Snaking

[Ron_5041484]

I have noticed recently that when overtaking 2 large lorries

that were travelling maybe 20-30 yards apart there seems to be

bad turbulance as i pass the first lorrie which seems to start

a snaking situation , this does not happen when overtaking a single lorry, I checked for correct nose weight and loading of the caravan ,which were perfect , I was towing with a BMW 530d

and the van was only 1300kg so I was well within 85% figures

This has happened on several occasions , but only when there are 2 lorries close together travelling at about 50mph, Its a bit frightening when you are committed in the middle lane of the M25

has anybody got any ideas as to what is happening,I have been towing for many years with various car caravan setups and it has never happened with them , I also used to drive articulated lorries so i am not new to towing , I would be interested in your views . what I forgot to mention was also using a stabiliser which has the correct torque ( R.J)

 

[Andy5Z]

Ron, whilst it certainly sounds like the lorries are the initial cause that sets this in motion, are you 100% sure that there is not some error in your outfit that adds to the effect, but on its own may be not enough to cause the initial motion.

This wouldn't just start to happen if its never happened before if you see what I'm saying. Things to check are tyres, condition and pressure. Jack van up and see if tyres spin ok and are not warped in any way. Is van loaded ok? Are its wheel bearings ok, brakes binding, is it sagging to one side and just about anything else you could think of really. I also have a 530d Touring and I increase its tyre pressures to the max specified when towing. Its surprising how much better it feels when I do this after collecting my van from storage with just normal pressures!

I'm sure you're well aware of all these things as you sound very competent, but it never hurts to check all again. Even if say tyres are new they could still be dodgy - it's happened before!

Has this only happened with this current van if maybe you haven't had it all that long? Maybe it's just got naff aerodynamics?

It's a difficult one to answer really, just try to eliminate causes one at a time, I'm sure you'll get there in the end.

Happy caravanning - Andy Palmer

 

[Lutz]

Even though, as you say, all the major factors which you, as the owner, are able to influence, like load distribution, noseweights, etc., are set correctly, there are a number of other potential sources of instability. Most of these are, however, design related and there is very little you can do to improve the situation. Here are a few:

1. The location of the aerodynamic centre of the caravan relative to the axle, i.e. whether a crosswind will simply try to push the caravan bodily sideways or will it try to rotate it at the same time.

2. Whether the suspension geometry of the caravan changes significantly as it is deflected by crosswind, thus effectively 'steering' it. Probably to a lesser extent this also applies to the towcar.

3. The rear overhang of the towcar from the centre of the rear axle to the towball.

4. The stiffness of the towbar and of the A-frame

5. The sidewall stiffness of the tyres, especially the tyres on the rear axle of the towcar (an increase in the tyre pressure can make a contribution towards a slight improvement here)

It is, however, difficult to establish the exact cause of your particular problem from a distance. One would need to carry out relatively sophisticated tests for a proper diagnosis.

 

[Ron_5041484]

Even though, as you say, all the major factors which you, as the owner, are able to influence, like load distribution, noseweights, etc., are set correctly, there are a number of other potential sources of instability. Most of these are, however, design related and there is very little you can do to improve the situation. Here are a few:

1. The location of the aerodynamic centre of the caravan relative to the axle, i.e. whether a crosswind will simply try to push the caravan bodily sideways or will it try to rotate it at the same time.

2. Whether the suspension geometry of the caravan changes significantly as it is deflected by crosswind, thus effectively 'steering' it. Probably to a lesser extent this also applies to the towcar.

3. The rear overhang of the towcar from the centre of the rear axle to the towball.

4. The stiffness of the towbar and of the A-frame

5. The sidewall stiffness of the tyres, especially the tyres on the rear axle of the towcar (an increase in the tyre pressure can make a contribution towards a slight improvement here)

It is, however, difficult to establish the exact cause of your particular problem from a distance. One would need to carry out relatively sophisticated tests for a proper diagnosis.

Thanks to Andy and Lutz for your comments I do increase the tyre

pressures to 45psi front and 50 rear , Both car and van have just had a full service ready for the new season I must say that although I like the layout of my van an Elldis Hurricane I am not impressed with the build Quality and the handeling , It's very poor workmanship with a lack of attention to detail , a shame really just needs better quality control . It's probably time to get another Bailey or go German ( R.J )

 

[Jim.M_5040716]

Hi Ron

This is quite sipmle when two wagons are that close you get a partial vaccum between them, this is why the drive that close it saves fuel, secondly you get the bough wave from the leading vehicle, so if you are aware of this factor there is nothing to wory about

good luck keep on trecking

Jim M

 

[Pete4]

Hi,

As well as the nose weight being right, you need to make sure that the weight behind the 'van wheels is kept as low as possible. If you have a heavy weight at the front of the van balanced by an equally heavy weight at the back, although the nose weight is correct, you will get a pendulem affect from the back of the van causing the snaking.

Pete

 

[VIP2006]

In 1990 I lost a van due to similar circumstances as reported.

I was going downhill well under 50mph and when being overtaken by 2 No.M&S Lorries I was buffetted and then sucked into the vacuum created by the first lorry the the same happenned again with the 2nd lorry and after they had passed my rvan started violently snaking forcing the van into the central barrier and then the van veered to the edge of the road up a small incline whence it turned the car to face the oncoming traffic before turning on its roof.

I always suspected that the incident was caused by the vacuum and turbulence reacting together.

 

[Lutz]

That would have been a case for an active electronic stabiliser system like the one I had fitted to my van. It reacts far quicker than the driver can hope to do and it doesn't have the performance limitations in extreme situations that the usual passive frictional stabilisers have.