Tow hitch dimension

[ATH]

I posted this question before on another thread about towing but it got lost somewhere and never got a response. The D4 handbook has a diagram showing the distance between the pin centre on the OEM receiver and the ball/spigot on the tongue as being 6"/150mm.
Anyone know if this is a vital figure as my present set up is longer than this by around 75mm and I want to get that changed if there is any danger at all although the vehicles tows brilliantly anyway.
Thnx. in advance.
AlanH.

[ATH]

It appears no one has an answer to this question. I suspect if I went to some of the upmarket alternative hitch providers they'd say theirs complies with it but wanted to hear from other towers first.
No one else ever read about that in the handbook? Has no one else ever bothered reading the handbook? [bigsmile1]
AlanH.

[entropy]

We need an engineer or physicist to respond to get any definitive answer, but logic says the longer the distance from centre of the rear axle to point of load on the tow hitch (moment arm?) the greater the down force applied through leverage. 75mm difference doesn't sound like much, but as we may be talking of up to 3500kg on the ball I'm guessing that 75mm of extra leverage (moment?) adds an undesirable amount of extra rotational down force.

Anyone out there that can do the math?

[LuckyLes]

I read that in the manual and my thoughts were that the hitch receiver has been been engineered to carry 350kg ( the max towball weight) at a distance of 150mm.
If you move that weight another 75mm out then you would effectively be loading the receiver with 525kgs because you have increased the leverage by 50%.
Les

[PeterJ]

Hi Alan, basically the previous posts are correct and I will try to keep this as straight forward as I can. Vehicle manufacturers will do strength and durability testing on the vehicle structure that supports the towbar / tow hitch and also the towbar itself or the components that form the mechanical connection between the vehicle and the trailer. Tow hitch strength and durability is covered by ADR_62 but mostly manufacturers will have their own in-house specifications that they test to and these will typically exceed the requirements for all markets and towing combinations that the vehicle is likely to be certified for. If you consider the ADR requirements for the towbar as a base line for this discussion and if we just consider the strength component for a ball coupling, the test is 1.5 x ATM, in this case 3500kg, hence 5250kg applied in a forward and rearward longitudinal direction as well as 0.5 x rated capacity transverse and vertical.
So, when you move the ball centre outside of the envelope specified by the manufacturer you are changing the effect of the static forces applied to the ball into the structure at the back of the vehicle. In this case 75mm (or 50%) further rearward will be increasing the stress in the body structure for vertical and transverse loads. For longitudinal horizontal loads it will be the same. Real world towing involves forces in all of these directions simultaneously and so manufacturers also conduct dynamic testing. This involves applying a cyclic load (push / pull) onto the towball at an angle downward at a load calculated around the design towing mass, this load is applied 2 million times. Your +75mm towball location will have a significant effect on the stress into the back of the vehicle structure by way of increased bending moment and increased shear stress. This test may or may not be required by the ADR depending on vehicle classification but manufacturers will do it so that they can prove the durability of the towbar attaching points on the vehicle structure.

Hopefully I have not confused you too much, bottom line is I would not do it.

Peter

[RoverLander]

Hi Alan, basically the previous posts are correct and I will try to keep this as straight forward as I can. Vehicle manufacturers will do strength and durability testing on the vehicle structure that supports the towbar / tow hitch and also the towbar itself or the components that form the mechanical connection between the vehicle and the trailer. Tow hitch strength and durability is covered by ADR_62 but mostly manufacturers will have their own in-house specifications that they test to and these will typically exceed the requirements for all markets and towing combinations that the vehicle is likely to be certified for. If you consider the ADR requirements for the towbar as a base line for this discussion and if we just consider the strength component for a ball coupling, the test is 1.5 x ATM, in this case 3500kg, hence 5250kg applied in a forward and rearward longitudinal direction as well as 0.5 x rated capacity transverse and vertical.
So, when you move the ball centre outside of the envelope specified by the manufacturer you are changing the effect of the static forces applied to the ball into the structure at the back of the vehicle. In this case 75mm (or 50%) further rearward will be increasing the stress in the body structure for vertical and transverse loads. For longitudinal horizontal loads it will be the same. Real world towing involves forces in all of these directions simultaneously and so manufacturers also conduct dynamic testing. This involves applying a cyclic load (push / pull) onto the towball at an angle downward at a load calculated around the design towing mass, this load is applied 2 million times. Your +75mm towball location will have a significant effect on the stress into the back of the vehicle structure by way of increased bending moment and increased shear stress. This test may or may not be required by the ADR depending on vehicle classification but manufacturers will do it so that they can prove the durability of the towbar attaching points on the vehicle structure.

Hopefully I have not confused you too much, bottom line is I would not do it.

Peter

Great original question as I have been wondering the same thing about my aftermarket hitch and the further distance of the towball. Also a great answer and sort of what I suspected (but with great detail and knowledge).

I also wonder if the vehicle and trailer stability software in the D4 are programmed based on a trailer pivot point? Would this also be impacted by moving this further to the rear. I have had no problems while towing a heave caravan but I do try to prevent problems before they happen!

Peter

[ATH]

Some good info there and thnx. to you all. My ball weight is way under 350kg but I'm going to alter the tow hitch so the centre of the ball/spigot will be around the 150mm mark.
Although the car rides superbly towing the van I'd hate to be caught out by something breaking and finding I was at fault..... and possibly no insurance if it caused an accident. Even worse if someone was hurt because I was negligent in this matter.
Cheers.
Alan.