Perfect Base Car For Super Aero Car/EV?

[The Toecutter]

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

[Mighty Mira]

Originally Posted by The Toecutter

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?

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.

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 Toecutter]

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.

[Mighty Mira]

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???



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.

[Gary Palmer]

Originally Posted by Mighty Mira

Could you post a diagram for that?

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.

[Mighty Mira]

Originally Posted by The Toecutter

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?

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:


Here is the Ford Capri:


This site shows how it might be accomplished:

[Gary Palmer]

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?

[Mighty Mira]

Originally Posted by Gary Palmer

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.

[ZugyNA]

Originally Posted by Mighty Mira

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.

[Mighty Mira]

Originally Posted by ZugyNA

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?

It looks like it increases the height and therefore the area by roughly 15%.

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

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.