Twin Axle Noseweights - What about axle Load ?

[JohnnyV]

Hi all,

We continually have a lot of Questions about Noseweights, with the general advice being "as heavy as possible within the limits of the Cars maximum permissable (75 - 100kg)" - magazine articles also add comments like 'with a generous 120kg hitch load capacity this car.....'

Now with a Single Axle I totally understand the advice, however I tow a twin axle with a normal noseweight of about 90kg - My question is surely that as you increase the noseweight you increase the Axle load on the Front Axle, and reduce it on the Rear axle ??

Is this too insignificant to worry about, or should we be thinking more about this rather than just 'loading the nose'??

I am sure someone will have a definitive answer.

Jon

 

[Lutz]

Within realistic limits of change of noseweight, the change in axle load between the front and rear pair of a twin axle is insignificant and not an issue.

 

[602]

Hi,

Al weights should be measured with the vehicles on a horizontal surface.

I suppose the are times when one or the other of the trailer axles is hanging in mid-air.

My twin axle boat trailer (2500kg)would jerk the back end of my Land Rover up and down when the trailer wheels went over a WHITE LINE.

INDESPENSION said that tests on a pair of 1000kg suspension units revealed occasional, brief, loading up to 4000kg.

602

 

[Gafferbill]

........yes and the same is true of noseweights.

Vehicles, towbars and trailer hitches are built to cope with shock loads.

For this reason the practice of setting the static noseweight to precisely meet manufactures recommendations is pedantic.

A ballpark figure is sufficient.

With a twin-axle, if the rear axle is "hanging in mid air".....

the noseweight can be momentary negative!!

 

[ProfJohnL]

Hello gafferbill

I must contest your assertion that "a ball park figure is sufficient"

Given that nose loading has legal criteria applied to it, and it is an offence to overload the tow bar of the tow vehicle, if a 'ball park' figure happens to be above the permitted maximum for that outfit, it can have a detrimental effect on your licence.

You must not exceed the static vertical nose load as specified by your outfit.

 

[Dustydog]

Gafferbill / John L

In my Physics study days I was told a static load is completely different from a dynamic load . Obviously there is a relationship but I wonder just how much safety margin is actually given by the towbar and vehicle manufacturer in the event the stated load is exceeded?

In safety engineering terms is it possible that the margin is a factor of say 2?

I think John is technically correct but clearly there must be merit in Gafferbill's "Ball park figure".

Safety critical items have been discussed a lot here recently. Should we all now carry torque wrenches and nose weight guages?

Cheers

Dustydog

 

[Lutz]

John only referred to the possible implications of exceeding the noseweight limit. Nobody has suggested that the value has zero tolerance, but this tolerance will always be an 'equal-to-or-less-than' one.

 

[ProfJohnL]

Hello Dusty,

Lutz has interpreted my point correctly.

The reason that the regulations measure the static vertical load, is because it is repeatable, and because it is practically quite difficult to measure the dynamic load that occurs with the vehicle in motion. You would have to define the motion, and I don't think VOSA would want to build special test tracks every where just to check nose loads.

The actual dynamic loading factor can be much more than just 2 x the static. From test run at MIRA pave track on a car towing a caravan, peak accelerations of 8G were recorded at the hitch, but these were for very short durations, and the more common place figure is 2 to 3G.

The car manufactures should be very aware of these dynamic effects and de rate the load capacity accordingly to cover themselves.

 

[Dustydog]

Thanks John for clarifying that.

I'd forgotten the legal aspect and was more focussed on the structure aspect.

Thus it seems to me the carrying of a nose weight guage is an essential.

Cheers

Dustydog

 

[Gafferbill]

John L and Lutz. we are in total agreement.. my ballpark figure was in the range legal maximum to minus 5 kilograms.

My contribution was a comment on posts on this forum, that some caravaners seem to spend a huge amount of time setting the nose weight exactly to the legal limit of the vehicles towbar.

They also seem to unduly worry if they do not get this precise match.

Pitching is a characteristic of caravans... by definition a downward movement at the hitch must be because of an increase in noseweight and vice versa.

As John L says .. " The static load can be easily measured and legislated for" so this is what we concern ourselves with.

However we should not forget what is happening when the outfit is in motion.