Noseweight checking

[JohnM20]

To make life a little easier when checking my caravan's noseweight I came up with the following idea, but are my thoughts/calculations correct.
Although I normally use a noseweight gauge these aren't particularly accurate. Using bathroom scales is more accurate but involves having a piece of wood pushed into the towhitch and onto the scales and I have heard of instances where the wood has flipped out. I also worry about possible damage to the stabiliser pads.
My idea is to weigh the 'van using the jockey wheel onto the bathroom scales. I know that being a foot or so behind the towhitch will produce a slightly lower reading on the scales but what if I first of all weigh the 'van at the towhitch and then weigh it again at the jockey wheel. I would then know that let's say 90kg on the towhitch = 85kg at the jockey wheel (not actual figures - just for illustration). Knowing what this difference was would allow me to make a fairly accurate assessment of what the actual towhitch weight was and I could keep a note of what the maximum and minimum weights should be at the jockey wheel position.
I don't think there would be any need to make sure the jockey wheel was pointing in the same direction each time I weighed the caravan (although that would be no problem) as the actual load will be down the jockeywheel screw which remains in a constant position. Please correct me if I'm wrong.
Any comments will be welcome.
JohnM

 

[Lutz]

If you are going to stand the jockey wheel on the bathroom scales, then it will make a slight difference if the wheel is pointing forward or rearward. If it is pointing forward it will be closer to the hitch and consequently closer to the actual noseweight at the hitch, although one would still have to correct for the difference. Because the point of contact with the bathroom scales is not directly vertically under the jockey wheel tube the scales do not measure the force in the tube, as you seem to think, but the force in the jockey wheel itself, and this is not the same.
Therefore, if you are going to use a formula to correct for the position of the jockey wheel, the jockey wheel will always have to point in the same direction.

 

[JohnM20]

Hi Lutz
Thanks for your reply. I guessed you would be one, if not the first one, to reply.
I wasn't absolutely sure about the jockey wheel pointing in the same direction each time but I thought you would know the maths. Is the rest of the idea basically sound ?
JohnM

 

[Lutz]

There is nothing basically wrong with using the jockey wheel as the support for measuring noseweight, so long as you correct for the difference in leverage from the fulcrum point (the caravan's axle).
Most proprietary jockey wheels with integrated noseweight gauges, like those that AlKo make, already correct for the offset between hitch and jockey wheel location (at least by close approximation). However, to minimise error, you do need to observe the corresponding operating instructions, though. They may state which way round the jockey wheel has to point.

 

[ProfJohnL]

Hello John

I fully agree with Lutz, but there is one further point where you were in error, the load as measured at the jockey wheel will actually be greater than the load at the hitch. (L1xF1= L2xF2)

By all means do the comparison weights, as that will establish the correction factor for your caravan e.g. Actual Nose load divided by load at the jockey wheel will give a result that is less than one. In future when you measure the jockey wheel load you simply multiply the reading by the correction factor to calculate the load imposed at the hitch.

May I also re-emphasise the need to always measure the load with the jockey wheel pointing in the same direction relative to the caravan, and that the hitch must be adjusted to be the same height as when it is attached to the car with all luggage and passengers. Fortunately it is relatively easy to make the necessary height adjustment with the jockey wheel.

 

[TD42]

as allready said by Lutz & John M .provided that the jockey wheel is always in the same direction & the van is level then you should have no problems.I found my van weighed at the jockey wheel was 10kg lighter than when weighed at the hitch

TD

 

[Lutz]

Two corrections to the above. It's not the caravan that should be level but the ground on which it stands. The caravan should stand at the same attitude as when it is attached to the towcar. If it is nose down in that state, the noseweight should also be measured nose down. The opposite obviously applies if it is nose up.
By the laws of physics, a load measured at the jockey wheel will always be higher than at the towball, not lower as you mention.

