Noseweight when parked on slope

[Anita_5109950]

We have a problem in that when the caravan is parked at home, finishing off loading for a journey, our caravan is parked on a slope.

All noseweight advice I've ever seen says that the caravan must be parked on a level surface to read the correct noseweight, so there's the problem.

If the caravan is parked with nose downhill it's not so bad as we can judge the gradient and put blocks under the noseweight gauge to compensate, but obviously this is not possible when the caravan is parked with nose up the slope, which is the normal position on our drive. I can think of no level parking area within at least 5 miles of home which would be a suitable place to check noseweight, even our storage facility is on a slope.

Any help or advice please.

 

[ProfJohnL]

Hello Anita,

The problem with attempting to measure the nose weight of a caravan on a slope is that down force changes depending on the slope.

To avoid this variation the method used to for legal purposes is to measure on the flat with the tow hitch of the caravan at the same height as the tow ball of the car.

Technically and because the centre of gravity of the caravan lies above and a little forward of the main axle, the turning force this generates is the nose weight. However, if the caravan is already nose down, then the centre of gravity is moved further forward of the axle, and so it creates more down force, conversely, if the caravan is nose up, then the down force reduces. To illustrate the point, in some cases a tow hitch can be lifted up and caravan can be tipped back. Because the Centre of gravity has moved to a point directly above or even just behind the axle, the caravan will stay tipped up.

To be absolutely sure of the nose weight, you must check on level ground, but for obtaining a reasonable approximation, I assume the caravan is pointing down the slope. Chock the main wheel so they cannot roll, and release the hand brake. Put the gauge in the tow hitch, and raise the gauge until the caravan is level. When the caravan is level the down force can be read on the gauge. Do not for get to re-apply the hand brake after wards.

 

[Lutz]

John L's method of ensuring that the caravan is standing level on the slope before measuring is fine but you will need a spirit level because it is difficult to gauge by eyesight whether it is truly level or not. And, of course, it only works if the slope is not so severe that the back end of the caravan is touching the ground when you've levelled it.

As he correctly points out, it is also only an approximation because the caravan should actually be standing at the same angle as when it is hitched to the car on level ground.

As the centre of gravity of a caravan is relatively high up, measuring noseweight accurately is a problem because even small changes in attitude result in big changes in noseweight.

 

[gary]

I agree with John and Lutz having tested this on the quite gentle slope of my own drive, the nose weight increased by some 30% while being parallel to the ground facing downhill.

Your question though was about measuring the van pointing uphill while keeping the van level, this not allowing room for the nose weight gauge to fit. So given all the above, why not use bathroom scales?.

 

[RogerL]

It won't help Anita, but it's the ground that should be level for accurate measurement, not the caravan.

The caravan should be on level ground with the tow hitch at the same height as the car's towball will be with the caravan hitched. So if the caravan normally tows slightly nose-down, the nose weight should be measured at the same angle.

Only in this way can the static downforce on the car's towball be measured (in the absence of expensive scientific instruments)

 

[MikeA]

A query for the physics experts among you. If you park the van facing downhill and measure the noseweight; then turn it round to face uphill and measure again - is the average measurment between the two anaccurate one?

Mike A

 

[RogerL]

It's probably close enough for practical purposes but it won't be exactly accurate without including angle function calculations.

Next time you've loaded up and hitched up for a trip, stop at a safe, convenient place where the road is level. Place a builders spirit level on the caravan floor over the axle and note the exact position of the bubble. Then when you're loading up set the caravan at that same angle using the same spirit level, regardless of your sloping drive, and measure the noseweight using bathroom scales under the jockey wheel.

This method doesn't have complete scientific accuracy but is good enough for practical purposes.

 

[Lutz]

As normal noseweight gauges only allow you to measure without the caravan being hitched up to the car little would be gained by taking two readings, one uphill and the other downhill, and then calculating the average. One might just as well measure with the caravan standing level (or to be absolutely accurate, with the caravan standing at the same attitude as if it were hitched up to the car on level ground).

Theoretically it would be possible to measure the noseweight with the caravan hitched up to the car but I don't know of any gauge on the market that does that. Even then, taking the average would only be correct if the car has linear springs on the rear suspension. If the springs are progressive, it would give an incorrect result because the downhill reading would be too low.

 

[MikeA]

Hi Lutz,

Not quite sure why you are bringing the car into the equation. Like Anita, I have the problem that convenient level ground is scarce in our neighbourhood. The driveway is a consistent slope and we have a motor mover fitted to the van, so it would be much easier to swap the van round from downhill to uphill for two measurements than it would be to go and find some level ground.

Of course I could just wait until we are pitched on a level site, but that would involve remembering to take the noseweight guage with me and so far I keep forgetting.

Cheers,

Mike A

 

[RogerL]

It's a good idea to keep the noseweight gauge as part of the caravan equipment.

It's surprising how the noseweight varies by 5-10kg just by marginally different packing, particularly between going and coming.

I check the noseweight every time I hitch up and adjust accordingly.

