# Aerodynamics



## Coley (Jul 26, 2007)

Nothing really, as I only run around, under 40 mph.

Might consider a sleeker body next time.


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## KiwiEV (Jul 26, 2007)

Me too. My MitsuBrickshi will be a town-only car with the option of highway speed so aerodynamics aren't an issue. 
This is a good thing too as I don't know where I'd get the money to sleek-en my car. It's about as aerodynamic as an old television. Hehehe


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## Greenflight (Sep 13, 2007)

Well, my truck is kind of a total loss when it comes to aerodynamics...  I did add a homemade bed cover, which should help a little bit. Just a couple days ago, I also lowered the front a few inches, which is more of a looks thing but may also help with aerodynamics. 

My Tercel is a fairly sleek car as is, but I'll probably add some body work to it once I get it going. May put a belly pan on it. We'll see.


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## Mr. Sharkey (Jul 26, 2007)

I'm not sure how useful a poll is going to be unless it's yes/no, as there are many components to enhancing aerodynamics that would make for a very convoluted poll tally.

In my case, I installed the factory GTI air dam and wheel arch flares, which are purported to lower the Cd by .4 alone. My car also has the grille blocked off by plastic inserts, and I replaced the open vents behind the front bumper with factory blanks. I've installed later model bumper mounts on the front that move the bumper closer to the body, and installed a factory valance between the bumper and the bottom of the grille which closes off the gap, lowering air circulation in that area.

Since the Pusher needs cooling air to enter the grille, I've fabricated custom mounts and cut down the rear air deflector from a 1980's Oldsmobile station wagon to catch a bit of the air coming off the rear of the roof and direct it down the slope of the rear hatch. No scientific data on that modification, but anything that smooths the air flow behind the car can't be bad.

Once, when this topic came up on the EVDL, it was suggested that installing factory fender liners from a Cabriolet could help reduce turbulence inside the wheel wells. I've obtained a set, but not yet installed them.

If I haven't already posted it here somewhere else, here's one possibility for improving front aerodynamics:










Looks a bit like Darth Vader or Dick Cheney, but it's cheap (foam insulation panels, duct tape and drywall screws), and it retains all of the factory safety equipment associated with the shock absorbing bumper, etc.

Finally, doing some research on a well-established platform might allow one to dig up images such as these:




























You'll note that the Golf model involved is a European model, with the small metal bumpers and a "duckbill" spoiler under the front lip. It would be interesting to comare this with a typical U.S. issue DOT-approved shock absorber type bumper and no spoiler and/or a GTI set...


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## mattW (Sep 14, 2007)

Hey sharkey,
Nice post! Yeah the poll was just to get an idea of how far people were willing to go, and to spark some discussion on the topic. I think its general enough not to get too complex and you can select more than one option. You've done some nice work on your rabbit, have you got any photos of the rear air deflector? I don't know how much that darth vader body kit will do for EV PR... that thing is huge! And i doubt it would be as effective as just a nice rounded front like the EV1 or a streamliner. You want your car to be as close to a tear drop shape as possible. There is a nice quick outline of what the insight designers did for aerodynamics at http://www.insightcentral.net/encyclopedia/enaero.html


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## Mr. Sharkey (Jul 26, 2007)

Although the deflector is visible on several photos I have of the car, none show it very well. I drove the car today for the first time in six months, an afternoon jaunt into town to meet my sister and brother-in-law for coffee. I took the camera along to catch a shot of the car and the deflector, but it got late, and after a double shot of espresso, I was absolutely frying and couldn't stand still long enough to pull the car over at some picturesque location and frame up a photo. For that matter, I'm still buzzing from the caffeine five hours later...

Maybe tomorrow...

In the meantime, here's a shot of the tucked-in front bumper, trim valance, custom headlight guards and the GTI spoiler, just visible at the bottom...


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## veperformance (Aug 17, 2007)

i dont think it will make a big difference but i will install lexan cover in front of the fog lamp area and cover underneat , that shoud give me good aerodynamic and protec the motor and electronics.(i tryed to attache a picture but i'm not shure it worked.)


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## Greenflight (Sep 13, 2007)

OK, I got to take my truck for a test drive with the front lowered. I didn't notice much difference below 30, but I think I picked up about 3-4 mph in third gear.  

Oh yeah, that was a good post Sharkey!


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## Mr. Sharkey (Jul 26, 2007)

Don't know if anyone is still interested, but I did say that I'd post some pics of the rear deflector.

