Aerodynamics

WAK

Mar 14, 2005
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I tow with a car which has a low nose wieght allowance of 60kgs, used it for a couple of seasons now and it performs very well, although it is a pain keeping the nose weight down, if anything the cars only downfall. Just recently, over a few drinks this conversation came up and caused a very in depth debate to which nobody knew the answer, so here is the question: When towing, the nose of the caravan is lifted due to the draught travelling underneath, because the nose is lifted the downward force is less, so are you actually keeping 60kgs of weight there or is it less, if it is less could you start with 65kgs noseweight to establish the 60kgs in motion. Although the noseweight will return to its original figure when you slow or stop, anway i hope it gets you thinking like we did.
 
Mar 14, 2005
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Hello Wak, The quoted nose weight limits are stated for stationary vehicles, and thus the static loading must be observed for legality. The actual load will vary for lots of reasons whlst in motion, but not only will the load be diminished by aerodynamics, it will be increased by braking, adnother factors too many to mention.
 

Damian

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Mar 14, 2005
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Wak, very good reading, hope you dont go down that road though, if you were to have an accident and it was found you had overloaded the cars stated towball weight, then it woul dinvalidate your insurance, and probably make you liable for prosecution .
 
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lutzschelisch.wix.com
You are correct in saying that aerodynamics have the effect of reducing the noseweight with increasing speed. In fact, at higher speeds this effect can be so extreme that the caravan is actually trying to lift the back end of the car up. Unfortunately, however, the law only specifies maximum permissible static values and makes no allowance for any dynamic changes. Ideally, therefore, the load distribution of the caravan should be dynamically adjusted to compensate for the aerodynamic lift. This could be accomplished by moving the axle of the caravan rearward as the speed increases, for example - theoretically possible but such a high-tech feature would be extremely elaborate and expensive.
 
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Im not sure that the profile for a caravan trully represents that of an aerofoil section and therefore the amount of lift produced is probably minimal. Lift forces have to overcome the downward forces to produce any upward lift which means for a 1500kg van you would need to produce in excess of 1500kg of lift needing some speed. I can let you have the equations if needed! You also need to take into account the amount of drag produced, which is probably rather high! Not to mention the fact that the designers have produced a shape that minimises any lift - otherwise we'd all be towing aircraft! Nice thought but I'd recomend sticking to the 60kg nose weight. Have fun.
 
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lutzschelisch.wix.com
Because the caravan is relatively tall, the aerodynamic forces are acting on a point that's way above axle height. Therefore, it's the drag that's causing the front end to lift.
 
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This an interesting thought. If you look at the profile of a modern caravan, it is indeed possible that it could create lift - this is because the path for air over the top is longer than that underneath, the air over the top accelerates and therefore results in a lower pressure than that under the van. The lift is not generated by the air under it, unless you consider the kite effect if the van is towed nose high, which is actually caused by drag and is very unstable. Anyone who can make a van take off like that has a lot of horsepower - and no sense.

Any genuine lift generated would probably be centred somewhere near the axle, and would need to be considerable to make a noticeable change to the noseweight - although if it did occur the noseweight would be reduced.

To scupper the entire argument:-

1.Lutz knows something we don't.

2. The car attached to the caravan will probably provide enough turbulance to destroy the laminar airflow required for lift.
 
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lutzschelisch.wix.com
I wasn't suggesting that airflow over or under the caravan is creating the lift but the shear aerodynamic pressure acting on its frontal area. As the towing vehicle is providing some sort of wind protection to the lower frontal area of the caravan, most of the aerodynamic forces acting on the caravan will be high up, above the roof line of the car. The resultant moment about the axle will cause the front end to lift.
 
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There was awing like stabiliser on the market a couple of years ago that fitted on the car towbar to shoot the air over the top of the caravan.It looked a bit like the rear of a Formula 1 car.On e of the claimed plus points was the reduction of the reduced noseweight effect that you describe.It was quite convincing but the stabiliser seems to unavailable now.
 

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