Aluminum foil as ebike construction material ???

[John in CR]

So I'm building a bike out of 18650 batteries, and already have the strings soldered end to end. The plan was to roll them up in fiberglass cloth & epoxy. I've done the first 7p10s string, but fiberglass is a bit too flexy as a frame material. Sure I could go to an outer layer of carbon fiber, but can't source any locally, and I'm cheap.

Aluminum makes quite stiff frames, and at about a 3.5" diameter even very thin AL can be really stiff if you avoid the "beer can effect" and this will be solid core, so no beer can effect here. I was trying to plan how to make my roll up layer of glass nice and smooth anyway. Then I thought if I get one of those commercial rolls of aluminum foil, I could just roll the wet epoxy fiberglass log up in aluminum foil, say 6-8 turns thick, and it would add tremendous stiffness at that diameter. Plus that length would now be waterproof too. The AL wouldn't be the exterior layer, so I wouldn't have to worry about toughness of the AL foil, and I could just finish with a 2 turn layer of glass. The final glass layer would be starting on a nice smooth uniform surface, so no sanding beforehand, and still allow an easy smoothly finished result. Also, there would be no waiting for expensive and harder to work carbon fiber to arrive, though I will probably still need uni-directional carbon to make the strong end connections, but that's for later. I'll probably use these "batt tubes" in a different manner before building them into a frame anyway.

So what do you think, is aluminum foil used in this manner a good idea for an ebike construction material? If so, how thick should I go, since the solid filled fiberglass tubes will already be very strong and all I really want to do is add some stiffness? This also make construction even simpler, which is a nice plus for a lazy "easy way" type.

John

[999zip999]

Johncr that almost sounds like a tube battery I ask you about, but the string would have been long but could follow the triangle tube like a six shooter and I found some plastic gutters that 5p's snap into. But yea end to end 2p to series. Those A123's ?

[Chalo]

Glass is a lot stiffer than aluminum, for what it's worth.

[John in CR]

No, I thought about doing one with A123's, but the tubes would have been too long for 23s. These are self balancing Konion VT's all capacity tested at 1.3-1.35ah that I've been saving for this project. No snapping them in, they will be an integral part of the frame. Each tube will be 7p in a 3+4 stack 10 cells long with terminations at the end, and then 7p10s coming back, so I will have the choice of using each as either 14p10s for a 37V18ah battery or a 74V9ah battery, though I am setting it up for minimum extra wiring at 74V. The two 7p segments will overlap like bricks, so the cells themselves will add to the strength. I need stiff as possible so no stress is put on the tab welds. I'll have 2 of these tubes once I'm done for a nice fairly high power almost 1.4kwh battery that's the ultimate in stealth, since the top tube and down tube will be the battery.

John

[John in CR]

Glass is a lot stiffer than aluminum, for what it's worth.

If that's the case, then it's definitely not a good idea, and I'll just source some carbon cloth for the outer layer. I don't know though, the stuff I look at says aluminum is significantly stiffer, and this seems like an idea way to put that to use since the core of the tube is solid. Fiberglass is pretty darn flexible in tube shape...thing fishing poles, pole vaulting poles, fiberglass antennae, etc.

John

[dogman]

Suprised you haven't discovered the hot duct tape yet. Aluminum foil with heat resistant sticky on one side.

Perfect stuff for temporarily taping over the vent holes in a motor while you ride through the salt spray, among other things.

[kfong]

To get stiffness, get several layers of the glass. Be sure the weave is at 45degrees from the previous layer.

[Chalo]

Glass is a lot stiffer than aluminum, for what it's worth.

If that's the case, then it's definitely not a good idea, and I'll just source some carbon cloth for the outer layer. I don't know though, the stuff I look at says aluminum is significantly stiffer, and this seems like an idea way to put that to use since the core of the tube is solid. Fiberglass is pretty darn flexible in tube shape...thing fishing poles, pole vaulting poles, fiberglass antennae, etc.

Solid aluminum is stiffer than fiberglass/resin layup, but you're not talking about using solid aluminum. You're talking about using it in a layup, like the glass. I looked up the Young's moduli of aluminum and glass, and it turns out that because glass can consist of a whole lot of different materials, it varies from less stiff to more stiff than aluminum. The Young's modulus (inherent stiffness) of aluminum is 69 GPa. Glass ranges from 50-90 GPa depending on what's in it. Fused quartz (silica glass) has a Young's modulus of 72 GPa, or about the same as aluminum.

