John in CR's monocoque composite frame build log

While I bide my time before dry season gets in full swing and I try some high speed runs, it's time to get busy building some ebikes. I want to do some stuff more presentable. I won't chase an AJ type finish, but it's time to end the ugly experiments. I figure if I do a build log, then I'm committed to getting it finished in short order.

Where to put the batteries? Inside the bike, of course. Sure some guys have built some bikes that fit some batteries, but I like big capacity packs for better battery life. My Konions have lost some capacity but are still going strong after 23 months of daily use, so big capacity packs work. I'm way over the manufacturer's rated cycle life, and I started with used cells. To meet my needs I don't need a bike with a battery compartment, I need a bike that has a battery cavern!

Here's a quick drawing of my general idea.


I'm not ready for a carbon fiber build, because I don't know enough to be confident about the critical stress areas, so my materials will be steel, balsa wood, 3mm plywood, fiberglass, and epoxy. I'm using 3/4"x1/8" and 1/2"x1/8" flat steel with lots of triangulation for strength in the plane of the bike, with double thickness at the high stress points at the headset, BB, and rear suspension. 2 flat steel skeletons will create the shape in the drawing above. Thick epoxy impregnated balsa will go around the perimeter for longitudinal rigidity. 3mm ply will go on the sides as side covers with fiberglass bonding it to the steel. Access to the inside will be through the rear, which will bolt on. The rest will be permanently attached. The balsa and plywood will make smooth shaping and finishing a breeze. Overall, it should be a fairly lightweight frame.

The length at what would be the top tube on a normal bike is 55". The width open on the inside will be just under 4", so 3 wide A123 M1 26650 cells or 4 wide 18650 Konions will fit with room for padding and securing the packs. To give you an idea of how much space I have to work with, I can fit 1.2wkh of Konion cells just in the first 2.75" of height along the top, and that's with lots of room to spare. I should be able to fit 3kwh or more of batteries inside the bike if I want.

Here's the first day of work, which should give a better idea of my plan. Note in the first pic that is a meter stick along the top to give size reference.

View attachment 2

View attachment 1

Just in case ya screw up, the balsa Trees I had originally wanted to get for you, are now down on the ground. LOTS of room to 'speramint, so, I gotcha covered.

The skeleton from more angles.




Before going any further with the triangulation and reinforcement, I need to build a flat work surface big enough to work on, because I can't do the permanent welding on a spaghetti structure without a surface to keep it true. That means probably no more frame work until Thursday.

Harold in CR said:

Just in case ya screw up, the balsa Trees I had originally wanted to get for you, are now down on the ground. LOTS of room to 'speramint, so, I gotcha covered.

Click to expand...

Thanks. Actually the cut balsa lumber you gave me is going to work perfect for this, and I have enough for several bikes. Still haven't touched the balsa logs though. Maybe this bike will get a balsa fairing.

John

One thing I forgot to mention. Everyone uses tubing to build frames, but not me on this build for 3 reasons:

1. Tubing wastes too much space, and a lot of it's directional strength is wasted because bikes just don't handle much force in the lateral direction.

2. I can't weld tubing for shlt, and this 1/8" steel welds so easily that even I can do an effective job.

3. I'm impatient and love to just hack things together. Tubing takes time to cut and fit properly, so using flat steel will save a ton of time.

I believe I can make a stronger frame in the directions that strength counts, with lots more space for batteries, and a lot less weight. If I can pull this off, anyone can do it too.

John

Neat idea. I've often thought that a monocoque 'frame' would be ideal for an ebike, for just the same reason as you.

Somewhere back in the old threads here we discussed it a bit and I remember mentioning some experiments I did using balsa for structural aircraft parts. I copied an idea by an Aussie boat builder, Michael Storer, who built a canoe from 1/4" thick balsa planks, then covered it both sides with epoxy resin and glass cloth (here's a link to it: http://www.storerboatplans.com/Balsacanoe/Balsacanoe.html). The picture that sums up using balsa like this for me is this one:



I made up some test samples a few years ago and was astounded at how stiff, strong and light the structure was. Even without any carbon fibre it was more than strong enough to make the single seat aircraft fuselage I was working on at the time, and didn't need any added structural members - the skin quite happily took all the flight loads.

