The single entry is probably the most controversial feature of the CarBEN EV -- it has to do with weight savings and surrounding safety structure.
It's not like the the benefits aren't well worth the minor sacrifice: the CarBEN EV could well be the most efficient car yet made, and it could be one of the first electric cars to have a range of 400 miles (or more) on a single charge. If I was able to take part in the X-Prize, the CarBEN EV would have held the most people of any car in the contest. It might have a Cd under 0.14 and weigh less than a ton; hopefully less than a ton with the driver onboard.
I'm serious about these goals, and I have to make choices that save weight, while not diminishing safety, and yes, body gaps add aerodynamic drag. The Bionic increased the Cd from 0.095 (the early blue clay model that I am starting with) up to 0.19. The main reasons for much of this increase is the uncovered wheels and the cooling for the diesel engine.
Since about 97% of all accidents involve impacts on the front and sides of the car, I want to have maximum protection in those areas.
Since a square encloses the most area with the least perimeter (except for a circle, which is hard to make work for a car), the best shape to make a car with a given frontal area, and get the most usable interior volume, the Mercedes Bionic/Boxfish model provides an amazing opportunity: it combines an amazingly low coefficient of drag (Cd) in a shape that is nearly a square in the frontal area. This makes it possible to have comfortable seating for 5 people in a car less than 14 feet long. A compact car can be much lighter and stronger, and still keep the frontal area down to ~25 sq ft (2.323 sq m). If the Cd of CarBEN EV is 0.14, then the effective frontal area (CdA) is 3.5 sq ft (0.325 sq m). And it is possible to get the Cd as low as 0.11 or so, and that would lower the CdA to 2.75 sq ft (0.255 sq m).
These would be unprecedented drag numbers for any car, let alone one that seats up to 5 people. Having an electric drive train also contributes a lot to this packaging efficiency: the electric motor is much smaller than an equivalent ICE and it's transmission (an electric motor only needs a reduction gear -- or can be direct drive!) and they need just a fraction of the cooling air flow.
And here's one of the reasons where the aero and the aero shape enter in: since truncating the back of the shape (called a Kamm back) makes the vehicle makes it much more practical, and has a very small increase in drag (and the Boxfish model achieves it's staggering Cd of 0.095 with a Kamm back), and this is where a small fraction of the accidents occur anyway, this is where I chose to put the main entry door.
Side doors add weight and reduce the safety; by cutting big holes in the structure (think about a large box beam web) which then has to be reinforced all around the perimeter, and the door itself has to have a similar frame all around the perimeter, and you add the hinges and if you want to have as much strength as possible, you need 2-4 latches (instead of the usual 1). Adding the latches, means that you gain back some/much of the strength you had with no side doors, but it will weight more.
Since I would need a rear hatch door *anyway* if I put in a side door; I can save a lot of weight and get the safety even better than most cars.
Another aspect of the aero that affects many aspects, including the seating arrangement: the tapered shape required for ultra low drag means that conventional rows of seats is not the best way to fit everything in. Since the electric motor is so compact, the driver can be moved forward between the front wheels, opening up more room. And the staggered seating means that even more legroom is available by angling your legs off to the side. So, the CarBEN fits 5 comfortably, in a package that most cars fit 4 less comfortably. The mesh seats are also a big part of this.
I think I've shown that the choices I've made so far, are aimed at achieving unprecedented ultra-efficiency, in a compact, very practical people moving machine. Since the most import part of that function is just that: moving people with safety, the small inconveniences of slightly more effort getting in and out of the car are more than offset, if I can get anywhere near the performance I think are possible. Form follows function, and I think the CarBEN EV can function at a very high level, indeed.
As Oliver Kuttner says: you must get the physics right to get to higher efficiency; and all design choices affect the efficiency. Using less energy is my focus, and that is where I cannot compromise.
After I get a prototype and running, I hope to experiment with rigid wheels and solid (non-inflatable) tires and regenerative shock absorbers. The solid tires and rigid wheels could be much lighter weight (which counts double to weight losses anywhere else), and they could have vanishingly low rolling resistance, and they would pass along most of the energy to the regenerative shocks; making their effect greater than it would be with conventional tires.
The ride quality could actually be better than with conventional tires, since light wheels makes the system more compliant (they move rather than moving the car), and the suspension can be fully tuned and damped to match the wheels.
This could help get the energy consumption even lower than 100Wh/mile, and that could extend the range; as well as recharging the batteries (a bit) from the energy regained from the shock absorbers (instead of wasting it as heat). Every little bit counts.
I have been looking at your design some more and have a few questions and concerns. I used the 2-D drawing you have posted to create a quick solidworks model of just the tires. First, what tire size are you planning on using? From the drawing I got that they are 24.5" OD and 6.1" wide, thats a pretty tall and skinny tire, would be a hard to find size.
