Perfect Base Car For Super Aero Car/EV?
 

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:

https://www.cg-car-reviews.com/images...1990308MED.jpg

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.

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...

https://img140.imageshack.us/img140/6...ivsmira4ia.jpg

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...

https://img134.imageshack.us/img134/1...chopped4lb.jpg

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.

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.

https://www.centennialofflight.gov/es...oef/TH14G4.jpg

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.
https://www.basiliscus.com/ProaSectio...D/NACA0012.jpg

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

https://www.aerospaceweb.org/question...0012-drag1.gif

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.

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.

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.

Could you post a diagram for that?

Here's what I meant by the crosswinds:
https://i2.tinypic.com/1zokcox.jpg

Note how in a crosswind, the car actually might get more drag than in a direct headwind, especially with a boattail! And since it is not going to be feasible to put boattails on the side to make the vehicle look like a manta ray (it will take up both lanes of traffic), it probably doesn't pay to taper the sides of the vehicle, only the top and bottom (since vertical wind is rather rare). Because if you taper the sides, a crosswind will develop boundary layer separation that much easier on the lee side of the boattail.

This is discussed here.

Thanks to Greg Locock for pointing me in the right direction.

Thus, if crosswinds are a factor, the flounder or stingray points the direction to a low drag design. It makes sense, since both of these animals need to be able to hover over an area when currents are changing direction. A shark or fish doesn't need to be able to do this, because it is more like an aircraft than a plane - it orients itself so that it is always hitting the air head on. The only way this could be accomplished with a car is if it had four wheel steering and could somehow point itself in the right direction. Which wouldn't work in gusty conditions.

https://www.castlecomfortdivelodge.co...s/flounder.jpg

Hence, the ideal shape for an EV will be flat as flat as possible.

https://www.greenspeed.com.au/Honda%20WSC%20384.jpg

Of course, knowing the average wind conditions will surely help. If crosswinds are a factor, then a more shark-like design will be better. Suffice to say, something like the Capri will naturally be better than a Mira as a base for an EV. And the above example car is going to be impractical. But tall vehicles, in general, we be penalized, hard. It also means that side skirts are going to hurt with a crosswind.

A practical car is therefore going to be somewhat elliptical in cross section, including the boattail. Hence the ev-1. Looks like those engineers really had it figured out. If you wanted to "roll your own" EV, the most logical starting point would be something like a capri. Ah well, looks like I may be selling the Mira down the track. I mainly bought it for the engine. It would be interesting to stick the Mira engine in a Capri, but probably better to convert the Capri to an EV. Oh well.

https://www.ev1.org/toyota/ev1con8.jpg

Mira: another approach to the crosswind issue, perhaps not practical, but certainly very creative, is 4-wheel steering: angle the car while underway to make best use of its "straight ahead" aerodynamics.

I first read the idea here:

https://privatenrg.com/#RearWheelSteer

Just FYI...

A university student in Ontario was killed last year driving his school's competition solar car shaped much like the image posted above. The solar car went out of its lane suddenly on a 2-lane highway and was struck head-on by a mini van coming the other direction.

One of the questions raised in the aftermath - trying to determine the cause of the loss of control - was the car's shape. It's similar enough to an aerofoil that high-speed stability is an issue: that shape generates lift, and road-holding and steering stability drops as speed climbs. If memory serves, other student drivers had experienced the effect, especially in gusts, and commented on it.

Thanks for the link! That's very cool.

If you read what I wrote closely, you see that I came to this conclusion too:

Say no more! It's obviously too late for me to be trying to read anything with any detail. :o

Going to bed now.

Very good point. I think the key is to make the bottom as curved as the top, or at least, to angle the end upwards (slightly). I think the latter is the better solution, to be honest.

It makes sense if you realize that Bernoulli's explanation is not really the proper explanation for why lift is generated. See here. The reason lift is generated is that air sticks to the surface, and momentum is conserved. Hence whatever direction the air tends to go after it leaves the vehicle is opposite to the direction the vehicle will be propelled. And provided that the slope the air is attempting to stick to is not steeper than a certain grade, you can direct it in any direction you choose.

In fact, now that I know about it, teaching the Bernoulli effect as the primary cause for lift is really doing a disservice to aerodynamics understanding.

