Yup... Another Pajero Ball Weight Post

[Moriarty]

I have talked about this issue with an engineer in Brisbane who does all sorts of mod plates for unusual vehicles and requirements. He said he would not be willing to do a TBM upgrade for the Pajero because the limitation seemed to be to do with the actual structure of the back end of the vehicle. The reasoning is easy to understand - the bit that is hard to understand is why Mitsubishi did not do a linear derating or a stepped derating like the Nissan Patrol wagon of 2000's did.
The reasoning is as follows:
The restriction seems to be to do with dynamic loads. If it was simply static weight on the tow ball then the actual trailer mass would not be relevant. Since the TBM restriction is related to the trailer mass (not just downforce) it may possibly have something to do with the moment coupling into the monocoque body tow bar hard point that comes from lateral forces on the fore/aft axis of the towbar under braking or acceleration. The hitch level is ~200 mm lower than the hard point so linear thrust could perhaps twist the hard point connection up or down with acceleration or braking. Under heavy braking (worst case with Trailer Brakes not functioning) the moment induced by the thrust from the trailer would add to the moment caused by the down force on the hitch. His assessment after looking at the configuration of the towbar and the page from the Pajero Owners manual was that he would not cover removing the TBM restriction by simply upgrading rear suspension. He would do a GVM upgrade for me but he would note that that the 180 KG TBM would still apply. As a retired engineer I understand his reasoning. I assume that the companies offering such an upgrade in other States have more information on the structural design on the vehicle.

[old Jack]

I have talked about this issue with an engineer in Brisbane who does all sorts of mod plates for unusual vehicles and requirements. He said he would not be willing to do a TBM upgrade for the Pajero because the limitation seemed to be to do with the actual structure of the back end of the vehicle. The reasoning is easy to understand - the bit that is hard to understand is why Mitsubishi did not do a linear derating or a stepped derating like the Nissan Patrol wagon of 2000's did.
The reasoning is as follows:
The restriction seems to be to do with dynamic loads. If it was simply static weight on the tow ball then the actual trailer mass would not be relevant. Since the TBM restriction is related to the trailer mass (not just downforce) it may possibly have something to do with the moment coupling into the monocoque body tow bar hard point that comes from lateral forces on the fore/aft axis of the towbar under braking or acceleration. The hitch level is ~200 mm lower than the hard point so linear thrust could perhaps twist the hard point connection up or down with acceleration or braking. Under heavy braking (worst case with Trailer Brakes not functioning) the moment induced by the thrust from the trailer would add to the moment caused by the down force on the hitch. His assessment after looking at the configuration of the towbar and the page from the Pajero Owners manual was that he would not cover removing the TBM restriction by simply upgrading rear suspension. He would do a GVM upgrade for me but he would note that that the 180 KG TBM would still apply. As a retired engineer I understand his reasoning. I assume that the companies offering such an upgrade in other States have more information on the structural design on the vehicle.

This is a very interesting opinion indeed.
Lovells in Adelaide in conjunction with Motiv Engineering have an approved GVM upgrade to 3110kg and 3000kg tow capacity with 300kg tow ball load with the only change being coils and shockers. In overseas markets Mitsubishi rate the towing capacity up to 3500kg.

The fore and aft lateral forces applied during heavy braking of a 3000kg loaded trailer with electric brakes will be less than those applied by a trailer weighing 2000kg with override mechanical brakes. Yes if the brakes fail then the forces will be significant more, no argument there but how much will more be decided by the rate of deceleration and the mass of the trailer.
The difference in the tow ball height relative the towbar attachment points on the rear structure, will only change the direction to which the load is applied to the rear structure, the loads to the fasteners will remain the same and in shear but the direction that these forces are applied will change, but the compression and tension loads in the rear structure will change direction but by a small amount.
A 200mm offset, from the centreline of the towbar attachment points, caused by a drop hitch and when applied to a towbar that is 600mm in length, measured from the towball, to the centre of the multiple attachment points, would increase the effective length of the lever by 40mm or 6.6%, and this full amount of extra mechanical advantage is only gained when the load is applied at 90 degrees to the angle of the revised lever, and in this instance the force would be about 25 degrees to 30 degrees off the vertical plane.

I agree that the arbatory instant decrease in allowable towball load that drops 70kg by exceeding just 1kg over 2500kg is absurd, it should be on a sliding scale at a rate of 1kg of towball mass for every 7kg of trailer mass over 2500kg.

I would like to see some computer modelling to prove or disprove my theory, very open for further opinions on this as a discussion topic.

OJ.