Here it is in a side view. I had my sheet metal guy fabricate some sheet aluminum brackets that fold under the hatch so that I didn't have to drill any holes on the exterior of the hatch:










(this view is of two Rabbits sitting side-by-side, the one in the foreground is powered by Biodiesel)

I didn't want a Rice-Racer picnic table sized fake wing apperance, so I made sure that it was pretty much tucked into the roofline of the car, and directed the airflow down over the hatch.

From the rear:










The Oldsmobile deflector ($10 at a wrecking yard) was much too wide for the VW, so I put it through the band saw, sculpting the ends to compliment the countours of the top of the roof at the sides. In this view, it looks fairly wide, but when you stand back a bit, it blends into the natural lines of the car.

A few holes to fill with Bondo, and a can of VW-matched touch up paint, some stainess steel fasteners, and I can at least pretend that it fills in some of the vacuum created behind a moving vehicle. Next time I have the backyard wind tunnel fired up, maybe I'll run some diagnostics...


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## John (Sep 11, 2007)

Since I plan on using my vehicle for a work commute most of my driving will be at 100 km/h. Aerodynamics will have a significant impact on the range of the vehicle. Whatever I do needs to be light weight as my vehicle will also be a bit weight challenged and operating quite close to it's GVWR. I'm considering degrilling the front (no point in all that cooling drag without an ICE), fitting a belly pan just under the engine (motor) compartment, fitting skirts to the rear wheel arches and turbulators over the rear window. I will try to verify each aero mod as an improvement somehow such as steady state motor amps on a known piece of road in calm conditions. The function of the turbulators could be verified by taping a ribbon to the roof of the car and driving down the road with a chase car to monitor the behaviour of the ribbon. A full belly pan would certainly help the aerodynamics but would weigh quite a bit.


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## mattW (Sep 14, 2007)

Check out this pick-up, sounds like its had the kind of treatment you are envisaging (except i don't know what turbulators are):








The owner says he got 28% better fuel economy for his efforts (story at http://www.evworld.com/article.cfm?storyid=870) so it sounds like its definately worth it. If it were an EV with a 40 mile (60km) range it would go up to 51 miles (82kms). I don't think you would get such a drastic change with a car but it shows you the difference aerodynamics can make.


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## mattW (Sep 14, 2007)

Well this is getting interesting, i've been reading more on aerodynamics at http://autospeed.com/cms/A_2455/article.html (really great article) and found this table:








As you can see the biggest factor affecting aerodynamics for this car (a volvo under development) is the cooling package! In an electric car (depending on the motor and controller) this is almost completely unnecessary since the motors are so much more effecient. By just blocking off the majority of the airflow into the engine with a nice rounded curve you could decrease drag by a third! That and the other aerodynamic adjustments discussed earlier i think would make a pretty reasonable increase in range. Enough to be worth the effort anyways.


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## veperformance (Aug 17, 2007)

Yep , front cooling trap do pick up lots of air , ev1 did quite a good job for that and thats one the reason i like my donor car.


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## mattW (Sep 14, 2007)

Mr. Sharkey said:


> Since the Pusher needs cooling air to enter the grille, I've fabricated custom mounts and cut down the rear air deflector from a 1980's Oldsmobile station wagon to catch a bit of the air coming off the rear of the roof and direct it down the slope of the rear hatch. No scientific data on that modification, but anything that smooths the air flow behind the car can't be bad.


Has your deflector made a noticable difference to your stability at high speed? A spoiler works by deflecting air upwards which results in a reaction force downwards on the back of the car. Your deflector is doing the opposite, deflecting air down and lifting the back up... Its possible to reduce the drag of a car but increase the lift which results in a effiecient but dangerous car. I don't mean to be critical but i don't want you spinning off the freeway at speed.


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## Mr. Sharkey (Jul 26, 2007)

With 500 Kg of batteries where the back seat and fuel tank used to be, any lift generated by the deflector would be negligible. 

What's laughable is all the asian import boy racers who put giant spoilers on the back of their Honda Civic and claim improvements in handling. Realistically, such attachments are eye candy only, as true aerodynamic ground effects don't really start to get generated until you are doing over 160 KpH.

In my case, the Pusher generates much more lift on the rear of the EV when it's providing power. I can and do feel the steering get a little bit lighter when I'm accelerating hard, but nothing that would cause the car to become unstable. I do have a fabulous amount of custom suspension work on the car, and it is rock solid on the road and in turns.