The best way to understand this is get an empty toothpaste tube and bend it in different shapes under running water. The water clings to the toothpaste in much the same way as air does. You can simulate the lifting effects that led to the death of that university student. You can simulate the downward pushing effect of a Porsche whale tail. Or you can bend it fairly severly and simulate the back end of most cars, which pulls your toothpaste tube DOWN (which translates to back, i.e. drag in the case of a car).

I would think that so long as air exits the rear of the car (or wherever a gust might go) at worst, parallel to the ground and not in a downward direction, you would be ok. If you look at their vehicle, no wonder it did what it did. It's shaped like a wing. With a good headwind, it will take off. Note the difference between that and the above honda solar car.

https://www.thestrand.ca/media/paper4...s/renf940h.jpg

I reiterate: teaching Bernoulli does the biggest disservice to young aerodynamicists imagineable.

Hey, don't worry about it. Thanks for pointing the stability thing out, you may have saved someone's life. Also, I did not know about the link you posted to me, which was also very much worthwhile. Cheers.

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:

https://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 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.

https://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.

https://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.

https://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:

https://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.

https://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.



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


https://www.evworld.com/images/pknox_toyota.jpg

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

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

I believe his name is aero1898head on this Yahoo group.

toecutter,

Thanks for the in depth reply. I will read and review it.

One thing about Phil Knox's truck is that it will get around what it did before in a crosswind. I didn't understand that for the longest time, but now I do.

Have a look at my ultimate aero shell for a vehicle. It should be capable of a ridiculously low Cd, and hence, need the same number of batteries as a regular EV but go 300+miles.

I had this brainstorm last night, and just got to realize it. I haven't yet given much thought to how I would make the shell, but I suspect that a ford/mercury capri (1991-1994) would make a good donor vehicle. Of course, to create a prototype would require composite techniques.

Having a third the number of batteries would enable it to accelerate at three times the rate of your truck, and not require such heavy springs etc.

If you can really achieve those range figures with that airfoil design, I will be very impressed. The big difficulty is keeping the airfoil practical.

The GM EV1 had a .19 drag coefficient and a frontal area of 19.5 square feet. With a 1,310 pound pack of Panasonic AGM lead acid batteries, it did 100 miles per charge at a steady 60 mph using 130 Wh/mile, but around 70 miles per charge in a normal driving cycle. Keep in mind the lead acid EV1 weighed a hefty 3,060 pounds, and used LRR tires with a Cr around .0065.

Trying to triple that will be very difficult, given that achieving under a .11 drag coefficient in a passenger car that uses turbulent flow in aerodynamics is theoretically impossible.

Now, using laminar flow and other techniques can really lower drag coefficient, but other difficulties arise.

I guess I'll have to try it then.

My intuition tells me that the best way to go about this project is first to build a projection of an airfoil in every direction that wind is expected to attack it, and go from there.

If it's possible to build one of these things for under $10k (hopefully less), make them identical, stick it in Walmart and see who wouldn't want to buy something that will do everything a regular car does but cost virtually nothing to run or maintain.

I mean, people spend $10k on a bike... why not have something that will do everything a car does, and not have to spend any money on it?

The best thing you could do is build the car and test it. How much batteries do yuo plan to use, and what are your performance goals?

It's definately possible to make a 100 mile range conversion of a conventional truck that reaches 80 mph top speed for under $10k. It is also possible to make an 80-100 mile range sports car conversion that does 0-60 mph in 6 seconds for around $9-11k. These prices include taxes and shipping and all that crap.

Figure out what your performance constraints are and how many batteries you need to use. Then I can outline a setup for you when I get back.

Don't be afraid to get multiple opinions. Especially, ask the EV list.

www.evdl.org

Hmmm. Let me have a think about this.

Range is ultimately going to be determined by weight in batteries/Cd*A. Which is why you chose the pickup truck; you can increase the battery weight much more than you can kill the CdA, unless you are prepared to do radical alterations.

The weight in batteries that your truck can carry is 1290kg. The CdA of the truck is estimated at 22* 0,25 = 5,5. Range proxy = weight batteries/CdA = 1290/5,5 = 234. (Ignore my hodgepodge of kg and feet). So, whatever vehicle I build has to be able to beat that. I'm estimating a frontal area of 17 square feet and we will use a Cd of 0.11 even though I think I can do better. That means the number of batteries I will need is
= Range proxy * CdA = 234 * 0.11 * 17 = 437kg.

