Snaking!!!

[glenno]

Gary.
Adding extra weight anywhere in front of the centre of gravity of the caravan will move the combined weight & cg fwd. This is a mathematical FACT.

 

[ProfJohnL]

Hello Gary;

I'm sorry but your contention that adding weight to nose does not change the caravans Cog is fundamentally wrong. It does change the the position of the CoG, and in fact moves it forward.

As far as the caravan chassis is concerned any mass attached to it and distributed on it forms part of its total mass, and thus contributes to the CoG. The hitch is not separate to the caravans structure so any change in mass at the hitch will affect the caravans CoG.

A caravan is more like a see-saw than you seem to think. Even on a see-saw. the CoG is affected by the mass of the people that sit on it, even to the extent that the CoG will be moved to some point is above the fulcrum. being above the fulcrum it will swing about the fulcrum as the see-saw moves just like a caravan. That is why when you have two equal bodies at either end, the see-saw tends to favour the one that is closer to the ground and it can leave the upper one helpless at the top of the arc.

However when considering caravans, simply adding extra mass to nose is not always the solution. usually it is only sufficient to relocate some of the pay load within the caravan to trim for the correct nose load rather than having to deliberately add mass.

Even though the overall mass remains the same the CoG is affected.

 

[joeby]

Yawwwwwnnn ZZzzzz

 

[Guest]

Hello Gary;
A caravan is more like a see-saw than you seem to think. Even on a see-saw. the CoG is affected by the mass of the people that sit on it, even to the extent that the CoG will be moved to some point is above the fulcrum. being above the fulcrum it will swing about the fulcrum as the see-saw moves just like a caravan. That is why when you have two equal bodies at either end, the see-saw tends to favour the one that is closer to the ground and it can leave the upper one helpless at the top of the arc.

Well John, what you say there is what I'm on about, 'it favours the lower'? any weight higher than the fulcrum is going to tip toward the lower so add to that side, and visa-versa?!
I know for a fact that nose up is unstable with a correct or greater noseweight, been there tied that, how I've explained it might not be understood, but by the simple fact getting the van level cured the problem proves I'm not wrong in the mechanics of it.
And I'm talking 32mm nose up, that's all!

 

[Lutz]

If you have experienced instability with an outfit that was nose up, then the reason for this instability was something other than the nose up attitude. From what you have described, if simply getting the caravan level cured the problem and you did nothing else, you effectively increased the noseweight.

 

[Jo-AnneFairway5402010]

Avondales are nose heavy anyway. The wheels are set quite far back. Check all your air pressures in your tyres on the van and car. I know it sounds mad but we dont put alot in the front locker also we used put alot of stuff at the back because of the way the wheels were set.
i dont think you was snaking because it happened to me whilst coming back from hillhead a few ago. people say accelerate to get out of it but to be honest i braked gently and wether the atc kicked in but it stopped snaking straight away. i thought we was a gonna the m5 almost came to a stand still. i got sucked in by a lorry. i will tow again but only on country roads and maybe dual carrage ways till i get my confidence back again.
i think yours was just an unsteady run by the sounds of it.
good luck and happy caravanning.

jo

 

[steve56616]

You maybe right Jo, will see how we go next weekend.
Blimey!! everyone seems to be an expert - I am more confused than ever and gave up reading most threads lol.

 

[xtrailman]

"seems to be an expert"

But actually not.

Constantly giving conflicting advise provided by organisations such as the NCC & CC, along with the caravan manufactures?

I have from personal experience found that stability IS affected by the nose attitude, so have others.

 

[Lutz]

If someone can come up with a technically plausible and verifiable explanation of why a nose up attitude alone should be any more unstable I would be gladly willing to give it due consideration, but no-one has. None of the reasons mentioned perviously in this thread stand up to serious scrutiny. Comments made on the basis of personal experience are of no help because no-one is able to produce comprehensive details of the conditions surrounding each case.