 

[Reg]

For 2 wheel (single axle) only:-
1 - Measure distance from centre of wheel to centre of coupling ("A") and multiply by required nose weight ("NW")
2 - Measure HORIZONTAL distance from centre of jockey wheel to centre of coupling ("a") and subtract from "A" then divide into "1" . Note that I have stressed that it MUST be the horizontal distance.
i.e.
"A" x "NW
---------------- = Nose Weight at Jockey Wheel.
"A" - "a"

An example - If "A" = 5.0m, "NW" noseweight at towball = 80Kg and HORIZONTAL dist from towball to centre of jockey wheel "a" = 20cm.

5m x 80Kg
-------------- = 83.33 Kg at Jockey wheel.
4.8m

As was previously said the jockey wheel is best in the most forward position and the nose weight should always be checked in that position.
I do not have any experience with twin axles but I would imagine that the same calculations would apply measured from the front axle.

 

[ProfJohnL]

Hello Reg,
Twin axles are far more complex, because the fulcrum point will change depending on the relative loads on each of the axles, and the degree of suspension compression.

To simplify the mechanics for the purpose of this explanation assume the trailer and its axles and wheels are rigid (i.e no suspension)

If the trailer hitch is very low, then the forward axle is fulcrum point,and the rear wheel will be off the ground but as you raise the hitch, the rear wheel comes into contact with the ground, and the fulcrum is transferred to the rear axle, IIn real life, the situation is more complex than that, because the fulcrum point will be affected by the complication of the trailers tyres and the suspension.
Depending on the design of the chassis The difference in the fulcrum point may be as much as a metre. That will certainly affect the results of any calaculated nose weight, and that is also the reason why TA must be set up very carefully, as the nose weight due to the height of the hitch does not have a a smooth transition but it can change very rapidly from a significant positive to a significant negative reading over a very small hitch height difference.

I have seen several a TA caravans where the hitch will just sit in the air if the jockey wheel was raised, in other words the caravan was actually balanced, but try to lift the hitch and it would only rise a short distance before it became too heavy to lift any further - Zero to 80Kg in matter of centimetres Its all down to the interaction of the two axles and suspension and the additional weight of any axle and wheels that change from carrying weight to actually dangling.

 

[Lutz]

The transfer of load from the front pair of wheels to the rear on a twin axle as you raise the front end (or vice versa when lowering) depends on how stiff or soft the suspension is (and, by the way, the tyre pressures), so this would be different for each caravan. There is therefore no simple formula for determining the exact fulcrum point.
The easiest way if you want to use the jockey wheel as the support on a twin axle is to measure the noseweight at the coupling first and then again at the jockey wheel with the caravan standing at the same height for both measurements. By applying such a practical test one does not have to rely on any theoretical formula.

 

[Reg]

The force (load x dist) at a certain height will be the same at the jockey wheel and ball hitch.
If you change the conditions for one or the other then of course you will get incorrect/different readings.
The position of the wheel, back or forward, will make a difference as you are adding (or subtracting) another lever.

But as you say Lutz if you are not sure then check it first at the hitch then the wheel.

 

[Rubix]

Blimey this is getting a bit heavy ! (bad pun I know)

I have a related question. I have measured my nose-weight on my drive with the 'van level, but my drive is on an incline (I park the 'van with the nose into the house) so the hitch height is less than it would be on level ground and at a height that is the same as when hitched to my car.

Is there a way of working out how I can compensate for this (the road outside my house is also on an incline) I can see there are many variables, but I would have to take my bathroom scales down to the nearest dual carriageway with a lay by, which is a bit excessive just to get an accurate measurement.

Which is more important : having the road level, the same hitch height (compared to your tug) or the colour of my roses in my next door neighbour's garden ?