I think I am starting to understand your logic. I'm not sure that stripping the ICE stuff will enable me to build a prototype and have room for 437kg of batteries .The reason why I wanted to experiment with something like a Capri is that I don't yet know how to work with composites. I don't particularly like the idea of hacking into metal panels to build a new car.

What I would like to do is rapidly prototype something basjoos style, with plastic (clear and coroplast), duct tape, etc, and test it.

However, now I am seeing the logic in starting with a truck chassis, even if it is an old datsun truck (depending on their GVM, of course).

Thanks for your input, you are indeed invaluable.

The big downside to the truck concept is that it will never be as cheap to operate or as efficient as a car though. But I do know it is a good surefire way to get range, if that is your ultimate goal.

Alan Cocconi of AC Propulsion has built lead acid electric Honda CRXs that get 140 miles range at 50 mph and ~110 miles at 60 mph. He got Cd down to about .25, and kept the battery pack thermally managed to stay at ~100 degrees to maximize battery capacity. But his first car was filled with batteries initially, even in the passenger seat essentially making it a car for one person. Eventually, that changed where the car could occupy two. The entire rear hatch area was full of batteries. He had about 1,100 pounds of Optima batteries in the car. Did 0-60 mph in 7.8 seconds, topped 80 mph, with no transmission. Due to the battery heating, they only lasted around 20,000 miles or so given that the heating would cause them to degrade within 2 years, when normally they should last 6-8 years without such heating. The car was hundreds of pounds over GVWR.

I imagine with Phil Knox's CRX mods to get Cd in the .19 region, a similar conversion to Cocconi's could be built with around 160 miles range at 60 mph using that battery pack. But the back seats would still be absent and the car would still be over GVWR. Get rid of the thermal management, and range at 60 mph might only be ~130 miles. Still very good though, and battery life would be greatly extended if they weren't kept heated(You might get 40,000-50,000 miles from them, instead of them degrading in 2 years). Such a car would be very cheap to operate.

I'm planning a similar conversion myself with my Triumph GT6. Simulations have me getting 100 miles range at 65 mph, if I get Cd down to .25, use LRR tires, extensively modify the car for efficiency, and use a pack of 25 Exide Orbital 50AH AGMs. In reality, I'll be happy with 30 miles, so it doesn't matter whether my goal is met or not, but it appears very feasible. But I realize it may not happen as the simulations predict.

A hobbyist building a light, efficient 100+ mile per charge EV using lead acid batteries with a conversion is treading new ground. It hasn't been done much.

The pickup concept has been proven repeatedly and without resorting to efficiency mods.

The efficient sports car concept is not proven on such a repeated basis as the pickup. Although doable in theory, this concept absolutely requires extensive efficiency mods.

I may be biased towards the pickup route if it comes to maximizing range, but my own personal vehicle preference is the sports car. That's the conversion I'm doing because that is ultimately what I wanted to have. I leaned more towards wanting to have fun and have a high performance car than wanting a long range science project.

Whatever route you choose to take, I wish you the best of luck and would like to help in any way I can.

Thank you TC. I appreciate the offer to help. You are very, very knowledgeable. I can already see that I can trust your figures as much as I would trust my own. It's like I had gone out and done the research myself. I'm very impressed.

And like you, I want to have my cake and eat it too. I want to go places, fast, not spend any money, and have room for myself and a few things, in any weather, and not get rained on. Is it too much to ask?

The solution basically comes down to supremely minimizing Cd*A, maximizing GVM, minimizing parasitic weight, maximizing battery weight/GVM.

Then it comes to proof of concept. In the absence of a wind tunnel, the only alternative is the coroplast mockup, in the dead of night on a country road, a computer timed slowdown test, and weight with a truck weighing device.

And as far as shape, I can't believe that if Phil's CRX can get 0.19, I can't kick the *** of that with a custom design. I mean, have a look at it. The whole rear of the car is staring at me, quoting the lines from Spaceballs: "Suck! Suck! Suck!". There is going to be a huge turbulent vacuum back there pulling the car back. It's at least half the car's frontal area. If you eliminated it, surely you'd see commensurate Cd benefits???

https://img228.imageshack.us/img228/7...cuumcrx5fw.jpg

After you've got a realistic Cd estimate, then it comes time to make a proof of concept. Obviously, this thing will be so far out there it may actually be easier to make a custom interior than try and use an existing truck/car interior to tack your "bodykit" on to.

