RV FEATURE
Caravans and their tow vehicles rarely jack-
knife and roll over. But when they do, the
consequences are severe.
While countless ‘explanations’ are rife, the
cause has long been known. It is not how rigs
behave in normal driving; it’s all to do with how
they behave in an emergency swerve, or when
subject to a high side wind gust. Excess speed,
however, is always a major factor.
When cornering or swerving hard, or hit by a
strong side wind gust, a vehicle does one of
two things. It will either take up a slightly wider
radius turn or, unless driver-corrected, it will
increasingly tighten up that turn and ultimately
jack-knife. These two conditions (see fig 1) are
called understeer (good) and oversteer (bad).
OVERSTEER
A caravan rollover results when its tow vehicle
is triggered into oversteer, particularly if at
speed. It is that simple. Its cause (and why
fifth wheelers are not affected) is tow hitch
overhang.
Early (pre-1920) heavy transport used caravan-
style trailers towed via a truck’s overhung
hitch. This worked well enough at the very low
speeds of that era but once they exceeded
about 30 km/h, such rigs increasingly jack-
knifed and rolled over. As speeds increase, so
do rollovers.
Consequent trials showed the issue could
only be totally resolved by eliminating hitch
overhang: i.e., locating the hitch directly over
the tow vehicles’ rear axle/s. The fifth wheeler
was thus born.
UNDER STEER
UNDER STEER
A conventional caravan and
tow vehicle can, if
the basic constraints
are understood (or
at least accepted),
be made realistically
stable at speeds up to
about 100 km/h.
Fig 1.
OVER STEER
OVER STEER
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