Snaking!!!

[Guest]

Simply John, the problem was my original van was designed around cross-ply tyres, with 80% radials fitted it towed nose up, it was also designed with a 55 to 65kg nose weight due to to a very long hitch to wheels section leaving a relatively short rear overhang, this was balanced out by the rear kitchen/shower room.
Being on 'spring-over' dampers the very simple solution was to add machined spacers to their top mounts and correct the missing height exactly, nothing then to do with altering the towball height.
Beforehand and while nose up it would easily become very unstable especially going downhill, nightmare at times, I can tell you even at lowly speeds, I added up to a total of about 90kg to the nose with little effect.
After and towing slightly nose down, it towed a dream in all conditions, absolutely unfazed by anything, even crosswinds, the long front to wheel distance, 65/35, being responsible I don't doubt.
The point being John, I learnt quick and the hard way, there was indeed other issues like the cars rear tyres at 50mph, but that was a different matter and what i originally posted was the possible cause of the OP question? How we got on to this attitude thing I don't know?!
However there is no doubt whatsoever in my mind, nose up was and is a big problem to stability.

As for a see-saw then, if you draw a level one with two equal weight bods sat on it, the forces they exert are vertical down straight through their heads, now tip the see-saw and the still vertical down force exerted by the lower one has moved behind him and therefore further from the fulcrum the opposite being true of the higher one.
To correct this the lower bod can move closer to the fulcrum, but, he must move up for himself and also to correct the other bods force being already closer, even though he's not actually moved. Trouble now is as the see-saw moves back toward level, things revert to how they were but now the once higher guy is going to crash down to the ground as the new lever lengths act doubly or is that x4? in his favour!
So if as in my case 55-65kg noseweight was correct level, it cannot be correct towing nose up, this was why I found 90kg was not sufficient, because the lever lengths have changed.

Now, there are indeed upper and lower limits for towball and hitch head height, these are construction and use regulations to govern manufacturing parameters, given the thousands of different cars (and at one time caravan chassis's), it's hardly surprising the tolerance is large to say the least
To suggest then that, as long as towball and hitch both comply is any indication that the van will tow in a stable manner, is absolute cods wallop!!
If anything, it's only to ensure the hitch will not bind or jam, causing it to be damaged and/or come adrift

 

[ProfJohnL]

Hello again Gary,

From the information you give, I can’t help feeling more was changed than the attitude of the caravan. Again based on your description of the caravan it seems likely that the distance from your hitch to the main axle will be 2.5M or greater. Based on 2.5M and the 32mm change in the height of the caravan over the axle (because of your new spacers, the effect on the attitude of the caravan will only be in the order of 0.7 degrees. If your hitch to axle distance is any greater the angular difference will be less!) It is very hard to accept that such a small difference causes such aerodynamic differences on such a relatively slow moving vehicle.

How you describe the action of the see saw is also true except the mechanical advantage is not as dramatic as you suggest, in fact two equal people can actually cause a see saw to oscillate with relatively small movements of the upper torso.
The see saw actually relies on the ability of the users to move to affect the systems CoG about the fulcrum. One must not also forget that traditionally the users also use their legs to push to add additional forces to overcome excessive imbalances.

Whilst the principals that cause a see saw to operate are the same that generate a caravans nose load, there is a difference in that a see saw is designed to oscillate, and in doing so moves through about 20 to 30 degrees, but a caravan is not designed to oscillate. It is also the case that the nose load should be set with the caravan in its towing attitude, and thus the CoG will be sufficiently biased forward to prevent it from being thrown back behind the axle under normal towing conditions.

So whilst I am not denying that you had a problem with stability, based on the information that you have provided, I cannot make the same conclusion about the root cause that you do.

 

[DerektheSecond]

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 have you towed with the caravan since this incident? what sort of roads were you on, was it very windy and as a previous member has touched on is the car ok, tyre pressures bearings etc is the caravan new are its bearings etc ok, How about asking some one else to tow it either with the same car or another tow vehicle, at least this way you can physically do something about the problem yourself

 

[Surfer]

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

You gotta to be kidding. Horse trailers can weigh a lot more than a caravan with a horse in it!

