An Investigation into Wind Deflectors

[colin-yorkshire]

I think most of the points have been said already but I thought they did work [sort of] when they first came out. although it was in a different era.
I have never had a dedicated tow car. it was always tow with what I drive. fair enough you took into consideration the fact you towed so would buy the 1600 or 2 litre instead of the 1300 model but apart from that no real concessions were made.
but back then thing were vastly different. most cars had a low profile and rounded roof line. at the same time as caravans had high flat fronts on them. the wind drag was noticeable on the few motorways there were.
a deflector seemed to make it better if positioned properly for angle and distance from the front of the van.
over the years things have moved on both in car and caravan design. cars are higher in the roof line more aerodynamic and visually bigger . ever seen an old classic ford or vauxhall from that era and thought how small they looked.
caravan design also altered to the lower front sloping position and weight loss to make them easier to tow with a lighter car. all made a difference to the drag factor while towing.
still have one somewhere rusting away on a shelf in the garage. I will say one thing though it saved the caravan front window a couple of times with debris flying over the car hitting the deflector and bouncing over the top of the van instead of hitting it full on.

 

[chrisn7]

Well the old style deflectors were little more than an oblong board with a splitter in the middle attached to the cars roof gutters, as mentioned already. I should imagine that with some proper research a much improved device might come about, but there would have to some way of calibrating it to a given outfit, which I suspect would be difficult for a universal-fit device.
Today of course there are no car roof gutters, so that would mean an additional purchase of at least one roof bar. I don't know about others I always have plenty to do when setting off, and mounting/dismounting/storing another gadget is yet another task to do

 

[otherclive]

We have just returned from a few days near Leamington Spa and as we had dogs and granddaughter I fitted my Hapro Traxer roof box. Now I know that this would not stand strict scientific scrutiny but the reduction in mpg for the outfit was only around 4mpg yet if I use the roofbox solo as we did this week I lose around 8 mpg. We used the Fosse Way so cruising was 45-50 mph and numerous hills etc. But given so many caravan outfits use roof boxes perhaps Kevin might include any roof box effects too as my roofbox is an elegant aerodynamic shape which creates minimal wind noise solo or towing.

 

[ProfJohnL]

We have just returned from a few days near Leamington Spa and as we had dogs and granddaughter I fitted my Hapro Traxer roof box. Now I know that this would not stand strict scientific scrutiny but the reduction in mpg for the outfit was only around 4mpg yet if I use the roofbox solo as we did this week I lose around 8 mpg. We used the Fosse Way so cruising was 45-50 mph and numerous hills etc. But given so many caravan outfits use roof boxes perhaps Kevin might include any roof box effects too as my roofbox is an elegant aerodynamic shape which creates minimal wind noise solo or towing.

That's a very good point, and thank you.

 

[RayS]

Two observations.
Once when towing in zero wind conditions I noticed that the mpg figure on the car computer was showing a higher consumption than it should.. Reason turned out to be that 'someone' had forgotten to close the big Heki over the lounge. It was front hinged rather than side hinged and looked remarkably like a sind deflector although on top of the caravan rather than the car. This suggests that wind deflectors produce significant drag more or less regardless of where they are positioned and so any improvement in overall airflow would be to some extent negated by this.

Second, the effect of a roof box. Over the years I've used a Thule weekender roof box on various tow cars resulting in the rear of the box being at several different distances from the front of the caravan. The very unscientific observation of the spacing was that some was much better than the other judged by how clean the back of the car and the front of the caravan stayed when towing through mucky conditions.. In one configuration on a Merc ML270 the effect was dramatic - both car rear screen and front of caravan stayed very clean. It's a pity there are no public wind tunnels so we could have an hour trying various configurations.

 

[kevinG]

Hello everyone!

I'd just like to thank you all for your input, I referenced this thread as background information for my dissertation. For those who are interested, I will break down my findings for your curiosity. Firstly, a flat plate was created and attached it to an estate and SUV type vehicles with angles of attack of between 25-45 degrees. Computational fluid dynamics analysis was undertaken using Cham Polis Phoenics at 10 and 22m/s (36 and 80 km/h). The most considerable contributing phenomena to aerodynamic drag was the air flow that struck the front of the caravan and more so the low pressure wake that forms behind the body of the caravan. It was observed that in ideal conditions, fuel consumption can be reduced by 14% ( PurpleLine's Aeroplus claims 13%) .