I think you could bin all the steel, just make the 'frame' up from balsa sheet, glued together with anything quick (hot melt or cyanoacrylate?) then cover it inside and out with glass cloth and epoxy resin. The hard part would be fitting the headstock and swing arm mounts, but I reckon you could just bond in fibreglass, or maybe carbon fibre, tubes. You can buy CF tube in small quantities for not too bad a price, or maybe look to see if you could find an old windsurfer mast of the right sort of size. I guess a steel or alloy tube bonded inside it might make fitting bearings easier.

Certainly an interesting project - I look forward to seeing how it works out.

Jeremy

Hi John,

Nice project i'm curious to see the finish product, great works.

Good day!
Black Arrow

Hi John;
Your space frame looks great! Or should I say the monster battery box?

I have been thinking the same thing about building a battery structure that the headstock fits on. My thoughts were to bend 1/8" or less aluminum to minimize the amount of welding and use a little aircraft construction riveting for stiffeners. Whenever I have calculated the cost of any one-off structure, composite comes out much more expensive than metal. The main advantage I have found with wood/composite is light weight and complex curves. The other consideration of coarse is local materials being cheap. We are fortunate with wood to have an endless supply of worthless shattered cedar slabs at any logging site. Just a bit heavier than balsa but stronger for a given size so one can reduce the section modulus and come very close to the same weight.

I read the article on the balsa/epoxy canoe with much interest. Zenon has built a very serviceable and inexpensive punt boat from mahogany door skins with a coat of resin, PERIOD. No cloth, even on the seams. Reject 48" X 80" door skins cost @ $4.00 each. The flaw is usually just the color or a split edge.

Please keep the camera clicking.

Thanks;
Gordo

Thanks guys, I've been wanting to do this for quite a while. I finally came up with a layout that has the potential to look slick and can hold lots of batts. I have plenty of balsa lumber to build several complete bikes, and pound for pound balsa is the strongest wood, but it's not tough. I've got enough balsa lumber to build a number of bikes. My concern though is with the stress points. Sure I can fiberglass in metal or CF tubing, but I worry that the impact loads will make it a bit like setting a pole with concrete but inside a styrofoam block. I wouldn't know how deep to set the pole to get enough strength.

I'm not very concerned about bike weight, at least on this first attempt. I just want to know it's strong with lots of battery space, and look presentable when I'm done. The skeleton I have in the pics above is right at 4kg, and with about 1kg more steel in key areas I'll be comfortable that the remaining strength and rigidity can come from the composite materials. I'd rather err on the side of conservatism and have a bit of extra weight than worry about the frame folding up when I hit a pothole.

Speaking of canoes, I want to build a faring (or is it fairing?) using the approach of http://www.gaboats.com. That must be better and lighter than those built with corrugated plastic.

Gordo,

I'd love to get my hands on a few $K worth of door skin wood. I want to build a catamaran bad, and instead of cylinder forming using plywood, I want to make my own plywood formed directly into hulls with epoxy as the binder, and composite cloths in the mix.

Gordo said:

I have been thinking the same thing about building a battery structure that the headstock fits on. My thoughts were to bend 1/8" or less aluminum to minimize the amount of welding and use a little aircraft construction riveting for stiffeners. Whenever I have calculated the cost of any one-off structure, composite comes out much more expensive than metal.

Click to expand...

Very true, composite is expensive. Sheet alloy is a relatively cheap and easy way to do something like this, and needs no special skills, welding etc. You could use alloy aircraft construction techniques, using thin sheet alloy (monocoque light aircraft typically use anything between 20g and 28g sheet, rarely anything thicker) pop riveted together. Put a few curves in some of the panels and it will be very stiff, light and strong. You can pretty easily fit things like the tubes to take bearings by wrapping the tube with thin alloy sheet and pop riveting through the lot, provided you can keep enough clearance internally for things like the steerer tube. The aircraft I designed had alloy tube wing spars and the wing strut attachments (which take about 70% of the flight loads) were made this way, although the rivets I used were monel aircraft pop rivets, rather than plain domestic alloy ones. Each 1/8" pop rivet was good for around 600lbsf ultimate, as I recall. By using a lot of rivets, closely spaced, you can get a lot of joint strength in thin sheet. Easy to build too, as thin sheet is pretty easy to cut out with decent hand shears.

