The Impact of Aerodynamic Drag On Towing Range


Hi, I’m Chris. About Me

In one of my previous articles on can electric cars tow a caravan/trailer, I discussed that when towing with an electric car you should expect the range to be reduced by roughly 50% from its typical range when not towing. But why is that? Why is the range of an electric car impacted so considerably when towing? You will rightly state its the weight of the trailer/caravan being towed as a reason for the reduction in range. However, it might not actually be the most significant factor in range reduction. Aerodynamic drag/wind resistance may actually be the most significant factor in reducing towing range.

Towing Aerodynamics
I believe more attention is going (needs) to be paid to towing aerodynamics with electric tow cars: Image – Computationalfluiddynamics.com.au

This article is not intended to be a detailed science lesson on the aerodynamics of vehicles while towing, I’m not qualified or smart enough to do so. My intention is to make you aware of the issue of aerodynamic drag while towing so when it comes to purchasing your next trailer/caravan along with an electric tow car you are aware of the issue which may help you to make better purchasing decisions to get the towing range you’re after.

Introduction To Towing Aerodynamics

Its not as if the issue of increased aerodynamic drag when towing has only been thought up along with electric tow cars. Anyone who currently tows with an internal combustion engine car will have noticed the significant reduction in MPG while towing. Many caravan owners are aware that wind resistance/aerodynamic drag is a significant part of that MPG reduction. There have been various efforts to reduce the impact of aerodynamic drag while towing as I’ll reference below. However, their impact has never been that significant.

Up until this point, the aerodynamic drag of a caravan and its impact on MPG/efficiency has not been a significant consideration to many caravan owners. While you will see some caravan manufacturers making references to how aerodynamic their caravan is, its not widely discussed. I’ve been running my small caravan site Horton Common since 2014. When discussing new caravan purchases with my guests over the years, no one has ever stated that part of the reason they purchased a particular caravan was due to how aerodynamic it was stated to be over the competition.

A simulation of the air turbulence created by tow car and caravan

I think the reason is, with an ICE car, while towing may reduce MPG resulting in more frequent fuel stops with a petrol/diesel car it just takes a few minutes to fill up. Hence, the higher fuel costs while towing are just taken as given, and the actual disruption to the journey time is minimal. However, with an electric tow car, at least for several years to come, the impact of more frequent stops to top up with charge will be more significant.

Once battery/charging technology and charging infrastructure is more fully matured, potentially a full charge in a few minutes may be possible. However, we are not there yet, and a 30 minute stop (potentially longer) to get a reasonable charge with an EV will not be uncommon. Check out my posts on which electric tow cars charge the fastest and guide to fast/rapid charging an electric tow car to learn more. Hence, I believe more emphasis will be paid to towing aerodynamics to reduce the frequency of charging stops, as the benefits could be considerable.

How To Improve Towing Aerodynamics

So just to further emphasize this point, fluid dynamics is an extremely complicated area of physics/engineering. I only have a basic understanding of the topic myself, I just want to highlight some of the findings in basic terms from previous research carried out in past with regards to towing. Back in 1981, this research paper discussed placing a wind deflector on the roof of the tow car to direct the air over the roof of the caravan. The result of this research led to products coming onto the market such as the Aeroplus as you can see in the promotional video below from Purpleline.

While wind deflectors may help an electric tow car increase its towing range, I believe the ultimate solution lies in better aerodynamic vehicle/trailer designs

Now, while such devices can potentially help to reduce Aerodynamic drag while towing, there are those who have found such devices had no effect or even decreased their MPG further (example). The problem is there is such a wide range of tow vehicles on the market, the exact location of such a wind deflector and its setup are all important variables. Therefore, such aftermarket/add-on devices are not the best overall solution to aerodynamic drag when towing for electric tow cars. I believe the solution needs to come from better aerodynamic vehicle and trailer/caravan designs.

The Impact Of Reductions In Speed and Relative Heights

The image I included at the top of this article is from research conducted by Leap Australia. I would encourage you to read their full blog post on towing aerodynamics, its a short read but very interesting. The key findings from running their simulations of a tow car and caravan with regards to aerodynamic drag were the following. First keeping the speed down while towing will yield the biggest impact in improving MPG/range while towing.

