Perfect Base Car For Super Aero Car/EV?

[MetroMPG]

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

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.

[Mighty Mira]

Originally Posted by MetroMPG

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:

http://privatenrg.com/#RearWheelSteer



Just FYI...

Thanks for the link! That's very cool.

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

Originally Posted by Mighty Mira

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.

[MetroMPG]

Originally Posted by Mighty Mira

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.

Going to bed now.

[Mighty Mira]

Originally Posted by MetroMPG

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.

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.



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

[Mighty Mira]

Originally Posted by MetroMPG

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

Going to bed now.

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.

[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]

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.

[The Toecutter]

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.

[Mighty Mira]

Originally Posted by The Toecutter

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.

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?