Perfect Base Car For Super Aero Car/EV?

[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.

[The Toecutter]

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.

[Mighty Mira]

Originally Posted by The Toecutter

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.

What's so crude about it???

The only thing it discounts is rolling resistance, which is going to increase (but not dominate). Other than that, you basically have the range.

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.

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".

[ZugyNA]

"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?

[The Toecutter]

Huh? As far as I am aware, given the same weight, gradient should not affect friction from rolling resistance.

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.

[Mighty Mira]

Originally Posted by ZugyNA

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?

Yup.

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

Yup.

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.

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

* run some tests?

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.

[Mighty Mira]

Originally Posted by The Toecutter

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

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.

[ZugyNA]

Originally Posted by Mighty Mira

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

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.

[MetroMPG]

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

Cd .19

[Gary Palmer]

Originally Posted by Mighty Mira

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.

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.