According to Wikipedia, the modulus of a 70/30% layup of glass and polyester resin, along the grain, is 40-45 GPa. That's probably starting with relatively high-modulus glass, so the construction cuts the stiffness of the bulk glass material by almost half. You can expect the same with sheet aluminum, depending a lot on how much resin you can squeeze out of the layup.

Aluminum may make stiff bike frames, but it's only 1/3 as stiff as steel. It makes stiff frames because you can make it into bigger tubes without a weight penalty, and bigger tubes are much stiffer. So maybe you should start with D cells.

[Jeremy Harris]

Chalo's summed it up with regard to the material's inherent stiffness, given as said by it's Young's Modulus (the higher figures being stiffer).

What you're looking for in a bike frame though is the section modulus, which is a combination of the material used and the shape and size of the section. There's also weight to be taken into account.

The section modulus is proportional to the area moment of inertia of the section, which basically means that large diameter tubes of any material will be stiffer than small diameter tubes. It's mainly the material right at the outside edge of a tube that contributes to stiffness, the rest just stops the tube from buckling like a beer can. This means that if you want the stiffest composite tube of a given size you want to use the highest Young's Modulus material right on the outside face. If you want to save money you can use glass cloth on the inner layers to provide the necessary buckling resistance, at the expense of a bit of added weight over using carbon.

It's worth remembering that square tubes are stiffer for a given maximum dimension than round tubes, although they will be heavier. Some of the box section bikes that have been built, with battery boxes forming a monocoque in the frame centre, are probably very stiff indeed when compared to a standard bike frame.

I've often thought about making a lost foam composite frame like this, with a big battery compartment in the centre. It's pretty easy to get hold of the pink or blue XPS insulation foam, carve it to shape, wrap it in epoxy glass and epoxy carbon fibre, then just pour petrol (gasoline) into a hole and melt out the foam core. This leaves you with a nice hollow structure with room for batteries, controller, wiring etc inside. You can bond in metal fixings for things like the steerer tube and rear fork/suspension mounts.

[Kingfish]

FWIW, the Electra Townie Carbon Fiber Plastic Fender Set (26- Inch) contains an inner layer of aluminum foil. This was discovered during some “modifications” I made when adapting them to my ride last year. I thought that it was very interesting – this core of very thin aluminum sheet (foil really) instead of entirely made of composite. Without question, these fenders are very durable and resist cracking, even when overtly cut.

Also, there is the added benefit of RF shielding… in case you want to hide yer wallet or sensitive electronics within the frame, and so on and so forth…

Foiled by yet another, KF

[Stone Meadow]

Solid aluminum is stiffer than fiberglass/resin layup, but you're not talking about using solid aluminum. You're talking about using it in a layup, like the glass. I looked up the Young's moduli of aluminum and glass, and it turns out that because glass can consist of a whole lot of different materials, it varies from less stiff to more stiff than aluminum. The Young's modulus (inherent stiffness) of aluminum is 69 GPa. Glass ranges from 50-90 GPa depending on what's in it. Fused quartz (silica glass) has a Young's modulus of 72 GPa, or about the same as aluminum.

According to Wikipedia, the modulus of a 70/30% layup of glass and polyester resin, along the grain, is 40-45 GPa. That's probably starting with relatively high-modulus glass, so the construction cuts the stiffness of the bulk glass material by almost half. You can expect the same with sheet aluminum, depending a lot on how much resin you can squeeze out of the layup.

Aluminum may make stiff bike frames, but it's only 1/3 as stiff as steel. It makes stiff frames because you can make it into bigger tubes without a weight penalty, and bigger tubes are much stiffer. So maybe you should start with D cells.

Yes, but John is not talking about "glass"...he's talking about glass fiber in an epoxy resin matrix. Big difference...HUGE.

According to this table, the Young's modulus for 50/50 fiber/resin is 25. That's about as good a ratio a rank amateur can get with pure hand layup, and only then if it's post-cured above 100+ centigrade. One can hit 40 only with unidirectional fibers, which I suspect John will have a tough time getting hold of in Costa Rica. Fiberglass has many useful properties, but high Young's modulus is not among them...

Last year my business built 1.5mm carbon fiber test coupons for a client that hit 150. Nice, but significantly short of the 190 minimum needed to replace a 316 stainless component. Still, 150 is over twice as good as aluminum...

[dogman]

Aluminum honeycomb was at one point very common in snow skis. The foam filled foil honeycomb replaced wood strips. Borrowed from aerospace for skis. Nowdays it's more likely to be a carbon honeycomb.

