Towing - caravan bouncing car around on bumpy road

[ProfJohnL]

Let's say the distance from the hitch to the axle is about 3.5m. Let's go mad and say there's a one foot (30cm) height difference. That means the caravan will be tilting down (or up) at about 5 degrees. Therefore, the measured value will be about 99.6% (Cosine 5°) of the true value. For a measured value of 90kg, the real value will be 90.34kg.

You have neglected the height of the caravan's centre of gravity above the axle. If it is 500mm (I don't know anything better) then with a change of hitch height of 150mm (the Milenco spring range I understand) and an axle-to-hitch length of 3500mm, then the CG will move horizontally relative to the axle by 150*500/3500 mm = 21mm.
If the caravan weighs 1250kg, then this generates a moment of 1250*21 = 26,250 kg.mm
This will cause a change of reaction (ie weight) at the hitch point of 26,250/3500 = 7.5 kg
A change of, or error, of 7.5kg is certainly the sort of figure that people here are concerned about. For other caravan weights and axle-hitch distances you can simply factor the above figure.

I think in your calc you are talking about the shortening of the horizontal axle-hitch length due to the inclination. Yes, that would be tiny and I have neglected it.

Thank you for trying to put figures to it, and the figure of 7.5kg you have derived is certainly within the range I have expected and experienced. I have deliberately avoided publishing a change figure, becasue it will be different for each outfit, principally becasue of the way the caravan is loaded that will affect the height of the CofG. That is an unknown and its impractical for most caravanners to establish the height of the CoG to use in calculations.

I have come across some SA caravans where I would estimate the actual difference in nose load over a 150mm height difference to be in the region of 25 to 30kg So its far from a trivial matter.

But the biggest changes occur with TA caravans. Here the distance from the hitch to the effective pivot point changes when the caravan hitch is raised or lowered . That is becasue the suspension compression rates are not linear Consequently the calculation of teh change in nose load with hitch height for a TA is a mighty complex issue, and frankly its quicker and simpler just to measure the nose load correctly.

 

[Parksy]

Very bumpy B roads may be enough to account for the problem as speeds under 50 might be needed on some. Everything looks OK in theory so I would give it another outing and see how it goes.
The stabiliser pads may be worn if the caravan had done a lot of mileage or it could be they have been contaminated and are ineffective. I assume you had a new tow ball on the car, and if so it is essential to clean all the paint off this before use. If this was not done the pads will be contaminated and need at least cleaning with emery paper and brake cleaner to bring them back and if that does not work replacing.

I think that this question had been answered correctly in the first few replies.
Slow down over bumpy roads and take fewer items to cut down on weight.
The stabiliser pads won't prevent the caravan from pitching or bouncing the car around, a more careful approach is needed on uneven road surfaces no matter how you check your nose weight.

 

[KeefySher]

Very bumpy B roads may be enough to account for the problem as speeds under 50 might be needed on some. Everything looks OK in theory so I would give it another outing and see how it goes.
The stabiliser pads may be worn if the caravan had done a lot of mileage or it could be they have been contaminated and are ineffective. I assume you had a new tow ball on the car, and if so it is essential to clean all the paint off this before use. If this was not done the pads will be contaminated and need at least cleaning with emery paper and brake cleaner to bring them back and if that does not work replacing.

I think that this question had been answered correctly in the first few replies.
Slow down over bumpy roads and take fewer items to cut down on weight.
The stabiliser pads won't prevent the caravan from pitching or bouncing the car around, a more careful approach is needed on uneven road surfaces no matter how you check your nose weight.

Sage advice

 

[tadhgocuilleain]

A change of, or error, of 7.5kg is certainly the sort of figure that people here are concerned about.

This is my point exactly.
You're right that I didn't think about the CofG being higher than the axle - I just did a few rough calculations on the back of an envelope - but my point still stands. 7.5kg is neither here nor there. It's the weight of a bag of groceries; three litres of milk, a bag of flour, and a frozen chicken. That's not going to affect the handling of the vehicle.

Running straight and level, on a smooth road, the 7.5kg will have no effect. On a bumpy road, the weight of the caravan being thrown forward and backward with momentum is going to be far more significant than the 7.5kg inaccuracy.

People that only tow a caravan are the only ones I hear being pedantic about nose weight. Does a builder pull out the nose weight gauge before dropping a pallet of cement into a plant trailer? Or a farmer loading three tonnes of cattle into a cow box? How about when a big stallion decides to sit against the tailgate of a horsebox and your nose weight suddenly goes from 100kg to -200kg?

You load up your caravan as well as is practically possible and drive according to the conditions.

a more careful approach is needed on uneven road surfaces no matter how you check your nose weight.

This is basically what I'm trying to say.

 

[ProfJohnL]

Hello tadhgocuilleain

This is my point exactly.
You're right that I didn't think about the CofG being higher than the axle - I just did a few rough calculations on the back of an envelope - but my point still stands. 7.5kg is neither here nor there. It's the weight of a bag of groceries; three litres of milk, a bag of flour, and a frozen chicken. That's not going to affect the handling of the vehicle.

Running straight and level, on a smooth road, the 7.5kg will have no effect. On a bumpy road, the weight of the caravan being thrown forward and backward with momentum is going to be far more significant than the 7.5kg inaccuracy.

