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42 Posts
Discussion Starter · #1 · (Edited)
In hopes to reduce wind drag and also reduce rain water entry into the motor bay I fabricated and installed a belly pan. If anyone installs and finds any errors to the instructions please let me know and I will update.

  1. Fiat 500e abs belly pan fabrication and install approximately 2hrs to fab and install
  2. Modeled from this site: Fiat 500e Front Belly Pan ($55+ shipping)
  3. List of parts: $83
    1. ABS plastic sheet - 83cm x 64.5cm x 2.38mm (32.7" x 25.4" x 3/32") $73 for 4'x8' sheet (I plan to do the rear diffuser later, I had a lot left over)
    2. Bolts M6 - 1.00x25 or Lowes (part image looks wrong) you may only need two of these for the two front mounts. $2
    3. 1/4" x 1-1/4" faring washers $1
    4. Red Loctite - $7
  4. List of tools:
    1. Jig saw
    2. Drill
    3. Drill bit 15mm or 3/8"
    4. Jack & Jack stand or car ramps or drive one side of car up on the curb
    5. Wrench or Socket for the bolts
    6. Latex or similar gloves
  5. Instructions
    1. Summary, cut the abs mount under motor bay. See below measurements. There are two existing threaded mounts at the front of the car that the M6 bolts screw into (mine had nothing mounted to them). There is an existing plastic cover in the center of the car which has 5 bolts holding it on, you'll use the existing two bolts on the front of that cover to mount the belly pan under (closer to the ground so the wind flows over the existing cover). End of install should look like this:
    2. Use Jig saw to cut abs 83cm(front/back of fiat) x 64.5(side)
    3. Drill holes per below diagram, note: I measured twice but did not make two belly pans to confirm the diagram is 100% accurate (sorry if it's off, let me know and I'll update the measurements) if you want to be sure, drill the holes one at a time use the bolts to mount in place to make sure your holes line up. This is what I did and then measured the belly pan outside of the vehicle to create the below diagram.
    4. After drilling all 4 holes, mount with bolts and washers, use red Loctite on bolts. Apply the Loctite the length of the bolt (like ketchup on a hotdog)
    5. Jig diagram all in cm:
    6. CM to Inches:

2013 500e
77 Posts
I've been working on this for a bit, but not as a product. I'm very happy to see someone making this accessible for others. :)
In my version, I've completely closed off the bottom as well as the front wheelwell space.
For the goal of minimizing the mess inside the motor bay, it was important to close off the wheelwells. I got a lot of dirt/sand/gravel from there.
However, closing this off restricted the cooling airflow, which has turned out to be a significant issue. I've done a bunch of testing while looking at OBDII data.
My solution has been to add a vent to the hood, which has solved the cooling problem so far. I've tested it on hot, but not our hottest ( 100+F ) days.

2013 500e
77 Posts
Ok; first I'd like to qualify the pic. It's not the most blingy thing.. I'm planning on anodizing it to match the hood or accent colors.
With regards to aero mods, it's really easy to mess things up and have reverse flows, etc.
In this case there are a few issues that don't give much margin for error.
1) The engine bay is packed with stuff that restricts airflow through the heat exchangers. I've removed a few things, and it's helped with flow & cooling.
2) The hood is very short. If the vent is located too far back, the positive pressure of the hood/windshield area could cause reverse flow. If the vent is too far forward, the flow must reverse direction to make it out of the vent.
3) It's best to minimize the amount of internal structure that is cut for the vent hole.

I purchased my vents from RaceLouvers, a company that specializes in making vents for race cars. They actually do real wind tunnel testing to make sure their products work. They also have a set of louvers intended for Abarths.

