They have to stick out the back in the region of 300 - 500mm. The roof & walls would continue past the rear of the caravan.

iana wrote:

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Aero-Foils on the back roofs of caravans. God ! what next. Just can't remember the brand of van it was.

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 I have an eoro foil on my tug its a 3.5 m tinny and at the end of the day its use full I can go fishing what speeds are we doing to make this peice of #$%? to work.

Cheers

I find fitting mad flaps across the back prevents the vacuum effect . Sucking dirt up the back . They are not spoilers . They deflect bare down the back to prevent negative pressure building up behind van or vehicle. I have a a rubber mat between wheels behind diff on motorhome. Seems to help also .

When you drive along the road, your caravan acts as a dirty big brick behind you. The front of the van had pushed a lot of air out of the way for the caravan to go along the road. At the rear of the van, there is a large low pressure area - travel on a dusty road and where does the dust enter your van from? the rear of course. Now, if you can fill that low pressure area with some fresh air, this will reduce your aerodynamic drag.

A caravan or trailer has several components to the force required to move it. You have friction, which varies according to how much weight you are dragging. Reduce the weight and you reduce the drag. A lot of that drag is experienced in the tyres. The tyres have some toe-in (I cannot understand why, but was told that some toe-in is necessary for stability reasons - I don't believe that). But if you have toe-on (or toe-out as I have had with my van) the tyres are scrubbing as you drive along the road - more drag.

Of course, if you tow up and down hills, your fuel consumption increases according to the terrain at the time.

But then comes the big one - Aerodynamic drag... Aerodynamic drag is felt as the turbulence behind your can (or a large truck as it passes you). The forces associated with tis drag vary according to the SQUARE of the speed, so if you travel at twice the speed, you get 4 times the drag. Assume you drive at 100 km/h. You will generate a certain drag force. Now, increase the speed to 110 km/h, the drag force goes up to 1.21 time the force at 100 km/h. This is significant. Now, consider the POWER required to drive at that increased speed. Power - Force x velocity, so the power required varies according to the CUBE of the speed. So you need 1.33 times the power to drive your rig at 110 km/h instead of 100. 1.33 times the power equals a lot more fuel required....

Similarly, if you drop your speed to 90 km/h, the aerodynamic drag force lowers to 0.9 squared = 0.81, and the required power drops to 0.9 cubed = 0.73, so you need 27% less power to drive at 90 instead of 100. Now, throw in a headwind and all this goes out the window totally because a wind of 20 or 30 km/h is not that uncommon. Choose a tailwind and you win every time....

Anything you can do to reduce the drag will improve fuel consumption and trying to fill the void behind the van with some extra air will help. How much? I don't know, but even if you can fill only 10% of the displaced air, you would expect something close to the 33% extra drag to be recovered. With the shape of most caravans or motorhomes, I doubt that you could achieve the 10% reduction, but it would be worth trying. I recall some years ago there was a device like scoops on the sides of a van, but I never heard any reports as to how effective they were.

Look at large trucks - they have wind deflectors mounted on the roof of the cabin to improve airflow over the trailers which are much higher than the cabin. They do work and offer a reduction of about 5% in fuel consumption. On a truck, that is significant. They cannot do the deflector trick on the rear of the trailers because typically the trailers are already at maximum allowable dimensions. Regarding caravans, you used to see similar deflectors mounted on roof bars on the tugs of caravans. I never found out if they worked or not, but since most caravans are streamlined to a degree at the front of the van, the effect would not be as pronounced.

When I ordered my caravan, I asked for a shorter drawbar (to fit in my shed). I asked for 150 mm shorter, but they gave me 200 mm shorter. This has had unexpected consequences:
1 The towball weight is very heavy.

2 When I picked up the van, I turned full lock to get out of the dealer yard, and the gas bottle which was mounted on one of the arms hit the bumper of my tug. The first thing I had to do when I got home was to relocate the gas bottle to a new frame in the middle of the drawbar.

3 (most important) - I think that I accidentally have improved the airflow over my caravan (the front at least). I have a 4WD and my caravan (a poptop) is only about 100 mm higher than the roof of the tug. The combination of the shorter drawbar almost the same height seems to have improved the airflow. If I drive on a wet road, the rear door of my Pajero stays totally dry - not a drop of spray gets on the glass. When I stop, yes it gets wet, but once moving it is totally dry. This suggests to me that the airflow is going straight over the roof of the van. When I drive on a dusty road, I can see dust rising to about 50 cm above the ground whirling around between the tug and the van, but none gets any higher. Similarly, spray from the road doesn't get higher than about 1 m maximum.

So, by accident, I think I have achieved a good aerodynamic effect at the front of the van, but the rear still causes most of the drag.

Aus-Kiwi wrote:

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I find fitting mad flaps across the back prevents the vacuum effect . Sucking dirt up the back . They are not spoilers . They deflect bare down the back to prevent negative pressure building up behind van or vehicle. I have a a rubber mat between wheels behind diff on motorhome. Seems to help also .

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 Hi Graeme...You no doubt are aware that if said "mat" is too close to the diff,you are likely to have the diff running at a much higher temperature than "normal?" Cheers

Maybe if you gave us a HINT as to which model van we are searching for ....

Warren-Pat -----Exactly, that is more to the point.

Element Evolution is the model.

-- Edited by iana on Monday 26th of October 2020 12:21:17 PM

No Ian,
Have a look at "Aussie By Design" caravans. I'm not certain who the builder was.
They called their vans Humpback Smartvans, but have gone out of production.

When I built my van back in 2014 it had a square back. We did the east coast back in 2018 and whilst in Port Fairy, a fellow camper came by to say hi and promptly informed me that I had made one bad mistake in the build.

"A Square Back" wind drag etc, etc; and advised I needed something to break the flow of air over the back. All sounded like good advise.

So research/weight up the options (I'm on the road) and off to the Big Green Shed.

The "fix" which is still there, lowered my fuel consumption from 18lt/100 to 16.5lt/100. Im happy with the results. It may not look pretty but it works.

Regards.

Hetho