Now, if you are considering buying an air deflector, these are my recommendations; One, the more 'boxy' your car and caravan are, the more you stand to benefit from an air deflector. Two, the greater the difference in height between the roof of your car and the roof of the caravan is, the more you stand to benefit (low riding cars are ideal). Three, when you install your air deflector, the angle of attack should roughly be equal to the angle that exists between the rear most part of the car roof and front of the roof of the caravan.

In terms of the impacts on handling, the effects are difficult to quantify. When talking about handling, friction between the tyres and the road is paramount. The friction equation is ; friction = friction coefficient X ( Weight - Lift). The only variable the deflector really impacts is the lift value. This was about a 25N difference for the optimum angle of attack, and is really negligible when you have a 35 000N car and caravan, so no meaningful conclusion could be drawn on the impacts on handling.

In conclusion, air deflectors can help reduce drag significantly under the correct circumstances, with the right car and caravan combo. Be warned though! In many instances, the drag shot up significantly and may do serious harm to your fuel consumption if not set correctly. If I had a caravan, I would most likely not buy an air deflector as the time, energy and cost of installing an air deflector and getting it to work for your specific car-caravan configuration would just negate the benefits of it working properly.

If you have any questions, please feel free to ask.

 

[camel]

Kevin,
Did you have any input to your research from any outside organisations or individuals like the caravanning clubs and there members.

 

[CustardAvenger]

... if you are considering buying an air deflector, these are my recommendations; One, the more 'boxy' your car and caravan are, the more you stand to benefit from an air deflector. Two, the greater the difference in height between the roof of your car and the roof of the caravan is, the more you stand to benefit (low riding cars are ideal). Three, when you install your air deflector, the angle of attack should roughly be equal to the angle that exists between the rear most part of the car roof and front of the roof of the caravan.

Hi Kevin,

My anecdotal experience would tend to correlate with your research. I got into caravaning with a 1970s/80s CI Cadet; this is an old design of caravan with a very flat front, only a few degrees from vertical. The weight is almost negligible, under 500KG empty and you could literally forget it was there... except at speed. My original tow car was a Volvo V50 estate, which was quite low when compared to the height of the caravan, so the drag experienced at 50-60mph was noticeable - I often described it as towing a parachute.

However, when I switched to my current car, a Volvo XC60, the drag seemed much less; the car is quite a bit taller so seemed to deflect much more of the air from the caravan front.

Paul.

 

[ProfJohnL]

Hello Kevin,

First of all thank you for the brief summary of your findings. Its good to get feedback after a project has been completed. Did you obtain your degree? I Do hope so.

Your findings are no great surprise, as they confirm what I and others have instinctively believed for a long time. But its reassuring to know that some research has been carried out which does confirm our understanding of the matter.

Sometimes the public are critical when they sees a University project, that confirms what is generally accepted as "common sense" and they wonder why time and effort has been spent on proving what everybody knows, but whilst that may be the public feeling, it belies the importance of the experience of completing a research project.

Just occasionally such projects do throw up some unexpected results, but even when they don't they do put some certainty to what otherwise may be argued for a longtime by a few dissenters.

Practically over recent years caravan manufacturers have been working on caravan aerodynamics, which will tend to reduce any potential benefits of wind deflectors.

 

[kevinG]

Camel,
No it was all done independently, this was a bachelors of engineering dissertation, I tried to get into contact with air deflector manufacturers but they weren't helpful.

 

[kevinG]

CustardAvenger
Yes, this seemed to be the case and I knew the results before I performed the analysis, however, the important aspect of this project was the process rather than the results. Glad to see that there is a correlation between my findings and the opinions of users!

 

[kevinG]

ProfJohnL,
Thank you for your support. I have not received my degree yet but I have just finished my final exams and got feedback on this project, received quite a nice grade . Yes, as I'm sure you know, the results were of little importance but the methodology of an investigation like this is a crucial skill for engineers. Although, I have a new found interest in CFD and drag reduction, and see automotive aerodynamics in a new light.