I can envisage a design like John's, but with an oval cross section rather than rectangular, made from sheet alloy, pop riveted together. It's probably look pretty good if just polished up rather than painted.

Jeremy

John in CR said:

Speaking of canoes, I want to build a faring (or is it fairing?) using the approach of http://www.gaboats.com. That must be better and lighter than those built with corrugated plastic.

Click to expand...

You mean a bit like this, John:



I saw this in the flesh at the Beale Park boat show in June this year. It's not made of composites at all, that covering is flax, sealed and bonded with boiled linseed oil. The builder is a bit fanatical about using natural resources, rather than synthetics, so had some nice displays of stuff made from flax (including a flax boat rather like the GA design).

Jeremy

I like that flax job, though I tend to steer away from tedious builds. My advantage is that balsa is readily available. It's treated as junk wood here.

Regarding the shape on my current frame. I've been thinking I should lengthen that single cross brace at the point at the center. That will put a slight bow in the plywood sides, which will make it more rigid once I add a layer of fiberglass on the inside. It will also hide imperfections that are sure to show in the finish work if I went with perfectly flat side panels. Curves look better to me anyway, and I already planned to carve a nice curve in the balsa lumber for the top tube and down tube pieces.

John

Looks good so far. This should be interesting, I'm actually planning on building a CF monocoque at some point in the future, so I'll be interested to see the problems you run into. Looks like that design will be awesome for holding ridiculous amounts of batteries though.

x88x said:

Looks good so far. This should be interesting, I'm actually planning on building a CF monocoque at some point in the future, so I'll be interested to see the problems you run into. Looks like that design will be awesome for holding ridiculous amounts of batteries though.

Click to expand...

Thanks, yeah I like big batt packs, and this was the first layout I came up with that can hold multi-kwh without looking like a pregnant guppy. Just in the upper space from under and rearward of where a seat tube would be, I think I can fit 1kwh. That means I can even go with a non-hubmotor drive housed inside the lower section and still have room for a multi-kwh pack

I hit a big road block this morning. Torrential rains last night damaged a bridge and a primary water tube supplying almost 1 million people with water. That means no water for as much as a week, and the way I get dirty and sweat doing construction work, that I have to hold off until a shower is possible. The only good thing is that I can work out the plan details of the BB and rear suspension in the meantime instead of just winging it as I go.

John

You told me there was 8" of water in the pool How much you need to get wet :lol:

Would it be crazy to lay the batteries out to get the shape? It'd be irritating to find out that if it was 0.25cm larger, it'd fit another whole row of cells...

Then again, I tend to split hairs a bit too finely. Some of my woodworking is a little too fine, and as a result, only works properly when the humidity outside is low.

Some deplore my drive to shoe-horn that last one in there, but I can't seem to help it.

Katou

ps. I like your thinking on why use flat bar. Solid reasons.

Harold in CR said:

You told me there was 8" of water in the pool How much you need to get wet :lol:

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The pool had 6" already, so we have 14", but that water is loaded with chemicals to kill mosquitoes and algae, so bathing there isn't an option. At least it gives us toilet flushing water though, or things would get really ugly. If we get a good rain tonight, I'll take a shower outside under a downspout from one of the gutters. A multi-m3 water storage tank will definitely be included with the house I want to build. Solar panels, wind generator, water collector, and switch over to diesel as a backup genny (so I can go biodiesel if necessary), and I'll be set pretty well for 2012.

Sucks about the pipe, good luck with that.

John in CR said:

Solar panels, wind generator, water collector, and switch over to diesel as a backup genny (so I can go biodiesel if necessary), and I'll be set pretty well for 2012.

Click to expand...

Take that another step and you could integrate a methane collector into your septic tank and have a methane powered generator instead.

katou said:

Would it be crazy to lay the batteries out to get the shape? It'd be irritating to find out that if it was 0.25cm larger, it'd fit another whole row of cells...

Click to expand...