[John in CR]

Thanks everyone for your input,

While the other technical answers are also correct, I have to rate Stone Meadow's as the number one answer, so the foil wrap is a go, especially considering Kingfish's input showing that someone at a manufacturer had a similar idea.

Now the question is how much do I put. I think the package will tell me the foil thickness, so at a 3.5" diameter how many mil should I go? Apparently the typical heavy duty foil is .024mm, so 21 layers would get me to 0.5mm, which would have to add some serious stiffness at that diameter, with little weight and thinner than even a single layer of glass/epoxy. I may still go carbon as the outermost layer since I may have found a source for some free scraps, but at least the foil will give me a nice smooth finish to start the final layer.

John

[Thud]

John,
You may want to do a little research on aluminum surface preperation for bonding with epoxy's or polyesters.
I have used standard aluminum foil as a release barier for replicating parts. (pulling molds from antique 1960 "modern" furniture for a retro job)

Acicd etching is described on the West systems website. I did a large project for a university with glass doors that had an aluminum frame & suspension system we had to acid etch to insure bonding with the urathane glues employed.

I know anodizing is often recomended on solids thet will be assembled with bonding agents as opossed to mechanical fasteners.

A sample lay up tested to failure will be afordable & answer all questions.

[John in CR]

Just like I won't bond each aluminum layer to the next as I roll it up, I don't think a great bond to the aluminum is important. The inside will be permanent and essentially solid, not hollow to slide the batteries into. Fiberglass will be between each layer of the cells bonded directly to them, and then rolled as an outer layer around them. Then while the epoxy is still wet, I'll roll it up tightly in the foil, 15-21 layers in one continuous piece. Once that's cured, an outer layer of fiberglass or carbon will be rolled over the foil.

Here's an end view and side view of a portion of one pack. Note that the blue is fiberglass, and the thicker section between the two 4 wide portions will be 4oz cloth folded in half twice for 4 layers, so the 2 halves are electrically insulated from each other. I'll be able to terminate the 2 halves in parallel for a 14p10s 37V18ah pack, or in series for a 7p20s 74V9ah pack. I'll have 2 of these waterproof, self-balancing, medium power packs containing about 5.5kg of cells in each plus hopefully less than 1kg of packaging. The packs can then be used as part of the integral structure of a bike.

Batt tube drawing.JPG (43.8 KiB) Viewed 415 times

[Chalo]

According to this table, the Young's modulus for 50/50 fiber/resin is 25. That's about as good a ratio a rank amateur can get with pure hand layup, and only then if it's post-cured above 100+ centigrade. One can hit 40 only with unidirectional fibers, which I suspect John will have a tough time getting hold of in Costa Rica. Fiberglass has many useful properties, but high Young's modulus is not among them...

I'm willing to bet the modulus of aluminum foil in resin is no better. How could it be, if the bulk modulus of the aluminum is no higher than that of glass, and there is no path by which to squeeze out excess resin from between the aluminum sheets?

What John is proposing to build here is an aluminum piñata. And that's fine, if it works. But it won't be an improvement over simple fiberglass.

Chalo

[Chalo]

Just like I won't bond each aluminum layer to the next as I roll it up, I don't think a great bond to the aluminum is important.

I don't think you understand the problem you are trying to solve, then.

Chalo

[Stone Meadow]

What John is proposing to build here is an aluminum piñata. And that's fine, if it works. But it won't be an improvement over simple fiberglass.

Chalo

Fair enough, but think of the entertainment value!

[texaspyro]

Epoxy don't stick for shit to smooth surfaces... or aluminum for that matter.

[Hillhater]

Unless each layer of foil is bonded to its partner layers with something that has the same properties as aluminium, the end "sandwich" will have only the strength of the bonding agent.
Aluminium foil has zero strength, other than in pure tension, and only then if you can avoid any / all minor imperfections.

.. Acid etching 0.024 mm foil would be an interesting project !

[John in CR]

Just like I won't bond each aluminum layer to the next as I roll it up, I don't think a great bond to the aluminum is important.

I don't think you understand the problem you are trying to solve, then.

Chalo

I apparently understand the problem I'm trying to solve better than you do. I made one of the 7p halves the other day, which had 2 layers of 4oz epoxy impregnated fiberglass cloth wrapped around the exterior with 1 layer of between the 3p and 4p strings. Once it was fully cured I was disappointed by how much I could flex the 70cm trapezoidal shaped tube. I need it to be extremely stiff, because any flex in this tubular pack will put stress on the tabs since the batteries themselves aren't likely to flex compared to the connection between them.

