CarBEN EV5 Full Sized Prototype Construction - 5 Seat Electric Car From Scratch

[Vekke]

One layer is nowwhere near enough. He is using twill and if the weight is around 300 g/m2 6 layer is 1,6mm so one layer is ~0,27-0,35mm. Now imagine that you have so thin layer of glasfiber and you need to start sanding it smooth. Although the foam looks pretty smooth in the pictures there will come lots of holes to the capsule when you start sanding and adding plastic padding.

Holes in the capsule means water can get inside the structure and it will freeze during winter and cause lots of other problems. One layer might keep the body together if you driven nicely but if you want the rigidy of what regular cars have you need about 1.6-2mm wall thickness. plus extra on corners. You have to think about surviving in crashes althought that is not the main purpose of the vehicle, those can happen from many reasons.

I did my final thesis of that Citycab self supportin carbon fiber upper body



ERA has full carbon fiber monocoque. which was next car after Citycab and next after ERA has also similar structure.



On my lupo project I did a hood with 2 +2 layers of 300g/m2 twill and its flimsy. Its not the same as having that foam inside when you would have basicly 4+4 layers.


When using hand layup the structure wont become so stiff as when using vacuum pressure to suck the resin and pressure to push it againts the tool.

[NeilBlanchard]

Right, I'm planning on using fiberglass both on the inside and the outside. Several folks here on EM have used similar glue to what I am using with fine results. I will test it, and from what I have seen from the glue as I've used it so far, it will be fine.

[Vekke]

Have you done any strenght calculations to the body?
I might be able to try make some calculations if needed...


That rear trunk looks like it could use a support beam between rear roof corner. So some beam which would link those side panels together. It could be in the trunk so that would lock itself to those sides when in closed position.

[Rokeby]

Neil,

Even thought the major stresses/loading of you car will be carried by a steel frame, The long term integrity of the body cannot be overlooked.

You can think of a 'glass-foam-'glass structure as similar to an I-beam. All bending loads are resisted in the two 'glass skins. All the foam does is maintain separation between the skins which handle tension, compression and torsion.

The integrity of the structure is greatly compromised when either skin is broken or dented/kinked. While a relatively thin skin may be sufficient to handle pure compression and tensions loads, the skins have to be substantially thicker to provide puncture/tearing and or denting. The each skin has to be thick enough to resist puncturing/tearing on it's own. There is essentially no contribution to puncture/tearing by the foam.

Cracks/tears in the external skin are especially troublesome. In the short term the strength/integrity of the structure is compromised. There will be localized points of high stress which will lead directly to slow continuing tearing due flexing and vibration. Even if the skin does not tear further, it is almost a certainty that there will be an area where the skin and foam are no longer connected.

Any crack, tear or puncture, no matter how small, that allows water to penetrate through either skin has to be prevented. Once water gets into the structure it essentially can't be removed. As as result, the structure gets heavier over time. By itself, water will cause delamination of the 'glass-foam-'glass structure. Over time, small amounts of water will travel throughout the structure. When exposed to freezing temps it will turn to ice which will pry the laminate apart. In high temps, it will vaporize, turn to steam, and delaminate the structure. (Temps in the "micro-climate" at the surface of boat hulls painted dark colors on sunny days can exceed 250 degF. That's why boats are usually painted white.)

I think that you should be thinking of skin thicknesses of at least 1/16 in. and perhaps even 1/8 in. on the outside.

Books about cedar strip canoe/boat building can give you a preview of methods to get relatively consistent resin application in hand built laminations. To get a full=strength chemical bond between different layers of epoxy/cloth, the following layer has to be applied before the earlier one is fully cured… just a little bit sticky is fine. It's going to be a real challenge to do the inside. You might even end up cutting the body into upper/lower or left/right halves to get full/free access to work.

I am not saying that a 'glass-foam-'glass laminated structure won't or can't work. It surely can, but it has to be done very well and the very last thing you want are skins that are too thin to handle not only the routine expected loads, but considerable additional thickness, just in case.

Rokeby

[Jyden]

Originally Posted by Vekke

One layer is nowwhere near enough. He is using twill and if the weight is around 300 g/m2 6 layer is 1,6mm so one layer is ~0,27-0,35mm. Now imagine that you have so thin layer of glasfiber and you need to start sanding it smooth. Although the foam looks pretty smooth in the pictures there will come lots of holes to the capsule when you start sanding and adding plastic padding.

You forget he's making a sandwhich construction. And the fiberglas should NOT be sanded down. Wowen cloth should be used, and filled with epoxy, and then the epoxy sanded smooth.
You can NOT compare a sandwhich to a monoshell. Sandwich is MUCH stronger.

