Perfect Base Car For Super Aero Car/EV?

[Mighty Mira]

Fitting a boattail to a car is what will really bring a car's drag down to bare minimum, after the wheels, undertray, grille and rear view mirrors are taken care of. Since minimizing drag is what will ultimately enable an EV with high range with existing battery technology, I have been thinking about what sort of vehicles to use as a base.

Now, ideally we want a boattail that is as short as possible while still being functional (i.e. reducing drag, being used for storage). We need to know the maximum taper that the boattail needs. Then it is just a matter of finding a car shape that when you overlay the rear segment of an airfoil over the car in such a way that it only intercepts but not intersects the furthest points of the car, the point of the boattail is a minimum of distance from the rear of the car.

I was thinking about this earlier, and came to the conclusion that the Australian version of the Ford Capri would work great:



I suppose any other similar convertable would be ideal. The most ideal convertibles are cars where the rear of the last hard piece is relatively close to the front of the car.

If you look at the rear of a car, the cross section is a rough rectangle. Minimizing the length of the boat tail really comes down to finding the minimum side length of this rectangle. And thus something like the capri shows a lot of promise. A full boattail might only converge to 4 feet behind the car.

[Matt Timion]

I thought that convertibles had horrible drag coefficients though... is this not correct?

[Mighty Mira]

Originally Posted by Matt Timion

I thought that convertibles had horrible drag coefficients though... is this not correct?

Sure they do.

But as a base to put a boattail on...



I suspect it would be a LOT easier to accomplish with a convertible.

Of course, if you were willing to totally hack into the mira, remove or cut the rear window in order to start tapering early...



But the difference between the capri and the mira is 7 inches, and with the capri one could start probably a foot or two further into the car. Meaning that you can get away with a still smaller boattail with the capri.

[Mighty Mira]

I thought this was useful to understand drag minimization.

In the first two pictures, drag is minimal. As the angle of attack increases, more of the boundary layer separates, and the wing stalls. Essentially the rear of the car can be viewed as "stalling", and hence generating lots of drag.

This behavior is a function of angle of attack and reynolds number (i.e. speed for a given car). I suspect that my angle as drawn is a little generous, however, it could be easily tested with coroplast (corflute), duct tape and an instantaneous FE meter.

[Mighty Mira]

Note also that a typical reynolds number for a car is 2.4 million.

Here is the NACA 0012 airfoil. Note how the slope is roughly 1:8 at the rear of the foil. What is this angle? arctan 0.125 = 7.125 degrees.


And here is the drag coefficient for that airfoil at Reynolds number of 3 million:



Note how the drag begins to blow up around 13 degrees, but is still damn good for a car - 0.015. If we add this to 7 degrees, we get 20 degrees.

This site was helpful.


What good is this to someone who wants to design an FE car? Well, start with a good car, and then experimentally compute Cd for different configurations. Start with a fairly steep boattail, and make it shallower until the drag drops off at speeds that the car will be driven at.

[Mighty Mira]

Another thing to consider is crosswinds.

A strong wind according to the Beaufort scale is 50kph. If we assume that gale force winds are rare, then the maximum that the car's angle of attack should be is 26 degrees. Which should be enough to seriously impact the car's drag, especially if we design it so that it's just on the verge of stalling in regular conditions (no wind).

Ugh. I'm going to bed.

[Gary Palmer]

Mighty Mira: Since your driving a "box" shaped vehicle, I am going to share some background/experiences I've had with you, which I found very fascinating and curious. Maybe you will be able to use it with your car, maybe not.

I had a Plymouth Reliant Station Wagon, with a 2.4 L engine and an automatic transmission. We used it to go camping quite a lot, so I built a car top for it, the length of the top and about 20" tall. I built the front to match the angle of the windshield and just squared off the back.

In terms of packing and hauling it worked really well. I had to put air shocks on the car to get it back to level, but after that it worked great.

The only problem was that with the heavy load of people and equipment filling the entire car and car top, I would get up to about 56-58mph and then it would be like a hit a wall. If I had a really long run, I might have been able to get it up to 65mph.

One year, on a annual trip to Death Valley, in December, I wanted to take some fire wood with us, since you can't get any their. However, we were out of room, so I put a tarp on the top of the car top, stacked a pile of wood about 10" high and about 3 ' long, at the back end of the car top.