Out of curiosity just in case I missed something I just put the aluminum idea to the test. We had a roll of craptastic aluminum foil much thinner than the commercial stuff I need to get a wide enough roll for my 70cm+ tube, and with masking tape I taped the end to a 1.125" wooden rake handle, rolled it around the handle 21 times and secured it with masking tape. Guess what? The wrapped in foil section of the rake was quite significantly stiffer than the plain wood. Since that worked, I put it to the test on the battery stick I already had in the same manner, and guess what? That got stiffer too. The aluminum added less thickness than another layer of fiberglass would, but I'm sure even this cheap thin foil added more stiffness than another layer of glass.

I certainly hope you're not going to conclude that the masking tape is what made these items stiffer, though it did serve the same purpose as the epoxy inside the first turn of foil, and fiberglass+epoxy as an external layer. That purpose is to hold the foil in place as well as protect it from damage.

Aluminum pinata...LOL !

John

[John in CR]

Unless each layer of foil is bonded to its partner layers with something that has the same properties as aluminium, the end "sandwich" will have only the strength of the bonding agent.
Aluminium foil has zero strength, other than in pure tension, and only then if you can avoid any / all minor imperfections.

.. Acid etching 0.024 mm foil would be an interesting project !

Not true. It will be like 20 perfectly fitting very thin aluminum tubes one slid over the next one and end up quite similar to a .5mm tube of AL over a 3.5" solid core. Now if you guys think .5mm isn't enough, then I'm listening.

John

[John in CR]

Epoxy don't stick for shit to smooth surfaces... or aluminum for that matter.

It doesn't really need to just like the roll of AL foil makes the cardboard tube inside much stiffer, though mine won't be hollow. The outside layer of fiberglass+epoxy will just be a covering holding the foil in place and adding some stiffness itself, a bit like some of the cheapest foil that comes wrapped in plastic. Fill the inside of one of those with some foam and crack someone over the head with with it.

Haven't you guys ever seen a newspaper rolled up with a rubber band around it?

[Chalo]

Unless each layer of foil is bonded to its partner layers with something that has the same properties as aluminium, the end "sandwich" will have only the strength of the bonding agent.
Aluminium foil has zero strength, other than in pure tension, and only then if you can avoid any / all minor imperfections.

.. Acid etching 0.024 mm foil would be an interesting project !

Not true. It will be like 20 perfectly fitting very thin aluminum tubes one slid over the next one and end up quite similar to a .5mm tube of AL over a 3.5" solid core. Now if you guys think .5mm isn't enough, then I'm listening.

A tube under a bending load has a spectrum of forces on it. There is a line of pure tension on the outside of the bend, a line of pure compression on the inside of the bend, and a gradient of tension, compression, and shearing forces everywhere between the extremes.

The only part of this spectrum in which a bunch of layers has the same integrity as a single thickness is along the narrow line of pure tension. It should be obvious that layers of foil have no structural value in compression, but I'm saying it. Compressive strength is basically tensile strength perpendicular to the direction of compression, and a stack of foil sheets loaded against its edge doesn't have that.

It should also go without saying, but I'm saying it, that a stack of foil sheets puts up minimal resistance to shearing forces, compared to an equal single thickness.

I ask you to assess the difference in bending stiffness between a book and an equivalent thickness of wood or MDF fiberboard, which are basically the same material.

I suggest that a narrow .5mm thick strip of solid aluminum bonded or riveted to the top and bottom of your monocoque would be of more structural value than a cumulative .5mm of layered aluminum foil around the whole thing. Ribs and stringers, inside or out, would be of much more value yet.

Chalo

[John in CR]

Ribs and stringers?????????????This thing is essentially solid battery inside. The batteries will be in there permanently bonded to this container.

I'll concede your point about the compression side of the multi-layer not being as strong as a piece of solid tubing of equal thickness, because that seems plausible, but I think the inner and outer supporting layers of glass will counter it to a large extent. I have some 1mm AL sheet, but I don't think I can get as tight a fit, and the joint would be a major weakness, so the multi-layer approach seems better. I'm not totally against using say a 1.5 turn wrap of the 1mm sheet if would add a lot more stiffness compared to a full 1mm roll of the multi-layer that fits better and perfectly tight and smooth. Another advantage of the sheet is that I could prep it for a better bond with the epoxy inside and out.

I just want the thing exceedingly stiff, but don't want to add too much diameter with a lot of glass, so maybe I should wait until I know if I have the carbon fiber or not, since it would make the foil a mute point, and have no worries about layers becoming delaminated.

John