I would tend to agree with that assessment on the effect of a 7.5kg overload, but that has not been the main point of my argument. Its principally that it is within the capability of (Or should be in the capability of ) each caravanner to correctly measure the nose load they apply to their tow vehicles. It's important because it is generally the case that caravanners will tend to load close to the top of their available limits. This means it is equally possible to easily exceed the nose load limits. I stress I am referring to the static nose load limits, because that is the only practical way to measure nose loads. It is certainly the case that whilst towing the dynamic effects of motion ans inertia will change the applied nose by a a considerable margin beyond the static nose load limit. But that fact is well understood by the vehicle and towbar manufacturers and they will have incorporated the necessary loading safety margin in their designs. Even though the designs will indeed withstand several times the static load limit, you should not exceed the static nose load limit.

The apparent absence of recorded prosecutions for excess nose load is not an excuse to abuse the limit set by the manufacturer.

People that only tow a caravan are the only ones I hear being pedantic about nose weight. Does a builder pull out the nose weight gauge before dropping a pallet of cement into a plant trailer? Or a farmer loading three tonnes of cattle into a cow box? How about when a big stallion decides to sit against the tailgate of a horsebox and your nose weight suddenly goes from 100kg to -200kg?

The general non compliance demonstrated by others is not a reason to follow suit. Speeding is a classic example of this mentality.

 

[Hutch]

Tim. We are tryiny to help , newbies about the nose weight , "thing" t
The Prof and myself and others can open our own Websites about this , and argue, but we are trying to help the new caravannerers to get the best help without confusing them. .
I LIKE THE Bedroom scales, every six months to re affirm my caravan weights,
Hutch.

 

[Dustydog]

Tim. We are tryiny to help , newbies about the nose weight , "thing" t
The Prof and myself and others can open our own Websites about this , and argue, but we are trying to help the new caravannerers to get the best help without confusing them. .
I LIKE THE Bedroom scales, every six months to re affirm my caravan weights,
Hutch.

Are yours the same as the bathroom ones

 

[tadhgocuilleain]

Tim. We are tryiny to help , newbies about the nose weight , "thing" t
The Prof and myself and others can open our own Websites about this , and argue, but we are trying to help the new caravannerers to get the best help without confusing them. .
I LIKE THE Bedroom scales, every six months to re affirm my caravan weights,
Hutch.

I'm trying to help too but to be honest, I got carried away with the debate (I love a good debate).

The point I'm making is that people complicate the nose weight issue far more than is necessary. You measure it as well as you practically can, whether that's with a commercial nose weight gauge or the bathroom scales and a pile of books.

I wasn't suggesting in the examples above that people should disregard the legal limits. I was saying that most experienced towers aren't pedantic about nose weight. The recommended nose weights are only approximate figures, so there's no need to push it so close to the legal limit of the hitch that a few kilos of inaccuracy will push it over.

 

[Parksy]

Tim. We are tryiny to help , newbies about the nose weight , "thing" t
The Prof and myself and others can open our own Websites about this , and argue, but we are trying to help the new caravannerers to get the best help without confusing them. .

The incessant pontification at great length drives away quite a few new caravanners from this forum and who could blame them for moving elsewhere when a simple query receives the caravan equivalent of War and Peace :dry:

 

[Perry525]

Perhaps I am missing something?

My car can carry about 650 kilos, and I make sure that the load is as far a possible forward between the wheels. While the load is never anywhere near 650 kilos – it does vary. And therefore, so does the hight of the towball.

Pedantically, is it suggested that, having loaded the car, we sit inside, and then someone else measures the height of the towball after placing a load on it equall to the caravan nose weight?

Then we manufacture a piece of broom handle that is the exact length required to place its rounded top, when placed on top of the bathroom scales, at the towball height, when the caravan is connected.

As a side issue, do we then make sure our parking space is perfectly level? At all points related to where the wheels rest on the ground.

Keeping in mind that over 20 years of caravanning I have never seen anyone measure their caravan nose weight and that once on the road, we have road camber, bumps, holes, hills......

 

[Parksy]

I couldn't agree with you more Perry

 

[tuffty]

Well just as a final update, I did a reasonably long tow over to Norfolk for a week's holiday last week over a mix of dual carriageways and some bumpy A roads. Awning still in the boot due to weight (I checked, and it's well within the car's tolerance both for total weight and rear axle weight, even allowing 90kg for the noseweight), but I pumped up the car's tyres to full "towing" pressure this time and just drove at whatever speed felt comfortable. After the helpful advice here, I've pretty much decided that what I'm experiencing is completely normal and nothing to worry about, as the unit never felt out of control or dangerous.

I did drive away with my breakaway cable still attached, and manage to set my Cadac on fire, but those are stories for another day! :silly: Thanks very much one and all!

 

[WoodlandsCamper]

I did drive away with my breakaway cable still attached, and manage to set my Cadac on fire, but those are stories for another day! :silly: Thanks very much one and all!

A new Woosie member pending.

 

[otherclive]

"and manage to set my Cadac on fire"

Isn't that what Cadacs are supposed to do.
Glad you have sorted out your concerns regarding the caravan-car ride.