Overloading tow hitch and A-frame

[otherclive]

Out today I noticed an outfit parked up next to me in the services. The family of parents and two teenagers were all quite large and were sitting in the front lounge of the van having a break. They had lowered rear steadies but not the front ones, or nosewheel. Given that the current Mondeo has a noseweight of 90kg and the van's A-frame probably has a limit of 100kg I would reckon the family's weight added to the 'packed van's' noseweight would well exceed the towbar and a-frame limits. Has anybody experienced any physical results of such overloads, or are the factors of safety such that there won't be any?

 

[JonnyG]

never actually thought about those things before, and indeed i have entered the front of our van whilst its hooked up to the car, so I suppose thats another 88 kg on everything. but station weight and moving/ bouncing weight are so much different are they not?
I have always assumed that the weight on the tow ball /A frame ect ect will be much larger at times when moving along compared to its stationary weight?

 

[ProfJohnL]

Hi Jonny

Yes the dynamic loading during towing is very different to any nose load applied when stationary.

The tow hitch and its mounting points will have been designed with the dynamic loads in mind and consequently should be able to withstand considerably more load when stationary than the specified limit.

I have no actual figures to call on but I can well believe that vertical accelerations in the order of 2 to 3G are potentially possible on the hitch when towing which basically means it should withstand a static load of 2 to 3 times (or possibly more) the specification. To put this into perspective if the specified max nose load is 75Kg then I personally would be happy to load it up to about 200Kg WHILST STATIONARY.

DO NOT ATTEMPT TO MOVE THE OUTFIT WITH THESE SORTS OF LOADS.

In practice of course if you do enter a caravan whilst hitched, then your weight is shared by the hitch and the main wheels in proportion to where you are between these two support points.

The distance from the tow ball to the front edge of the seating in most caravans will be about 1.5M (allowing for gas bottles and rake of front screen) Generally the distance from that point to the axle will be about 2M, so at worst an person sitting at the from of the caravan would only exert a maximum of about 57% of their weight on the hitch

The further back from the hitch, the lower the percentage that gets transfered, and the same applies if the axle is nearer.

Now because it is impractical for joe public to measure the dynamic loads on the tow ball, the designers test and calculate what equivalent static load will remain within safe working limits when under towing conditions. That is why the manufacture nose loads must be adhered to when towing.

 

[Lutz]

The Prof has referred to dynamic loads which are in the order of 2 to 3G. Note that such loads are relatively frequent. In less frequent cases peaks of up to 10G can be experienced for fractions of a second.

 

[Dustydog]

The Prof has referred to dynamic loads which are in the order of 2 to 3G. Note that such loads are relatively frequent. In less frequent cases peaks of up to 10G can be experienced for fractions of a second.

Lutz
Is it possible I may pass out at 10G?

Very interesting how much load the towball and bar have to take. No wonder new Al-Ko Balls are so expensive.

 

[ProfJohnL]

Hello Dusty,

Would you pass out at the load or the cost?

 

[Lutz]

Lutz
Is it possible I may pass out at 10G?

Very interesting how much load the towball and bar have to take. No wonder new Al-Ko Balls are so expensive.

Under static conditions, a towball could easily carry the full weight of a fully laden caravan without breaking. One could run into problems if this were to occur very often, though. The EU regulations specify a durability test over 6 million cycles under normal loading conditions.