The other thing I noticed was the fact that the rear track width is less than the front, and less than 48". From a vehicle dynamics stand point, less track in the rear would tend to promote more oversteer, or drift. The extent to which this could become a problem would be dependent on the suspension setup. The seating arrangement would also complicate things because there are 3 seats on the left and only one on the right. This would mean the the left side's suspension would need to be stiffer to compensate for the uneven weight distribution, when loaded with 5 people. All this could add up to a vehicle that handles differently turning left vs. right. In any case a safe car is a predictable car when it comes to handing.
The real question needs to be what type of suspension are you thinking about for both the front and rear? Do you plan to make these components yourself or try to use existing car parts? I like your plan to build a steel tube frame and fiberglass body, it should make it much easier to get a prototype rolling and also keep things cheap. For the regen shocks you talked about, where can you get them? I've only heard a few stories about prototypes being made, but nothing in production.
A lot of questions I know, but I want to help things move along, cars aren't built on Cd alone.
Thanks for the questions. The tires are intended to be 15" 185's, though in the end, I hope to make my own rigid wheels with solid (non-inflating) tires. So they are close enough to stand in for now.
Yes, the rear track is narrower -- like the EV1 and Dave Cloud's Dolphin. This is the only way to drop the Cd below 0.2, I think? From what I understand, a tadpole trike would handle really nimbly, so the answer may well lie in getting the correct suspension geometry. I'm assuming at this point that the front suspension will be double wishbone (to avoid the height of a strut) and the rear would be independent trailing arms, without a beam axle. I've seen these set up with multiple linkage so that they are anti-squat and you can actually get passive "steering" out of the back suspension. But, I have not gotten into the details, yet. The aerodynamics are at the top of my priorities, and I have to figure out how to best build it!
I thought I put in a bit on the asymmetrical seating -- I've answered a similar question on Facebook earlier today. Basically, the most the weight would be unbalanced is about 300-350 pounds (the two "extra" seats are for shorter adult/kids), and that weight is on the higher part of the road crown; and away from the much rougher right side edge. I've only ever had to replace wheel bearings and the like on the right side of any of my cars. The battery pack in the floor is 800-900 pounds, and since most cars have the driver on the left -- and most often the driver is the only person in the car; so, most of the time the CarBEN will be in total balance! On the other hand, most cars are usually out of balance by up to 250 pounds (or more).
There is a company right nearby in Cambridge, MA called Levant Power, that was started by folks from MIT, and they make regen shocks that have micro turbines and little electrical generators in each shock -- and not only do they generate power, they are tunable. If they had ride height/leveling that would be very helpful for an aerodynamic car like CarBEN.
Okay, I now have all five 3D figures in the model, and the two passenger seats right behind the driver are splayed slightly to fit their legs in, while still having decent headroom. You can see this best in the plan section:
Longitudinal section views:
Do you want a copy of the SketchUp file? I may try to bring it into Banzai 3D (FormZ in new interface) and see if I can get a solid model.
These are the only pages I've seen that are so overloaded with data that my system chokes. Even with automatic resizing, I only get the top portion of most pictures.
What's the reason for the uneven weight and passenger distribution ?
With one passenger moved further forward, the roofline could come down lower and/or become narrower near the end, reducing the size of the wake.
Within the same dimensions, you could provide more leg- and elbow room for the passengers by seating them on both sides and staggering the seats.
What I'm missing in your design is dedicated luggage space.
Having luggage laying around in a car adds to the injuries in an accident.
So it needs to be stored somewhere secure / separated from the passengers.
You don't want side-doors as they introduce weak spots requiring heavy reinforcements but the huge gap in the roof and rear will require the very same reinforcements.
Loremo did something similar, but it has a rather large structural box between the front and rear seats up to above the lower window line.
With a heavy battery pack, no matter how low it's positioned, the structure must be strong enough to be tipped over or even rolled without too much damage.
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Last edited by euromodder; 09-30-2010 at 08:50 AM..
The asymmetry is so that there can be an aisle to get in from the back door. Also, since the driver in in the center that seat can be moved forward, and that ripples back through the seating plan. The tapered sides make it very hard to fit seats side by side -- even if they are staggered. The seat on the right side is staggered and no additional seats could be fit behind it.
Yes, the hatch will require reinforcing, but it would be in the car even with side doors, and the most likely crash impact zones are the front and sides. Your point about the Loremo is apt -- since that car was very low, and they also wanted to surround the passengers with a continuous structure, they were left with the option of (essentially) entering through the roof. The CarBEN entry is through the rear transom and the roof; since the height limitation is not as restrictive.
