One for our resident weight experts - Fifth wheel towing

[KnL]

I'm not in the market for one, but an article in this month's CC Magazine has one of these beasts tested on the back of a Nissan Navara.

The MIRO of the trailer is given as 3200kg and MTPLM of 4300kg, giving a user payload of 1100kg.

Am I missing something really obvious ?

If the max tow of the Navara is the usual 4x4 limit of 3500kg, then surely a user payload of 300kg could never be exceeded on public roads.

Which in a vehicle where it's load area is taken up by the coupling mechanism and a roof box is not an option due to the overhang of the trailer, does not leave much to play with.

I can't see how, but does this type of coupling remove the towing vehicle from the usual rules on weights etc., or does the equivalent noseweight of 500kg come in to the equation somewhere.

For your thoughts thanks,

Ken.

 

[Lutz]

The max. GVW of the Navarra is 3500kg, its kerbweight around 2100 to 2200kg depending on specification, so that gives you 1300 to 1400kg payload. Assuming a reasonable 400kg margin for passengers and other odds and ends, the fifth wheel trailer could apply about 900 to 1000kg to the coupling plate. The remainder of its MTPLM of 4300kg, i.e. approx. 3400kg would be supported by the fifth wheel axle(s). Although 3400kg exceeds the maximum permissible towload of 2305kg, the latter only applies to a load behind a conventional towbar, not on a fifth wheel coupling plate.

 

[RogerL]

According to Nissan's website, the towing limit for the Navara is 2700kg, the payload being 1050kg.

So 2700 + 1050 = 3750kg, subtract the 3200kg MIRO of the 5th wheeler give a total outfit payload of 550kg.

This 550kg has to cover the weight of hitch equipment fitted to the pickup, any passengers, their luggage in the pickup and the trailer payload - if the 230 litre tanks are full in the trailer that would leave just 320kg total payload.

This outfit may have been legal to tow with a single driver and no other payload but as a touring outfit it's illegally impractical.

A typical small car with small caravan would have payload of 650kg (500 + 150).

Once again the CC Magazine prints promotional material without engaging brain and considering the interests of CC members.

 

[RogerL]

The Navara will be plated with a Gross Train Weight - that cannot legally be exceeded, whatever the type of trailer.

 

[Lutz]

I don't follow you, Roger, even though my numbers may be wrong (they may have been for an older model). The towing limit of 2700kg that the Nissan website quotes applies only to towing with a towball on a conventional towing bracket. This may or may not be the same for a fifth wheel coupling. I was assuming it isn't because the determining factor for the towing limit is often a structural one and the structure of a fifth wheel coupling is completely different to that of a towing bracket. If the payload is 1050kg, then, assuming one would want to keep a minimum of 400kg for passengers and their luggage, the difference, i.e. 650kg, would be available for the fifth wheel to support. Consequently, the axles of the trailer would have to carry 4300 - 650 = 3650kg. As this is over 3500kg, the fifth wheel unit would have to have its own independent braking system, but that is normal anyway with a fifth wheel unit as there are no provisions for an overrun brake on a fifth wheel.

 

[RogerL]

I don't follow you, Roger, even though my numbers may be wrong (they may have been for an older model). The towing limit of 2700kg that the Nissan website quotes applies only to towing with a towball on a conventional towing bracket. This may or may not be the same for a fifth wheel coupling. I was assuming it isn't because the determining factor for the towing limit is often a structural one and the structure of a fifth wheel coupling is completely different to that of a towing bracket. If the payload is 1050kg, then, assuming one would want to keep a minimum of 400kg for passengers and their luggage, the difference, i.e. 650kg, would be available for the fifth wheel to support. Consequently, the axles of the trailer would have to carry 4300 - 650 = 3650kg. As this is over 3500kg, the fifth wheel unit would have to have its own independent braking system, but that is normal anyway with a fifth wheel unit as there are no provisions for an overrun brake on a fifth wheel.

I can only repeat my post directly above - the Gross Train Weight is an absolute figure.

 

[ProfJohnL]

Hi Ken.

