It all started at a Tesla service center in Vancouver BC in the summer of 2016 when I was talking to an employee about the wheels. I asked why there was a difference between the 19-inch wheels and the 21-inch wheels in terms of efficiency. The rep told me it had to do with the contact patch, as well as having more of a flat face on the 19 inch wheel. That got my mind going on this idea of covering the whole wheel to see what kind of benefit you get.
I started searching the internet to see if this kind of thing existed. I found out there were some existing products but they were not the best looking and did not fit 21 inch rims. After looking at some products used in the trucking industry I decided I wanted to make a cover that was as close to invisible as possible. I got some helpful hints from what I saw the truckers using and decided polycarbonate was the material I needed.
The next goal was to figure out how to attach a sheet of polycarbonate to the wheel. I did not want to have to modify the wheel at all, no drilling into the rims or drilling lug nuts required. After much thought on this subject, I decided the best way was to make a bracket that could be bolted on using the lug nuts that hold the wheel in place. I lucked out as there was just enough room to tuck some metal under the lug nuts and also leave enough room to still get your socket on the nuts for tightening and loosening them on and off. I found a 3D print shop and told them what I was thinking and they scanned my 21 inch rim and modeled up a bracket for me. They also 3D-printed a plastic model of the bracket. Plastic was not going to be strong enough, as the walls get pretty thin to pass the lug nuts.
Next was to find a CNC shop to machine 4 of these brackets out of aluminum for me. I contacted my uncle who works in this field and he directed me to a friend who he used to work with who had opened his own machine shop. After my uncle made some adjustments to the model he sent it to his friend’s shop and they cut 4 brackets for me out of billet aluminum. Before the brackets were ready, I used the 3D printed plastic prototype to get the polycarbonate sheets ready. I cut them to size using a band saw and a router. I had a spare 21 inch turbine wheel with no tire on it that I could use to router out the exact size of the rim. The 3D printed prototype was used to line up the holes for bolting the polycarbonate sheet to the bracket.
The CNC machined brackets cost about $250 Cdn each, the polycarbonate is about $140 Cdn a sheet and I can make two sets per sheet. The retention plates cost me $35 Cdn for a set of 4. Then $10 Cdn for hardware.
It took a while to get the brackets from the machine shop, but once I had them it was time to bolt them on the car and do some testing. The bracket turned out great, it bolts to the car nicely and gives a great surface to mount the clear face to. At first I just used bolts with washers to attach the clear face to the bracket and I was getting some cracks on the clear face. I needed larger washers or a plate that would hold the face more stable at high speeds. So I got my uncle to model a retention plate for me and I got him to put the letters ‘EV’ on them. EV works in two ways for me; first my name is Evan so it works for that but also, as we all know, EV is electric vehicle! Once I added this plate to the assembly the clear cover was held properly and there is no more cracking at high speeds.
Now it was testing time. Before any testing I would do a trip to warm the car up. Being in Canada, this is often needed. I am located about 2 kilometers from Highway 401 in southern Ontario, so I can easily get to the highway and then set the cruise control to do a proper test trip. I am also fortunate enough to have a supercharger about 63 kilometers to the west of me just off the 401 and another supercharger about 90 kilometers to the east of me also just off of the 401. So these locations became my halfway points for multiple test runs.
I would only have 2 kilometers of stop-and-go driving, then at least 63 kilometers of highway driving with the cruise control set at either 110 or 120 km/h. Then upon exiting the highway I would have maybe 1 kilometers of stop-and-go to get to the supercharger. I would note the wh/km that the car reports at the time of hitting the exit ramp and then again when I reached my destination, either the supercharger or back at home. Every test run would give four data points. I would do a run with the covers on to get my four data points and then I would do that same route with the covers off. This would give me four more data points.
There were times when I hit traffic and would have to slow down but for the majority of the drive I would keep the cruise set at my desired speed. I also made sure to use the same amount of auxiliary energy draw. I only had the radio on, no heat or AC and I kept the seat heat on for all the rides, windows and sunroof closed the whole time. Have a look at the chart for performance numbers.
21″ Covers to Port Hope Supercharger at 110km/h
|Test||Wh/km (covers)||Wh/km (no covers)||Improvement|
21″ Covers to Kingston Supercharger at 120km/h
|Test||Wh/km (covers)||Wh/km (no covers)||Improvement|
So from what I have gathered, these covers do improve efficiency considerably, which I expected since this is not a new idea. The new part of my idea is the fact that they are clear and are used on a fully electric vehicle.
We now know that the Model 3 will have aero covers on the base model, which Tesla claims will improve efficiency by up to 10 percent. The Tesla Semi also has wheels with a flat face to them. I wonder why?
During my testing, I noted the biggest improvement when driving into a headwind. I saw about 9 percent better efficiency in the headwind situation that day, so Tesla’s claims seem to be about right in my books.
I would say that yes one of the downsides to having these on is that the brake dust collects more on the wheel. However, you do not need to completely remove them to clean the wheel, you just need to remove the 5 small bolts, take the clear face off and you can clean the wheel and check your air pressure.
I hope you like the covers and learned a little about this DIY project. Thanks for reading!
TMC Member evogreen is a Model S owner based in Trenton, Ontario.