- They tend to be much stiffer than a car and this can cause higher crash forces on the occupants - even though the delta-V of the vehicle is probably lower the delta-V experienced by the 4WD occupant is higher (the ultimate example of this effect is the army tank - it is said that a head-on collision between two tanks at 20km/h is likely to be fatal for the occupants).
- Many crashes involve collisions with roadside objects or vehicles or similar or heavier mass so the mass-ratio advantages do not apply.
- 4WDs tend to roll more easily than cars and a rollover crash is more likely to be life-threatening
- Design of life-saving occupant restraint systems is typically less advanced with 4WDs
- 26 May 01 The Age: Ka the cheapest car to run, says RACV ($96 per week compared with $271 for a Landcruiser)
- they reduce the risk of a live (and kicking) animal from crashing through the windscreen - the kinematics don't support this and in some circumstances bullbars may make the situation worse.
- they reduce the risk of vehicle disablement after an animal strike - in some unusual circumstances, with some vulnerable vehicle designs, this might be the case but in other cases a bullbar could make things worse.
- The UK Dept of Environment of Transport has a Discussion Paper on bull-bars.
- TRL has a research report "Assessment and test procedures for bull bars" (for purchase)
- 24 Jan 2001 ATSB: Bull Bars and Road Trauma - new CR report but note that the source of data is not the best for identifying bull bar involvement in accidents.
- May 2009 CASR: The effect of bull bars on head impact kinematics in pedestrian crashes (3.4MB PDF)
Buying a 4WD? Do your homework first
(letter to Editor, Manly Daily 16 Aug 01)Vic Widman's comments (MD 9 Aug) about the safety of
4-wheel-drive
vehicles deserve some clarification. As part of my
professional work I
help
the NRMA and its partners in the assessment of the safety of
vehicles.
As
Vic points out, the NRMA ratings on crashworthiness of
vehicles do show
that some 4WDs perform well at protecting occupants,
compared with most
cars. This
is to be expected given the weight advantage of these
vehicles over
cars.
However, there is a big range in performance amongst 4WDs -
some are
very
poor at protecting occupants. The result is that, on
average, the group
is
no better at protecting its occupants than large and medium
cars. As a
group
however, large 4WDs are much more likely to seriously injure
the
occupants
of other vehicles. This is a concern for road safety
authorities around
the
world.
Other issues to consider when looking at buying a 4WD
compared with a
car
are: increased risk of rollover crashes, increased danger of
young
children
being run over in low speed accidents, the difficulty that
frail and
disabled people have getting into and out of these high
vehicles,
higher running costs (approximately double that of a small
car) and
much greater
environmental damage per kilometre travelled.
Vic runs a great 4WD training course and I can thoroughly
recommend
that
anyone contemplating a 4WD tour should attend that type of
course. I
found
his course very useful preparation for a Cape York tour
where we hired
a
Landcruiser from Cairns. Another motorist in our group who
drove her
4WD
from Melbourne to Cairns spent about the same in running,
servicing and
motel
costs as we spent on airfares and vehicle hire. And we
didn't have that
long,
risky drive back home after the exhausting (bone shaking)
holiday.
For some city people a 4WD is a great idea - all I am
saying
is do your homework
before deciding to buy one and reduce your chance of being
involved in
a
serious accident.
Michael Paine
Crashworthiness
Risk of fatality is related to change in velocity ("delta
V").
For
a two vehicle collision the delta-V is proportional to mass.
Therefore, IN THEORY, a vehicle that has twice the
mass of the other
vehicle should experience half the delta-V. However, IN
PRACTICE
many other factors come into play. Crash
tests
and studies
of
real world crashes suggest that, on average,
four-wheel-drives
are not as safe as large passenger cars even though they are
typically
much heavier
than cars. Some possible reasons:
Comparison of ownership versus rental of a 4WD
Consider a trip from Sydney to Cape York, at the top of Australia. The road distance from Sydney to Cairns is about 2700km and takes about 5 days driving each way. The air trip between Sydney and Cairns takes about 3 hours and costs about $500 per person (discount economy). In Cairns you can hire a Toyota Landcruiser for about $900 per week. Allow 2 weeks for the trip from Cairns to Cape York and back so the rental cost is about $1800. The fuel costs for this part of the trip are the same whether you own or rent the vehicle so they will be ignored.
Air fare for 4 adults = $2000
Rental for 2 weeks = $1800
Total = $3800
The drive from Sydney to Cairns and back will consume about $700 in fuel and tyres. Allow $100 per day for travelling costs such as accommodation (this is very low) so the cost for 10 days on the road is $1000.
Due to the water crossings on the Cape York trip you will need to arrange a major service on return to Sydney. Allow $1500 for this.
So far the trip in our own vehicle has cost $3200. Marginally ahead of renting.
