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ROAD-TRANSPORT-TECHNOLOGY  2005

ROAD-TRANSPORT-TECHNOLOGY 2005

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Subject:

Increased mass limits for two axle rigid trucks with a rear dual-tyred single axle

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Date:

Mon, 28 Feb 2005 15:58:15 +1000

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Members and Friends of IFRTT


Representatives from the local trucking industry have recently requested
Queensland Transport to revisit its position on increasing the mass limit
for two axle trucks with a rear dual-tyred single axle.  The request is
based upon a discrepancy in both domestic and international mass limits.
Queensland currently allows a limit of 9 tonnes, while Victoria allows 10
tonnes for vehicles fitted with road friendly suspensions.  European Union
limits, while adhering to Directive 96/53/EC, are higher again at up to
11.5t.

While practically all of Queensland's pavements are composed of an unbound
granular pavement with a thin bituminous seal, the principle of pavement
degradation neutrality under RFS is maintained.  Therefore dynamic
performance is viewed as the most prevalent issue, with static roll
threshold (SRT) being of the most concern.

Two reports have been previously commissioned by the National Transport
Commission (www.ntc.gov.au) in assessing the safety performance of these
vehicles at higher mass (Sweatman, 1999 and Pearson/Prem, 2002).  Computer
simulations showed that vehicle configurations which utilise dual-tyred
single axles can have relatively low levels of stability performance under
current single axle mass limits, especially in the case of two-axle rigid
trucks.  The key finding of the computer simulations was that air-suspended
axles at 10.0 tonnes provide stability significantly better than that
provided by mechanically-suspended axles at 9.0 tonnes.  This suggests
that, at face value, refusing an increase would be difficult to justify.
Please note that it was on the basis of these findings that Victoria
allowed the increase.

Queensland and other Australian states have maintained a 9 tonne limit.
While finalisation of Australian Performance-Based Standards for
infrastructure is relevant, Queensland has primarily maintained the 9 tonne
limit on safety grounds.  There are two major reasons for this.  Firstly,
there is no guarantee that RFS roll stiffness would be maintained to the
levels assumed in computer simulation.  Secondly the SRT values for both
9.0 tonnes/mechanical suspension and 10.0 tonnes/air suspension are well
below Australia's performance-based standard of 0.35g (even though
simulation showed an increase in SRT on air suspension).

In summary, the research appears to suggest that these vehicles are poor
dynamic performers.

However, if this in fact the case, the road freight industry is asking why
European Union member states allow limits of up to 11.5t. Therefore, as
part of reviewing literature in preparing a response to industry's request,
we are looking at the following aspects:

1. Are these vehicles over-represented in rollover accidents?  Ideally,
there would be some statistical correlation.
2. Are there more stringent driver competency requirements in the EU (or
other areas with higher limits such as Latin America) to compensate for
increased mass?
3. Are there any vehicle specific dimensional limits (constructed or loaded
height, width and so on) in place to improve the roll stability performance
of these vehicles?

We have already contacted and gratefully received further advice from Peter
Sweatman and Bob Pearson.  We would very much appreciate any advice on any
other studies that you may know of, or any other aspects we should be
covering.

Regards,


Les



Les Bruzsa (Principal Engineer)
Strategic Policy Team
Land Transport and Safety Division
Queensland Transport
PH (617) 3253 4205
Fax (617) 3253 4211



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