In previous years I would get about 17-18mpg, with the car top on. My gas mileage went up to about 22mpg. Additionally, when I got up to about 57mph, I didn't seem to hit the wall, like I had previously.

So'o I got some cardboard and fabricated a shape to put on the top of the car-top/carrier. The shape was a reverse airfoil, upper half, placed across the top of the car-top/carrier. With this shape, I was able to drive at a higher speed, I could pass other car's at 65 and so forth. I also was able to get 22mpg-25mpg.

I gave the car and car top to my parents. They made a trip with it, with 5 full size adult's and with the car packed with everything from the kitchen sink to who know's what. I asked my dad how he thought they had done on mileage and he said he had only checked the first tank of gas, which didn't make any sense, because he was always checking mileage, on trips. He said that on the first tank they got 25mgh and he was dumbfounded. He said he didn't think anyone could ever get that kind of mileage, with that kind of load, in a car with that size of engine. So'o he quit checking it because he didn't want to try to figure out how it could be getting that, in case it was because of some etheral experience or cause he might jinx or upset the balance of, if he tried to figure it out, to closely.

I believe what was happening is that the huge box shape on the back side was basically creating a huge vacume behind the car and that the reverse airfoil shape caused the air to be accelerated from the top of the car back down behind the car, breaking up or releasing the vacume effect's on the car. In either case, their was definitely a force beyond just frontal surface area, which was really dragging the car down, as the speed's went up.

Hope this information might be insightful or helpful.

[ZugyNA]

Originally Posted by Gary Palmer

I believe what was happening is that the huge box shape on the back side was basically creating a huge vacume behind the car and that the reverse airfoil shape caused the air to be accelerated from the top of the car back down behind the car, breaking up or releasing the vacume effect's on the car. In either case, their was definitely a force beyond just frontal surface area, which was really dragging the car down, as the speed's went up.

Hope this information might be insightful or helpful.

Very interesting! I took a trip where I had a roof rack on a wagon and a tarp tied over this...as expected the tarp ballooned up to create the kind of shape you describe. Had at least 600 lbs in the car. Mpg didn't espec improve though.

And then there is this:

http://www.max-mpg.com/html/tech/main.htm

You notice that the VW already has part of the the shape you describe...but is helped by the wing...which gives it a rear profile like the red car.

Overall it is shaped like your wagon at the back?

I'm thinking of designing a camper on a pickup and am trying to find practical ways of making it AERO...wondering what the ideal top curvature would be?

Would it be feasible to use something like a tarp as part of the design to allow an ideal shape to form naturally?

I might test this idea by tying a tarp on the Tercel's rack and testing a tank or two.

[The Toecutter]

The ultimate platform for an EV conversion and a gasoline car will be very different depending on your goals.

For an EV conversion, if you are trying to maximize range for minimal cost, I can offer no better recommendation than a small pickup, like a Toyota XTraCab, Datsun 1200 minitruck, or an 80s model Chevrolet S10.

It seems counter-intuitive, given that trucks have such poor aero. Where their strength rests is with their weight-bearing capacity.

However, they do lend themselves to modification. Phil Knox has a gas-powered Toyota T100 pickup. With aeromods, he brought highway mpg from 25 to 32.

http://www.evworld.com/view.cfm?sect...le&storyid=870

Apprantly, he cut the Cd from .44 to ~.25, which accountd for all of the gain. No LRR tires, no synthetic tranny oil, no weight reductions, no brake modification to zero brake drag, no alignment changes. All 100% aeromods. With those other mods, I'm rather confident he could get around 36-40 mpg highway.

I know of a few EV trucks that have done 120 miles highway range on golf cart batteries. They used the truck's weight hauling capacity to its maximimum potential and loaded up with about 2,400 pounds of batteries. Just filled the entire truck, bed and all.

You can see two notable ones described below.

