A couple of months ago, the Danish Association of EV Owners (FDEL) published an article on how towing a trailer and caravan affected the range of an EV, and I got to present the findings for the readers of CleanTechnica. Now, FDEL has been out in the cold weather testing some more and they asked if I would share these additional finding, which I am happy to do below (including a list at the end of EVs that can be delivered with a towbar):

Photo credit: Danish Association of EV Owners/FDEL

Size Matters — In More Ways Than One

Should the Easter holiday take place in a caravan? This time around, FDEL’s testers, Christian Nolsøe Nielsen and Martin Messer Thomsen, took the Audi e-tron on a trip and exposed it to both large and small caravans.

FDEL’s test from December 2020 with the Tesla Model 3, towing different trailers, made the test panel even more curious about the trailers’ influence on energy consumption and range.

The trailer test at the time showed that the height and shape of the trailer obviously influence wind resistance significantly. Model 3’s advantages as an almost aerodynamic marvel were abolished when we pulled a tall hauling trailer or a traditional caravan with an almost vertical surface. But what happens if you put a lighter and apparently more aerodynamic caravan after the Tesla? And what happens to the range if you put an electric car with a larger front area to tow the two different caravans?

Weather conditions during the test

• Wind: 5 m/s (direction from changing directions)
• Temperature: 0–5° Celsius (32–41° Fahrenheit)
• Tests were carried out at the end of January 2021. But similar temperatures and wind speeds can be experienced during the autumn, winter, and Easter holidays, when some people drive a caravan.

Model 3 vs. e-tron

The test panel went on tour again in late January. This time with the Tesla Model 3 towing a small Tab Model 320 caravan as well as the Audi e-tron 55 initially towing the large traditional caravan, switching later in the day to the smaller Tab caravan. The stretch was the same as for the trailer test in December, an almost flat stretch between Køge and Nr. Alslev (76 km/47 miles). The temperature was again around freezing point and the wind at 5 m/s from changing directions.

During the December test, the wind came from the south, and the energy consumption outbound and inbound was thus visibly different. As can be seen from the table below, wind direction during the January test had less influence on the difference in consumption on the outbound and inbound trip, respectively, because it came from changing directions.

Test Results for Tesla Model 3 LR with Large & Small Caravan

Photo credit: Danish Association of EV Owners (FDEL)

Towing Tab Model 320 caravan weighing 700 kg:

• Driving out in changing wind directions: 325 Wh/km — 215 km range (523 Wh/mile — 134 miles).
• Driving back in changing wind directions 336 Wh/km — 208 km range (541 Wh/mile — 129 miles).

Towing traditional caravan weighing 1000 kg:

• Driving out in headwind: 400 Wh/km — 175 km range (644 Wh/mile — 109 miles).
• Driving back in tailwind 337 Wh/km — 210 km range (542 Wh/mile — 130 miles).

Reference test on motorway at 110 km/h (68 mph) without towing:

• Driving from Brønshøj to Køge in headwind: 183 Wh/km — 380 km range (295 Wh/km — 236 miles).

(Range calculated from 70 kWh of available battery capacity.)

Energy consumption remains high on the Model 3, even though the Tab caravan is both lighter and more aerodynamic. The very different designs of the test caravans thus meant a surprisingly small difference in results.

Compared to the reference test without towing, the caravans almost cut range in half on the Model 3 with this load.

Test results for Audi e-tron 55 with large and small caravan

Photo credit: Danish Association of EV Owners (FDEL)

Towing Tab Model 320 caravan weighing 700 kg:

• Driving out in changing wind directions: 425 Wh/km — 200 km range (684 Wh/mile — 124 miles).
• Driving back in changing wind directions 402 Wh/km — 211 km range (647 Wh/mile — 131 miles).

Towing traditional caravan weighing 1000 kg:

• Driving out in changing wind directions: 431 Wh/km — 197 km range (694 Wh/mile — 122.4 miles).
• Driving back in changing wind directions 433 Wh/km — 196 km range (697 Wh/mile — 121.8 miles).

Reference test on motorway at 110 km/h (68 mph) without towing:

• Driving from Brønshøj to Køge in changing wind directions: 297 Wh/km — 286 km range (478 Wh/km — 178 miles).

(Range calculated from 85 kWh of available battery capacity.)

Caravans affect the SUV’s range relatively less than the sedan’s

As expected, consumption does not increase nearly as much when we hook a caravan on the Audi e-tron. The range only decreases by about a third. The e-tron has a relatively flat front to break the wind. Basically, the e-tron therefore already uses more energy than the Model 3, according to the reference test without towing.

However, with the e-tron, it comes as a surprises that there is no major difference in energy consumption whether towing a small and presumably more aerodynamic caravan compared to a large box-shaped caravan.

Also, a word on comfort: The Model 3 ride comfort and stability was more influenced by the heavier caravan than was the case with the larger and heavier e-tron.

Towing A Traditional Caravan Is Fine

Photo credit: Danish Association of EV Owners (FDEL)

As the results surprised us in relation to our expectations, we asked Jens Nørkær Sørensen, professor at DTU Wind Energy to explain:

• The fact that the “aerodynamic” caravan Tab model 320 isn’t more energy efficient than the “box-shaped” caravan is relatively easily explained from fundamental aerodynamics. (See below.)
• The problem with the Tab model 320 can be briefly explained by the fact that it has a combination of a small side to front aspect ratio (< 1) (see below), an aerodynamic 2-D cross section, and a flat cut of the two sides. Due to the thick profile cross section, a negative pressure will form on the upper side of the caravan, which pushes the air from the underside to the upper side. Thereby, two powerful vortices are formed along the sides of the wagon, which gives rise to an increased wind resistance, the so-called induced or parasitic resistance.
• In principle, this can be solved by either making a “chubby” side design and/or a much longer aerodynamically shaped rear end, so that the relative height becomes smaller. In practice, however, this is impossible, as it will result in a very long and/or very wide caravan. So the traditional design of a caravan is in fact not that bad.