So, it's either
a) find something that will fit the shell and give you a stock interior.
b) go with a strong chassis and build both the shell and the interior.

Perhaps something like a capri would be best from the perspective of testing. Since it's about as small a shape as imaginable, it would make a good test vehicle. Then you could either select something that was not bigger in dimensions than the bodykit you wanted to graft on and also had the right GVM, or go find a good GVM chassis, strip it bare and build your car from the ground up.

Mira: A artist, I am not. So, bearing that in mind, attached here is a "crude" sketch of the shape which I tried to describe.

TC,

I'm starting to think that something like a Datsun 1200 ute would be the ideal vehicle for my conversion. My requirements:
-(relatively)High GVM
-Cheap
-High GVM/chassis weight
-narrow wheelbase (needs it for the front end, as the rear is going to be near-Suburban width).

I suspect that's the only thing that really has everything covered. Although, since I would be planning on majorly modifying the truck to convert it to an aero shell, I suspect it might be good to get a cheap capri just to play with, as the existing shell should be able to fit inside an aero shell, whereas the 1200B might be a fraction too high.

Ultimately, I want to see what is possible with the aero. I think it should be possible to make something that is at least as practical as an econobox, or nearly so. Certainly as practical as a two seat sportscar.

It should be as good or eclipse the performance of a standard car.

It should have a 300 mile range.

It should do it on Pb Acid batteries.

As such, the only logical way I can see to do that is to concentrate on building the ultimate aero shell, with a few concessions to practicality. And then put in enough batteries until it goes a distance.

Here is the Datsun:
https://www.turbophile.com/feature_ca...0-ute-back.jpg

Here is the Ford Capri:
https://www.tonysauto.com.au/mediumimages/Capri.jpg

This site shows how it might be accomplished:

Mighty Mira: Did you ever get my etcha-sketch of the car top that I tried to describe. I thought I had attached a .bmp file with it, but I can't seem to locate it?

Yes, I did... although I have absolutely no idea how it managed to improve your fuel economy. It should both increase your frontal area and increase your Cd. I'd like to see some ABA testing on it with an instantaneous FE meter for verification.

I think the mpg gains he mentions are relative to mpg with the box on top....so the added airfoil shape doesn't increase frontal area that much?

The gradual curve at back helps the air to flow down into the low pressure area?

I'd really like to incorporate this into a camper I'm going to build....problem being how to make the curved shape.

It looks like it increases the height and therefore the area by roughly 15%.
Hmmm. I would expect to see flow separation not much further back than the maximum height of the design... and virtually the whole rear of the thing being a vacuum.

I wouldn't anticipate that reverse airfoil shape doing anything more than it does to a smart fortwo (Cd 0.37).

Of course, I am willing to be proven wrong. I'd just like to see some independent verification, that's all.

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.

https://www.xmotorsportz.com/projects/F350super/F350.jpg

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.

If I had some way of building a car from the ground up and range was my goal, I'd attempt to build a luxury car on an F350 chassis with lowered ride height.

Imagine being able to keep the frontal area around 25 square feet, and get around a .16-.18 Cd. You'd be able to load up with around 3,000 pounds of lead acid batteries in a car that would weigh around 4,500.

Using your crude method of estimating highway range, it would get 209 miles per charge at 70 mph with a .18 drag coefficient, 236 miles per charge with .16 drag coefficient.

Now we're talking! An energy-efficient Cadillac, if you could ever envision such a thing.

One thing to consider is that on a gradient, vehicle weight will become the dominating factor in your losses due to the dramatically increased rolling resistance. But most driving is relatively flat.

What's so crude about it??? :D

The only thing it discounts is rolling resistance, which is going to increase (but not dominate). Other than that, you basically have the range.
Huh? As far as I am aware, given the same weight, gradient should not affect friction from rolling resistance. Of course, range will be limited by battery technology, after some gains by maximizing battery weight/total vehicle weight.

BTW I figured out how to prototype the ULCDEV (Ultra Low Coefficient of Drag Electric Vehicle). Get a capri, get some pine boards, some clear plastic sheet and some rubber sheeting. Lay down rubber sheet over the top of the body. Lay the pine boards over the top and under the bottom of the vehicle. Bolt them together so that friction binds them but the rubber protects the paintwork. Construct a framework. Attach the plastic sheet. Then take it for a coast down test.