Oh, and high speed? "High speed" for this car is 100 KpH. If I was worried about the rear wheels coming off the ground due to the few pounds of lift that the deflector might generate, I'd put a couple of bricks in the boot.


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## mattW (Sep 14, 2007)

How good are E-meters and that kind of thing at measuring your effeciency? (i was going to say fuel economy but i guess it would be amp economy ). Or i guess the bigger question is; if i was making aerodynamic changes what would be the best (electrical) diagnostic tools to test their effectiveness. You would obviously want to do before and after trips on the same stretches of road at the same speed but what/how would you measure to tell the difference?


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## veperformance (Aug 17, 2007)

It is quite easy to mesure , you must run at 55mph for about 1 minute on a wide straight and leveled road and note voltage and amps every 15 seconds.you must also make shure there is no trafic close to you in any direction and make shure that tire pressure is maintained.with 4 mesures you can make an average that should be a good indication of the influence of the aerodynamic modifications. also when making your average convert your mesurements in watts ( volts * amps =watts).when your done can you post your results i'm looking forward to see them.


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## mattW (Sep 14, 2007)

Well it might be a while yet, i'm still in the research/learning/saving phase of EV construction but judging by the success of other people's aerodynamic adjustments i am definately planning to do some changes and will also try to post the before and after stats. If all goes to plan i should be starting with a honda CRX. Depending on how ambititious i am at the time i intend to work on the undercar airflow with an ABS plastic belly pan (similar to http://www.autospeed.com/cms/A_2456/article.html but bumper to bumper) possibly larger wheel guards or even rear wheel covers (aka insight) and maybe even a custom fiberglass bodykit that smooths out the front air intake and bumper (using the same method as http://www.geocities.com/silverkit98/BodyKit1.html but looking more ev1-like than street racer). These will obviously be the finishing touches to the car and may or may not happen but a big part of the appeal of electric cars for me is their efficiency and so things like aerodynamics and weight reduction really appeal to me.


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## Mr. Sharkey (Jul 26, 2007)

mattW said:


> How good are E-meters and that kind of thing at measuring your effeciency?


The E-Meter patent was purchased by Xantrex and has been renamed "Link 10" (Here's a link to a descriptive page about it from Energy Outfitters). If you were to use the method described by veperformance above, the Link 10 could be very useful, especially if you purchased it with the RS232 option, which provides a data port from which you can compile data at one second intervals. Running this data through Excel, it's possible to make some very useful graphs:









Chart of power usage while using the Pusher trailer: Following the blue line, we see significant discharge currents at the beginning of the trip, which represents surface street driving and acceleration when entering the Interstate highway. About 09:25, the Pusher trailer assumed the majority of the load while operating at a steady speed on the highway, and some amount of regenerative braking replenishes the power used earlier. Large upward spikes in this line of the chart represent regenerative braking used to decelerate the vehicle at stop signals and when exiting freeway off ramps. At 09:52, I exited the freeway and drove the remainder of the trip on rural roads with some traffic stops along the way.

This chart represents about 50 miles of driving. Note that the number of ampere-hours consumed from the battery pack (yellow line) totals about -20Ah.

You could generate similar charts while running a known course at a fixed speed and see the results of any changes you make fairly easily. A one second capture duration for the data would help insure accuracy. I'm no Excel whiz, but I'm sure the program is capable of some powerful equation processing that could average the data, making the results even more useful.

I've always monitored efficiency by reading from a dedicated kilowatt hour meter and writing down the number of kilowatt hours consumed by the car and cross referencing that to the miles covered. While not an instantaneous result of efficiency, and subject to many variables, over the course of several weeks or months, a very reliable result can be obtained.

Overall,the Link 10/E-Meter is very useful. I own three of them, and wouldn't run my car or my renewable energy power systems without them, or something similar.


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## Greenflight (Sep 13, 2007)

Man, this thread has really become a wealth of info...  Matt, those links are great. I'm definitely planning on putting a belly pan (at least up front) on my truck.


