Hi Les

Just a couple of minor comments from New Zealand.  First, please note our
drive axle limit is 8.2 tonnes so we have no experience of the higher load
limits you are considering but we do have a minimum SRT requirement in place
for all larger heavy vehicles including these.  

Two-axle trucks are not over-represented in our rollover statistics.  Most
rollovers (more than half) are the result of excessive speed through curves.
A further qurter or so of rollovers are the result of the vehicle going off
the edge of the seal onto soft shoulders etc. Consequently rollovers tend to
occur predominantly in open road situations rather than urban situations.
Two-axle trucks are used largely for local delivery functions with larger
combination vehicles being used for line-haul and other longer distance
operations.  Hence two-axle trucks do not have the same exposure to risk for
the most common types of rollover.

The increased roll stiffness of the RFS usually comes from a stabiliser bar
or similar device. The axle itself may act as a stabiliser bar.  The RFS
itself is usually less vertically stiff that the suspension it replaces and
without a stabiliser bar or equivalent would be less roll stiff and could
have lower rollover stability.  This raises two points.  Firstly there is no
need to have a stabiliser bar in order for a suspension to qualify as an RFS
and there are air suspension systems that have lower roll stiffness than
typical steel suspensions. Secondly the on-going roll stiffness of a
stabiliser bar in service is likely to be quite reliable.  Wear in the
bushes may introduce some lash but the underlying roll stiffness should be
maintained.  

My final comment is that in my experience with SRT calculations, the SRT of
a vehicle is determined by the front and rear suspensions of the vehicle
working together and the best performance is achieved when their roll
stiffness relative to the load they are carrying is approximately equal.
Generally speaking steer axles suspensions are much less stiff
proportionately than the drive axles and SRT can often be improved the most
by increasing the roll stiffness of the steer axle.  Modern European
vehicles often have substantial stabiliser bars fitted on the front
suspensions to provide this increased roll stiffness.

The New Zealand SRT requirement determines the relationship between load
height and gross mass to achieve the minimum acceptable SRT.  Thus less
stable vehicles are load height restricted at higher gross masses.  Thus a
particular two-axle truck may be allowed a load height of 4m when the gross
mass is 12 tonnes but only 3.2m when its gross mass is 15 tonnes. (this is a
made-up example and I have not configured a vehicle with these particular
characteristics).

I hope this information helps.

Regards,

John   

-----Original Message-----
From: Technical, operational and economic aspects of road freight
transportation [mailto:[log in to unmask]] On Behalf
Of [log in to unmask]
Sent: Monday, 28 February 2005 6:58 p.m.
To: [log in to unmask]
Subject: Increased mass limits for two axle rigid trucks with a rear
dual-tyred single axle


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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