"Red Beastie", a converted Toyota XTraCab with 40 Trojan T105 batteries, a pack weighing 2,440 pounds. With a DCP Raptor 1200 controller, batteries connected in two parallel 120V strings, and an Advanced DC 9" motor, it does 0-60 mph in ~19 seconds, tops out at 85 mph, and does 120 miles per charge at 60-65 mph speeds. John Wayland used this truck to make a 440 mile round trip from Portland, Oregon, to Seattle, Washington, and back. This was also a very practical vehicle, and could haul electric racecars 50 miles to the track at freeway speeds. Unfortunately, the truck was destroyed in a fire this month when a dumptruck's parking brake failed and rolled into the house of its owner, Tony Ascrizzi, destroying the truck in its entirity.

http://www.austinev.org/evalbum/37

"Polar Bear" is a converted Chevrolet S10 pickup, using 40 Trojan T125 batteries, pack weighing in at 2,640 pounds. It had a 9" Advanced DC motor and 600 amp DCP Raptor controller. The pack was again configured as two parallel strings of 120V. This truck did 120 miles per charge at 60-65 mph highway speeds, and topped out at 75 mph. It is also no more. It was destroyed in a collision. The ~$2,000 lead acid battery pack lasted 45,000 miles due to keeping the percentage discharge low for high cycle life. Basically, this truck was cheaper to operate than its gasoline counterpart so long as gas was over ~$1.30/gallon by my estimation. It too was quite practical with its range, although like "Red Beastie", the entire bed was filled with batteries.

http://www.austinev.org/evalbum/185

The setups for both of these trucks above could be duplicated for around $8,000-12,000 today. The flooded batteries could be swapped for AGMs and the Raptor swapped for a Zilla controller and add about $5k to the cost, while dramatically increasing acceleration performance to that of a musclecar(0-60 would be around 6-7 seconds with a Zilla 2k controller and twin 240V strings of group 31 size AGMs if the same motor is retained).

None of those trucks above had any significant efficiency modifications. "Polar Bear" had no aeromods, no LRR tires, no synthetic transmission oil, no machined brakes, no alignment adjustment. The only efficiency modification "Red Beastie" had was a bed cover, which would have negligable benefits.

So what if these trucks had aeromods out the *** like Phil Knox's gas truck to lower drag, had LRR tires, synthetic transmission oil, brakes machined to be round so that they don't drag, and a 0 degree camber, 0 degree toe-in, and 0-degree toe-out alignment, while maintaining the same battery, motor, and controller setup?

I've run simulations that say they would do between 180 and 250 miles per charge at 60 mph, depending on outside factors and on the type of small truck used. This is with cheap lead acid golf cart batteries, nothing fancy.

Please re-read that above. You read right.

It is entirely possible to build an EV that does 200+ mile range without advanced batteries. A hobbyist, provided with the budget and time, could accomplish this.

I'm saddened no one has yet made the attempt. But I'm nearly certain it can be done. All that needs combining is Dick Finley's "Red Beastie" concept, the last EV he designed before his death, Phil Knox's aeromods, and other tweaks GS members on these forums use to maximize fuel economy.

If I had that kind of cash? You ****ing damn well bet I would give it a go!

There is no possible way to convert a gasoline sedan, sports car, or compact to the above setup. There simply isn't enough load bearing capacity. A van won't work either due to the difficulty in implementing significant reductions in drag coefficient. To do 200 miles range with a conversion of a gas-powered automobile on lead acid batteries will require a small pickup truck, with low frontal area, good load bearing capacity, and great battery room.

My idea for a viable long-range conversion is this, in italics:

Build a lead acid powered EV that could meet ALL of the following constraints:

a) 0-60 mph in 18 seconds or less
b) Top speed 90 mph or greater
c) 200 miles range or greater at 60 mph
d) Capability to seat 4 or more adults
e) Under $20,000 total cost, including donor chassis and shipping for components

The performance parameters above are basically what an IC economy car could achieve in the late 1970s/early 1980s. It may not be fast at all, even could be described as anemic, but it is acceptable performance for keeping up with traffic. Even the cruising range is there.

Is it possible to achieve these parameters using flooded lead acid golf cart batteries? Without any advanced batteries needed? I'm about to explore this possibility.

Demonstrating this sort of performance would make an electric vehicle conversion practical and palatable to a much larger percentage of the population.

The following setup will be simulated, with costs and weight tallied and donor vehicle cost not included. Shipping is assumed to cost 10% of all components marked with *.

