Towbar torture

[BushWacker]

It does this 3 times per speed bump!

Chaz please go speak with your neighbor.

I want a Bosal replacement tow bar!

Have sent him 3 photos ( & will relay his response)
Before
Collapsed
'Gusseted' Collar
...

[LesseW]

In my opinion the fault was the Jurgens bush caravan, they are known to be heavy with a 140kg+ tow ball weight irrespective how well you try to pack the caravan to distribute the weight. Most “standard” tow hitches will eventually fail. You will need a tow hitch that is manufactured by Leo Prinsloo....


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[BushWacker]

My MiniMetalian is a short-ass in that the body behind
the axle is very short ... so difficult to get weight
behind the axle.
I have a spare wheel (wolf-rim) on a swing-arm
& that helps and Im going to plant the battery
at the back of the load bay
and maybe will put my watertank across the back
(instead of along 1 side) of the lower loading bay.

[HenriSteyn]

It does this 3 times per speed bump!

Chaz please go speak with your neighbor.

I want a Bosal replacement tow bar!

Have sent him 3 photos ( & will relay his response)
Before
Collapsed
'Gusseted' Collar
...

Carlos is retired now? 2 years.

I used to work with him, for a very short period at Towmaster.

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[HenriSteyn]

This picture shows the crap design. Absolutely no strength behind the square tubing. Those 2 gussets at 45 degrees do absolutely nothing.

Is that a factory towbar?

It’s not that the gusset does nothing in that position , it actually makes it more susceptible to cracking .

If it wasn’t there at all , it wouldn’t have broken.

Properly positioned it would have made it stronger .

As far as "Factory Towbar" is concerned anyones guess. Bought and fitted by the Mitsu agents. One would guess that is the best route to go considering comebacks.

If they fiddled me I at least have some recourse should it go that far.

If this event had been more disastrous and the van was written off, at least I think there would be compensation.

The bush repair consisted of 2 extra gussets (which should have been there if the design engineers did their job properly).

Photo of temp repair attached. Why i saty temp repair, those gussets need to be double thickness and extend around the back.

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[Apocalypse]

IMG_20180916_142020.jpg
This is a Towbar rated to tow 3.5t with a vertical load of 150kg. It mounts to the chassis with 6 m12 and 4 m16 bolts. The plate is 10mm thickness. You will also see there are no sharp corners in the design to prevent stress raisers.
It's a pity you can't fit something similar to your vehicle Henri.

Those towball extensions are specifically made for mounting the tow bar mounted bike racks on vehicles that have a Spare wheel above the tow hitch.

They are rated for 60kg.

[Apocalypse]

It would be interesting to weigh yours, Henri, just before you depart for a trip like NKZN.

Not disputing all the other comments of pendulum action etc. which caused the failure but as you said the caravan’s balance is probably out of spec for most of the tow bars made in SA.

Also, your tow bar doesn’t look like the one I had on both my Pajero’s which in my useless opinion is a stronger design to yours.

To take some nose weight off I always try to travel with a full water tank. The tank sits behind the axle and takes 90 litres.

Then I pack as much heavy stuff like solar panels tables tent bits and pieces on the bed or under the bed.

To weigh, well most domestic scales only go to 120kg. I think. I need to go look.

Bottom line is not much more can be done other than use on moerse strong towbar.

Seen many Pajeros towing Xplorers so never thought of having a problem.

Always a first time....

The problem with static towball weight is that it's not the real force that the towball is subjected too in a dynamic situation.

That 90kg behind the axle still creates a massive inertial force when it rocks over the fulcrum (axle) - an empty trailer with 150kg nose weight exerts a lot less dynamic force than a full laden caravan with the same static nose weight.

The other problem is, of course that a Pajero chassis was never really designed around having a massive Drawn capacity, and there is no real place to mount a decent tow hitch.

[HenriSteyn]

... long knecked towbar also considerably compromises
departure angle ... and I would venture that
it has probably previously experienced
a few hefty grazing cracks & bangs ?
Was it Archimedes who said:
' ... give me a lever & I will move the world ... ' ?

