Add the extra weight of a spare tyre, or any
heavy object really, to the back, and that
bounce will be magnified. Perhaps not to the
point where cupboard doors fall off or
benchtops delaminate, but I wouldn’t bet on it.
EFFECT ON BALL WEIGHT
Locating that second spare tyre at the rear of
your caravan reduces your van’s towball
weight. As you’d know, every caravan needs
to impose a certain percentage of its overall
weight onto the towball in order to remain
stable when under tow. A rear-heavy caravan
is an all-but-certain recipe for disaster.
To find out how much a second spare wheel
could reduce ball weight, I visited the RV
Repair Centre in Bayswater, Vic. A single-axle
van with a single spare wheel and room for a
second was waiting for me.
We weighed the ball weight without the
second spare wheel and found it had a ball
weight of 263kg. We also weighed the
wheel: 36kg.
WHY?
Collyn Rivers, an RV book publisher and exresearch
engineer for Vauxhall Bedford,
explains that all caravans, to a greater or
lesser degree, will have a tendency to sway.
This could be due to crosswinds, a passing
road train, or changes in the road’s surface.
Too much weight at either the front or rear of
a caravan has the potential to create ‘yaw
inertia’. Because these two areas of mass are
further away from the pivot point (the axle/s),
they will tend to increase the oscillation of
the caravan, rather than be damped by the
tow vehicle.
To overcome this, it is essential that as much
mass as possible is kept near the centre of the
pivot point, on the floor or around the chassis
wherever possible. Heavy items should
thereafter be gradually loaded towards the
front of the van.
After mounting this tyre to the rear of the van,
we re-weighed the ball weight. It had reduced
by 17kg to 246kg.
Now, 17kg might not sound like much, but in
essence we had added 36kg to the back and
reduced the nose weight by 17kg, and in
doing so substantially reduced the caravan’s
towing stability and maximum towing speed,
and increased the possibility the rig could
jack-knife.
The spare wheel weighed 36kg –
surprisingly heavy.