I know what 4 wide Konions need and 3 wide A123s, and I added a bit of extra width that I was comfortable with and still not get in the way of pedaling for me. 6 wide Konions (my stuff is mostly 2p) and 4 wide A123s would be too wide for me. Laying the cells down sideways is too narrow, so I couldn't accommodate the maximum I'd like. I also wanted to leave extra space to fit as yet unknown packs of other cells or chemistries. Basically what I did was go for the maximum exterior width I thought was okay at almost no weight penalty for going wider than the minimum necessary. The extra width with current pack plans with be used to permit some air flow through the housing, and it give me flexibility going forward.

I did consider building the bike to a specific pack and use fiberglass and epoxy to use the battery pack itself as an integral part of the support structure. Most of my Konions are a bit too worn for that approach, and the balancing needs of the A123's make that a risky move, because what happens in the event of a dead cell or wire problem? That is something I want to try at some point. For this build the batteries will only help the structure from the standpoint of preventing crushing or folding like an aluminum can (sorry I forget the engineering terminology for that).

John

x88x said:

Sucks about the pipe, good luck with that.

John in CR said:

Solar panels, wind generator, water collector, and switch over to diesel as a backup genny (so I can go biodiesel if necessary), and I'll be set pretty well for 2012.

Click to expand...

Take that another step and you could integrate a methane collector into your septic tank and have a methane powered generator instead.

Click to expand...

Isn't that really only beneficial if used for making heat, which we don't need here? I can't imagine our family being able to produce enough methane to generate useful amounts of electricity. Lot's of hot air, but that's not flammable.

John

John in CR said:

Isn't that really only beneficial if used for making heat, which we don't need here? I can't imagine our family being able to produce enough methane to generate useful amounts of electricity. Lot's of hot air, but that's not flammable.

Click to expand...

Well, I know ICE engines can be converted to run on compressed methane, but idk how much would be needed to run one for any amount of time... I figure though, if the generator is only for emergencies then you could just have an automated system constantly collecting and compressing methane, then you should have enough to run it for a while by the time an emergency crops up.

I can handle occasional emergencies with my gas genny. I'd like to become grid independent and ready for long-term emergencies. The current cheap price of solar cells + a biodiesel genny make going off grid economically viable in the short term.

Jeremy Harris said:

Gordo said:

I have been thinking the same thing about building a battery structure that the headstock fits on. My thoughts were to bend 1/8" or less aluminum to minimize the amount of welding and use a little aircraft construction riveting for stiffeners. Whenever I have calculated the cost of any one-off structure, composite comes out much more expensive than metal.

Click to expand...

Very true, composite is expensive. Sheet alloy is a relatively cheap and easy way to do something like this, and needs no special skills, welding etc. You could use alloy aircraft construction techniques, using thin sheet alloy (monocoque light aircraft typically use anything between 20g and 28g sheet, rarely anything thicker) pop riveted together. Put a few curves in some of the panels and it will be very stiff, light and strong. You can pretty easily fit things like the tubes to take bearings by wrapping the tube with thin alloy sheet and pop riveting through the lot, provided you can keep enough clearance internally for things like the steerer tube. The aircraft I designed had alloy tube wing spars and the wing strut attachments (which take about 70% of the flight loads) were made this way, although the rivets I used were monel aircraft pop rivets, rather than plain domestic alloy ones. Each 1/8" pop rivet was good for around 600lbsf ultimate, as I recall. By using a lot of rivets, closely spaced, you can get a lot of joint strength in thin sheet. Easy to build too, as thin sheet is pretty easy to cut out with decent hand shears.

I can envisage a design like John's, but with an oval cross section rather than rectangular, made from sheet alloy, pop riveted together. It's probably look pretty good if just polished up rather than painted.

Jeremy

Click to expand...

I'm sure you know 3 bucked rivets are equal to 5 of the best pop rivets money can buy. I have buckets of cleco's and just love to play with my pliers. I have a metal cutting band saw, 2hp table saw, 250 MIller with one pound spool gun, all overkill for bike stuff. Must get back to welding pie plates, with tig before I build something. I figure it takes 8 hours before the welds look good and I never weld for more than a few minutes each month so I can not do a good job like John's. I will bet his build is twice as strong as need be? But then I don't know CR pot holes intimately. Enough hijacking this thread.
Sorry.