 

[Lutz]

The reason why I brought the car into the equation, Mike, is because noseweight should always be measured with the coupling of the caravan at the same height as if it were hitched up to the car. If the noseweight is low, the towball will inevitably be higher than if the noseweight is high (due to the spring rate of the car's rear suspension). Now, if you measure once uphill and again downhill and the spring rates are linear, averaging the two results will cancel the effect of the towball being once higher and once lower from the ground, depending on the tilt. However, if the spring rates are progressive, the towball will be higher with a high noseweight than with linear springs. Maybe the difference is small and perhaps it is negligible in practice but to be 100% accurate, it would have to be taken into consideration in the equation.

 

[mike_5041102]

It is not only ground slope that is a concern, do not forget nose weight gauges are notoriously inaccurate, mine reads 15kg too low.

 

[Lutz]

By holding the front end of the caravan by hand at the right height I reckon that with a bit of practice one can estimate the noseload to the same accuracy as the average noseweight gauge. I find that 75kg is about the most that I can comfortably hold for 5 to 10 seconds, so that's what I do. I would agree, however, that others might find it uncomfortable and again others really easy, so it's perhaps not a method that works for everyone.

 

[RogerL]

By holding the front end of the caravan by hand at the right height I reckon that with a bit of practice one can estimate the noseload to the same accuracy as the average noseweight gauge. I find that 75kg is about the most that I can comfortably hold for 5 to 10 seconds, so that's what I do. I would agree, however, that others might find it uncomfortable and again others really easy, so it's perhaps not a method that works for everyone.

Those of us who are less mobile struggle with much more than 30kg so lifting 75kg is out of the question.

I originally calibrated my noseweight gauge on the bathroom scales by applying 75kg (as registered on the scales) downwards while my wife marked the noseweight gauge. Couldn't do it now as I've lost 20kg since then!

 

[runnerphil]

I normally level the van and then use blocks to sit the nose weight on. Is this not right?

 

[Colinn]

Surely Lutz, lifting/holding capability is dependent on your fitness at the time and will change as you exercise or age (!). In my experience what seems light and easy one day can be nye on impossible on another! I'm sticking with the bathroom scales...

 

[mike_5041102]

Checking a 75kg nose weight by lifting is also an excellent method to determine the condition of the spinal joints. Could be a bit of a bummer thou if the test is negative and the intention was to start a holiday!

 

[Lutz]

Nothing wrong with bathroom scales but I would challenge anyone to tell the difference between the stability of their outfit at, say, 65kg and 75kg noseweight, all other conditions being equal. I reckon that would have to go down to 50kg or lower before the average person would notice an appreciable change. That being the case, my rough-and-ready method is good enough for me.

 

[cooky1]

I don't think you can expect to measure the noseweight by hand with any degree of accuracy. I beg to differ but I think a noseweight difference of 10kg does make a difference and it is noticable when towing, ecpecially in not so favourable conditions. Why do you think manufacturers state a maximun drawbar weight on your towing vehicle. Also why do manufacturers sell noseweight gauges.

 

[Lutz]

There's no denying that the higher the noseweight, the better the stability of the outfit. The maximum specified by the car manufacturer is based purely on structural constraints. If all underbodies and towbars were designed to take 100kg, there would be no cars with a 75kg limit. Obviously, a deviation of 10kg is going to have less of an effect at 100kg than at 75kg.

Have you tried towing with different noseweights under otherwise identical conditions? If so, when did you feel that you noticed an appreciable difference in the way the outfit handled?

 

[cooky1]

I have found in the past that when the noseweight was a bit on the light side the caravan tended to move more from side to side ( not quite snaking). I stopped and moved some items to slightly increase the noseweight and the stability of the outfit improved.

By the laws of physics if you work out the difference in downward force on a drawbar at 65kg and 75kg taking into account the distance the tow hook is from the van wheels then it is noticable.

I don't think you will ever get identical conditions to test the theory but surely accurately measuring weights and sticking to manufacturers guidelines can't be a bad thing. I am sure the manufacturers engineers know whay they are talking about.

 

[Lutz]

I fully endorse the bottom line of your statement, Graeme. It's just that, legal implications aside, I don't see the rest quite as dogmatic.

 

[cooky1]

OK, maybee I am just being too cautious.

 

[Guest]

Just out of interest what is the 'safety' tolerance used by towbar manufacturers in their products. I appreciate that if a figure of 75kg is given then it should be adhered to as closely as possible, but I also doubt that at 76kg the towbar will fail. For instance Volvo state the load on their own towbars should be 'approximately 75kg'. What does approximately really mean in terms of kg? Also the stresses of road conditions will mean that the efrective weight on the bar will lighten, or increase quite a lot. I admit to being a cynic, but I suspect that there are probably more outfits out there over the limit than are acknowledged. Fortunately, failures of towbars seem to be rare.

 

[Lutz]

I think you will appreciate that the 75kg figure, or whatever it is, is a purely static specification. Of course, no towbar will fail under 76kg or even 100kg static load. It's the dynamic endurance load that counts. The standards call for 2 million cycles under specified conditions which take not only the vertical noseweight but also the horizontal towload into account. Such dynamic forces would be equivalent to a very much larger static force.