I located my vent on the passenger side, where the air would be forced to flow over the charger & help cooling. The charger is last in the cooling loop, so even when it's not being used it heats up from motor/inverter generated heat. Also, when the car is charging, the vent provides an effective cooling boost that reduces cooling system loads.
If more cooling is necessary, I can add the opposite side louver. At present I'm looking into improving the flow in front of the heat exchangers and reducing the inlet size.
Here is a pic of the vent after I got done with pressure/tuft testing:


Bottom line: The motor/inverter stay the same temperature as they did before I added the bottom/fenderwell covers. My guess o' meter's range prediction has extended to 90miles - and this is a 2013 car with some very non-aero performance mods.

2013 500e
77 Posts
Thanks, & now I'm curious what "very non-aero performance mods" you have!
I'm going to try to not pull this thread off topic; I'll post some links to other threads where I detail them.
In general, wheel & suspension mods. Aerodynamically, open spoked wheels pull air from underneath the car, swirl it, then release it out the sides, destroying the smooth flow of air down the side of the car. This is why EV wheels tend to have minimal openings.

2013 500e
77 Posts
More on topic, I have a couple suggestions to improve the underside. I see that this setup is essentially the same as EV-upgrades product. Because this can be a start from scratch effort, it could be much better.
1) Cover the entire midsection between the wheels. The battery supports are aerodynamic disasters. They are essentially 13mm high square edge ribs on an otherwise reasonably smooth surface. This will cause enough turbulence to make any diffuser in the back ineffective. The air entering the diffuser must be reasonably attached smooth flow.
2) Reduce the diffuser angle and maybe cover more of the rear suspension. The sources I've read on the topic (including texts on racecar aerodynamics and wind tunnel tests) suggest a diffuser angle of approximately 7 degrees for minimum drag. The angle can be increased slightly (if there is good flow) for improved rear downforce, but the air must stay attached to the diffuser, or it becomes ineffective. You can see why most "diffusers" in the back of sporty cars are just bling.

2013 500e
77 Posts
Here is how I started things. An Aussie blog on automotive aerodynamics suggested trying stuff with simple/cheap prototypes. My first front covers were made of a foam/cardboard material:

Believe it or not it survived almost a year. :)
I did learn that it was worth working on a better version, and that it would be important to keep the gravel and sand out. Even though it was just held on with the standard Fiat bottom panel bolts and was flexed around my cross-brace, there was no problem with it flapping in the wind.

So rev2 was made of decent plastic. Making it out of ABS like Jimmy0000 is doing would be much better than this stuff I had sitting around. This white stuff is the shower surround material from HomeDepot. It's both heavier and not as impact resistant as ABS.

The bottom is connected to wheelwell side covers, which really helps stiffen both the bottom cover and the wheelwell side covers.
Here is a shot inside the front wheelwell. The covers were made of 1/16" ABS that was heat formed to both make necessary corners and stiffen the pieces. Heat forming is really easy with a heat gun (hair dryer on steroids...)
I also covered the vent in the front. The side cover was made of 3 separate pieces to enable fitment around stuff like my chassis brace, the driveshaft, etc.

Here's a closeup of the front part, fitting around the chassis brace:

Here's a closeup of the back part. I used rubber to get close to covering moving stuff like the driveshaft, swaybar, and steering link:

It's holding together very well so far, and I have no problems with the sand and gravel piles. It's also removable if I need to fix something.

2013 500e
77 Posts
Wow! This belly-pan stuff is really working! Most folks around here are talking about the range reducing over time.
My range is actually increasing. I'm not kidding.
Before cleaning up some of the bottom, my range was about 88 miles, according to the guess o meter. After doing the mods posted above, it actually started improving. I had some serious doubts about that, so I said nothing. But here it is, at 95% charged.
Speedometer Automotive tire Gauge Steering part Vehicle

The books and other sources I have about aerodynamics say it is possible to improve the aerodynamic drag by about 19% to 20% by smoothing the bottom. By completely covering the bottom and making a good diffuser, it might be possible to increase the range to over 100 miles! :)
I'm inspired; I'm building that now.

2013 FIAT 500e
3,175 Posts
That's great!

GOM miles aren't a very reliable source though, so we just have to take your word for it if you're using less %, driving the same speed on your usual route(s) at about the same outside temperature.