 

[VinnyE]

Most likely too late to this party - as usual!
I thought that you would gain most aerodynamically by cleaning up the air flow leaving the caravan rather than the air leaving the tow car?
Having said this, I would be concerned about adding a wing or roof spoiler to the caravan as it would increase down force on the back of the van and hence relieve weight on the tow hitch?

 

[ProfJohnL]

Most likely too late to this party - as usual!
I thought that you would gain most aerodynamically by cleaning up the air flow leaving the caravan rather than the air leaving the tow car?
Having said this, I would be concerned about adding a wing or roof spoiler to the caravan as it would increase down force on the back of the van and hence relieve weight on the tow hitch?

Hello Vinny,

Aerodynamics on vehicles are adjusted for three basic reasons. The first is to minimise drag and allow the vehicle to pass as easily as possible through the air. The second reasons is only really used by performance vehicles where the aero package is designed to create down force which allows the vehicle to grip the road better for improved traction. and the third again relates to performance vehicle where the device may provide additional braking. Cars with caravans don't really need extra down force or extra breaking, so the main aim is to make the outfit as slippy as possible.

Most modern cars have been engineered to be quite slippy as a solo unit, but put a big slab fronted caravan behind them and the picture changes. Usually the biggest issue is the gap between the car and the caravan which traps the air that has flowed over the car, which becasue of the presence of the caravan and its covered A frame prevents the air flow returning to near laminar flow behind teh car. Instead it is again split, some will flow under the caravan, and the rest has to fight the caravans front panel, shedding it so some will rise over the caravan and some split sideways round the sides. Its almost certain there will be vortexes created between the car and caravan which drastically affects the smooth air flow and thus creates extra drag.

Wind deflectors mounted on the car MAY help to clean the air flow up, but they have to be carefully positioned and of teh optimum size and shape. Get any of those three things wrong and the wing will increase drag.

I cant see putting as up turned wing on a caravan would have any beneficial effects, you are effectively making the frontal area of the caravan even bigger like my third category for air breaking.

I won't be trying it. :dry:

 

[Dustydog]

For those interested in railway engineering aerodynamics William Stanier and more so Nigel Gresley developed the famous streamliner locomotives, the Coronation Pacific and A4 Pacific. Both had streamlined fronts to allegedly cut through the air.
Whilst very attractive and futuristic for the 30s the main effect was smoke deflection away from the driver’s cab window.
They discovered much later the real drag was caused by the air travelling along the sides and roofs of the carriages.
Is that relevant to the sides and roof of our caravans

 

[Raywood]

I think the difference hear is the length of a train and a car and caravan and the lack of large gaps between carriages on a train.
Trains are several times longer than a car and caravan and so the drag created by air passing over the sides is far less of a problem for us. Ten or twelve carriages produce a lot of drag down the sides but up to 7 meters of caravan is negligible in comparison.
Basically a train height is the same as the carriages behind and they are close coupled so the drag from the gaps is fairly low. With a caravan you have a substantial gap between the car and caravan and then the near vertical front on the caravan. This will produce a lot of drag and must be the main area of drag. To illustrate this there was a trip from Glasgow down to Chester into a head wind and the car recorded 24 mpg on the trip down. A few days later we returned with a tail wind roughly the same and got 35 mpg recorded. I would normally have expected 30-32 mpg from that combination.
I know people query the accuracy of a cars figures, but it does show the effect of wind on fuel consumption as any error should be consistent across the trip.
If it helps I did once read an article where they had tested clean and dirty trucks for wind resistance, slightly dirty ones produced the best figures, suggesting the air passes over the slightly dirty vehicle better then the clean one. They concluded that the dirt trapped air between the bits and gave a smoother passage through the atmosphere. An excuse not to wash the car,

 

[ProfJohnL]

In a similar vein, back in the 1970's I had to drive a fully laden Transit van with a Luton body from the Midlands to Estuary Caravans of Felixstowe. There was a strong Westerly wind, and on the outward fully laden trip I average about 25MPG, but on the homeward (Empty) run I only managed about 18mpg, becasue of the strong winds.