This is serious stuff. Don’t be gung-ho, or casual. Or worse, a casualty
Let’s face it, any accident with a jack is likely to be nasty. Changing a wheel or tyre is also likely to be in the most unhelpful location and when you’re possibly stressed. Not ideal. People make mistakes when they’re stressed, or not equipped with the right knowledge and the right gear. There have been reports of some jacks having their lifting mechanism unexpectedly fail, resulting in the jack instantly collapsing – not good.
If you know the correct, safe procedure, then the other causes of jacking accidents are clearly the jack:
(a) Not being completely suitable for that particular caravan, and;
(b) Not having its top positive-located so as to prevent it sliding in any horizontal direction.
So here’s a guide to help you.
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 also:
- 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 tyre
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 caravan.
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 five percent.
The relationship between a jack’s Load (lifting force) and its Travel, is a basic physics topic of Moments … which is Force x 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 x LM = J x LJ
The greater the LJ Distance, the lesser the J Force can be … and vice-versa.
Caution: 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 that you know exactly what to do if you have the misfortune of getting a punctured tyre on your travels.
- Do you know where everything you will need is stored … and how to use everything?
- Does the wheel brace fit the wheel nuts, and do you have the strength to loosen the tightened nuts (especially if fitted using a tyre shop’s rattle gun)?
- Does the top of the jack correctly suit the jacking points on the underside of the caravan chassis rails?
- Does the jack have sufficient lifting capacity (Load Rating)?
- Does the jack have sufficient travel … and is 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 for clarification.
Typical (Conservative) Example:
Total Mass of caravan: 2000kg
Ball Loading: 100kg (5 percent of Total Mass)
Therefore effective Load = 1900kg
Heavier side of caravan M : 1000kg
Lighter side of caravan: 900kg
LM = 2m
Therefore M x LM = 2000kg.m
Therefore J x LJ must equal (or exceed) 2000kg.m
If LJ is 3m, J must exceed 667kg
If LJ is 4m, J must exceed 500kg
A typical locating device positively locates the top of the jack, to prevent it slipping in any horizontal direction.