 

[ProfJohnL]

Whether a caravan tows with it's nose up, down or level is determined by the height of the tow ball on the towing vehicle.

For saloon cars the EU stipulates the maximum height of a loaded towball should be no more than 420mm from the centre of the ball to the ground with a lower limit of 350mm. Caravan manufacturers are aware of these limits and will design their caravans to tow with this height of hitch.

If a caravan assumes a nose up attitude with a hitch at 420mm, then it is a clear indication the caravan manufactures believe it to be a satisfactory attitude. If it is not a satisfactory attitude then the caravan manufacture would be guilty of producing a design with an inherent fault, and the selling dealer would fall foul of the sale of goods act by selling it.

Assuming all legal limits are adhered to, then towing with a nose up attitude is no worse than any other attitude.

However as with so many other aspects of towing concentrating on one specific aspect to the exclusion of all others will not guarantee a safe or a good tow. The good tower will assess all aspects and and make any adjustments to reach a legal compromise.

 

[Lutz]

To add to the comment above, the requirements not only state that the towball shall be between 350 and 420mm but also that the hitch shall be between 395 and 465mm when the caravan is standing level (all dimensions apply to the fully laden condition). Hence, a towball at top limit (420mm) and a hitch on bottom limit (395mm) will result in a legally acceptable nose up attitude. In addition, there will be the changes in height that inevitably occur when either towcar or caravan, or both, are not fully laden.

 

[Philspadders]

centre of mass ( there is no such thing as cenbtre of gravity) nose up, nose down, all these factors will affect the way the caravan performs.
I have towed a well balanced van with a very pronounced nose up attitude , and the handling was affected. ( ihad to move my van but due to a breakdown had to borrow an isuzu crewcab to do it). The turning moment is in effect raised and its this that causes the problem. In a very nose down attitude handling will also be affected. I would be surprised if it caused a snake but due the the change of pivot point height in relation to the rotational centre of the axle, the towing vehicle could suffer some rather unpleasant pitching.
Lutz wrote "(all dimensions apply to the fully laden condition). Hence, a towball at top limit (420mm) and a hitch on bottom limit (395mm) will result in a legally acceptable nose up attitude."
having measured my hitch hieghts i must be illegal as my swift charisma 565 is slightly nose down when measured at the front and rear of the chassis.
We really need to draw some vector diagrams to explain the motion and the forces acting upon the towing vehicle.
However as we all know that a correctl laden outfit is the safest outfit and i for hitch height i would go for level or certainly slightly nose down.

There will always be lots of opinions on a forum , some correct , some not, but often we are blinded to new ideas and new observations simply by our own arrogance ... " ive been doing this for years so i know best" ..... and the moons made of cheese dont you know.

Have fun and keep on disagreeing , but im right and you are all wrong.

 

[Surfer]

On the one or two occasions I have had a snake, as it started I slowed down and then accelerated which cured the snake but doing this depends on circumstances in both cases was on a motorway travelling at about 55mph.
A nose up situation on a caravan could cause tail wagging, so not a good idea. Too much weight on the nose lifts the front of the towing vehicle giving less grip to the road surface. Not a good idea either.

 

[Lutz]

Why should your outfit be illegal, Philspadders? If the towball height is at bottom limit and the hitch on top limit, the outfit would be well nose down and still be legal.
There is no need for vector diagrams as, for a given outfit, only the location of the centre of gravity (or mass) and hence the noseweight determines stability. Whether correct noseweight is achieved by raising, lowering or redistributing load is immaterial (within reason).
Because the front wheels of the towcar are so far away from the fulcrum point (the rear axle) compared with the towball, excessive noseweight is unlikely to noticeably affect the steering, unless it is so excessive as to be blatantly obvious.