2013 500e
77 Posts
Totally agree about the GOM's accuracy. For what it's worth, the route hasn't changed and the average speed over the route is approximately the same. The main thing that might be different is a higher temperature, as this has been a pretty hot summer. The route is roughly 50% rural rolling hills, 45% freeway, and 5% city. We live on a hill, and there's a 1000ft elevation gain on the way home.

I plan to do some reasonable "there and back" testing using the power output gauge on a reasonably flat road on a reasonably calm day. I would like to test diffuser angles and a few other details. Separation is a major issue with diffusers and I want to ensure it's actually working.

2016 500e
43 Posts
Very cool stuff! I am in a hilly area as well. Just got my car Thurs so will be following what you find. Any chance you would produce kits?

2013 500e
77 Posts
Sorry, I wasn't planning on making a kit. This is purely for sharing knowledge.
The main reason is shipping. The two middle sections (which I will show later) are quite large - about 3' X 5'. Shipping big stuff gets pretty expensive.
Secondly, I'm not really set up for production cutting and processing large sheets. I'm doing it on a DIY level, with a circular saw and a small bandsaw. There is a lot of filing rough edges involved, which takes a surprising amount of time. Also, to do this right one needs to be good at thermoforming.
I do think the effort is worth it; and would be willing to share dimensions, patterns, etc with someone who is set up for processing sheets of ABS, like vortexblue

2013 500e
77 Posts
Here are some aerodynamic details for the following insanity. :p
First, some inspiration, or why bother doing this??
I have quite a few sources about aerodynamics, both books and web files. I hope this is helpful.
To start, here is a page from a lecture by Dr. Drew Landman: "Ground Vehicle Aerodynamics"
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Folks in the racing & high performance world have known this for years. When done correctly, you can get both downforce and reduced drag.

In Joseph Katz's book, "Race Car Aerodynamics, Designing for Speed" there are two graphics that apply directly to diffuser angles and cooling drag:

I think this information was based on 2d simulations. As I'll show later, real world testing in wind tunnels reveals some limitations. However, note that even with this data, there appears to be a practical range of diffuser angles between 5 and 12 degrees.
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Here is the graphic about cooling drag. Most cars are designed with either A or B. The 500e appears to use method B along with a bottom exit.
If you look at the info about the vent I'm using at racelouvers you'll see it's possible to both reduce drag and provide downforce with method C.
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Here are some shots of high performance road cars, taken from the book "Modifying the Aerodynamics of Your Road Car" by Julian Edgar:
The first is the bottom of an Audi R8. The bottom isn't completely smooth, but it is effective.
Font Gas Auto part Automotive tire Machine

Next, a Lamborghini Aventador. Notice how completely smooth the bottom is. There is no reason an EV can't have a smooth bottom like this.
Product Automotive tire Camera accessory Vehicle Auto part

In Simon McBeath's book "Competition Car Aerodynamics, a Practical Handbook" there is an excellent test of a full size Nissan 350Z in the Mira wind tunnel:
In the test, they used an adjustable diffuser to analyze downforce and drag. The diffuser was set between 8 and 14 degrees. The only downside of this test is the Mira wind tunnel has a fixed floor, so there is some separation that wouldn't exist on a track.
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Note how there appears to be a maximum downforce between 10 and 11 degrees. There is probably an issue with separation at angles greater than 11 degrees. Also note how drag increases slightly between 8 and 10 degrees.
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In addition to adjusting the diffuser angle, they tested the angle of the flat bottom. In the tests, the angle was adjusted between 0.3 and 1.3 degrees. The diffuser was fixed at 10 degrees for these tests.
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Just as a note about the 500e, the battery is lower (closer to the ground) in the back. When I set up my suspension, I made sure that it was level. This required the a significant forward rake. I think it would be difficult to get anything more than a 0.5 degree rake of the bottom; I'm shooting for 0.3 degrees.

