Rear Axle Load Caused by Ball Weight

No, Rod, I only worked it out today.

This doesn't change as you load the vehicle - just adds to it.

Loads behind the rear axle will work the same way, ie add more than their own weight to the rear axle.

Loads in front of ghe rear axle will share their weight between front and rear axles.

Cheers

I believe danger exists only if you don't know what's happening or if you do, ignore it.

How was the Ranger dangerous in your boss's application?

Cheers

It was very overweight at the rear and about 230 Kgs light at the front and the chassis flexed like it was going to bend any minute. We have all seen the result of that set up.

72 kg is one thing.
230 is quite another!
Cheers

Just 2 questions,

1. If you are following the manufacturer recommendations for example the tow ball weight max is 350 KG and you keep to that then what is the problem?
2. The above example but with a weight distribution hitch added and adjusted correctly.

I know that the GVM cannot be exceeded but as far as engineering is concerned surely following the vehicle recommendations shouldn't be a problem.
That is the towing equipment is compliant.

Redbakk
While the GVM might be OK, the rear axle might well be over the design limit.
Take a typical modern ute towing a caravan with a 350kg ball weight: because the hitch is ~1.5m back from the rear axle, this results in a 525kg load on the rear axle. Add the distributed load from 2 people - probably 100kg - and then a bit of gear, maybe a fridge, in the tray to take it up to GVM and it would be easy to be over the rear axle design load.
While a WDH may well help in some cases, some modern vehicles with anti-sway technology actually advise against fitting a WDH.
Cheers
Andrew

Exactly, Andrew.

And some drawbars on some vans cannot accommodate a WDH, mine included.

Cheers
Frank

One of the BIG problems in towing circles is the over simplistic view many have of towing capacities .
There is a lot more to the vehicles specified limitations than towing capacity and permitted ball weight.
The result is a great many vehicle combinations on the road operating outside of manufacturers specifcations, while the operators believe they are all good.


On the matter of weight distributing hitches.
While the WD hitch may redistribute weight, it does not reduce the weight on the ball or the stress on the coupling and the chassis that carries it. ..... in fact a WD hitch considerably increases the stress on the coupling, the tow bar and its attachments.
Many of the new vehicles either advise against WD hitches or explicity forbid them.

The specification may be rear axle loading ....... but what is that specificaton seeking to limit .......
The tyres, wheels, axle assembly and springs may well be able to carry a great deal more than the specified rear axle load limit ....... but the failures we are seeing, indicate that the chassis and its various attachments are the weak points.

So redistributing the load may produce a result within specifcations, but still be ill advised.

cheers

Thanks 671. Schoolboy physics from 50odd years ago dragged out of dim dark recesses of memory. The brothers would be pleased that some of what they taught actually stuck :-)

Cheers

671 is right on the mark. Aeons ago, when Bedford trucks ruled the road, GMH went to great pains in the operator manual to explain the dangers of overloading your Bedford truck.

In essence, they pointed out that any overload translates to double the overload weight, when you shock-load the chassis and axle via hitting a pothole, a sunken culvert, an out-of-level bridge approach, and the 100 other uneven road conditions that your suspension and chassis copes with daily.

So if you overload an axle by 100kgs, that translates to a 200kg overload when any of the above are encountered. Speed exacerbates the impact.

I can recall a highly embarrassing episode for GMH when the HR Holden and the Vauxhall Viva were new, in 1967.
Lap-sash seatbelts had only recently been fitted as standard to the GMH range, and GMH were keen to get a lot of media exposure about the superb strength of their seatbelts.

To this end, they invited a media pack and all their photographers to a stunning demonstration, where they were going to lift cars by their seatbelts!

They rolled out a new HR holden and slipped the seatbelt tongues out the open front windows - the crane driver hooked onto the seatbelt tongues - and he steadily lifted the HR Holden sedan 4 feet (1.2M) off the ground!!

The media pack oohed-and-ahhhed and the photographers clicked away merrily.

Then came the piece-de-resistance! The crane driver hoisted the HR Holden up about 6 feet (1.8M) and the GMH boys rolled a brand-new Vauxhall Viva under the HR, slipped its seatbelt tongues out the open windows, and hooked them to the underside of the HR!

The crane driver then hoisted both the HR Holden AND the Vauxhall Viva a good 8 feet (2.5M) off the ground - and the media went wild!
Photographers clicked away, while the GMH boys explained how the seatbelt tongues were designed to hold a load of 4480 lbs (2033kgs) for at least 30 seconds.
The HR weighed 1180 kgs and the Vauxhall Viva weighed 700 kgs (approx.), for a combined weight of 1880 kgs.

After about 10 mins of this, the crane driver got pretty bored, and he was moving around inside the crane cabin, gawking at something else.
In doing so, he accidentally bumped the lift/lower control lever with an elbow - just a fleeting bump.

The cars dropped about 2 inches (50mm) at most, with the lever bump - but almost instantly, the HR Holdens seatbelt tongues snapped! - and both cars crashed to the ground!!

Needless to say, the GMH blokes were appalled - and the media pack took a whole lot MORE pictures!!

There was a very hurried examination and discussion by all the GMH heads and engineering chiefs - and they promptly re-assembled the media pack to explain exactly what had gone wrong.

The chief engineer of GMH explained how the 2" (50mm) sudden load drop by the crane - which seemed like virtually nothing to the onlookers - had actually nearly DOUBLED the load on the seatbelt tongues!

This seemingly-slight impact from the drop, was enough to exceed the seatbelt tongues designed and engineered strength by a large margin! - thus resulting in the seatbelt tongues failure - and a very expensive media stunt!

Virtually the same effect occurs on axles when a heavily-loaded axle receives a heavy road impact - from something regarded as relatively minor - say, in the shape of a 50mm deep pothole.

The impact can send the axle load into the danger zone - and even though engineers design in shock-loading resistance into their axle designs - if you're running at maximum axle loading - or even just near it, it doesn't take much by way of repeated heavy road impacts, to result in axle damage.

Cheers, Ron.

G'day Frank

As an old dog engineer who's played with this sort of calculation for more years than would normally be considered reasonable, you've done well old son......... 10/10

" where does the 250kg get transfered to???? shared between front axle and trailer axle???"

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Yes they will take a lot of weight off the rear axle but unfortunately the number of places where you can use them off road is very limited.

Yes but an additional 250KG of load and stress on the coupling, drawbar and vehicle attachments.

cheers