Makes me think of my swottings.
Lasarm en kragarm

There must be serious forces added with the length of that towbar.

Think an aftermarket rear bumper will be the best solution.

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Thanks Christo.

The Paj bumper is pure tupperware and really has no serious mounting points.

The Paj design allows for 4 x 12mm bolts on each side, pretty strong.

The real issue is the actual square bar and extention design.

I am on 2 x Pajero FB pages and 2 x Xplorers FB for 3 years and no such incident has been reported. I would thus take it this is a singular issue with this towbar and not the Paj chassis itself to be able to handle loads.

One person (CATS) on the 4x4 other forum has had his Xplorer for 13 years and towed until recently with 2 Pajeros, no issues, through seriously rough terrain.

Cheers

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[Apocalypse]

I shall demonstrate why a 'gusset' that lands in the middle of a tube weakens the structure , not strengthens it.

So, in the 2 diagrams above we have a simplified cross section of the tow hitch.

In the top one, the 60x60x3mm Carrying tube is welded directly to the plate.

This is very very simplified

the entire tube flexes with load (every time it bounces) and 'pulls' to the right on the red weld. the weakest point there is the flat section just below the weld - smallest cross section with the highest load

thats 60mm x 3mm (or should be) and the tensile strength of mild steel is around 400N/mm^2 - so, that will take a load in the direction of load of 60x3x400 = 72 000N or 7,2 tons before it stretches and becomes thinner and brittle


In the lower one, there is a 5mm plate welded down the middle of the tube.

This causes the tube to flex only as far as the tip of the 'gusset' under load and redirects the force to down and right and into the unsupported centre of the tube where flex can occur.

effectively a good portion of the load is transferred to the point at the end of the gusset (the red weld) because it will flex there and not over the full length of the tube to the plate.

the weak point there is that the exposure to load is about 5mm x 3mm - or 5mmx3mmx400N/mm^2 = 6000N or 600kg before the steel is worked. less than 10% of the value of the fuel weld 50mm behind it.

Thats means the steel at that point will fail far sooner than the tube would have failed at had the 'gusset' not been installed

once there is a crack at that point, it flexes more and more easily, and continues to crack along the weak point (the end of the crack) until it gives up completely.

The idea of a structure is to spread the load into the structure, not direct it to a point load that cannot take the load!

[HenriSteyn]

It would be interesting to weigh yours, Henri, just before you depart for a trip like NKZN.

Not disputing all the other comments of pendulum action etc. which caused the failure but as you said the caravan’s balance is probably out of spec for most of the tow bars made in SA.

Also, your tow bar doesn’t look like the one I had on both my Pajero’s which in my useless opinion is a stronger design to yours.

To take some nose weight off I always try to travel with a full water tank. The tank sits behind the axle and takes 90 litres.

Then I pack as much heavy stuff like solar panels tables tent bits and pieces on the bed or under the bed.

To weigh, well most domestic scales only go to 120kg. I think. I need to go look.

Bottom line is not much more can be done other than use on moerse strong towbar.

Seen many Pajeros towing Xplorers so never thought of having a problem.

Always a first time....

The problem with static towball weight is that it's not the real force that the towball is subjected too in a dynamic situation.

That 90kg behind the axle still creates a massive inertial force when it rocks over the fulcrum (axle) - an empty trailer with 150kg nose weight exerts a lot less dynamic force than a full laden caravan with the same static nose weight.

The other problem is, of course that a Pajero chassis was never really designed around having a massive Drawn capacity, and there is no real place to mount a decent tow hitch.

Good point Apoc.

And those sixty gazilion speed bumps on the R22 are right next to the 60 kph sign, so due warning is supplied in true African fashion. There is no way one can slow down sufficiently without seriously standing on the brakes and activating ABS to avoid going over them too fast.

And the profile of these bumps are in competition with Mount Everest. KZN roads department could have donated 3/4 of the tar to the Free State to speed up their road rehab process!

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[HenriSteyn]

I shall demonstrate why a 'gusset' that lands in the middle of a tube weakens the structure , not strengthens it.