 

[Surfer]

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.

Many cars are allowed to tow well above their kerbweight legally. I have no issue towing above my kerbweight but then i woudl eb very careful and restrict my speed to road conditions. If you are silly enough to exceed the maximum towing limit impsoed by the manufacturer and then have a incident, the insurance should not be void.
I also thought the same until I checked up on it. An insurance company will also pay out if the driver is unlicenced and that is something I did not knwo either. I assume that they probably will pay and then come after you. It all depends on their terms and conditions I guess.

 

[Philspadders]

it occured, its a crew cab ( the isuzu is smalle and lighter than a toyot hi lux, hardly commercial), you are clutching at straws.
Please disprove me through either a mathematical model or empirically. I dont thinl you can do either.

CoG does not exist, Centre of Mass does exist.

Now its see saws..

this thread will go on and on and on.

time to lock it perhaps.

 

[extrucker1]

hi steve, are you sure it was snaking and not just moving around because of road suface, wind etc,. i have had a couple of avondales, i have one now and have found them very stable. ray.

 

[Lutz]

There is nothing than can be proved or disproved because we don't know all the parameters surrounding your claim. From the little that we do know one can only say that it doesn't make technical sense, but maybe we are missing out on something important. The object of continuing this thread was to try to establish why something has happened which, from the information that has been provided, cannot be logically explained.
Arguing about terminology whether Centre of Gravity or Centre of Mass is bit like splitting hairs. Of course you are correct when you talk about Centre of Mass, but everybody knows that the same is meant when Centre of Gravity is mentioned.

 

[Parksy]

The object of continuing this thread was to try to establish why something has happened which, from the information that has been provided, cannot be logically explained.
Arguing about terminology whether Centre of Gravity or Centre of Mass is bit like splitting hairs. Of course you are correct when you talk about Centre of Mass, but everybody knows that the same is meant when Centre of Gravity is mentioned.

The originator of this topic simply wanted some ideas as to why his newly bought caravan is snaking.

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

Not enough noseweight could be the problem and when asked if he'd checked it the o.p replied that he hadn't but that his caravan 'looked level'.
Since then the thread has neen hi jacked with this debate about how the towing attitude of a unit affects noseweight which as far as the o.p's problem is concerned is largely irrelevant.
I'm beginning to understand now why this forum is struggling to attract and retain new members.
Most would have given up following this topic way back and tried another forum where their query might have had some answers relating to them.

 

[Dustydog]

The snag is Parksy the OP hasn't responded to any of the more simple suggestions so no one knows if he still has a problem. Others who may come on here with similar problems will hopefully take heart that some of the more mundane suggestions may actually be a fix!

 

[Dustydog]

Thank goodness for Wikipedia:-
Polar moment of inertia is a quantity used to predict an object's ability to resist torsion, in objects (or segments of objects) with an invariant circular cross section and no significant warping or out-of-plane deformation.[1] It is used to calculate the angular displacement of an object subjected to a torque. It is analogous to the area moment of inertia, which characterizes an object's ability to resist bending and is required to calculate displacement.
The larger the polar moment of inertia, the less the beam will twist, when subjected to a given torque.
Polar moment of inertia should not be confused with moment of inertia, which characterizes an object's angular acceleration due to a torque. See moment (physics).

On reflection I think it was his bypass modulator valve that caused the snake.LOL

 

[Parksy]

Thank you Lutz, at least the o.p will have some suggestions to consider

The snag is Parksy the OP hasn't responded to any of the more simple suggestions so no one knows if he still has a problem. Others who may come on here with similar problems will hopefully take heart that some of the more mundane suggestions may actually be a fix!

I think that we lost him sometime around last weekend DD. He responded early on but then he wrote

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.

I can understand why members have hotly debated the nose up / down issue but it might have been better if a separate topic which dealt with it had been started.
Newbies would be put off from posting if they thought that their questions would provoke intense debates about physics every time that they asked a fairly straightforward question.