-WarP 9'' series DC motor x1 160 pounds $1,575 (EV Source)*
-Trojan T145 flooded lead acid Golf Cart battery x40 2,840 pounds $5,140 (Trojan Battery)*
-Godzilla Controller(72-300V DC, 1,000 amp max, HEPI) x1 16 pounds $2,555 (EV Source)*
-PFC 20 Charger x1 20 pounds $1,525 (EV Source)*
-Vicor DC-DC converter (300Vmax, 12Vout, 200W) x1 8 pounds $700 (Vicor)*
-E-Meter x1 $229 (Xantrex)*
-Solid-State Ceramic Heater Core x1 $75 (Grassroots EV)*
-Adaptor Plate x1 15 pounds $800*
-Miscallaneous components(Heat shrink tubing, fuses, steel for battery racks, ect.) 150 pounds $1,500*
-Donor Vehicle 1980s Mazda B2000 pickup truck w/extended cab 2,600 pounds $1,000
-Sheet metal, plastic, fiberglass, and other components for aerodynamic modifications 30 pounds $100
-Nokian 205/70R15 LRR tires x4 $268 (Nokian)*
-Leaf Springs x4 $300 (Renegade Hybrids)*
-Redline MTL synthetic transmission oil $15
-alignment correction to 0 camber, 0 toe is free with tire replacement

Roughly 600 pounds of IC related components can be removed from the donor. The donor has an estimated drag coefficient of .45 and an estimated frontal area of 22 square feet. With aerodynamic modifications, the drag coefficient is expected to be reduced to .25. The donor vehicle cost was an estimate. It is assumed normally 2 passengers will occupy the vehicle, so 350 pounds will be added to account for 2 passengers and any other onboard items.

The Nokian NRT2 LRR 205/70R15 tires chosen have a .0085 rolling resistance coefficient. The specific model was chosen for its ability to handle a 1,480 pound payload per tire. This allows room for a gross vehicle weight of 5,920 pounds, or 681 pounds of passengers and luggage. These tires are also rated to 118 mph. The tires are assumed to have no weight change over stock tires. This is an incorrect assumption, but the weight of the stock tires is unknown.

Total Cost: $17,249
Total Weight with two occupants and luggage: 5,589 pounds
Max Weight: 5,920 pounds

In order to maximize range, the following aerodynamic modifications could be done with sheetmetal, plastic, and fiberglass:

-aeroshell, a tapered bed cover made of fiberglass
-underbelly, made of corrugated plastic
-grille block, made of corrugated plastic
-rear wheel skirts, made of sheet metal
-front air dam, made of sheet metal
-side skirts, made of sheet metal
-rear diffuser, made of sheet metal
-wheel covers, made of corrugated plastic
-build shaved door handles from parts found in junkyard, weld a sheetmetal backing plate to where the door handles were

This would get the drag coefficient down to an estimated .25, similar to Phil Knox's pickup truck.

Further, the brakes can be adjusted so that they don't drag.

So the following truck will be modeled:

Weight: 5,589 pounds
Drag Coefficient: .25 (from aero mods)
Frontal Area: 22 square feet
Drivetrain efficiency: 93% (slight boost from synthetic oil)
Tires: 205/70R15, which means a tire diameter of 25.34 inches.

The Mazda B2000 pickup has the following gear ratios:

1- 3.622
2- 2.186
3- 1.419
4- 1
5- .858
F- 3.909

The batteries would be arranged in a single 240V string. The Zilla would be configured to limit maximum current draw to 450 amps, maximum motor current to 1,000 amps, and maximum motor potential to 170V. At 450 amps, the Trojan T105 batteries would sag to roughly 4.5V, allowing a maximum of 122 horsepower from the batteries. The 500A limit is imposed to prevent battery damage.

Thus modeling the 9" motor, we get the following torque versus speed curve and power versus speed curve under maximum acceleration:

0 RPM 220 lb-ft 0 HP
1000 RPM 220 lb-ft 42 HP
1500 RPM 220 lb-ft 63 HP
1750 RPM 220 lb-ft 73 HP
2000 RPM 195 lb-ft 74 HP
2500 RPM 178 lb-ft 85 HP
3000 RPM 164 lb-ft 94 HP *peak motor horsepower, limited by battery pack*
3500 RPM 132 lb-ft 88 HP
4000 RPM 105 lb-ft 80 HP
4500 RPM 83 lb-ft 71 HP
5000 RPM 69 lb-ft 66 HP
5500 RPM 55 lb-ft 58 HP
6000 RPM 44 lb-ft 50 HP

A motor redline of 6,000 RPM was chosen to prevent motor damage. The batteries are the limiting factor in acceleration and power that the motor can deliver. Stiffer AGMs would extend the torque curve out more dramatically improving acceleration but add greatly to the cost.

