This page discusses some of the aerodynamic improvements I have completed on my scion xB EV conversion.
Aerodynamic drag is one of the two big factors affecting the efficiency of an EV. (The other being weight.) Aerodynamic drag is a function of speed, density of the air, and the size and shape of the car body. Of these variables:
- You can control speed within some limits (you can drive on the freeway at 60mph instead of 70mph for example, but it might not be a good idea to drive at 45mph). Driving at 60 instead of 70 will reduce aerodynamic drag by over 25% for example.
- You can select the vehicle body for its aerodynamic properties. The best way to get an aerodynamic EV conversion is to start with an aerodynamic ICE vehicle. It is much easier to start with an aerodynamic car than to try to make it better later.
- While it is best to start with an aerodynamic vehicle, you can make alterations to the vehicle body to improve its aerodynamics, within practical limits. However without extreme changes you are unlikely to improve overall aerodynamic drag by more than 10-20% on any given chassis.
The effect of drag caused by the shape of a car is defined by its drag coefficient Cd and frontal surface area A. Lower is better, meaning less drag given any particular speed and air density. Aerodynamic drag always goes up with the square of speed, but that term will always be proportional to CdA of the car. This means a car with 1/2 the CdA of another will experience half the aerodynamic drag at any speed. For most cars, aerodynamic drag is the major force acting against it at freeway speeds, so you can see why choosing for aerodynamic drag is important. Further Information regarding automotive drag coefficient is at http://en.wikipedia.org/wiki/Automobile_drag_coefficient. Futher information on calculating aerodynamic drag from speed, CdA and other factors is at http://en.wikipedia.org/wiki/Drag_equation
There are many things that can be done to a car body to improve its aerodynamics. Some things are fairly simple and others are quite extreme. For a good sample of mild to wild examples of this, look at the http://www.ecomodder.com site, which isn’t a purely EV site but instead is dedicated to mileage improvements to ICE and other vehicle types, many of the changes are based on aerodynamic changes.
Common Aerodynamic improvements
- Remove Roof Rack. If your car has a roof rack, this is probably the single biggest thing you can do to easily improve mileage. Take it off unless you actually need it.
- Remove the spoiler, if the car has one. A lot of sporty sedans and sports cars have OEM spoilers. These do absolutely nothing for the car at the sort of speeds they are normally driven. All they do is increase drag and weight.
- Remove any other body effects which protrude out from the sides or top of the car, such as flared fenders. The only body effects you would want to keep is a front air dam, if the car has a functional OEM one (as opposed to an ornamental aftermarket one full of vents and holes).
- Use the narrowest wheels and tires that fit the car, and use wheels (or hubcaps) with a smooth surface instead of “mag” wheels. You can buy “moon disk” hub caps if you have steel wheels, or in many cases you can find OEM made alloy wheels that are reasonably narrow and not bad aerodynamically. You don’t have to limit your wheel search to those made by the manufacturer of your car. Wheels are standardized and many other OEM wheels will fit. Just figure out what the bolt pattern and offset of your wheels are, and go from that.
- Lower the car. This has the effect of reducing the frontal surface area especially when coupled with a decent front air dam. This also allows less air to go underneath the vehicle, reducing turbulence and drag from the underbody. Lowering springs are available for many modern cars, some kits are even OEM provided.
- Block front grilles. This reduces the amount of air going into the engine compartment, which then goes under the vehicle causing turbulence and drag. While you are limited in how much you can do this with an ICE car due to cooling requirements, you can pretty much block off everything with EV conversions.
- Remove the passenger side rear view mirror, if visibility will not be impaired. If you need both mirrors, look and see if there are aftermarket mirrors available that are smaller or more aerodynamic than the OEM ones.
- Add a belly pan. You are limited in what you can do with an ICE vehicle here due to the heat of the exhaust system, but with an EV you can cover up a good portion of the underbody without any harm. This will further reduce drag from any air getting under the car at speed as well as protecting cables and battery boxes.
- Add wheel skirts. These are covers that go over the open fenders at the rear of the car. Some classic cars came with these as they were in style for a time, but no modern cars have anything similar. However drag from the open wheel wells is significant, so covering the rear wheels will improve drag. You just need to make sure the skirts can be removed easily to facilitate access to the tires.
All the above changes, maybe with the exception of the belly pan depending on how extensive you try to make it and how the car underbody is made, can be completed in a day or less with the right bits and pieces present, and some are free and most are fairly inexpensive. It is possible to go much further (see ecomodder link) but you will be spending a lot more time and money. All of these changes involve substantial body work. At some point with all of this, you would have been better off just starting with a more aerodyamic vehicle to begin with.