In addition to this test, he also showed the results when the diffuser is set at to large of an angle or doesn't have non-turbulent air entering it. Here is a test in the same wind tunnel with a Lotus Exige with an aftermarket diffuser. Notice how the exit air is actually blowing INTO the diffuser! 馃お
Wheel Tire Automotive tail & brake light Vehicle Car

2013 500e
77 Posts
Another good reference is Carroll Smith's "Tune to Win." It has a lot of practical information about internal/external aerodynamics, suspension tuning, etc.
In the external aerodynamics section, he references some tests done by NACA before WWII.
If you plan to do mods like this, there are a lot of details that should be considered.
For example, when connecting two panels, what is the joint construction? Before I looked at this table, I was planning to use method 2 with the air moving right to left. However, I changed to last method; it's very low drag and allows quite a bit of "reality screw up".
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Also important is the fasteners used on a project. I was hoping to use all flush rivets and Dzus fasteners, but that pretty much requires aluminum to form the holes for the flush rivets. I compromised by using flanged button head cap screws, which are similar to brazier head rivets. The rivets I used are all brazier heads.
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Last but not least is what happens when a hole is left open? NACA tested access ports on airplane wings and found the entire area behind the access port PLUS an area that increases at a 7 degree angle is turbulent and provides no lift. So, let's say you have a hole just before the diffuser for suspension or something. This area will be turbulent and that part of the diffuser will not work - similar to the Lotus picture in the previous post. I've seen computer simulations that confirm this exact situation.
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2013 500e
77 Posts
Here is some stuff that needs to be removed/taken into consideration if you are planning to install a complete bottom:

One of the original side covers. This piece covers some of the area between the rocker panels and the battery. As far as I'm concerned it's useless weight that will get in the way.
They do have one useful aspect: Note the clips (silver) near the bottom of the pic. They connect to three tabs on the battery. The tabs will provide connection points for the new covers.
Wood Automotive exterior Gas Composite material Bumper

Here is one of those tabs on the battery. I used riv-nuts extensively in this project to fasten the bottom. There were many inaccessible nuts and I wanted to make the covers removable. Another option would be Dzus fittings, but I decided fast removals weren't necessary.
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If, for some reason you might want to lift the car (flat tire, work on bottom, etc) it will be necessary to provide removable panels around the jack points. You can see the little triangles near the wheels on the rocker panels. Here is a pick of the driver's front side jack point. My jack stand is supporting the place a shop would use for an overhead lift.
Note the body seam (outlined in red). I use a low profile jack with a slotted spacer to lift the car.
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2013 500e
77 Posts
Here is another view of the jack point, this time looking out from the battery toward the driver's front tire. Sorry about the poor quality.
The red circle indicates the body seam.
The green circle points out a handy detail about the rocker panel. There are two parts, and a little space between them that allows a thin sheet of material to fit. This can be used as one of the bottom panel mounting details. This gap runs the entire length of the rocker panels on both sides.
The blue circle shows one of the four spaces I used to allow a riv-nut on the panel to fit in that gap between the rocker panel parts.

I'll show it later, but my two middle cover panels are captured between these gaps in the rocker panels. Along with the bolts on the battery and rocker panels, they are securely fastened to the car.

Wood Arthropod Electrical wiring Wire Insect

2013 500e
77 Posts
The driver's front wheelwell as I started this phase of the project. Note that all the cover plates I made worked fine and there was no issue with things vibrating loose. These panels were added to block the cooling air exit out of the wheelswells to reduce the amount of turbulent air that would disrupt airflow on the car's sides.
The panels were made of ~ 0.063 ABS; bending them with thermoforming stiffened them considerably. The rubber used to cover the space around moving suspension parts also worked well.
A very important note regarding cooling: The wheelwells and bottom are the cooling exits on the standard setup. I've blocked the bottom with a complete cover panel and the wheelwells with these covers. There absolutely must be a hood vent or two to provide cooling air exits if you decide to do this.
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