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Seems a bit illogical to me, if I am reading the above as written.

 In principle, the higher the towing capabilities of the tow car, the lower the speed raring. As written, the restriction is independant of the weight being towed. So in theory, a car with a low towing capacity (under 3.5t car plus rated towing limit), pulling the biggest twin axle on the planet, can happily tow at 130 kph.

 Towing capacities are usually different for braked or unbraked trailers, so maybe I would be better to cut the brake cable on the van, then i might get away with a higher speed limit.

All sounds a bit daft  to me, but it would be nice to get a truly definitive answer to the question, because you can bet your boots it will arrive in UK soon.

FM

Greetings Again,

Angus and Frogman, You and me are in the same boat.

Iv'e an Audi A6 Avant with a 2.5TDi engine.

Brute of a car, pulls like a train, weighs over 1700kgs but and its a BIG, BIG but.

Because its got the carrying capacity of a dozen sherpas, it doesn't matter what I put on the back, I'm restricted.

Mind you, 90Kph is enough for me but I dont have to go back to work after my hols.

What has to be remembered is that, compared to specially designed articulated lorries, a car and caravan outfit is a compromise.  Because of the very short wheelbase of a HGV tractor unit compared to the very long distance to the rear wheels, an HGV is far more stable from sway and other side-forces. Notice how they can sharply change lanes; something a wise caravanner would never dare to do.

With a caravan, the distance from the car's rear axle is very similar to the wheelbase of the car itself.  This means that, if snaking commences, the caravan has the mechanical advantage to swing the car, as well as itself, from side to side.

No matter how well matched and weight-balanced an outfit may be, excessive speed will make it easier for the caravan to take advantage.  This fact is proven by how the reduction of speed in strong side winds can greatly increase stability.  When driving too fast, it isn't just side winds that are a problem; passing a high-sided vehicle will create drag factors which react to both vehicles.  Because the high-sided vehicle will probably be a heavily laden HGV, the HGV, simply because of its weight, will not deviate from its path. Therefore, all the sideways effects will be transferred to the caravan, and the faster the outfit is travelling, then the easier it is for the HGV to pull it sideways; and there is one reason for the start of your snake.

Lower limits for caravans and similar towed outfits were not issued for fun.  Although the stability of caravan has improved over the years, the laws of physics impose a limit to what can be regarded as safe.

Bertie.

Whilst Berties observations have some merit, the comparison with HGVs ( or LGVs as they have now been designated) is lost a little when comparisons with a wagon and drag arrangement are made. As these are now becoming more popular for a variety of reasons, it is strange that an unladen `wagon` can pull a fully laden `drag` (drawbar trailer) without any other restrictions. The axle spacings can easily mirror an up-scaled car/caravan combination, yet it is perfectly legal to drive a wagon and drag where the trailer could weigh double that of the towing unit.

A recipe for `tail wagging the dog`? Most certainly, and as the laws of physics still apply, the potential for disaster is many times greater than when towing your `van. Tell a haulier he shouldn`t `drag` more  85% of the towing vehicles kerb weight  and you may end up wearing a spanner.

To Bertie (2 tops)

Nice argument, and I for one am not against speed limits, BUT I have a Terracan (2.3 T) which has a towing limit on a braked trailer of around 3.5 t. By this legislation I am seriously at risk if I pull an empty 5 cwt luggage trailer at more than 90 kph in France --------(60 mph as existing in UK). How about applying physics to that?

My point is that if the law is seen to be an ass, then it brings the law into disrepute. Is it beyond the wit of law makers to think matters through properly, and formulate a law which is logical, and thereby if it is visibly sensible, then people will respect it. All "sensible" people will act in a sensible manner irrespective of any laws, but for those who need "guidance" surely it is better that the law also makes sense?

FM

I can think of a few reasons why a wagon and drag trailer arrangement is very different:

• Whole outfit designed as one, ie tractor unit designed for towing.
• Heavy duty tyres, not the pathetic minimalist tyres fitted to caravans
• Air brakes on the trailer not small cable operated drum brakes
• Air suspension and dampers, not rubber suspension.

I'm sure there is more to it than just this, and you generally don't see these things waving about in the wind like some caravans do.