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## mattW (Sep 14, 2007)

I am also curious to ask people who already have working EVs how hot does your motor and controller get? Can you touch them after a period of driving? how much airflow do you think is necessary to keep them from overheating? If decent airflow was required it would definately affect how far you could go with fitting a belly pan because if air is to flow through the engine bay there needs to be somewhere for it to go, whether its air vents on the hood or from underneath the car (most cars vent under the car). So a belly pan would prevent air from flowing through the motor compartment unless you made room for vents in the pan. You can read about airflow through the engine bay here, the site is to do with getting the most out of your turbo but it has implications for electric car design. Basically we can assume that if 85% of an ICE is lost as heat compared to 15% in an electric car then there is probably 5-6 times more airflow in our cars than is required to keep the components cool. For the most aerodynamic car we want to minimise the flow to the minimum possible level without the motor or components overheating. Adding a small belly pan, only on the front section of an already aerodynamic prius in the site i linked in a previous post increased fuel economy by by 10% at a cost of just AUD$66. I hoping this thread will help people realise that they can increase their range and top speed considerably without the cost and weight of more or bigger batteries.


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## Greenflight (Sep 13, 2007)

I happen to have a little experience with cooling... My motor burned out at about 500 miles due to overheating.

After that experience, I decided I didn't want it to happen again, so I ducted an electric radiator fan to the motor endbell. With the fan operating whenever the ignition is on, the motor gets warm to the touch, but not quite "hot," to use technical terms.  

During the summer, when it's hot and humid, the motor still gets a little hotter than I'd like, but not nearly as hot as before the fan.

The controller stays cool as a cucumber.  

I'd say your estimate of about 5-6 times more airflow than needed is about right for a good quality motor. To be safe, I'd avoid blocking more than 2/3 or 3/4 of available airflow.

I think the ultimate solution is the belly pan under the front only, with small guides by the rear wheels, and maybe a few mods on the back end. But that's just my uneducated guess.


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## John (Sep 11, 2007)

One way to minimise you cooling air requirement would be to duct it from your cold air intake at the front of the car to where it is needed. I would say the majority would be needed by the motor. Those with direct drive would even benefit from fan assist for this particularly when travelling slowly up hills. I have a liquid cooled controller so it will be easy to duct air to the tiny little radiator. Those with air cooled controllers could build a shroud to duct air over the heat sink where it is needed. 

I'm thinking about making circular ports at the stagnation point on the front bumper passing into a conical diffuser designed for maximum pressure recovery dumping into an air box to slow the flow and allow dust and water to fall out and drain away. The ducting would connect that air box to the motor and controller. 

Even with a belly pan under the entire engine compartment there would still be openings at the transmission tunnel and drive shaft and steering rack ports without needing to create special vents through the pan. I suppose I could duct the hot air from the motor out of the compartment to prevent it elevating the temperature. Then I would only need to supply a tiny amount of ventilation air to the compartment to remove a small amount of heat and some hydrogen from the batteries.


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## Mr. Sharkey (Jul 26, 2007)

In my case, I have ducted air from the grille of the car with blower assist.

The controller in my car handles only 10 amperes of current, so heat dissipation is negligible there.

Cooling for the motor is collected from an opening in the grille, which is accomplished via a custom-made aluminum box with a removable tray that holds a 4x6" sheet of blue polyester filter media to collect dirt and bugs. The output of this filter box feeds a 4", 12 volt DC squirrel cage blower, which in turn feeds the motor shroud from the brush end. Spent cooling air is exhausted from the shaft end of the motor.

The blower is connected to a PWM speed controller, and runs at about 1/4 speed anytime the ignition switch is on. When motor temperature gets up to about 75 degrees C, a temperature sensor signals the PWM controller to feed 100% duty cycle to the blower motor, bringing it up to full speed.

The combination of positive air flow from the motion of the car and the blower running at low speed seems to be more than enough to keep the motor at a low temperature on city streets. The blower usually only runs 100% at highway speeds, and usually throttles back to 25% by the time I have exited the freeway and driven home to park (based on my previously living in the city).


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## Greenflight (Sep 13, 2007)

OK Sharkey, fill me in here. HOW does your car's controller only handle 10A?

John, I'm thinking if you place the ports at the stagnation point you won't get much flow for the drag produced. If you don't need much air, wouldn't it be logical to use flush scoops in attached (heck, maybe even laminar, if you have a smooth car) flow locations?


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## John (Sep 11, 2007)

Placing the ports at the stagnation point will cause the stagnation point to move somewhere else on the surface. My theory is that by putting the ports there (basically at the apex of the vehicles shape) the flow will have travelled the minimum distance across the surface of the vehicle possible before entering the port causing minimum boundary layer growth for maximum diffuser efficiency and pressure recovery.