There will be at least one "hoop" of a roll cage just behind the driver, and it will be supported by two tubes that run from the floor up to the roof. Another advantage of no side doors is the window pillars only span the windows themselves, making them much stronger than in most other cars. So the roof on CarBEN will be quite robust, I think.
Think about the 'B' pillar on a typical 4 door sedan: it alone spans from the floor to the roof, with no stiffening panels. In the event of a side crash impact, that one pillar takes virtually all of the force. Even with the anti-intrusion bars in the doors, the force exerted on them transfers as point load onto the 'B' pillar.
The anti-intrusion bumpers (there are probably going to be two) in the sides of the CarBEN are continuous all the way from the back of the car on one side, around the front and continuous to the back of the other side. These will be supported by the roll bar(s) up over the roof, and the front will have a subframe crush zone. The floor will have to be a box beam, around the battery pack and this supports the base of the roll bars.
Here's some images of the battery pack I modeled today (all 768 cells!):
This would be about 55kWh! :-O I'm thinking a few cells cane be moved from the very back and added to the side flanks? Or, I'll worry about the exact layout when I get the chassis built...
As I see it, the rear entry is actually one of your most expensive features in terms of aerodynamic drag.
Adding one side door and switching to three rows of side-by-side seating would allow you to slice a foot out of the middle of the car - absolutely huge from a frontal area perspective (six square feet!), and from a Cd perspective as well.
And again, there's no need to allow the batteries to contribute to the frontal area. I see room to remove a few square feet of frontal area there.
It looks to me like these two basic changes could cut your aerodynamic drag by about a third! Don't you agree?
The asymmetry is so that there can be an aisle to get in from the back door.
Looking at the enlarged plan view, if the rear seat was moved to the right and the seats were to be spaced further apart (lengthwise), I figure you'd still have a zig-zag aisle wide enough to pass through.
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There will be at least one "hoop" of a roll cage just behind the driver, and it will be supported by two tubes that run from the floor up to the roof.
CarBEN can have A and B pillar / ring structures like any car.
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Think about the 'B' pillar on a typical 4 door sedan: it alone spans from the floor to the roof, with no stiffening panels.
Yes, but it is a "circular" load-bearing structure going all the way around the car.
The B-pillar is helped by the C (and D) pillars in a sedan / station wagon / van design.
In CarBEN, the huge rear hatch opening prevents any such continuous structure behind the hatch's hingeline, and hat's going to cost you in weight and reduced stiffness.
In order to make the roof robust, these structures will have to be continued into the hatch, with an interlocking system locking the hatch to the sides.
That's going to be heavier than a continuous structure, and the extra weight is going to be high up on the vehicle.
There are huge pay-offs in flexibility from having multiple entry/exit points.
In a tail-end collision, your passengers are effectively blocked-in.
It could also become a problem in a low parking lot.
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In the event of a side crash impact, that one pillar takes virtually all of the force. Even with the anti-intrusion bars in the doors, the force exerted on them transfers as point load onto the 'B' pillar.
And from there on the load spreads all around the B-ring structure and to anything connected to it.
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The floor will have to be a box beam, around the battery pack and this supports the base of the roll bars.
The floor box would actually be a good structure where you could offload impact forces from the B-pillar ... or from stiffeners around the side-doors.
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I'm thinking a few cells cane be moved from the very back and added to the side flanks? Or, I'll worry about the exact layout when I get the chassis built...
Keep an eye on weight distribution early on in the design process because it gets more difficult to correct later on.
Don't get me wrong, I like your idea of designing a radically efficient and different car - and I like it better than the Miastrada design (sorry Jim ) as it is far more down to earth. I think that even in this form, it's also more practical than the cramped Loremo.
But don't make a design radical just to be radical.
There has to be a really, really good reason for any radical design feature to catch on.
The shape is radical with good reason, and means the traditional seating pattern is out - but the chosen access option is taking up an enormous % of the cockpit floorpan area (roughly 30% maybe ?) and luggage will interfere with access.
Design ideas :
- drop the huge hatch idea
- add side doors abeam/in front of the front passengers
- rearward sliding doors would hardly protrude from the body as the doors are at the widest section, and the design narrows going aft.
- move the rear seat to the right and further forward, though keeping the staggered layout as it reduces width.
- provide access to the rear seats between the front passenger seats (driver and front passengers already have to get inbetween these seats in the current design)
- the above will provide ample dedicated luggage space at the rear, and an even greater useful volume when the rear seats are made to be stowable in the sides or floor (could interfere with the batteries) ; alternatively, the design cold narrow or slope down even more at the rear if that's aerodynamically useful.
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) as it is far more down to earth. I think that even in this form, it's also more practical than the cramped Loremo. 