That should give you Cd*A.

Then you can determine the weight of batteries required, using my "crude method".:D

"Hmmm. I would expect to see flow separation not much further back than the maximum height of the design... and virtually the whole rear of the thing being a vacuum.

I wouldn't anticipate that reverse airfoil shape doing anything more than it does to a smart fortwo (Cd 0.37).

Of course, I am willing to be proven wrong. I'd just like to see some independent verification, that's all."


What the whole thing tells me is that an increase in frontal area is not that important IF you have a very gradual slope...and that the low pressure area at the back is the big issue as far as reducing drag?

Main thing is to get a good flow of air into that low pressure area?

For a camper (espec one used off road) a boat tail might be a little awkward...so you need to find what helps flow relative to a basic box? Think over the road trucks.


DIY wind tunnel?

* old vacuum cleaner

* make a tunnel accessible from the side

* make a sliding chassis in the tunnel to afix the models to...with a sensitive "weight scale"

* run some tests?

I've seen rolling resistance expressed as the force rolling + force gradient.

Force Rolling = Coefficient Resistance * Weight

Force Gradient = Weight * Sin(Angle)

There are other factors as well, but this is a simple experession. Force rolling does actually vary with speed due to the coefficient of resistance changing with speed, although at normal highway speeds the changes are negligible from city speeds.

Yup.
Yup.
Well, for one, if you can't do a boattail, then the only other real option is to make the front contoured.

The trouble with going small is that you need to increase the airspeed until the Reynolds numbers match. A vacuum isn't going to cut it. However, if you can go full scale with something that you can sell for near the price you paid (or might want anyway), spend <$200 in materials, you really aren't that badly off. Certainly cheaper than renting wind tunnel time.

Ahhh, I thought that's what you might be thinking.

I don't normally like to group the two. One you lose to heat forever, and the other you have whenever you have to come back down the hill. If the energy is lost it is due to (having to) choose a route that forces you to brake instead of using EOC or at least coasting.

However, it will certainly make a difference if the only reason you need the range is to visit great aunt Betsy who lives in Boulder 300 miles away, and your EV was configured to make 300 miles exactly.

I disagree with this. There are many options less effective than a boattail...but better than a square blunt back end.

For a camper or truck used for hauling "stuff".....the basic cube form is most functional...espec for the truck.

Since the truck essentially needs to be functional first...you have to work the aero stuff around that.

To custom build a camper...I have more options...but the more I try to go aero...the more complex the job is and the more problems I'll have with structural strength.

If the reversed airfoil idea is sound...it is doable. Sure would like to see some research on it though.

For a camper/truck shape, seems like Mercedes Benz has already done your homework. Copy the Bionic car.

Cd .19

https://www.ae-plus.com/Car%20compani...05C2472_04.jpg

When I built the cartop, my impression was that it was primarily the frontal area breaking into the wind, which created most of the power requirements for moving a vehicle forward. The fact that this reverse airfoil shape which technically increases frontal area, actually caused a very large loss of power required to move the vehicle forward, made me reconsider some of that as a unique, high factor.

From what I could tell, it causes the air to start moving back down so that it is moving towards the back of the vehicle, where their would be a void of airflow, causing that to be freed to some extent. Before the airfoil shape, if I drove down a dirt road the back window would get all dusty. After the airfoil shape the window would be comparatively clear, with almost no dust on it.

I've wanted/planned to try to put this shape on my van, my wagovan and so forth, but I can't seem to catch up to myself. It would be really helpful to have a scantool type of device to test it with, as well, but at the moment I don't have the means to acquire one, so I am stuck with full tank testing, which has some degree of variability to it. If someone wants to build a shape to try, I built the first one out of cardboard boxes and duct tape, "which is the real reason the shape worked, at all, of course!" and I drove the car home from Death Valley and got the similar results.

I built mine out of a piece of 1/8" luan board. I made the sides out of 3/4" wood and routed a grove into the side of the wood, to hold the luan. The sides hold the wood and the bend siffens the luan, so that it becomes a very light, very stable shape.

I know it seems to go against what you might expect, but it did work and that's why I shared it with you, for your consideration.