Tow ball weight effectsn on tow vehicle and van

G'day Steve

From the way I see it, you only need to consider the tow ball weight when working out how much you can carry in the vehicle i.e. it becomes a component of the weight of your total load carrying capacity.

When it comes to calculating your combined total you certainly wouldn't add it twice

Hence total combined GVM = Vehicle GVM + Van GVM - Tow ball weight (if you included it in your Vehicle GVM).

Steve,

My understanding is:-

The gross combined mass (GCM) for your tow vehicle is established from the vehicle GVM and maximum towing capacity both of which are nominated by the manufacturer.

However what is important is the following:-

The gross vehicle mass (GVM) is the measured weight of the vehicle which if towing includes the tow ball weight, everything loaded on board (including fuel) and of course the mass of you car.

The gross trailer mass (GTM) included weight of the van and everything you have loaded into it, however this figure does not include the ball weight

The aggregate total mass (ATM) is the total mass of the fully loaded caravan not hooked to the car. i.e. GTM plus tow-ball weight

The way the actual figures are established i.e. measured when you are inspected on the road (or when you take it to a weigh bridge) is by driving the vehicle and caravan onto scales that measure the axle weights for tow vehicle and caravan. The measured GVM must not exceed the manufactures nominated figure (usually quoted in the manual) and similarly the caravan must not exceed the manufactures GTM. This figure and the ATM are nominated on the compliance plates attached to the caravan chassis and can't be exceeded.

I not sure what you are trying to achieve by
"I would love to be able to subtract the tow ball load off the van weight this would be great or am I just wishfull thinking??"

Hope this helps.

Rob

Steve

Does this picture help - I am assuming it is still the current definition as it still seems to be used on a few sites.

http://www.caravandealer.com.au/things-to-know/weights-masses-and-payload-weights

If you know your ATM and Tare weights your max Payload is the difference between them full stop.

This takes into account unloaded ball weight (Tare (no payload weight) = tyre load + ball weight) and this is independent of being attached to the tow vehicle. I would suggest the max payload is probably a structural design constraint with safety margin.

Hope the following is correct and makes sense.

This ball weight (as part of Tare weight) is also unloaded weight - once you add payload the ball weight wil increase or decrease depending on how you distribute the load, but does not change the manufacturers limits of ATM, GTM or Payload.

I think a confusing thing about ball weight is that what is specified on or calculated from the VIN plate is an unloaded weight as determined by the manufacturer after building the caravan. Payload will change the ball weight and tyre load, but ATM and GTM must still be observed.

There seems to be no fixed specification for ball weight after loading other than the 10% rule of thumb for safe towing and not exceeding maximum ball weight for the tow bar. And I dare say the optimum ball weight would differ between tow vehicles for the same caravan. But none of this changes the manufacturer limits they specify.

Steve,
If you have a weight distributing hitch which restores the rear of the tow the same height that it was before hitching up, then some of the tow ball weight will have been redistributed to the front wheels of the tow, with the rest being distributed to the wheels of the van. None of it will be on the rear axle of the tow.

The split will be roughly proportional to the distance between the tow ball and the front axle of the car and the mid point of the axles on the van.

If the front axle is 4 m from the tow ball and the mid point of the van axles if, say 6 metres, 40% of the tow ball weight will be distributed to the front axle and 60% to the van axles. The combined rig will of course weigh the same, but the towing vehicle will not carry all of the ball weight.

I hope this makes sense. Running each axle over a weighbridge might make it all clearer.
Keith