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## mattW (Sep 14, 2007)

If you place a scoop on an area with attached flow you create an area of turbulance behind it, look at the area behind the hood scoop of this WRX undergoing DIY aero testing. I think your idea sounds good john, will you be using the existing grill or creating your own ports? Also it would be good to get a photo of the front end of your car so we can see what you mean. Sharkey do you have a photo of under the hood that would show the ducting or are there batteries in the way?


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## Mr. Sharkey (Jul 26, 2007)

Greenflight said:


> OK Sharkey, fill me in here. HOW does your car's controller only handle 10A?


Simple: Shunt Wound Motor. The controller's only load is the field coils in the traction motor. A single TO-3 high current/high voltage transistor handles the current. It's mounted on a small aluminum plate, enclosed within the larger controller enclosure. No heat sinks, cooling fans or atmospheric radiation necessary.

The armature in the motor of my car is connected directly to the battery pack through a high current contactor. The fields receive 100% PWM duty cycle from the controller when the motor is idling (and during full regen), and the field is weakened (lower PWM cycle) for acceleration.

Matt:

I was looking for a decent image showing my filter box and ducting, but don't think I have one, perhaps I can go out and snap one tomorrow when it's not storming. Only one battery under the hood, a 26 Ah gel cell for 12 volt accessories. Lots of room to view and move under there.


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## Greenflight (Sep 13, 2007)

I see... so, what are the downsides? Sounds really cool...  

I was thinking more along the lines of a flush scoop- one that draws the air down from the surface without projecting into the boundary layer. This type of scoop is designed to cause the least possible disturbance.

It seems like the stagnation point would split in two around the port (just guessing here  ). Since the greatest loss of boundary layer energy generally occurs in the area on either side of the stagnation point, it seems as though you would end up with an excessively thick boundary layer at the front of the car, causing flow detachment sooner at the rear.

Anyway, just my $0.02...


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## TexomaEV (Jul 26, 2007)

I've added a belly pan under the the frontal area of the car (blackened Coroplast), mainly to protect the electric motor, and components from splashing water/debris. I suppose it could help with Aerodynamics as well.


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## Greenflight (Sep 13, 2007)

Well, I used up almost a whole roll of duct tape and a couple sheets of pink (I know, I know...) insulation foam today, and I now have something that could possibly be described as a belly pan. It goes all the way back to the front axle in the middle, and it also covers the area in front of the wheels.

One of the pics also shows a lexan grill cover I put on a few days ago.

The results? Well, they don't seem to make a whole lot of difference. If I had to guess, I'd say they reduce drag a little. I can't really say for sure.

When I go to test stuff like this, I usually go about a mile one way on the flat stretch of road in front of my house, and then return, shifting into third and allowing it to reach top speed.

I noticed _maybe_ a 1 mph increase, but it's hard to say...  

Well, it can't hurt anything, right?  

I'm going to do a little more work on the area in front of the wheels. I think I can get some improvement with a little tweaking.


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## John (Sep 11, 2007)

What you’re talking about is an NACA submerged scoop. They don't suffer from the loss of efficiency you would expect from a regular scoop due to it ingesting the boundary layer. Something about a vortex generated by the flow over the leading edge of the NACA scoop ejecting the boundary layer and causing it to ingest clean air. 

Boundary layer growth might be greatest at the stagnation point but the boundary layer would be thinnest at that point due to that being the first point of contact of the airflow. Placing an intake there wouldn't cause an increase in boundary layer growth. First air flow contact would simply move out to the edges of the intake.


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## mattW (Sep 14, 2007)

Greenflight you could also try to measure your power output at the same speed before and after. Has your overall top speed improved? If your speed is limited by the revs of the motor it probably wont make a difference but if you were struggling to get enough torque to get to higher speed then you should have a higher top speed with the changes you made. You could also try getting up to a specific speed (the higher the better) and then let the vehicle coast to measure the deceleration. This experiment and the maths needed are described in detail here. You definitely need to be doing testing to make sure what you have done will make a practical improvement to your efficiency. Aerodynamics is a hard thing to predict and sometimes is very counter intuitive. I would say that if you can't find a measurable improvement after adding something then its probably not worth the extra weight.


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## Greenflight (Sep 13, 2007)

Yup, NACA submerged scoop. Words weren't coming to me.  I guess the concept of intakes at the stagnation point just sounds weird.  Probably works fine. I'll have to look into it some more.