 

[Soozeeg]

Newbies would be put off from posting if they thought that their questions would provoke intense debates about physics every time that they asked a fairly straightforward question.

Not only newbies.....it all went right over my head, and once it descended into petty squabbles about the centre of something or other it just wasn't worth reading anymore.

 

[Lutz]

I quite understand, Parksy, and I think most of us have genuinely tried to help, but the trouble is that a theory has been put forward which is, to put it mildly, controversial. Much more likely are one or more of the following:
1. Inadequate noseweight (by general consensus this seems to be the most likely cause)
2. Inadequate tyre pressures, either on the caravan or the rear wheels of the towcar (I don't think anyone would dispute this, either)
3. Road and, in particular, weather conditions (crosswinds, downhill gradients, etc.)
4. Tyre bounce due to weak or worn shock absorbers or no shock absorbers fitted
5. Poor load distribution as a result of:
5.1. Lots of relatively heavy items stored in the overhead lockers, i.e. payload not distributed low enough in the caravan, thus resulting in an unnessarily high Centre of Mass
5.2. Big difference between wheel load on the caravan from left to right, i.e. Centre of Mass not along the centreline of the caravan. This would offset the Centre of Mass from the aerodynamic centrepoint of the caravan making it more susceptible to yaw under crosswind and/or aerodynamic drag at higher speeds.
5.3. Too many heavy items at the ends. This would increase the Moment of Inertia which also influences stability

 

[Lutz]

Apologies, Dustydog, for my incorrect use of terminology which I have since put right. Perhaps I got a bit carried away. I will try to avoid making the same mistake in future, but I think, on the whole, my list of possible causes basically still applies.

 

[TheTravellingRooster]

Hi to you all out there,and especially the Newbies.Your, are the future of this Forum unless you give-up or it is shut down first. It is becoming a little like:- The Gun Dog that becomes Skittery.They usually (I once heard) are taken to a quiet corner; given a biscuit with one hand and an end with the other!.I digress.
I have followed this post from the beginning and I have had the courage/audacity (I am still relatively new myself)to submit a post,#3. It has of late,judging by some of the latter contributions;become a rather gobble-de-**** laden apparent attempt at regurgitating the subjects of two back to back lectures,each of which might well have been 'Another View of Pure Maths' & The Physics of Inertia'. The lectures attend whilst on a Higher Plain induced by a Cocktail of LSD,Wackey Backey and a Triple Atlantic Roller to wash down the Peanuts ( Atlantic Roller Constituents:- by request ).
There must be a number of contributors to this post with Spinning Heads & A Crumbling Gray Mater Zone,caused entirely by information overload.
What started as a simple input & question has escalated into a Perfect Example of Bulls Excreta Baffles Brains. Might I suggest with a Tongue-in Cheek audacity;that there is a much simpler,likely explanation;by way of a question and then a possible answer.
Is it possible that Steve has upset somebody in the recent past & they are quite envious of his New Wealth /Acquisitions?.If this is the case, then they could be tampering with his tyre pressures unknown to him.

 

[ProfJohnL]

Yes rooster,
i'm on all of that and more.....

Granted the OP was a simple question, and it was basically answered within a few posts, but then someone made a statement that goes against all know physics, and it needed correcting, not only to hopefully educate the misguided soul, but also to ensure that new readers don't get the wrong information.

Interestingly caravanning colloquialisms abound, and so many of them are actually wrong - but don't get me started cause I'll never stop!

 

[colin-yorkshire1]

hi all
I kept away from this tread but read it with interest I do love it when the teccies get going and all the "male bovine excreta" flows with exuberance.
now the thread seems to be dying a bit I thought I would have a coulple of cents worth :- first the centre of gravity is not the same as the centre of mass, not when it is in linier movement its not otherwise a motorbike would fall over at speed.
second the lenght of the towing load from the ball hitch is in my view the greatest factor in instability the bigger the van the less stable it is afterall you dont see many 12ft caravans overturned on the motorway do you.
I have no science degree or technical data to quote just common sense and observation, science is based on theory and provable facts as neither can be tested based on one incedent any comment is therefore just speculation.

