THERE are too many "nasty accidents" when jacks are being used to
change wheels on caravans and camper-trailers, according to the Caravan
Council of Australia.
"There have been reports of some jacks having their lifting mechanism
unexpectedly failing, causing them
to instantly collapse," founder and general manager Colin Young
He attribute the main causes as being jacks not being completely suitable
for the particular caravan and not having their tops positively
located to prevent horizontal sliding.
"It is essential that a caravan jack is appropriately approved and has a
sufficient load rating and lifting range," Mr Young stressed.
He has now issued the following guide on the
safe use of jacks.
Caravan jacks must:
Be approved to the Australian Standard
Have the top positively located - by a mating locating recess - to
prevent any horizontal slippage
Be operational for when the caravan is both empty and fully-loaded
Be positioned on a firm base
Have a sufficient load rating
Have a sufficient length of travel
A caravan jack must:
When on a firm base be low enough to engage a
locating recess when any tyre is fully-deflated
Have sufficient travel to enable a fully-inflated tyre to replace that
Allowance must be made for the suspension "droop"
when the caravan is raised.
Allowance must be made for the probability that the tyres(s) on one side
of the caravan will be loaded greater than on the other side. Typically,
this may be around 10 percent.
A conservative assumption must be made on the actual ball-loading.
It is of course much more preferable to have had the ball-loading
measured for the particular loading of the
While a typical caravan may have a ball-loading
of around 10 percent of the actual mass of the
loaded caravan, the actual ball-loading
may only be around 5 percent.
The relationship between a jacks load (lifting
force) and its travel is a basic physics topic
which is force
multiplied by distance.
With the ball as the pivot, it is a
"balance" or a
"compromise" for any
given effective load (M) times its
distance (LM) from the pivot and the jack's
available force (J) times its
distance (LJ) from the pivot.
The jack's available travel
(extended length minus
retracted length) must
be sufficient to enable a tyre to be safely replaced. M
times LM equals J
The greater the LJ distance, the lesser the J
force can be
Never rely on a jack
tragic accidents have
occurred when a jack has failed or has slipped.
It is most important to practice changing a
wheel/tyre before heading off on a trip so you know exactly what to do if
you have the misfortune to have a punctured tyre on your travels.
Do you know where everything you will need is stored
and how to use
Does the wheel brace
fit the wheel nuts, and do you have the strength
to loosen tightened nuts?
Does the top of the jack correctly suit the jacking points on the
underside of the chassis rails?
Does the jack have sufficient lifting capacity (load
Does the jack have sufficient travel
the minimum height low enough?
Always carefully read the warning
notice and operating
Instructions supplied with the jack.
If something is not clear to you, contact the manufacturer
Typical (conservative) example:
All-up mass of
caravan: 2000 kg
Ball-loading: 100kg (5
percent of all-up
Therefore effective load = 1900kg
Heavier side of caravan M: 1000kg
Lighter side of caravan: 900kg
LM = 2.00m
Therefore M x LM = 2000kg.m
Therefore J x LJ must equal (or exceed) 2000kg.m
If LJ is 3.0 m, J must exceed 667kg
If LJ is 4.0m, J must exceed 500kg
Typical Locating Device
locates the top of the jack
to prevent it slipping in any horizontal direction.