Sorry, freeatlast, you are confusing a wagon and drag with a tractor unit and semi-trailer. A wagon and drag is simply a rigid with a tow hitch fitted (yes the arrangement is different to a car/caravan one) and is not `designed as one`. You can purchase a rigid and have towing apparatus fitted retrospectively, like you would on a car.

The `heavy duty tyres` are adequate for the loads carried, there is no mammoth safety margin built in as implied.

Braking system - likewise. How much retardation can you want/use? Cable operated drum brakes on a caravan are capable of locking the wheels, which is more retardation than you can use. It doesn`t allow for an ABS system though.

Air suspension systems are fitted to comply with the need to have `road-friendly` suspension in order to carry greater axle-loads than `non-road-friendly` axle load limits will permit.

The reason you don`t see the things `waving about in the wind` is due more to the skill an application of the drivers than any inherent design advantages. How many tankers do you see wobbling about all over the place, yet they are potentially more unstable than the flatbeds and curtainsiders you see everyday. This is because if the driver didn`t know what he was dong the thing would be on its side at the first roundabout.

I know LGV drivers get some stick on here (sometimes with some justification), but the fact is that if your average `vanner  drove with half the skill and awareness of 99% of truckers we wouldn`t get the bad press we do. 

In the case of "wagon and drag", the outfits that I have seen are four-wheeled, with the front pair of wheels being steered via their towframe. In this situation, because the wheelbase of the drag spans most of the drag's length, the drag cannot snake or sway in the way that a caravan is able to. Even twin-axle caravans have their two axles so closely arranged that they still can snake. Obviously, this close arrangement is necessary if both pairs of wheels are non-stering.

Bertie.

Am I looking at the ones with two axles?  If so, are these axles rigid, or are they allowed to "steer" themselves in a similar way that a car does when towed on an "A" frame behind a motorhome?  If so, or if they are rigid axles with a substantial wheelbase, i can imagine them being a bit dodgy.

Bertie.

sat 8 th aug on m6 going south 55mph campers caravans passing far to quick

and lorries as well buses well over 70mph so were are not all bad caravans, 60mph the boys in blue are watching so are cameras

Quote: Originally posted by Angus1215 on 14/8/2009

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Sorry, freeatlast, you are confusing a wagon and drag with a tractor unit and semi-trailer. A wagon and drag is simply a rigid with a tow hitch fitted (yes the arrangement is different to a car/caravan one) and is not `designed as one`. You can purchase a rigid and have towing apparatus fitted retrospectively, like you would on a car.

The `heavy duty tyres` are adequate for the loads carried, there is no mammoth safety margin built in as implied.

Braking system - likewise. How much retardation can you want/use? Cable operated drum brakes on a caravan are capable of locking the wheels, which is more retardation than you can use. It doesn`t allow for an ABS system though.

Air suspension systems are fitted to comply with the need to have `road-friendly` suspension in order to carry greater axle-loads than `non-road-friendly` axle load limits will permit.

The reason you don`t see the things `waving about in the wind` is due more to the skill an application of the drivers than any inherent design advantages. How many tankers do you see wobbling about all over the place, yet they are potentially more unstable than the flatbeds and curtainsiders you see everyday. This is because if the driver didn`t know what he was dong the thing would be on its side at the first roundabout.

I know LGV drivers get some stick on here (sometimes with some justification), but the fact is that if your average `vanner  drove with half the skill and awareness of 99% of truckers we wouldn`t get the bad press we do. 

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So, if I read this right, all caravanners should become HGL (LGV) drivers.

Right then, that's me OK, and yes, I've driven tankers too.

Quote: Originally posted by Fat Belly on 14/8/2009Cameras won't be triggered by a car and caravan doing 70 on a motorway or 60 on a single carriageway - in both cases they will be doing 10 mph over the limit - only mr plod would get them - and where are they when you want them eh!

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Post last edited on 15/08/2009 09:13:25

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Apparently there are now speed cameras that can differentiate between a car and a caravan or large commercial vehicle.  I don't know how the system works, but I have been advised by someone with knowledge in this field that it is so.

Bertie.