupling vertical loading (+/-) of 40kN, the axle spacing needs to be in accordance with the following graph (blue line):

For drawbar lengths ≥ 3500mm, a maximum axle spacing of 1450mm is recommended: The ATA-ITC Project converter dolly has a drawbar length of 4100mm and axle spacing of 1350mm. Fifth Wheel: The fifth wheel should be positioned on the theoretical centreline of the axle group and be free for fore/aft articulation (like the specification as would be used on a prime mover).

12501450

16501850 1950 1950

1250 1350 1350 1350 1350 1350

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3000 3500 4000 4500 5000 5500Max

imum

axl

e gr

oup

spac

ing

(mm

)

Theoretical drawbar length (mm)

Axle Spacing versus Drawbar Length - 40kN Static Vertical

Calculated

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