Now here come the crunch. If you owned a Toyota Corolla instead of the heavy Landcruiser you would save about $4000 per year in standing and running costs (based on NRMA estimates for private ownership and 15,000km per year).
Fuel and tyres: Sydney-Cairns return $700
Travelling costs: Sydney-Cairns return $1000
Major service: $1500
Extra annual cost of 4WD compared with small sedan: $4000
Total: $7200
Difference: $3400
In other words, you and your family could just about take a second trip to Cape York (or maybe the Kimberlies) with the money you save by only renting the 4WD when you actually need it!
Plus you haven't spent 10 days of your precious holidays on
the
road just getting to and from the place where the adventure
starts, with
the added risk of a serious crash.
Bull bars
Bull-bars or "crash protection bars" are very popular on commercial and 4WD vehicles in Australia. This situation has resulted largely from myths:Myth Number 1 - Bullbars will protect me in a crash
As mentioned above, modern vehicle designs include a "crumple zone" at the front. This is designed to crush in a controlled manner to cushion the impact and reduce the loads on the vehicle occupants when they are thrown forward. Without this crumple zone the occupants could be exposed to life-threatening loads in a moderate crash. There is also a greater risk that the passenger compartment will collapse around the occupants.
The problem with most bullbars designs is that they bridge this crumple zone with a rigid structure. This means the vehicle cannot crumple in the manner in which it was designed. This exposes the occupants to a much greater risk of injury.
-
.
.
.
.
.
.
In the case of forward control vans the bullbar can
transfer
the
crash forces into the cabin of the vehicle, rather than
the chassis.
..
.
who was trapped by his legs in a car crash" - say no more!
.
.
.
.
.
.
This situation is even worse when life-saving airbags are fitted. Vehicle manufacturers conduct considerable research in order to "tune" the airbag to the crush characteristics of the vehicle. When these characteristics are changed by a bullbar the airbag might be rendered useless because it fires too early or too late.
Myth 2 - Bullbars protect my vehicle from damage in a low speed collision
Tests by the NRMA show that a bullbar can increase the cost of repairs in a typical low speed collision. Firstly the cost of a replacement bullbar must be added to the repair cost. Secondly, the bullbar may transfer the crash forces deeper into the vehicle structure, causing more damage than would occur without a bullbar.
Bullbars might help reduce 'cosmetic' damage in the event of a very light collision, such as a car park accident but in more severe crashes they are likely to increase the cost of repairs. There are, and safer, ways to protect a vehicle form 'cosmetic' damage.
Other highly questionable reasons for fitting bullbars are that
Increased danger to other road users
The biggest concern about bullbars is the effect they have in a collision with a pedestrian, cyclist or motorcyclist. The extra rigidity of the bullbar will cause greater direct injury to these vulnerable road users. Children are particularly at risk because their head or chest is at the same height as the bullbar.
Recent tests by the University of Adelaide found that, in a test simulating a pedestrian's head striking the front of a Toyota Prado at 40km/h, a steel bullbar produced head decelerations typically 5 times greater than the "unprotected" vehicle and injuries would have been fatal. The aluminium bull-bar was not much better than the steel one.
However, a new design of moulded foam plastic bullbar performed exceptionally well - let us hope that this idea takes off! (The paper by the University of Adelaide will shortly be published in the proceedings of a joint Staysafe/SAE-Australasia conference "Developments in Safer Motor Vehicles"). The foam plastic bullbar is made by Team Poly Products Phone +61 8 8326 2256 Fax +61 8 8326 2257. See also the 1997 Australian Design Awards.
Another problem is the way that a pedestrian is typically thrown violently when struck by a vehicle with a bull bar - due to this effect even a low speed collision is more likely to be fatal if a bullbar is fitted.
Sequence from the Video "The Physics of Car Crashes" by the
Roads and Traffic
Authority of NSW.
(Marketing Dept phone 02 92186315, fax 02 92183510)
A bullbar is also likely to cause more injury if the vehicle
to
which it is fitted hits the side of another vehicle. The
impact forces
from a bullbar
will usually be higher up on the struck vehicle, closer to the
chest
and
head of the occupant of that vehicle.
Fuel consumption and pollution
Bullbars affect the smooth flow of air around the front of the vehicle and add weight. Both of these factors will increase fuel consumption and pollution.
It is estimated that a bullbar will increase fuel consumption by about 6% due to these effects. This equates to an extra running cost of at least $150 per year for a typical 15,000 annual kilometres.
Added to this is the financial cost of the bullbar purchase -
say
$50 per year - giving a total extra cost of $200 per year for
the
"privilege" of having a bullbar at the front of your vehicle.
Top of page
Conclusion
Don't fit a bullbar! You will save money and you are less
likely to be involved in a fatal accident.
This web page prepared by Michael
Paine
Last update 1999!
These notes are the author's views and do
not
represent the views or policy of any
organisation.