I run out of torque long before I run out of revs. Top speed in 3rd is pretty predictable, around 45. After I lowered the front, it went up to about 47, which is about where it is now. Tomorrow, if I get the chance, I'll add some deflectors in front of the wheels and see what happens. I'm going to try those wedge-shape deflectors. If I can get them to funnel some air towards the center of the car (where the pan extends furthest back), the pan should help more. We'll see. I'm also going to try gap sealing the top edge of the bumper to the grill, with a large fillet. Should help flow around that area a bit.

I'll let you know what I find out.

All this duct tape is creeping me out...  Great for trying out mods, though.


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## Greenflight (Sep 13, 2007)

I finally got the chance to throw together some deflectors for the front wheels. They seem to improve performance slightly, but it's hard to tell because it's pretty windy out. Calm conditions would be much better.  

From what I can tell thus far, these deflectors are probably worth their weight. I think it would be worth making a nice fiberglass (maybe even carbon fiber?  ) belly pan with molded-in deflectors. Another project for when I get bored.

If I were to lay up a belly pan, it would be easy to incorporate a new bumper cover as well, possibly picking up some more speed there.

I'm going to take it for a spin again this evening when the winds have (hopefully) calmed down and I'll post some more results, along with pics.


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## John (Sep 11, 2007)

I've been wondering about the effect of the incidence of the belly pan to the road. The belly pan slopes down from the front towards the road. This would cause the air flowing under the car to accelerate towards the back of the pan (to a rearward velocity relative to the road) which would in turn cause the pressure to drop below atmospheric pressure sucking the pan (and the car it's attached to) towards the road. The down force generated might have a negative effect on the overall aerodynamic drag. If the belly pan wasn't ridged enough the effect would exaggerate it self as the pan flexed towards the road. The high velocity flow is then dumped when it reaches the back of the front axle into a large open bottom cavity formed by the rest of the engine compartment where it mixes with dead air. There is then no chance of diffusing the flow to pick up some forward pressure to mitigate the drag.

I think Kevlar would be a better choice for a pan than carbon fibre. It has a similar strength to weight and is much tougher it is just not as ridged.


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## Greenflight (Sep 13, 2007)

True- except the decrease in pressure would probably be compensated by upwash at the rear of the belly pan, plus the increase in pressure that would likely be caused by the sudden slowing of the air. Hard to say, though.

It is true that a lower angle of incidence would probably improve performance significantly. In fact I was just thinking about that. If I were to build a custom bumper cover I'd compensate by lowering the front of the bumper low enough to flatten out the pan. Plus it would reduce overall airflow underneath the car, which would probably also have a pronounced effect.

Kevlar might be a better choice... I just don't know much about it. I'll probably just end up using fiberglass anyway.

I drove the car a little more and concluded that there was a slight increase in performance from the deflector. A better design would probably have a much more noticeable improvement.

I like these ideas! Good, cheap ways to improve performance... Gotta love it.


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## John (Sep 11, 2007)

I was thinking that effectively dumping the high velocity flow into a large cavity containing dead air would prevent the low pressure area from going further back than the back of the belly pan and would reduce any up wash effect. Pressure recovery in this area without a diffuser would be quite poor don't you think.

I would prefer the notion of not creating any negative lift. My car also has sloping front pan. I'm guessing it was built that way for a clearance angle for steep driveways and the like.


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## John (Sep 11, 2007)

Your lower front bumper (spoiler) doesn't appear to have a very pronounced lip. I imagine it would spill quite a lot of air into the underside flow especially considering how flat over all the front of the vehicle is with a lack of tapering off at the corners.


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## mattW (Sep 14, 2007)

Ok I don't think I'm understanding this properly, as far as I know force occurs due to a pressure differential and a sloping belly pan would have positive pressure since it would be deflecting air downwards, and since there would be less pressure across the bonnet (/hood) and behind the pan the total force would be upwards and backwards. I try to explain my understanding of it visually in this dodgy flow diagram attached, the green line is where i would image the force to be acting. Is there something I am missing? what would be creating down-force?


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## John (Sep 11, 2007)

A lot to do with aerodynamics is counter intuitive. A stream of flowing air possesses a few different forms of energy. The dominant forms to consider are pressure energy (also known as flow energy) and kinetic energy as they are flow dependant. As the kinetic energy increases due to the flow accelerating to a higher velocity the pressure energy decreases and the converse is also true. The air duct formed by the underside of the belly pan and the road decreases in cross sectional area at the back forcing the flow to accelerate to pass the same volume of air as it must this in turn causes a reduction in pressure via this energy exchange.