Now it is time to simulate acceleration and top speed. The following acceleration calculator was chosen for its ease of use and accessability:

http://www.nightrider.com/biketech/accel_sim.htm

The proper gear ratios, torque versus RPM, weight, drag coefficient, and tire rolling resistance parameters were input. A drivetrain loss was estimated at 7%, which would account for a slight efficiency boost from synthetic transmission oil. It is estimated the front/rear weight distribution will entail 70% rear, 30% front due to the bed being loaded with batteries, and wheelbase was estimated at 110 inches. In order to prevent the program from committing an error, a launch RPM of 100 was chosen.

The optimum shift points for maximum acceleration were 3,920 rpm for 1st to 2nd gear, 3,810 rpm for 2nd to 3rd gear, 3,650 rpm for 3rd to 4th gear, and 3,300 rpm for 4th to 5th gear. A shift duration of ? second was assumed.

We get the following estimations:

0-30 mph acceleration: 4.6 seconds
0-50 mph: 12.7 seconds
0-60 mph: 17.7 seconds
Top speed: 111 mph
1/8 mile drag race: 12.7 seconds @ 50 mph
? mile drag race: 20.5 seconds @ 64 mph

This meets the specified performance parameters. It's about as fast as a typical gasoline powered car from 0-30 mph, and from 0-60 mph, about as fast as an 80s model pickup truck with an anemic 4 cylinder engine. It wouldn't be fast, but it would be able to safely merge with traffic. An added perk from the Zilla is that it would easily smoke its tires and pull tree stumps.

For range, a simulation is going to be performed with Uve's Calculator. The above parameters will be entered, along with a brake/steering drag coefficient of .002 to account for corrected alignment and machined brakes. A relative wind factor of 1.2 was chosen to represent an aerodynamic vehicle, and a wind speed of 7 mph was chosen to represent outside wind conditions in average weather.

http://www.geocities.com/hempev/EVCalculator.html

The following results were obtained:

Range at 50 mph was 377 miles in 3rd gear.
Range at 60 mph was 216 miles in 3rd gear.
Range at 70 mph was 162 miles in 4rd gear.

And just for curiosity's sake, range at 90 mph was calculated at 102 miles in 4th gear.

This is within the constraints outlined above.


In theory, such a vehicle is possible. In practice, no one has tried it. The closest to it are John Wayland's ?Red Beastie? and Brian Methany's ?Polar Bear?, two trucks that have achieved 120 miles highway range on similarly large battery packs. Neither truck has extended cab, so they could only seat 2 or 3 adults.

This truck I outlined would be a passenger vehicle capable of seating 4 adults, accelerating from 0-60 mph in under 17.8 seconds, topping out at 111 mph, and doing 200 miles per charge at 60 mph. This would require the proper efficiency modifications to achieve this range and top speed. Without the efficiency modifications, range and top speed would be comparable to the two conversions referenced above.

Such a vehicle as I outlined would not only be beneficial in demonstrating that advanced batteries are not needed for a conversion to compete with gasoline powered cars in range and top speed, but it would also serve as a viable platform for a conversion business to harvest ideas from.


If the $17,000 component price is too high, performance could be sacrificed for a significant cost reduction. A lower voltage setup with two battery strings in parallel, a cheaper charger and controller, and less luxuries such as heating could result in a conversion with similar range and a price tag around $8,000. But 0-60 acceleration would increase to around 40 seconds with a 120V, 400A Curtis controller and the batteries split in 2 parallel strings. Lower cost Trojan T105 or T125 batteries could be substituted for a loss of about 10-15% of the range but greatly reduced cost.


For a few thousand dollars greater than the projected $17k concept, AGMs and regs could be put in place of the flooded batteries, allowing performance comparable to the new cars of today. But this would bring costs near $20,000. A Zilla 2k would add even greater costs, but allow rapid acceleration to compete with today's $30,000 sports cars.



