EV Chronicles #61 - Efficiency Impacts from a Thule Roof Box

Right after getting back from Florida, and while the Polestar 3 was in the shop for service on the AC charger and the HVAC vent controllers (that all went very well by the way), I decided to order the Thule Roof rails with the feet and fit kit for the Polestar 3. I currently have no way to mount my bike(s) on the Polestar and I have some old roof rail bike racks as well. I have been using a hitch mounted rear bike rack system for years, but the Polestar 3 as I ordered it, doesn't have a hitch. Additionally, we have a trip to the FLX (Finger Lakes, NY) Region and then also back to Florida this fall. On the way to Florida, and maybe to FLX, we'll have 4 adults and all our stuff in the car with us. I think if we pack lightly, and leave a few things behind we'll be able to easily fit without having to pack things around our back seat passengers with this setup.

I like to avoid anyone being a sardine on our road trips, which is why I have owned a Thule Roof Box of some kind for the last 25+ years. My current box is a Thule Pulse M (I think). I have had this one a couple of years and actually haven't used it yet. Prior to this box, I had a Thule "Rocket Box" which I have had since around 2000....it was big and long and it swallowed stuff! It could easily hold 4 sets of golf clubs along with a bunch of other stuff packed in around them. It was awesome and we used it often for family vacations in our GAS vehicles, particularly on camping trips. Unfortunately, after 20 years of service, the mounting hardware needed a full replacement, so about 3 years ago, I bought this new, shorter Thule Pulse M. I found out after buying it that the Model Y roof rails were spaced too far apart from one another (by about an inch) for the box attachment points to fit those rails. After doing some research and ensuring that the spacing of the roof rails on the Polestar would be close enough for this box, I pulled the trigger on getting the rails. Now it is time to nerd up and do some drag analysis on this setup like any good engineer would do!

In all my years of driving gas vehicles with a Box on top, sometimes bikes on top too, and often times, MORE bikes on a hitch or trunk mounted rack (4-5 bikes total), fuel efficiency definitely took a significant hit every time we traveled. I would estimate that our MPG in these cases were often 10-20% less. I remember one trip to Bar Harbor, we had our 2011 Toyota Venza LOADED. I think we got about 19 MPG and the 4 CYL engine in that car struggled to hold 75 MPH on I-95 due to the massive aerodynamic drag. I would generally get about 24-26 mpg on highway trips in that car without all that stuff mounted on the outside car (I wish I had a picture of that car loaded up). In a gas vehicle though, who cares? The only real impact is that you pay more for gas for the trip right? You don't really need to stop more frequently, at least no more frequently than our need for food and bathrooms, maybe adding an additional stop at a gas pump with one of the bathroom stops. Fast forward to the EV era of my vehicle ownership experience, and now it matters, or at least that is what I thought.

The question is, HOW MUCH DOES IT MATTER? I really want to know the answer to this question, because this might end up being a deal breaker for putting the roof box on the Polestar 3 for a LONG trip like our one to Florida. I am concerned that the extra drag might result in extra charging stops or at least longer charging stops which could add maybe a couple extra hours of charging to the trip, maybe more?

So, before I present you with a PHD level discussion on the Equations of Aerodynamic Drag, some real world data that I have collected, and the analysis I have performed using ABRP with a variable amount of increased energy consumption, let me summarize by saying, at least for a Polestar 3:

IT DOESN'T MATTER.

Aerodynamic Drag - Equations and Engineering Analysis

The equation of Aerodynamic Drag (force):

F_drag = 0.5* ρ  A * C_d * V²

Where:

ρ = Air Density

A = Frontal Area

C_d = Coefficient of Drag

V = Velocity

First, let's start with Air Density. It varies as a function of atmospheric conditions like temperature, humidity, and altitude. Of these three items Temperature is the one environmental variable that can impact air density the most in conditions we typically drive in. Air Density can be 10-15% higher at temperatures below freezing as compared to the air density of air near 100F. In an EV, this comes with a double whammy, because in an EV, additional energy will be needed for keeping the cabin warm and in some extreme cases, the battery warm enough to send and receive electrons.

For a more detailed understanding of Temperature and Humidity impacts on Air Density, see this graphic.

https://commons.wikimedia.org/wiki/File:Air_density_dependence_on_temperature_and_relative_humidity.svg#/media/File:Air_density_dependence_on_temperature_and_relative_humidity.svg

Next, lets' look at Area and the Coefficient of Drag. When a vehicle owner adds anything to the top or back of their vehicle, they potentially change two quantities in the equation above, Frontal Area and/or the Coefficient of Drag. Without performing a complex CFD (Computational Fluid Dynamics) analysis using very powerful computers and expensive software with very detailed 3D CAD models of a vehicle and the items that get attached to it, or putting the car into a wind tunnel (which has as much as 5% error on the measurement anyway), figuring out the impact to the drag coefficient is very difficult to do.

In the case of adding the cargo box on top of the vehicle though, it is probably a safe assumption that the drag coefficient isn't going to change all that much given these boxes are designed to be fairly aerodynamic. The dominating factor then becomes the change in the frontal area that results when adding a cargo box like the one I added shown in the picture below.

