Measuring Nose-weight

[Rubix]

How important is the angle of the "road" compared to that of the caravan chassis & hitch height (as it should be the same as when it is attached to the tow-car) ?

By that I mean what sort of deviation in nose-weight figure is likely with a 5% difference in either angle or hitch height ?

Does anyone have an accurate figure for these, or a way of calculating them.

 

[rob_jax]

No and it's impossible to calculate or estimate - because it depends how much weight you have "high up" in your caravan.

If all the weight in a caravan was mounted level with the centre point of the wheels - then you could.

The problem arises because a caravan is a big white box - and there's a fair amount of weight high up (bodywork, lockers, stuff in lockers, etc) And that'll differ from van to van, and from load to load.

Probably the way forward for you is to do a simple experiment - measure the nose-weight with your caravan level, then nose-down in the towing position, then nose-up by the same amount. If you detect a significant change in weight across the three positions - then yes it is important. If not - then forget it.

Robert

 

[ProfJohnL]

Hello again Rubix,

The simple answer to your question is no. There are too many variable that by them selves are difficult to measure. It is far simpler to measure the actual nose load, which is the characteristic that the towing regulations specify.

Whilst the physical dimensions of the caravan don't change, the way you load it does. You can see that directly affects the mass of the caravan, but it also affects the position of the caravans Centre of gravity. The nose load is the result of the mass of the caravan acting vertically down through the centre of gravity, and how that load is shared between the main wheels and the tow hitch.

Anything that changes the turning moments, such as the horizontal distance between the centre of gravity and the main axle (I.e. tipping the caravan relative to the true horizontal) changes the nose load.

Any error or allowance that will cause the official nose load to exceed the limits for the car, hitch or caravan must be avoided. The regulations specify how the upper limit is established, and that is a real limit. If you exceed it you are breaking the regulations, with all the knock on effects that has for your licence, insurance and vehicle grantees etc.

 

[Rubix]

Thanks John & Robert,

I did a "structures" module when I was at school which in lay-mans terms was very basic civil-engineering where we learnt about calculating moments of bridges etc.

It didn't quite go to this level so I really didn't realise that it was that complex.

I didn't realise a different centre of gravity could change these calcluations, but I will accept that you know more than me and appreciate it would take far to long to explian.

My reason for asking is my drive is not level and so when I have measured nose-weight appreciate that my numbers aren't 100% accurate and was hoping for an approximate devaiation so I could try and be more accuarate. I also appreciate that my measuring device (bathroom scales & a bit of wood) are not going to be higjly accurate either.

Now why don't you pull the 'van off your driveway and measure it on the road I hear you cry, well that too isn't level, so I would have to drive the 'van 1/2 miel down the road to a layby that IS level.

I have always set up the van to be level (with a spirit level) so the ground to hitch is less than it should be as I have an uphill sloping drive way (hitch to the house).

I will look into this a bit more myself - the wonders of the internet.

Thanks again.

 

[Lutz]

If, by sheer coincidence, the caravan is level when hitched up to the car, you could measure the noseweight on a slope if you level it first using your spirit level. However, this only works for a single axle. If you have a twin axle, the difference between the axle load on the front and rear pair of wheels would falsify the result.

 

[Rubix]

I'm not sure I understand you Lutz.

I understand the problems with a TA, but what you are saying contradicts what others (on here & magazines/books) say

Can you elaborate please.

 

[Lutz]

Noseweight should always be measured with the caravan at the same attitude as when it is hitched up to the car and both car and caravan are standing on level ground. If, in that case, the caravan happens to be standing level, too, then you can also measure with it standing level even on a slope, but you will have to use a spirit level.

 

[ProfJohnL]

Hello Rubix,

The problem is almost as difficult as the cube you invented!

The regulations require the nose load of a trailer to be greater than 25Kg (EU specified minimum) but less than the lowest upper limit specified by the the car, tow ball and caravan manufactures. It is generally accepted that towing stability is maximised when a nose load close to the permissible limit is achieved.

The other important EU specification says that the when attached to the tow vehicle and both are loaded and ready to tow, the centre of the tow ball must fall between 350mm and 420mm above the the surface of level (Horizontal) ground. As these regulations are law, and they specify limits, there is no margin for error beyond those limits.

The consequence of being outside the limits is that by definition the outfit is legally not road worthy.

I can understand your frustration on this matter. The fact is, it is far quicker and easier to measure the actual nose load than to try and calculate it. The use of bathroom scales with a suitable piece of wood (and spreader to avoid damage to the scales) is a perfectly acceptable method provided you can allow for the uncertainty of measurement of the scales.

But lets try to be practical here and find a solution for you.

For reasons that have be explained elsewhere in the forum, the nose load of the trailer will vary depending on how high the hitch is compared to the road wheels. The nose load increases if the hitch is lowered, and decreases if the hitch is raised. The degree of change is difficult to calculate, but it is significant if it means that under regulation measuring conditions the nose exceeds the permitted maximum.

The regulation condition require the caravan to be on a level (Horizontal) surface. The hitch height is measured vertically from the horizontal to the centre of the ball. So relying on the fact that nose load greatest when it is at it lowest height, then if the load gauge is set to give the minimum hitch height of 350mm then at all other regulation hitch heights the nose load will be slightly lower.

As the hitch height has to measured from the horizontal line from the road contact of the wheels, any slope of the ground compromises the use of a fixed 350 mm length gauge. What you need to do is to establish how much height is lost or gained as a result of the slope over the distance between the hitch and the wheels. If you then change the length of the gauge by this variance, when the hitch is lowered onto the gauge the required 350mm elevation above the horizontal will be restored, and the nose load measurement will regain its accuracy.

The gauge will only remain accurate provide the measurement is taken on exactly the same slope, and in the same direction, and with a caravan that has the same hitch to wheel dimension.

I hope this helps.