As is stated in their blog post, “drag goes up four-fold, for a two-fold increase in speed. Put another way, the drag you experience at 30kmph is a quarter of the drag that hits you at 60kmph.” Therefore, with an electric tow car which may take 30-40 minutes to charge reducing speed may actually get you to your destination quicker than driving faster. Their second key finding is that a tow car and caravan/trailer at the same relative height can have a significant reduction in fuel/energy consumption.

Relative Heights & Towing Aerodynamics
Keeping the tow car/trailer at the same relative height increased efficiency by 48%: Image – Computationalfluiddynamics.com.au

The findings above on the benefits of relative heights between the tow car and trailer/caravan match the benefits in principle of devices such as the wind deflector referenced above. However, a lower/short caravan is obviously less practical (reduced interior head height). Who knows, perhaps pop-up roofs in caravans may become more popular as a means to improve towing ranges with electric cars.

However, it should be noted the image above with the outfit which achieved a 48% increase in fuel economy had another design change other than its reduction in height. There is also a reduction in the gap between the back of the tow car and the front of the caravan. This reduced the air turbulence coming off the roof of the car, reducing drag. This is an example of where I think caravan/trailer designs will change to improve aerodynamic performance and thus improve electric tow car ranges.

Real-World Example – Tesla Model X Towing An Airstream

Research papers and computer simulations are all well and good, but what about a real-world example of the impact of aerodynamic drag on the range of an electric car while towing? Well, luckily I already have one example I can reference, and there will hopefully be more in the future. Below is a video from the All Electric Family on their experience towing an Airstream caravan behind their Tesla Model X. The video is quite long, though I would encourage you to watch it all if you can. If not, jump forward to the stats breakdown at 12.51.

Evidence that the shape/profile of your caravan may very well significantly impact your range while towing with an electric car

As they discuss in the video above, the family were testing the towing performance of the Airstream against their own camper an Apex Nano seen in the image below. In the video above efficiency stats/comparisons are provided in watt-hours per mile, and I’ll be discussing how electric car efficiencies are referenced and compared in a later post. The most important point is obviously the difference in range between towing the Airstream caravan vs their Apex Nano camper.

Apex Nano Camper
This is the Apex Nano camper the family usually tows behind their Tesla Model X. Not that dissimilar in design/aerodynamic profile to many typical UK touring caravans: Image – Coachmenrv.com

The family achieved a towing range with their Apex Nano camper of 108 miles but a range of 177 miles with the Airstream caravan. The result was a 69% increase in range with the more aerodynamic Airstream caravan, hence a considerable range increase. However, if they drove slower the range nearly doubled, with a 200-mile towing range possible with the Airstream! Bear in mind this was not a scientifically controlled test. However, the difference is so significant with two caravans of similar weight, the impact of reducing aerodynamic drag is obviously considerable.

Conclusions On Aerodynamics & Towing Range

So what are the takeaway lessons from the above information? Should everyone who wants to tow with an electric car be buying only Airstream caravans? Not exactly, not that an Airstream is not worth considering, but they are notably far more expensive than other typical caravans for the same size etc. The point to take away from this is that more aerodynamic designs for UK caravans are likely to be coming due to the impact on range while towing with an electric car. Therefore, when you are next in the market for a new caravan, look out for any (proven) information/facts on the improved aerodynamics of a particular caravan.

The second key takeaway is the impact of speed on aerodynamic drag and hence the range of an electric car while towing. While your particular caravan may not have the most aerodynamic profile, reducing speed while towing will lead to a longer range. Given how long it takes to charge an electric car, as discussed above, driving slower may actually result in a shorter total journey time as strange as that sounds.

Chris

Hi, I’m Chris. I own and write all the content on ElectricTowCars.co.uk. The content above is purely my own personal thoughts/opinions/research and should not be treated as professional advice. I hope you find the content above useful to help you find your future ideal electric tow car or truck.

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