I have a Bachelor's and Master's in Mechanical Engineering and 30+ years work experience as an Aerospace engineer (orbits, not air breathers), and there is one aspect of this equation that I am not 100% sure how to apply to a moving vehicle. Ground Clearance. The Polestar 3 has 6 inches of ground clearance and there are 4 inches of space between the bottom of the cargo box and the roof of the car. I am not 100% certain if those areas should be included in the frontal area of the vehicle or not. So, as a good engineer, I ran through various methods to compute the increase in the frontal area of the Polestar 3 (blue shade above) caused by the cargo box (yellow shade above). The increase in frontal area ends up being somewhere between 13-18% depending on how I go about including or not including the ground clearance spaces in the geometry above.

Real World Data

Overall, the data I have collected so far suggests that in the real world, the impact of having the cargo box on the vehicle is about 20% increase worst case on the ENERGY consumption per mile of the Polestar 3. This equates to a 17% reduction in real world range (1/1.20 = 83%). The conditions I have analyzed so far:

• Highway Driving (75-80 mph)

  • This is based on our typical 100 mile trip to visit family with the box on the car for our last trip, past trips for this route, and our recent trip to Florida

  • Trip Computer in Car:

    • 400 Wh/mile vs. 485 Wh/mile = 21% Increase

  • SOC used for 100 mile trip to visit family

    • 33% usage is typical (+/- 1%) vs 40% going and 36% coming back (5 mph slower) with the box on during our last visit = 21% increase

• Local Driving (30 mph average:

  • Car Trip Computer - ~330 Wh/mile vs 360 Wh/mile = 10% Increase

  • This makes sense because overall average speeds are much slower. About 30 mph is my local average with all the traffic we have to deal with here.

• Overall Mixed Driving - From my Charging Data

  • 480 Wh/mile vs. 555 Wh/mile = 16% Increase (includes charging inefficiencies and phantom drain in the numbers)

    
    

So, based on the Theoretical analysis and the real world data I have collected so far, I feel that it is safe to say that MY SETUP (vehicle and box combination) will experience a maximum of a 21% energy increase and about a 17% range reduction with the cargo box on the Polestar 3. Other EVs and cargo box combinations will likely experience similar results but will obviously vary some. Hopefully the theoretically explanation above is helpful to others to get a rough estimate of the impact for your own setup.

ABRP Analysis

So, now that we have established the energy consumption impact of the Polestar 3 with the cargo box on top, what does that really mean to travelers embarking on a 250-400 mile trip? How about 800 miles in a single day or 1000 miles over two days. The best way to determine the answer to this question (without just hoping in the car and doing these trips, twice) I have found is using a website/APP call AbbetterRoutePlanner.com (or ABRP) one of the best tools out there to conduct these "what if" analyses.

In ABRP, there are multiple assumptions about the type of vehicle, the stopping strategy, the ambient temperatures, and speed traveled thinks they could acchieve just to name a few of the variables that can be adjusted in the tool. Power consumption at a speed of 65mph is yet another input available to the user. Therefore, based on the analysis I have presented so far, I can run trip "simulations" that assumes a baseline set of assumptions for all of these parameters, and then I can increase the power usage by 21% ONLY and compare the overall trip time and impact to the number and total duration of charging stops.

Every EV owner can use this tool to perform there own power consumption sensitivity analysis for their own setups. I utilized a set of assumptions that very closely modeled our real world door to door time for our recent trip from our home in SE PA to Florida. I was shocked to find that for a total of 18 hours of travel (about 16 hours of driving, 2 hours stopped), that increasing our energy consumption by 21% only adds 30-40 minutes to the overall trip depending on the exact charging strategy that one employs. But, if you think about it, increase energy will equate to an equivalent increase in charging time. 120 minutes turns into about 145 minutes for example, and then if you have to add an extra charging stop throughout the day, then add a couple more minutes for overhead getting off and back on the highway.

Even if you are assuming 30% increase in energy consumption, we are talking about adding a maximum of 60 minutes to the trip. If you factor in hitting one storm, one rush hour, one fender bender along I-95, or just changing up your charging plan as compared to the "perfect", most optimal plan, and your trip is going to change by this much time or more. I did the same analysis with my Model Y assumptions and the impact was as much as an hour increase in overall travel time under worst case conditions. It is my take that the impact of having a travel box on an EV is dependent on the EV, but it is still essentially "in the noise" given all of the many other variables that can impact your overall trip duration.

I have performed the same analysis in more detail below for our trip the FLX, and the conclusion is the same. In that case, the final charging stop needs to be a little longer, perhaps as much as 5 minutes longer for a 5 hour trip to arrive at the same SOC in all cases analyzed. But again, it just doesn't matter. The truth is, without the box, I don't need two 10 minute stops to arrive with 50% SOC. I can actually do it with as little as 17 minutes of total charging, but my bladder will need two stops, so we will be stopping twice for about 20 minutes total no matter what is on top of the car. In my experience, our bathroom stops average about 10 minutes in duration. The case with the maximum charging time resulted in 23 minutes of total charging. Three extra minutes total is basically nothing.

The results actually surprised me. I thought the time impact would be larger on our trips. This comes from my experiences analyzing many trips in ABRP with my Model Y LR assumptions. I think the smaller battery pack of the Model Y really makes a difference when doing these comparisons. I also think that the Model Y is overall, generally a more aerodynamic vehicle than the Polestar 3. As such, I think adding the same box to the top of a Model Y will have a bigger impact by percentage impact on the range than that same box will have on the Polestar.