Screenshot 2021-05-07 at 10.26.28.png

So, in the 2 diagrams above we have a simplified cross section of the tow hitch.

In the top one, the 60x60x3mm Carrying tube is welded directly to the plate.

This is very very simplified

the entire tube flexes with load (every time it bounces) and 'pulls' to the right on the red weld. the weakest point there is the flat section just below the weld - smallest cross section with the highest load

thats 60mm x 3mm (or should be) and the tensile strength of mild steel is around 400N/mm^2 - so, that will take a load in the direction of load of 60x3x400 = 72 000N or 7,2 tons before it stretches and becomes thinner and brittle


In the lower one, there is a 5mm plate welded down the middle of the tube.

This causes the tube to flex only as far as the tip of the 'gusset' under load and redirects the force to down and right and into the unsupported centre of the tube where flex can occur.

effectively a good portion of the load is transferred to the point at the end of the gusset (the red weld) because it will flex there and not over the full length of the tube to the plate.

the weak point there is that the exposure to load is about 5mm x 3mm - or 5mmx3mmx400N/mm^2 = 6000N or 600kg before the steel is worked. less than 10% of the value of the fuel weld 50mm behind it.

Thats means the steel at that point will fail far sooner than the tube would have failed at had the 'gusset' not been installed

once there is a crack at that point, it flexes more and more easily, and continues to crack along the weak point (the end of the crack) until it gives up completely.

The idea of a structure is to spread the load into the structure, not direct it to a point load that cannot take the load!

Eggzackery what happened.....

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[BushWacker]

... just thinking if you put double swing arms
for your 2 spares at the back of your caravan
you will reduce the nose load at the hitch
and ... not have a spare overhanging cars tow hitch,
on the long haul, wont need extensions etc
Once caravan unhitched & pitched
you'd have to put 1 spare back on the car ...

[HenriSteyn]

To take some nose weight off I always try to travel with a full water tank. The tank sits behind the axle and takes 90 litres.

Then I pack as much heavy stuff like solar panels tables tent bits and pieces on the bed or under the bed.

To weigh, well most domestic scales only go to 120kg. I think. I need to go look.

Bottom line is not much more can be done other than use on moerse strong towbar.

Seen many Pajeros towing Xplorers so never thought of having a problem.

Always a first time....

The problem with static towball weight is that it's not the real force that the towball is subjected too in a dynamic situation.

That 90kg behind the axle still creates a massive inertial force when it rocks over the fulcrum (axle) - an empty trailer with 150kg nose weight exerts a lot less dynamic force than a full laden caravan with the same static nose weight.

The other problem is, of course that a Pajero chassis was never really designed around having a massive Drawn capacity, and there is no real place to mount a decent tow hitch.

Good point Apoc.

And those sixty gazilion speed bumps on the R22 are right next to the 60 kph sign, so due warning is supplied in true African fashion. There is no way one can slow down sufficiently without seriously standing on the brakes and activating ABS to avoid going over them too fast.

And the profile of these bumps are in competition with Mount Everest. KZN roads department could have donated 3/4 of the tar to the Free State to speed up their road rehab process!

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"before the steel is worked. less than 10% of the value of the fuel weld 50mm behind it"

Diesel or Petrol weld?

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[HenriSteyn]

In my opinion the fault was the Jurgens bush caravan, they are known to be heavy with a 140kg+ tow ball weight irrespective how well you try to pack the caravan to distribute the weight. Most “standard” tow hitches will eventually fail. You will need a tow hitch that is manufactured by Leo Prinsloo....


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Not going to argue with you.

'Coz you are the chiefstoelsitterperson.

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[HenriSteyn]

... just thinking if you put double swing arms
for your 2 spares at the back of your caravan
you will reduce the nose load at the hitch
and ... not have a spare overhanging cars tow hitch,
on the long haul, wont need extensions etc
Once caravan unhitched & pitched
you'd have to put 1 spare back on the car ...

Good idea Chaz.

But the stats on broken towbars on Pajeros dont indicate such drastic action.

I have seen only one report of where the towbar ripped off the chassis. In that case only 4 bolts were used instead of 8, recipe for disaster.

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