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## mattW (Sep 14, 2007)

I understand how that would work with a gust of wind at the front of the car; Pressure would build at the front compared to the back but then the pressure difference would 'suck' the excess air from the high pressure to low pressure zone. But when the car is moving the pressure at the front of the car is being constantly renewed by new air crashing into it. The only reason the air would accelerate would be because the pressure differential was maintained which means that there is still high pressure at the front. I'm not trying to argue, I'm just struggling to understand how it works...


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## Greenflight (Sep 13, 2007)

Matt, try thinking of it this way (and please, somebody smack me upside the head if I'm getting this wrong). 

Every action has an equal and opposite reaction. So each time the car acts upon the airflow to change it's direction, a force is exerted on the car as well as the air. So when the car deflects air upwards, the air exerts a downward force on the car, and vice versa. And each action/reaction takes energy. So even if the net force is zero, if the air has to go up and back down, it takes energy. 

When working with air flow at subsonic speeds (I think we can all agree that this is subsonic) it is generally considered incompressible. This is not to say that air is incompressible- it's just that the effects of compression in these situations are small enough they're worth ignoring. 

So anyway, when the air passes under the car, the belly pan deflects it downward (note energy loss) and then flattens it out (note energy loss). As John was saying, this reduces the cross sectional area of the "tunnel" underneath the car. Since the forces aren't enough to compress the air a noticeable amount, the only option the air has is to speed up. 

Think of it as the proverbial water in a pipe. If the pipe becomes smaller, the water speeds up. Same diff.

And as far as the air behind the belly pan, it becomes a tremendously disorganized flow that my feeble mind can only guess at.

Way not to speak to the question, huh?  

And BTW, thanks for that glamorous pic of my truck. To be honest, I never really looked at it that way before.


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## John (Sep 11, 2007)

To elaborate further the laws of the conservation of energy and the steady flow energy equation apply. The two energy forms considered are kinetic energy and pressure energy due to the dynamic exchange of form that occurs between the two due to the conservation of energy. 

*Work* is defined as equalling force times distance (f.d), in SI units Newtons times Meters (N.m or a Joule). 

*Power* (watt's) equals work per time (f.d/t). You will recognise (d/t) distance per time as being speed so power also equals force times speed (N.m/s). 

*Energy* equals power multiplied by time (W.h, watt hours) or (f.d/t.t) so time cancels out of the equation so it ends up as (f.d) or the equivalent of work which in SI units was Newton meters. 

*Newton*. In SI units gravity will produce an acceleration of 9.81 meters per second per second (9.81 m/s/s or 9.81 m/s^2) or a force of 9.81 Newtons per kilogram (9.81 N/kg) so N = kg.m/s^2 (force equals mass times acceleration). This is what quantifies a Newton.

*Kinetic energy* equals half mass times velocity squared (KE=1/2.m.V^2). V=d/t so KE=1/2.m.d^2/t^2 or in terms of units KE=kg.m^2/s^2 and substituting N for the term kg.m/s^2 results in KE=N.m. Carefull not to confuse mass with meters (both are m).

*Flow energy* equals pressure time’s volume. Pressure equals force per area so in terms of units QE=N/m^2.m^3. So this cancels down to QE=N.m. Flow energy is what does the external work in a hydraulic or for that mater pneumatic cylinder.

There ends the definitions and derivations. 

Flow over curved surfaces.
An object in uniform rotary motion i.e. scribing a circular path at a constant velocity is in fact continuously accelerating toward the centre of the circle. There must be a continuous force pulling the object towards the centre of the circle. Fluid flowing over a curved surface at uniform velocity will similarly be accelerating towards the centre of curvature. The forces required to generate this acceleration will cause changes in the pressure at the surface the fluid is flowing over. Flow over a convex surface will require the air to accelerate towards the surface. This accelerative force will cause a reduction in pressure at the surface conversely a concave surface will cause a pressure rise at the surface as the air accelerates outward from the surface toward the center of the bend. This is why you get a pressure rise at the base of a cars windscreen. The bend in the flow there requires an abrupt acceleration outward from the surface.


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## Greenflight (Sep 13, 2007)

OK, so here's what I'm thinking of wasting duct tape on now.  

Check out the pics of this car (BTW, the batteries in this car are AMAZING): http://www.austinev.org/evalbum/464

See that front bumper? I'm thinking of rigging up something like that. I would be able to lower the bottom edge of the bumper, reducing the amount of air that passes under the car, and also duct it further out to the sides, away from the front wheels. I could also bring the front of the fender closer to the wheel, reducing the excessively large gap that currently exists. I could also flatten out the belly pan quite a bit.