[3-Wheeler]

Originally Posted by Jyden

You forget he's making a sandwhich construction. And the fiberglas should NOT be sanded down. Wowen cloth should be used, and filled with epoxy, and then the epoxy sanded smooth.
You can NOT compare a sandwhich to a monoshell. Sandwich is MUCH stronger.

Yes, the sandwich construction should not have to be sanded much at all after layup and such.

I found on the tail, and a motorcycle fairing made 20 years ago, that the foam should be as smooth as needed for a "show car". That is the secret to light-weight fabrication. Get the foam smooth first.

Then start the lay-up. Use any technique to keep overlapping layers of cloth very flat with hardly any finish sanding required.

Use epoxy/micro mixes to fill in low areas. Do not sand away at the glass lay-up, as that is where all the strength is.

Use multiple layers of glass at any mounting points to the frame, as this is where higher stresses are.

Look closely at the many pictures of my tail construction project.

Neil is correct, in the fact that he is maintaining the foam as a composite structure in the overall body construction. Removing the foam would result in a much weaker and heavier structure if reinforced with more glass, due to the absence of the foam.

Note: there are airplane designs out there that make use of fiberglass composite techniques. These airplane structures are found to out-perform the old aluminum skin/riveted/ribbed structures similar to the Cessna series planes built many years ago. A properly executed foam composite structure is actually stronger and more resistant to buckling than the aluminum structures.

I have a DVD video of a Burt Rutan designed wing shape being placed on two blocks and being stood on by a full size male and the fiberglass out-performs the aluminum in this case.

Jim.

[2007 ion2]

niel, epoxy isn't simply a "glue". the epoxy resin (key word resin) hardens to form plastic. a glue is simply meant to hold two surfaces together, and lacks any strength independent of the material being glued together. epoxy is used because, even though you still need to use care to leech out as much as possible to attain optimal strength, it properly bonds with the fiberglass, carbon or kevlar fiber, as well as the substrate to form a proper structurally sound layer.

I agree, he would need much more than one layer to allow a: sanding without loss of too much material in the finished product, b: enough strength and rigidity to function as a car, and c: a surface that can easily be repaired in the eventual case that damage occurs, and resist damage.

[NeilBlanchard]

The glue that I want to use will be able to impregnate the fiberglass. Last year I made a wheel cover with an old sheet (flannel) and this glue, and it has help up very well so far.

I will be experimenting with smooth XPS foam and also with a smooth surface that is then scored and see if that has noticeably better bond, and if the surface can be smooth. If I need to do peel ply then I will - but folks I am not necessarily trying for a super smooth surface. The most important thing for this prototype is to see how efficient it is.

I will likely make the next one quite differently; either with aluminum honeycomb sandwich board as the interior surfaces with foam added to form the outer shape; or this car will be used as a plug to make molds, and then make body panels that go around a metal structure.

The main challenge I have is securely attaching the suspension and subframe(s) to the fiberglass composite sandwich. The can be metal reinforced (possibly inside the main battery channels in the floor) but if it is strong enough with just the suspension through attachments and gussets, then I'll test it that way.

This foam is pretty strong stuff - it is about 25psi compression, and it flexes under bending up to a snapping point. The glue is actually somewhat flexible when dry, too, which I like. The fiberglass will make this car very strong in bending and torsion. It is only really the point loads at attachment points where I need to be really concerned with. The suspension will need to cradle the chassis, and I think it will be the hardst in the front.

[Jyden]

Regarding attachment points. The way to do it, is to make a slightly bigger hole than needed. Fill it with resin and some filler, like sawdust, and when hardend, then make the hole in the resin thats fills the hole. The foam has no strenght when it comes to attachments. The arear can be made stronger with extra glass all around to distribute the forces.

[Galane]

Originally Posted by Vekke

When using hand layup the structure wont become so stiff as when using vacuum pressure to suck the resin and pressure to push it againts the tool.

"Vacuum pressure"? What's that? No such thing! There's vacuum OR pressure.
Which it is depends on if it's relatively higher or lower than ambient pressure.
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I'm with the rest on not relying on glue to provide structural strength with fiberglass. I'd paint a coat of epoxy over the foam first, let it cure, then sand it to reveal any low spots. Mix more epoxy, add microballoons and use that as filler. If you're going to use this as a plug to make molds, save yourself major headaches by making it as fair a surface as you can now.

For attachment points you'll want the glass and resin quite thick and gently tapered to the thickness of the main skin area. Metal tubes or threaded inserts are also a very good thing for attaching stuff to composite structures.

Don't make sharp corners in stressed areas or near areas subject to vibrations.