Thanks for your help

 

[Lutz]

Load x distance is not force, Reg, but moment.
Rubix, one can compensate for a sloping drive. Using a spirit level, check how the caravan stands when hitched up to the car on level ground and then make sure the spirit level indicates the same when the caravan is standing on the slope. Keeping the caravan like that you can determine the actual length of support that you need for the bathroom scales (which will be different to the length on level ground and consequently only of use on your driveway.) Don't forget to chock the wheels and release the handbrake first or else you will get an incorrect reading of the noseweight.

 

[Jennifer2]

Has anyone tried the newish Reich thing that checks the noseweight? Not the weight control, rather the one that you use on your towball.
There are single and twin axle models.

Does it actually give an accurate reading for a twin axle?

 

[Lutz]

It cannot be absolutely accurate because it goes between the coupling and the towball and therefore raises the front end of the caravan by the same amount. It's just an easy and convenient way of quickly measuring an approximate value.

 

[Reg]

Hi Lutz
Quote
"In physics, measure of the turning effect, or torque, produced by a force acting on a body. It is equal to the product of the force and the perpendicular distance from its line of action to the point, or pivot, about which the body will turn. The turning force around the pivot is called the moment. Its unit is the newton metre."
The " Turning Force" is called the " Moment"
So I think we can both claim a point!!
It;s good keeping the grey matter working still.

Regards
Reg

 

[Jennifer2]

So possibly worth having?

 

[Lutz]

Difficult to say. It depends whether you are willing to pay for the ease and convenience at the expense of accuracy.

 

[Lutz]

I'm not at all happy with that definition that you have dug up. The first sentence is alright by stating that a force is a force and a moment is the product of a force and a distance, but there is no such thing as a turning force.

 

[ProfJohnL]

Bathroom scales are probably cheaper, and likely to be more accurate, and they can be used for other things as well such as seeing if you had too many burgers the night before.

 

[ProfJohnL]

Hi Rubix,
The important bit, Why would you have your roses in next doors garden in September 2010, I though everyone know that was only allowed if the neighbours husband was away on the first Wednesday evening of the month!

For a number of good and long-winded reasons the nose load varies according to the height of the hitch, It is the real height of the hitch relative to the axle that is important, which is why it is simplest to use a level (horizontal surface) as this minimises variables that can affect the measurement.

As Lutz points out it can be done on a slope, but it becomes fairly complex to compensate for the incline. Whilst you could arrange one occasion to use some level ground and set up the caravan, and then check what the reading is when you get the caravan home and on the slope. Then calculate percentage correction difference.

You could then use the correction to modify any results you get when loading for different trips. Again as Lutz suggests it is important to chock the wheels and release the brakes before taking a reading.

 

[Philspadders]

rotational dynamics, torque, turning moments , its getting very technical next will come deceleration effects , velocity time graphs, structural calculations.
a caravan is essentially a beam , not uniformly loaded so calculating the turning moment or actual load on the hitch would require some detailed measurements of the caravan , the contents, the mass of the contents, the distribution of the contents , and the centre of the hubs would be used as the datum or "0" point.

a caravan is essentially a beam , not uniformly loaded so calculating the turning moment or actual load on the hitch would require some detailed measurements of the caravan , the contents, the mass of the contents, the distribution of the contents , and the centre of the hubs would be used as the datum or "0" point.
So stick it on a set of bathroom scales with the hitch at towing height , weather its on a slope or not matters not one jot , but the height of the hitch does. just chock the wheels to ensure that it will not move down the slope, and the unit will rotate about the hubs.

Bathroom scales are not that accurate by the way . they are mass produced and un calibrated. they can be up to 5kg out.
Mx =

(

)2 dx =

x dx =

x2

=

My =

x

dx =

x3/2 dx =

x5/2

=

=

=

=

=

=

=

the above may help you out a bit if you have some time.

 

[Lutz]

I don't know what to make of the previous reply, but standing on an incline certainly does make a difference to the reading of the noseweight. Obviously, it does depend how steep, though. We have been given no indication of how steep the one is in the case in question, so it is difficult to judge whether the difference is likely to be significant.