Charging With A Caravan

Not many charging stations are designed with electric cars towing things in mind. Check out this video at the fast charging stations at Karlslunde Syd and on Farø:

How much range per charge is enough for your holidays?

According to the test, there is not much range to gain by choosing the Tab Model 320 caravan, about 200 km (124 miles) of the range on a flat stretch in Denmark on a cold winter day. You have to plan some charging if you want to go further than that. And expect even shorter range if you experience headwinds.

The pandemic sets a limit to our freedom of movement, and Easter and summer holidays are approaching. Maybe 2021 will be the year when you reduce the family’s CO2 footprint by going on vacation closer to home, perhaps in an electric car with a caravan?

If you want to see what stretches in a more hilly terrain does to the range, take a look at these articles from the Norwegian Electric Car Association: Historical test: Almost 140 km with three electric cars and caravan and Long range test: 2,250 km with electric car and caravan.

A Little Bit Of Aerodynamic Theory

by Jens Nørkær Sørensen, DTU Wind Energy

The wind resistance of an object is proportional to the speed of the 2nd power, and the effect of the speed of the 3rd power. That is, a doubling of the speed gives a 4-fold increase in power and an 8-fold increase in power. The resistance force is usually written as FD = ½ * rho * A * V ^ 2 * CD, where rho is the density of the air, A is the cross-sectional area (width * height), V ^ 2 is the velocity in the 2nd power, and CD is an experimentally determined resistance coefficient. If we assume the same values ​​for everything except CD, then it is solely its value that gives the aerodynamic difference between the two types of caravans.

CD contains three contributions: 1) friction loss; 2) Shape resistance and 3) Induced resistance; CD = CDf + CDp + CDi.

1) For “clumsy” bodies one can disregard the friction loss, CDf, as it is usually vanishing compared to the other losses.

2) The size of the CDp is mainly about the 2-dimensional design of the body (cross-section), and its influence on the wake (waken), which is formed behind the vehicle. The smaller the wake, the less resistance.

3) The induced resistor, CDi, is the joker in the game here. This resistance, which is related to the formation of two strong vortices on the sides, occurs on bodies that have a large buoyancy, for example flight wings, and it is proportional to the buoyancy coefficient (CL) of the second power and inversely proportional to the aspect ratio (width divided length ). It can be calculated with good accuracy from the formula:

CDi = CL ^ 2 / (pi * b / h). A quick estimate of the shape of Tab model 320 gives the following values: CL = 0.5 and b / h = 0.7, This gives CDi = 0.5 ^ 2 / (pi * 0.7) = 0.11.

Typical values ​​for a square vehicle with rounded edges (similar to the “classic” caravan) are CD = 0.45–0.5. A VW Beetle (reminiscent of the Tab Model 320) typically has a CD = 0.35. If you add CDi to this, you get that the two models have comparable resistance coefficients.

(Note from first-in-line editor Jesper: I didn’t get any of that. Give Jens a call at DTU if you need his professional assistance….)

• The Tesla Model 3 and the large caravan are privately owned.
• The Audi e-tron was kindly lent by the Audi importer and the small Tab caravan was kindly lent by tagboksudlejning.dk.
• All expenses related to the test were borne by the testers.
• Photos: copyright Danish Association of EV Owners FDEL.

Electric cars with towbars: What options do you have in 2021?

Photo credit: Danish Association of EV Owners (FDEL)

Denmark is known as a country where many have towbars installed on their car. What options do you have as a consumer if you need a towbar for your small trailer or your large caravan and at the same time want to buy, lease, or rent an electric car? FDEL decided to find out.

Do you frequently go on holiday with a caravan or need to drive a load of garden waste to the recycling station with a larger or smaller trailer every now and then? It is widely believed that one cannot settle for one car in the household if the car is an electric car. Several motorists opt out of the electric car when they have to invest in a new car on the grounds that the electric car and towbar simply cannot be combined. But is this true?

There are a number of electric cars on the market already that can tow a large or small trailer, and the number of options are increasing fast.

Below you can see a selection of electric cars with the option of towing:

Towing 1000 kg and more:

(Model and max. towing with/without assisted brakes)

• Audi e-tron: 1,800 kg/750 kg
• Hyundai IONIQ 5 (AWD), late 2021: 1,600 kg/NA
• Nissan Ariya (late 2021): 1,500 kg/NA
• Skoda Enyaq (late 2021): 1,200 kg/NA
• Mercedes-Benz EQC: 1,800 kg/750 kg
• Tesla Model 3 LR: 1,000 kg/750 kg
• Tesla Model X LR: 2,250 kg/750 kg
• Tesla Model Y (late 2021) 1,600 kg/NA
• Volkswagen ID.4: 1,000 kg/750 kg

Towing less than 1000 kg:

(Model and max. towing with/without assisted brakes)

• BMW iX3: NA/750 kg
• Ford Mustang Mach-E: 750 kg/NA
• Hyundai Kona: 300 kg/300 kg
• Jaguar i-Pace: 750 kg/750 kg
• Kia e-Niro: 300 kg/300 kg
• Maxus Euniq: NA/400 kg

Photo credit: Danish Association of EV Owners (FDEL)