- Add a boat tail. This basically means narrowing the rear of the body of the car. The most aerodynamic body is a tear drop shape, and a boat tail starts to add that shape. A boat tail can be added to the back of the existing car body, or the existing car body can be narrowed. The former approach can impact practicality since it increases the length of the vehicle, as well as the weight, and can make it difficult to access the trunk. The latter approach requires extensive body modifications. If the boat tail is extreme you might need to narrow the rear axle of the car. A lot of work, basically.
- “Chop” the roof. This is a common hot rod body modification, so it might be the best of the extreme body changes to consider. A “Chop” job involves taking the roof off the car, taking a couple inches off of all the window pillars, and reattaching the roof. The roof is either lengthened slightly, or the windshield tilted back, to make it all mate up again. Many cars you could take a couple inches off of the roof easily without drastically affecting driveability. This will directly reduce frontal surface area, so should have a decent impact. However, it is a lot of work as it involves not only substantial body and sheet metal work and a full repaint, but also glass cutting and interior upholstery rework.
Scion xB Aerodynamic Improvements
The first generation scion xB has a CdA (in m^2) of 0.8 m^2. While it is boxy meaning a relatively high Cd, it is also small meaning a relatively small frontal surface area, so overall it isn’t too bad. However, it is certainly worse than some cars, such as my old 1985 MR2, which had a CdA of 0.54 m^2, one of the main reasons I picked it. But to put the 0.8 CdA in perspective, other cars I considered, such as the Pontiac Aztek and Honda Element, were almost twice as bad as the xB, and the xB is better than many sedans and sports cars are.
I have done a few of the above things to the scion. Overall I figure I may have improved drag by as much as 10% but it is hard to tell for sure without A/B comparisons under otherwise identical driving conditions. I did most of my early freeway driving without these mods in the winter in less than great weather often, while many of the mods I did came along as the weather started improving.
I got lucky and found an xB that already had OEM lowering springs. These lower the car by about 1.5″ over stock. I had to find new heavier springs in the back due to the battery weight, but the front springs are the TRD lowering springs that came with the car. Given that the car is five feet or so wide, I can say this reduces my frontal surface area by around 90 square inches, or over half a square foot vs. stock height springs. If you ever rolled down your window at freeway speed and held you hand out in the wind, you know that even that amount of surface area hitting 60mph wind is a lot of drag you are now avoiding.
I was able to find smaller, aerodynamic aftermarket mirrors that fit the scion. Their build quality certainly isn’t OEM, but after some fiddling around they fit and look OK. They have about 70% of the surface area of the OEM mirrors, and are shaped to allow for less drag. I figure these probably reduce my frontal surface area by another 1/4 of a square foot.
It was quite easy to make grill blocks for the xB. There are two open grilles on the front of the car. The upper one I just made the grill block out of a piece of flat black plastic I had to fit over the top of the existing grille, and zip tied it in place. The lower one I removed the original grill and made a couple simple brackets to hold it, then zip tied everything in. It is fairly subtle as the black grill blocks look very similar to the original black grilles, so unless you look closely you might not realize the changes are there. The “perfect” approach would have been to fully fill in the grille recesses so the front of the car was smooth, but that was too much work.
This was a bit more work, but once I got all the design and attachments figured out It was done in a day. I got lucky in that I found a large section of plastic dumped in a wooded area that was perfect for the job so that is when I decided to do it. The plastic was a portion of the outer rim of one of those collapsible above ground swimming pools. The plastic is 1/8″ thick by 42″ wide ABS plastic and it was 25 feet long. This is the same kind of plastic they use for car underbody components so is pretty much perfect for the job, perhaps just a little on the heavy side. It is rigid, but flexible enough to not crack or break when struck or bent.
The bottom of the scion from behind the front axle to just ahead of the rear axle has integrated frame rails that were very easy to attach a belly pan to, so I built the pan to fit that though it would be beneficial to have the pan cover at least some of the engine compartment area as well. The overall dimensions of the pan are about 3 feet wide by 5 feet long. The sides of the scion underbody are already pretty smooth so I did not make the pan go all the way out to the edges of the body.
An added advantage is that the pan protects a large section of the high voltage battery conduit.
I was able to make the pan to fit existing holes in the car body, so I did not need to drill a single hole in the scion underbody. The pan attaches to two existing bolt holes on the front cross member, and to eight large drain holes in the bottom of the frame rails using 5/8″ size “well nuts” (rubber collapsible anchors) and with a few smaller plastic automotive body rivets at the back going into other parts of the underbody, and finally with an existing reinforcing metal bracket that attaches across the floor tunnel of the scion body under the back seat area.