 

[Philspadders]

ive been there, done it and it the handling was affected. even with a correctly laden vehicle the attitude does have an effect. i did not image it , i did not dream it. it was affected. the pitching movement was horrendous. yet towing the van later, laden exactly the same way with my own vehicle and just slightly nose down , the pitching effect was no longer evident.
i cannot provide mathmatical evidence only the empirical observations made when actually towing a carava, correctly laden and balanced with a nose up attitude.

i can only suggest that you try it.

ps , towing a unit a 60mph with a nose up attitude of say 5 degrees will see an increase of pressure and flow on the underside of the unit. to fully understand and measure the effect would require some very complex modelling and a wind tunnel. There will be a degree of lift simply due to the angle of attack of the underside of the unit. The lift forces could be quiet considerable. Evn though the front of the unit is effected as well , the loads would need to be measured.
There could be a slight "wing" effect that may generate lift, as contrary to what many people think mushc of the lift generated by a wing is due an airstream beinbg pushed down from thenderside of the wing from the centre of pressure. The caravan would develop thesame centre of pressure an soome lift would be generateow much i dont know , but neither does anyone else ...

Net timyou drive at 60 stick your hand out of the window ansee what occurs

 

[Lutz]

I'm not suggesting that you imagined anything, but I am saying that the actual root cause was not what you think it was. However, without knowing all the details, it's difficult to hazard a guess as to what it could have been. What is clear, though, is that a relatively minor change to the attitude of the caravan cannot have such a big effect. The fact that you say that the pitching motion was horrendous also indicates that the cause was something else, as attitude itself cannot directly affect pitching.
Your example of a 5° change of attitude is totally unrealistic. For an average sized caravan, 5° would result in approximately 300mm nose-up at the hitch, which would be way out of the legal tolerance and probably result in its back end already scraping the ground. Besides, the relatively large frontal area of the caravan combined with its boxy shape creates an aerodynamic drag which far exceeds any effect that a lesser change of angle can possibly have.

 

[Parksy]

We collected our new van last week and brought it home with no problems. After taking it out on our first trip on Friday it kept snaking every time I reached 49 - 50mph.
The van is a Avondale Ospray and our tow car is a 2.5 X type Jag.
It was loaded evenly I thought. Awning across axle, water roll and watermaster in the back bathroom. Clothes even all round and that was about it. All water was emptied from all tanks and toilet etc.
Any ideas what I may be doing wrong?

Steve

Everyone I have spoken to don't seem to check it other than make sure it looks level. Not been caravanning very long as had to take our Luna back to the dealers so got this new one now. So not very experienced to be honest.

To get back to the original question you need to get your noseweight as close as possible to the upper limit for your car and caravan whichever is the lower figure of the two.
Check that the caravan tyre pressures are correct and it might pay to increase the pressure in the rear tyres of the towing vehicle, ask at a tyre fitting centre for advice as to how much.

 

[Surfer]

Parksy, should that read get the noseweight as close as possible to the noseweight of the towing vehicle? This would also allow you to tow in excess of your kerbweight where the manufacturer has a higher braked towing limit than the kerbweight. I do not condone this practice but is done every day by people towing horse boxes.

 

[Parksy]

Parksy, should that read get the noseweight as close as possible to the noseweight of the towing vehicle? This would also allow you to tow in excess of your kerbweight where the manufacturer has a higher braked towing limit than the kerbweight. I do not condone this practice but is done every day by people towing horse boxes.

Both Alko and Winterhof set noseweight limits of 100kg for their hitches which might be lower than the permitted limit for the towing vehicle.
I have noticed that VOSA are beginning to take an interest in caravans and trailers and have set up checkpoints at motorway service stations and last year they reported an increase inoverloaded caravans. This presumably means that there will be more checks in future which may catch out horse box owners who exceed their limits.

 

[Philspadders]

lutz wrote"but I am saying that the actual root cause was not what you think it was."

Yes it was.

Unless you can mathematically disprove this or have empirical data to disprove it whats the point in trying to argue against it.
It occured , many others have reported similar handling issues in similar circumstances yet you continue to refute that poor handling can result from a nose up attitude, that is of course your unproven viewpoint.

