Using a roof box when towing

[Buckman]

I appreciate that if you drive around solo with a roof box, there probably will be increased consumption even if the box is empty. Most roof boxes are aerodynamically shaped for good airflow over them minimising and drag.
However if you are towing will the roof box decreased drag on the caravan in a similar manner when people drive in the slipstream of a large vehicle thus keeping fuel consumption about the same when towing whether or not you have the roof box?

 

[otherclive]

I have a Hapro Traxer a medium 350 litre roofbox. It seemed to make minimal difference when towing my last caravan with either a Superb estate or Subaru Forester. But the caravan front profile was quite rounded and aerodynamic compared to some previous caravans. However since the roofbox was put on the car as a necessity on certain trips it was irrelevant to me what it did for fuel consumption as just easing back by a smidgeon would recoup any losses if I were so inclined.

 

[Darren.H]

I tow with a petrol car, fuel consumption isn't the best.

On the occasions we have gone away without the top box I havnt noticed any difference.

But

It would be a difficult definate answer to give as the variables on consumption vary every trip.

 

[JTQ]

IMO there is no definitive answer that fits all cases; it depends on a lot of details and can fall either way, together with varying with the unit's speed, to either "help or hinder".

A big factor I suspect will be the closeness of the box to the van's front, where typically the closer the better. Another the relative heights, again I suspect with a "squat" tow vehicle the less chance of there being any benefit, on a "Transit" ilk the more.

We have quite a challenge in that cars get squatter, few even estates have viable bodywork for a really aft placed "box", van fronts tend to follow the raked back style, and for optimising stability a preference to lengthen the "A" frame.
Thus, we tend towards having a very large gap, "box" to the van's front end where the airflow impacts, way from the best arrangement to yield help.

 

[Jaydug]

I haven't used a roof box when towing for twelve years or so. When I did use one, I found it made little difference to fuel consumption when towing. However, if I left the box on the roof of the car after arrival on site., driving solo with the box caused the fuel consumption to rise considerably.

 

[RogerL]

The hole in the air is still the same size, ie that of the caravan, so unlikely to make any difference to the towing consumption.

 

[ProfJohnL]

There has been a number of previous threads about the value of "windjammers" and their effect on towing MPG etc, and there is no clear evidence either for or against them.

For much the same reasons I expect the effect of roof boxes on drag reduction to be just as uncertain, and are likely to be as marginal and could be attributable to road conditions and drivers inability to precisely repeat the journey and actions such as accelerating speed and breaking.

What is also highly likely is how if the the exact positioning of a wind jammer or roof box is changed by as little 10cm , it could change from being helpful to unhelpful or vice versa.

There is probably a good reason why roof mounted wind jammers for caravans are no longer manufactured.

 

[Dustydog]

In the late 70s early 80s windjammers, aerofoils , were a popular caravanners accessory.
I never bothered. Then ,and I agree now ,there was / is no evidence they added anything constructive to the outfit’s economy or performance. A top box I place in the same category.

 

[RogerL]

In the late 70s early 80s windjammers, aerofoils , were a popular caravanners accessory.
I never bothered. Then ,and I agree now ,there was / is no evidence they added anything constructive to the outfit’s economy or performance. A top box I place in the same category.

I seem to recall there was some evidence it was effective if placed right at the back of an estate with vertical tailgate, eg Volvo 245, but ineffective if placed further forward as most cars would force it to be.

The Purpleline Wind Deflector is still on sale in Australia - but their caravans are taller with more right angles rather than the rounded corners on European/British caravans.

 

[Buckman]

A number of SUVs have a low profile windjammer as standard at the very rear of the vehicle. They would deflect the air upwards slightly.

 

[Raywood]

I doubt they would make any difference with a caravan in tow as they are not big enough. They are there to improve the aero dynamics of the car itself, and possibly because the makers think they look good.