Quote:
Originally Posted by drmiller100
Back seat seating. Yes, there are issues without great solutions.
Do I want the car narrow? Yes.
Do I want a 1.5 inch roll bar above the driver? Yes.
Do I want a back seat? Yes.
Do I want side impact protection? Yes.
Do I want to build a custom door? No.
Do I want the front door to open inside of a standard parking spot? Yes.
Which one of the above am I willing to sacrifice?
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Narrowness: You definitely will not want to narrow it at the front, because if you do, you will have to make the suspension brutally stiff to have enough roll resistance for good handling. (And as you point out, narrowing the powertrain is effectively out of the question, anyway.) Even with its incredibly wide track and (eventually) lowered stance, the Aptera rolled too much with suspension geared for comfort. Just to get through a simple handling test that any sedan can pass easily, (in the X Prize) they had to stiffen the suspension. The T-rex has much wider track than you do, for this reason.
BTW, don't repeat all the Aptera mistakes with suspension geometry. Unless you intend to give your wife a wild ride, go for consistent understeer. That means the front wheels must tilt at the same rate as the rear wheel as the car corners. The original Beetle geometry is good in this respect. Ditto 2CV and DS series. The Civic geometry would be expected to cause dramatic oversteer, when applied to a car in which the rear wheel tilts adversely under cornering loads.
1.5 inch roll bar: This should be 1.75" .120 DOM. Even at that, it will need to be triangulated to function correctly. If you figure a 4 G load, and a loaded weight of 2000 lb (1500 empty vehicle + 500 passengers and groceries) then you need to resist 8000 lb at the center of the roll bar. You can test this with an instrumented portipower. I'd guess that your current structure will permanently deform easily at fraction of that load. The narrowness of your cabin helps, because the span from left to right is less than it otherwise would be.
I gather you plan to provide roll protection for the back seat person too. The current longitudinals are too long to have meaningful stiffness in bending, so you will put a roll bar behind her head, I assume.
Side impact protection: This is another reason to use larger tubing. Loads in side impacts go up to about 50 G (almost 100G in the worst-engineered cars). Engineered cars have very sophisticated structures that are impossible to match weight-wise with a ladder frame. (This is one reason that some SUVs and trucks have had atrocious side impact protection.) A properly designed fully-triangulated space frame can be both light and strong, but encroaches on passenger space. In most modern cars, even the windshield glass is used as a load bearing element, providing both torsional and bending stiffness. Monocoque is hard to beat.
I'm sure you do not fall into this category, but many people think that a fiberglass body hung on a frame makes a light structure. The old Corvettes show that this is not the case at all, with a mid 70's Corvette weight of about 3500lb. This is unbelievably heavy for such a small two-seater car. (The five seat Accord of 1976 was about 2000 lb, due to its safer and more sophisticated structure.)
Custom Door: I agree. Building a door is a hassle, and the doors from your hatchback are already longer, if I recall, than those for the sedan. Getting into your back seat should not be more difficult than for the Civic Hatchback it you keep the door location relative to the front seat tracks the same. The Civic already has about the minimum headroom, so you may need to raise the door sill and roof height somewhat (or lower the floor to a point below your shop floor). No need to reinvent the wheel here: I'd just duplicate the arrangements in the Civic: the hatchback version was fairly low outside, if I recall, and rear seat headroom was just barely adequate. An extra few inches height will mean almost nothing in terms of aerodynamics, but will ensure marital bliss.
The stock hatchback should have a (structural) advantage in length, because the rear wheels can be further forward relative to the back passenger backrest. This, in combination with the larger circumference of the hatch back body, gives the hatchback a huge advantage in weight-to-stiffness ratio. If you are willing to put up with some handling imprecision and creaks and stress cracks in the body, perhaps you can match the weight of the Civic. Otherwise, the expectation would be that your vehicle will be heavier, due to the inefficiency of a long, thin structure, and the lack of monocoque advantages. But even matching Civic weight will be an accomplishment to be proud of, with a home-built car.
It will be interesting to see what you have planned to avoid wakes behind the front wheels. (Perhaps Aptera style wheel fairings... you might need to park with the wheels cocked to one side to clear the door?) Going from full Civic width to the door width (within only a few inches of car length) without creating huge wakes will be a challenge. Perhaps the door hinge could be out where it was in the Civic, but the doors could taper a bit more toward the back of the car. If you can somehow handle the transition from front width to side width, you may be able to get better-than-Civic aero. It would be interesting to know if the dimpling on the back of the Corbin sparrow fenders actually had any effect in keeping the flow from being completely separated and turbulent -- I doubt it. But perhaps you can dig up CFD studies on it -- or put a Sparrow in a wind tunnel... or borrow one to drive around with tufts. I have a couple friends that have them, so let me know it you are thinking along those lines.
I'm really impressed with the progress you've made! It will take really extensive engine mods to get close to 50 mpg combined (as measured today), but you will have a very cool car, nevertheless. The current Accord V6 is variable displacement, and that can help a little -- maybe 1 or even 2 mpg, vs standard VTEC. (The variable displacement system on the Accord includes audio from the sound system to cancel the odd sounds of an engine running on few cylinders... pretty cool stuff.)
It's already starting to look like a car! Very Cool.