Lutz and Roger have tried to give answers, but I am not sure of the actual legalities here, but, there are some fundamental differences between a 5th wheel trailer and a more conventional caravan on a tow bar.

Because the 5th wheel trailers tend to have their road wheels well to the rear of the trailer, the trailers weight is more evenly shared between the hitch and the axles. Also because the 5th wheel plate is located within the wheel base of tractor, the 'nose' load is shared between both axles on the tug, rather than producing an opposing axle load between the front and rear axles.

For this reason above it is even more important to consider all the axle loadings. A significant proportion of the trailers mass will be supported by the 5th wheel, which reduces the towed mass on the trailers axles. The 5th wheel load will be shared between the tugs axles. This is good news because as the trailers load increases so does the tugs 'weight', which sticks it more firmly to the road.

By using the axle load measurement method, it automatically includes the weight of the 5th wheel gear and all payloads etc.

As with any mechanical system, none of the vehicles capacities should be exceeded. So consideration must be given to axle loadings, Gross train weights, pay load in the cargo bay, and how it is anchored etc. Provided all loads are kept within the specified limits, then there should not be a problem.

 

[KnL]

Thank you all for your replies,

Ken.

 

[Zafiral]

As the Nissan would have to be significantly altered to allow the coupling to be fitted, could it be replated at this time?

 

[RogerL]

Civical - yes it's possible that Nissan UK have re-plated it after fitting the necessary bits in the loadbed.

Neither a dealer, nor owner, can re-plate a vehicle.

 

[Zafiral]

RogerL

Owing to the amendments necessary to the pick up my guess is that it came specially from Datsun or if it was altered then the differences were so major it required the V5 to be altered.

Would you be able to replate a vehicle if you applied for SVA (single vehicle approval) the same as you would do if you imported the vehicle.

 

[RogerL]

Civical - Fitting fifth wheel turntables is normally done by third party specialists, not manufacturers.

I can't ignore the possibility that Nissan have re-plated this Navara, but I doubt it.

The original GTW would have been set as high as possible for marketing purposes so considerable engineering changes would be needed to increase the GTW.

It's just not as simple as a caravan manufacturer fitting a 1500kg chassis but plating it for 1400kg and allowing upgrades to 1500kg with no technical changes.

 

[gary]

A 5th wheel is always going to be more stable but the combined weight of GTW is nothing to do with that, it's purely to protect the vehicles mechanics from self destruction. Under those circumstances I don't see how it can be raised?

GTW is surely then cast in stone and whatever is stated on the truck is all that is allowed by law, no matter if it's on a 5th wheel, on a tow hook or piled on top!

 

[weejock]

Most comercial vehicle may be ordered with diffrent weight axles and many have an svo operation that carries out these modifactions once the vehicles have left the production line. These vehicle are then plated accordingly.

This is by no means an unsual thing and if you think of any land rover that has been converted for BT or power companies, Police vehicles. Transits that have platform lifters on the back, Ambulances the list is pretty endless.

Even in my line of work which is not special at all apart from a fridge on the front of vans we specify an upgraded front axle.

So without actually seeing the plating certificate any speculation about weights is just that meaningless speculation.

 

[ProfJohnL]

I have seen many of these 5th wheel arrangements for pick up's in Canada. Usually the necessary brackets and supports for the 5th wheel are designed and bolt/clamp to existing structures designed for this or other bolt on features. Things like horizontal platforms to make a flat bed, hydraulic access lifts, breakdown hoists, camper cabins, cabling winches, water or fuel bowsers etc

I don't know if the pick ups available here have the same forward thinking provision for add ons.

 

[weejock]

John it really depends upon the manufacturer. Virtually all chassis cabs have a "body builders guide" for fixing points etc and for electrical connections.

What I should also have said in my previous post was on a comercial vehicle it is very easy to change the plated weight.

If you venture a look at some tax discs you may see a vehicle that is say clearly a 12 tonner but is only licensed for 10. The operator has merely had the vehicle downrated. A lot of supermarket 40 tonners are replated to 32 tons etc.