Unless you can prove otherwise.

Full stop. nothing more to say really.

.

 

[Dustydog]

Steve 56616
What year is your Osprey? More recent ones were fitted with the Al-ko ATC anti snake system. If you have this are you happy it is working correctly?
I assume the Jag's suspension is in good order?

Have you had another drive increasing the noseweight as discussed in the previous posts?

 

[ProfJohnL]

Hello Gary

You are correct that the person on the low end of a see saw is favoured, but in my example I stated the masses were of equal size, and that would mean if the see saw was level there forces would be balanced and the beam would not move.

A caravan is not “balanced” it is deliberately out of balance so it will create a nose load. If you tip the caravan then the actual nose load changes, for the reasons you describe.

So if you set a nose load of x Kg with the caravan level. But when it is hitched the caravan is nose up then the nose load will be less than x. kg. Before you tow you must re trim the caravan so the hitch produces the correct nose load of x. kg but at the actual towing height.

This is why it is so important to be able to measure the nose load at the actual towing height.
I am left wondering how you managed to drop your tow ball height by 32mm?

 

[ProfJohnL]

Hello Phil

I think you know that I have the same opinion about nose up attitude as Lutz.

I recognise the fact that none of us have the necessary equipment to prove or disprove the opposing views on the subject, but I must raise a few questions about the incident that you refer to support your contention.

In you post of 5th sept 9:29 am

You state that you had to move your well balanced van, but need to use a different tow vehicle, which when hitched produced a nose up attitude.

When you say the caravan was well balanced, I assume this referred to your normal towing vehicle. As has been described extensively over several threads, the nose load a caravan produces changes according to the height of the hitch above the ground.

If your well-balanced caravan has its hitch raised higher, and you do not make any changes to the way it is loaded, then its nose load will be reduced, and that will certainly change the way it behaves.

The other factor that cannot be simply ignored is the change of vehicle. There have instances in this forum where a contributor has found that certain combinations of car and caravan that on paper should be a good match, simple don’t go well together, yet changing one or the other and the problems deminishes.

So I have to cast some doubts on your evidence.

Yes there have been forum reports of instability with a nose up attitude, but there have been forum reports of instability with a nose level or down attitude also.

Equally there must be many thousands of caravanners who tow with a nose up attitude without incident, but because there is no incident they don’t feel the need to
report it.

How about the other factors we do agree about, tyres and suspension, Industrial/commercial vehicles have very different suspension characteristics to cars, What about the surface you were driving over.?
There are too many variables in your experience with attributes that are known to cause difficulties with towing to be able to conclusively state the differences you found were simply down to the caravan being nose up.

 

[TheTravellingRooster]

Towing above the published manufacturers Maximum Tow Weight is only going to end up in one direction if and when an accident occurs that cannot be sneaked away from;and worse still there is an injury which involves the police are God Forgive:-A Fatality. The insurance (if any is in place)is useless.Then the CPS will step in with an 'invitation' from the Police.The Insurance will be Void, by a contravention and a stupid act.Towing above a vehicles Actual Kerbweight as per (EU Directive 95/48/EC)or better still from a certificated Weighbridge Exercise is just as nuts( Tail Wagging The Dog )unless you really know what your at; and then even that is very subjective. I have been 'Banging On' for long enough now about the EU Directive (long before it was being published in Practical Caravan Magazine:-Caravan Basics/Glossary).Towing at 100% of Kerbweight is one thing(for the experience)but Towing above the Maximum Publish Towing weight is irresponsible to say the very least.An accident with a fatality than is not you!,could well land you in an HMPS Hotel,never mind not having enough wonga to meet the Law Suits.

 

[susiep]

Horse trailers can not be included in this subject as most are twin axle, made of nothing and weigh very little. They do not have all the heavy mod cons fitted in caravans. Good luck sorting this problem, happened to me only once with caravan and it was pretty scary