but the best quote I have heard for a while came from professor richard dawkins who said "anyone who doubts the theory of gravity should try and jump from a 10 story window".the outcome will allways be the same. this is not the case with instability if it was it would no longer be a problem and stabilisers would not be required there are so many different factors involved most but not all covered by LUTZ in his list,
the only real answer is by trial and error untill the problem is found maths on its own will not solve the problem.

colin

 

[DerektheSecond]

hi all
I kept away from this tread but read it with interest I do love it when the teccies get going and all the "male bovine excreta" flows with exuberance.
now the thread seems to be dying a bit I thought I would have a coulple of cents worth :- first the centre of gravity is not the same as the centre of mass, not when it is in linier movement its not otherwise a motorbike would fall over at speed.
second the lenght of the towing load from the ball hitch is in my view the greatest factor in instability the bigger the van the less stable it is afterall you dont see many 12ft caravans overturned on the motorway do you.
I have no science degree or technical data to quote just common sense and observation, science is based on theory and provable facts as neither can be tested based on one incedent any comment is therefore just speculation.

but the best quote I have heard for a while came from professor richard dawkins who said "anyone who doubts the theory of gravity should try and jump from a 10 story window".the outcome will allways be the same. this is not the case with instability if it was it would no longer be a problem and stabilisers would not be required there are so many different factors involved most but not all covered by LUTZ in his list,
the only real answer is by trial and error untill the problem is found maths on its own will not solve the problem.

colin

Look what youve done!!! youve started the thread up again

 

[Lutz]

Sorry to have to comment on Colin's reply.
It seems to confuse size with weight (or mass). The reason why you don't see many 12' caravans overturned on the motorway is because they are generally very light and have a low moment of inertia. Actually, the longer the caravan, the more stable it is. The ideal caravan would probably be a folding top 26' with an MTPLM of not more than 1000kg.

 

[Soozeeg]

Are you lot still going on???? Yes, I know I don't have to read it - and I haven't. Just can't believe that you all want the last word on the subject.

 

[colin-yorkshire1]

Sorry to have to comment on Colin's reply.
It seems to confuse size with weight (or mass). The reason why you don't see many 12' caravans overturned on the motorway is because they are generally very light and have a low moment of inertia. Actually, the longer the caravan, the more stable it is. The ideal caravan would probably be a folding top 26' with an MTPLM of not more than 1000kg.

dont be sorry LUTZ I said I had read the thread with interest. I assumed that the lenght/weight ratio was obvious so did not mention it. however if your last statment is correct the worst senario is a heavy short trailer so the double occupied high sided short horse box must be totally unstable, thing is never seen one on its side either. funny that.

colin

 

[TheTravellingRooster]

Hi to you all out there.I certainly hope that I am not the one that brings this Thread to ' Frayed Finish ';pardon the pun-I am a retired upholsterer.This thread has become a little like a Tired & Lack-Luster Business.It fails to bring in ' New Investment',in the case of a Forum that could equate to no new input on the Subject Matter.Many business's in my industry Died on Friday opened up again on Monday,some even crocked on Monday & breathed fresh air on Tuesday;because it had only just been discovered!.This thread might benefit from having a Liberal amount of Cyanide Gas,rather than a refreshing Gas & Air revival.
Perhaps in a more:- caravanning associated reference; maybe it was time it 'Snaked Away' into one of the heaps of upturned cars & caravans and re-emerged as a new topic.Perhaps as, " Reasons why your Caravan(on the back of your vehicle) 'if it's still there' won't behave its' self & with a sub-link:- Is it Fit for Purpose?.Being Towed;that is!.

 

[Soozeeg]

Well, it was (hopefully) dying a death until you bumped it up to the top again! lol ;o)

 

[Lutz]

Perhaps because there aren't very many on the roads, LOL, and those that there are drive very carefully knowing that they've got a precious load on board (their horses).
If everyone drove with the same amount of due care and attention to suit the prevailing conditions even the most potentially unstable outfit can be handled quite safely.