Plus it would be really easy to test out.  

So, what do you guys think?


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## John (Sep 11, 2007)

I would love to see the results of such a test. It would vindicate or debunk the theory as to why the previous belly pan didn't produce a more positive result. It has occurred to me that there might be some other large airbrake on the truck which was making the gains a smaller part of the whole drag picture making the gains difficult to see.

To see what sort of down force you might expect I ran through a quick calculation with some guestimated values. Assuming sea level standard air density and pressure and a flow velocity of 60 mph (27m/s). Assuming the belly pan reduced the cross sectional area to half doubling the flow velocity. Using 1m^3 for flow volume.
KE1+QE1=KE2+QE2 so QE2=KE1+QE1-KE2
Density of air is 1.225kg/m^3 and air pressure is 101325N/m^2
QE2=(1.225/2*27^2)+(101325*1)-(1.225/2*54^2)
 =99985
Pressure drop=101325-99985=-1340N/m^2
Assuming a belly pan area of 1m^2 and a uniform pressure drop across the length of the belly pan the average pressure drop would be half or 670N/m^2 for a force of 670N or about 68kg of down force. Note how small the pressure drop is, 1.3% of atmospheric pressure. This still generates significant down force.


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## Greenflight (Sep 13, 2007)

That's a noticeable effect. Very interesting.

I'll probably work on the fairing tomorrow if I have time. We'll see. I can imagine reducing the overall flow underneath the car by that much would help quite a bit.

I'll post the results when I'm done duct taping.


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## mattW (Sep 14, 2007)

Hey guys I was thinking we could start a Wiki topic about aerodynamics. One section on the theoretical side of things and the other on practical projects for people to undertake. I just realised that this topic is approaching 50 posts and appreciate how much information is on here, it would be good to get it into an easily accessible form. Would you guys be willing to add some of your wealth of knowledge?


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## Greenflight (Sep 13, 2007)

Woohoo the 50th post is mine!!!!  

Good idea Matt. I'd be happy to add some pics of my experiments, along with results, etc., as long as I could figure out how.  Never actually used a wiki before.


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## Greenflight (Sep 13, 2007)

OK, I had some time to kill today (finally), so I started working on a bumper cover for my truck. As soon as I can get it to the point where I'm confident it will withstand 50 mph winds, I'll try it out.

I'll post some pics of this ridiculous looking contraption as soon as I get the chance.

And I keep meaning to add to that wiki but I can never seem to get around to it!


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## Greenflight (Sep 13, 2007)

OK guys, here's the final rendition (sorry for the bad pic). I decided I wasn't comfortable having a huge chunk of foam on the front of the truck, so I made one out of lexan instead. Not the most elegant design ever, but hey- it works!

I might have picked up 1-2 mph. I should probably try making a new belly pan that attaches to this thing, rather than just "existing" behind it. But at any rate, the fact that it didn't slow the car down is a good sign.


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## John (Sep 11, 2007)

To put this in perspective drag increases with the square of speed so if your top speed went from 55 mph to 57 mph that would be equivalent to a 7% reduction in drag or going from a cd of 0.45 (guestimated) to 0.42. Assuming that the aerodynamic drag makes up about half of your total drag, by driving at a similar speed as before you could convert that into approximately a 3.5% increase in range. Top speed is not a particularly fine measure of the gain. I'm not sure what would make a better measure.


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## Greenflight (Sep 13, 2007)

Hmmm. 3.5%. That's kinda depressing.  

Either way, this stuff is fun to play with. Plus I think it makes my truck a little more bearable for the people behind me.  

Just because I'm curious- You seem pretty familiar with aerodynamics... Is that your line of work?


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## mattW (Sep 14, 2007)

Hey guys, we're been doing all wrong, trying to make subtle changes while keeping with the style of the car... we should be ashamed. THIS is what we need to do:









And look at the European syling on the front here:









i'm...just...speechless!


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## mattW (Sep 14, 2007)

But on a more serious note I just found a forum for people trying to up their mpg with a whole section on aerodynamics with 11 pages worth of topics. Check it out here, looks like a good resource.


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## Greenflight (Sep 13, 2007)

Okay, don't know how I missed that post before, but I seriously almost fell out of my chair when I saw that...  Do you know if the guy got any better mileage after, uh, "tweaking" it?

I like the rear hatch... nice touch.


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