Roof Racks, Boat Tails, and other Aerodynamic Oddities

Jan 12, 2019

A comparative analysis of five Coefficient of Drag Graphs (up to 100 mph) in Cd = 0.05 increments from Cd = 0.13 up to Cd = 0.33. This includes the Tesla Model 3 at Cd = 0.023 and a aerodynamically modified Tesla down to Cd = 0.179. Also included is an ideal body form of Cd = 0.13 and a Nissan Leaf body form at Cd = 0.28. Finally we have a Cd = 0.33 body form which is the shape of most cars on the market approximately.

Roof Racks, Boat Tails, and other Aerodynamic Oddities It has been a strange month for Aerostealth Works shooting You Tube Videos and for aerodynamic exposes in general. If you have been following any of my recent blog post here at EV World or my Vlogs on You Tube you may have noted Phil Knox and I have been working on explaining aerodynamic graphs of various coefficient of drag forms in Cd = 0.05 increments.

We first started with a graph of a Cd = 0.179 form which showed that a Tesla Model 3 could do 100 mpg-e at 100 mph. This graph was based upon the principle of taking a stock Tesla Model 3 and adding a boat tail to the car plus some rear wheel well spats to reduce the drag from Cd = 0.23 down to 0.179. The addition of these aerodynamic appliances would increase the range from 310 miles to 377 miles at normal highway speeds and increased the performance at other speeds.

We needed a comparison Cd graph for a stock Model 3 as well for comparison purposes. Then noticing that the spacing was a 0.05 increment and that we could produce a graph of an ideal body form of Cd = 0.13 for comparison purposes. This total body redesign would have an outrageous capability of 122 mpg-e at 100 mph as well as a 379 to 418 mile range at normal highway speeds.

We decided to go up the Cd ladder 0.05 to Cd = 0.28 to display a typical body form of a Nissan Leaf. This turned out to very illuminating as it showed why EV’s like the Nissan Leaf do not perform as well as a Tesla Model 3. While a stock Model 3 is capable of 86 mpg-e at 100 mph a Nissan body reduces this to 72 mpg at 100 mph. The range is only approximately 273 miles at normal highway speeds. All this presumes that the Nissan Leaf body (with a similar frontal area) is put on top of a Tesla Model 3 drive train and battery.

Then I ran across a Vlog about installing the new roof rack kit on a Tesla Model 3 by a Canadian Vlogger “The Kootenay Family” so I commented on just how undesirable it was to carry items on the roof of the car where the air speed was the highest. Kootenay who is an “Environmental Engineer” apparently became concerned about just what the energy penalty was for carrying his ski pod on the roof of his car and he performed some test and shot a video about the results.

The video basically showed him driving and his son along for the ride keeping track of some numbers. The test was performed on a minus 4 degree Celsius day at 60 mph on 1.5 kilometer two way legs for a total of 3 kilometers. He did the test three times, for each of the three iterations with no roof rack, an empty roof rack, and a test with the ski pod on the roof. The long and short of the test was only a small penalty for the roof rack itself, but a 11% penalty for carrying the pod which seemed to have about 2 square feet of frontal area.

Kootenay called all this acceptable but I thought it was appalling and said so in the comments section. I think roof racks and the general idea of carrying anything on the roof of your car should be against the law. I wondered that as far as skis were concerned if you couldn’t have an upside down rack inside the car to carry them in the head space over the center console.

Upon seeing Kootenay’s results I wondered just what this 11% penalty meant in terms of drag, after all I had some graphs for that. The roof rack had in this instance increased the power requirements from 184 watt hours per kilometer to 207 watt hours per kilometer. After doing some translating to miles and looking at the percentage of increased energy requirements between the Tesla Model 3 and the Cd = 0.28 Nissan body energy requirements I could discern that they were in almost perfect agreement.

In other words the Kootenay roof rack and pod had increased the drag on his Tesla Model 3 so much that he was now driving a Nissan Leaf body. The millions of dollars spent by Tesla to refine the shape of their Model 3 body to have one of the lowest drag coefficients on the market is all for nothing if you carry stuff on the roof. It seems that this doesn’t mean much of anything to most people as I have discovered in my encounters with roof rack mania.

Finally I felt we needed a Cd = 0.33 graph for comparison of the body forms where most cars live which is what I told Phil. The graph arrived this week and I shot a video about this new graph a few days ago. At this point I had five graphs lined up on my white board. Three graphs were on the board and one each was clipped to both sides as outriggers. I think we have reached our limit on the number of graphs that can be displayed at once and discussed in any meaningful manner on video.

The range from Cd = 0.13 (lowest drag form) and Cd = 0.33 (where most cars live) is an astonishing 0.20 Cd span. Telling numbers are that while the low drag form can do 122 mpg-e at 100 mph the Cd = 0.33 body form is only capable of 64 mpg-e at 100 mph, At 70 mph this body only gets 108 mpg versus the 170 mpg of the low drag form.

We think the Cd- 0.179 modified Tesla body is the way to go as it doesn’t involve a total exchange of the body for a new body but rather only involves the addition of some appliances to an existing stock Tesla Model 3 to move the drag from 0.23 to 0.179 to improve the performance envelop by 10 mph.

By that we mean that most of the results at highway speeds can be achieved at a speed 10 mph higher than can be achieved with a stock Model 3. For example a modified Tesla Model 3 could achieve 153 mpg-e at 70 mph for 350 mile range while a stock Tesla is only capable of 133 mpg-e at 70 mph for 305 miles.

For Christmas my son in law knowing my interest in Tesla got me die cast model about 6 inches long of a Tesla Model X. I sent the model to Phil Knox who is building a detachable boat tail and wheel skirts for it now. When he gets this model project done he is going to send it back to me so I can shoot a video of it for you on You Tube. This should help people with visualizing what we are talking about when we speak about aerodynamic appliances and add on boat tails.

With a hitch or a bicycle rack added to the rear of the car it should be possible to add a boat tail to a Tesla Model 3 by using ratchet straps to tighten the boat tail against the body (with a rubber interface to protect the paint) and use it on road trips. It would be removed for daily driving and thus it would not impact the utility of the vehicle for everyday use. Rear wheel well covers would complete the appliance package. We are still up in the air about any mirror mods and feel that a slightly longer boat tail (by a few inches) could compensate for mirror drag.

Since the possibility of me buying a Tesla Model 3 is still at least some 23 months off such a project would not be feasible by me until at least the spring of 2021. I would need Phil Knox to help me build it and we would probably have to get together for a week or two to complete the project. Given this is the most reasonable timeline we will have to put this project on the back burner for now unless someone has a Tesla Model 3 and wants to attempt this project with us. I have to tell you 100 miles per gallon at 100 miles per hour capability looks mighty tasty from where I am sitting.

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