So, what do you think of this idea? Criticisms? Suggestions?



If designing a car from the ground up, a custom built midsize or luxury car based on a pickup truck chassis would have similar carrying capacity, increased passenger and cargo room, but also significantly less weight and frontal area compared with the fully outlined conversion concept of the Mazda B2000. Purpose built as an EV, the same battery pack could be fit into the car concept. This reduced weight and frontal area would result in dramatically increased acceleration performance and increased range provided the same attention is paid to efficiency and drag coefficient is kept down to the .18-.20 level, which is very feasible. Perhaps in a purpose built car with this setup, a 0-60 acceleration time of 14 seconds(comparable to a 1st generation Toyota Prius) a range of 250 miles at 60 mph, 200 miles at 70 mph, and top speed in excess of 120 mph could be achieved. Swap the flooded batteries for Group 31 size AGMs and upgrade to a Zilla 2k controller, and a $25,000-30,000 electric musclecar that does 0-60 mph in 6 seconds and tops out at 200+ mph that retains the same utility and range may be within the realm of possibility(albeit a governor may be needed well below theoretical top speed if stability becomes an issue).


Still no advanced batteries needed.

I'm thinking of designing a camper on a pickup and am trying to find practical ways of making it AERO...wondering what the ideal top curvature would be?

Look at this photo. Go to the EV World link above and read about Phil Knox's truck.




You can get ahold of Phil Knox for help at the maxmpg Yahoo group.

http://autos.groups.yahoo.com/group/maxmpg/

I believe his name is aero1898head on this Yahoo group.

[Mighty Mira]

Originally Posted by The Toecutter

So, what do you think of this idea? Criticisms? Suggestions?

You know what your idea made me think of?

An EV hummer.

Range is basically equivalent to Battery Weight/CdA (for the same battery type).

In fact, let's work it out, shall we? I am going to use the 70mph spec at 162miles, because I think that travelling at 50 or 60mph is unrealistic.

If Range = Fudge Factor * PbAcid Batter Weight/ Cd*A,

162 = FF * 2840 / (0.25 * 22) = FF * 516, hence FF = .314.

So... Range = .314 * Battery Weight / CdA.

If you use an F350, that's 0.314* 5000/CdA. Say that there is 44 square feet of frontal area, and you can get the Cd down to 0.25.

That's Range = .314 * 5000/(0.25*44) = 142 miles. Yuck.

You'd have to cut the height. No way around it, you'd have to make a custom job.

So... make a bigger, longer, truck. Put electric motors on every wheel, or just really powerful electric motors for one or two wheels. In fact, you could eliminate a diff by computer controlling the speeds of the different back wheels.

Load the thing chock full of batteries at all points between the wheels. Now you have a super low center of gravity, this truck will corner like it's on rails. In fact, upgrade the suspension so that it carries enough batteries to outweigh gas powered SUVs. And make it so that it has enough power to accelerate at least as far as an SUV, either through supercapacitors or larger motors, whatever happens to be the limiting factor.

Range: You've got it.

Maintenance: Once every X years, change the batteries. In fact, make it so that it can't be run down past a certain amount, that way the owner can't claim the batteries were faulty. And tyre and brake changes.

Safety: Active safety, hardly anything will outcorner it. Passive safety - all you have to do is weld a nice cage inside the crumple zones, that way anything that runs into it will go flying, even other SUVs. Ever played Grand Theft Auto? Think what happens when you are driving around an SUV and run into small cars, now you can do the same thing to other SUVs.

Running Costs: It should cost something like an ordinary car, but since the countries with cars have lots of coal reserves...

Effect on the Environment: Once everyone has one, those 350 years of coal might last another 50.



Now, let's do a calculation with something capri sized. Assume I can get the Cd down to 0.11. A Capri has a frontal area of 1.93 square metres (estimation from height and width spec), * 10.74 = 20.7 square feet. Well, actually it's 18 from UVE's converter. Hell yes!

So, we can figure out the battery load for this car:
Battery Weight
= Range * CdA / Fudge Factor
= 300 * 0.11 * 18 / .314
= 1891lbs of batteries, or 900kg.

I have no idea how much in the way of components would be removed. At the very least, I would think that if the Capri didn't work then something like the Datsun 1200 could be configured to do the job.