Cold molded plywood/fiberglass ebike build

Submitted for the approval of the ebike society . . .


Hello all. Glad to have this forum for inspiration and advice. My love affair with ebikes is recent but hot. After seeing a plywood minibike built by a user on Reddit (https://www.reddit.com/r/Bikeporn/comments/gtjvxg/plywood_mini_bike_i_built/), and with years of sailboat repair experience under my belt, I decided to try my own hand at a cold molded ebike.

Said Reddit guy was kind enough to send me his CAD files, so the first step was to learn how to use a CAD program. After about a week of measuring and tweaking I came up with a shape I liked. Then I transferred it to paper, then cardboard, then the plywood.

The frame is made from 1/2" birch ply with fiberglass and epoxy sheathing for strength/sealing and a varnish coat for UV protection of the epoxy. I added extra layers of glass to the dropout area and rear triangle for additional strength and stiffness.

The side panels cover a cutout that allowed me to place the battery in the top spar without bending the ply too far or making the seat too wide. There's a horizontal plywood "spine" joining the two frames on the underside of the top spar and strengthening the area I cut out.

The head tube, fork, bottom bracket, and dropouts come from a Jamis I found on CL that had decent components but too small a frame for me. I cut triangular steel torque plates to go between the dropouts and wood frame and filled the insides of the dropouts with thickened epoxy to prevent collapse. It was a real bonus to preserve the rear derailleur and disc brake tabs.

I scavenged the brake/shifter cables and sheaths from the Jamis too and ran them through the frame using little wood tabs with notches cut into them, then glued to the frame notch side down so I could thread a zip tie through. Maybe not the most elegant, but I was working with what I got.

The motor is a 26" 1000w Voilamart, and the battery is the 48v from my Radwagon. Not that I need two ebikes right now, but I'm hoping if I get a second battery my wife will take to the wagon...

The seat is softish packing foam and gorilla tape. I think I need a better solution because in a leaned forward riding position it is not all that fun for my perineum. I'm thinking about taking springs from a bike seat and suspending the plank the cushion is attached to.

I've taken it around town the last few days and have dared to get it as fast as ~29 mph. I'm not sure how much it weighs (other than it's significantly lighter than my wagon), but it's a pretty zippy little ride. I'm still working on finishing touches like fenders, lights, some extra bracing, etc., but I figured it was far enough along to share.

What do you think?

Nice Job , There is someone in San Jose , California that makes wooden bikes , works of art .

I look forward to your next frame ... with Longer Wheelbase , Rear Suspension , and much more rake on the front headtube/fork . ( the modern All Mountain/Enduro/Down Hill bikes are now going to 60.5, 61, and 62 degree head tube angles .

For your next frame, remember ... Long , Low , and Slack .

Thanks. I struggled with the wheelbase and rake decisions and take your point. This one has a 48" base, which was between my large frame Marin hybrid (44") and my long tail cargo electric Radwagon (54") so I figured I was in safe territory. A little more rake might be welcome, but I wasn't sure what the trade-off would be in terms of stress at the fork/frame joint or other factors.

That is the beauty of this build though that I could make frames like suits of clothes and swap the components fairly easily. I think I'll give it some time though and let my builder's urge replenish.

And on the rear suspension, I've already thought about eventually chopping the triangle, installing a lower pivot, and installing a single point shock. My first cutout had a separate floating rear triangle, but ultimately it was a little ambitious for a first build when I hadn't yet proven the basics. Stay tuned.

The tweaks and enhancements begin.

Here I've added suspension to the seat and changed the padding, which totally solved my comfort problem.

Then, in an attempt to use the whole animal, I repurposed a leftover donor frame section, combined with some parts from an old lawn chair, to make a seat for my boy. He likes riding in front on the Radwagon, so we'll see how he likes this one.

Finally I shaved down the rear leg a touch so my chain won't rub on my highest gear.

Beautification, to the extent possible, is still down the road. In other words, please don't judge the drips in my varnish.

Dang, how did I miss this up to now? It never occurred to me I could spend the lockdown doing that.

Let's see, did you hotcoat the wood? You don't necessarily even have to fiberglas it, you mix your polyester resin up to 50% acetone and it soaks into the wood. I learned to go to 3% on the catalyst but I never understood it was so important to go extra. The outside can still seem like wood at that point.

What was your cloth? I'd think a 10 ounce if it's just a sacrificial ply. At the risk of nitpicking, what I see is alternative starved/slushed, which is too little where the threads of the cloth are showing and too much where it pooled and is thick. BOTH are weaker, where you have them meet is the worse. Your plywood is plenty strong enough so it probably doesn't matter, but a gelcoat would make it look better.

Thanks for chiming in. It's true, the lockdown was a contributor to this what I can only call obsessive pursuit. The real driver though was curiosity.

The glass is S-2 5.6 oz fabric from TAP, which is supposed to be somewhat stronger than average and is said to be suited for aerospace and surfboards. The resin is epoxy, not polyester, so I think there are different processes and mechanics at play than what you describe. I did do a neat coat on the roughed up wood for good adhesion before laying the first layer, and there are multiple layers of glass in the area shown, so you are only seeing the weave of the outermost.

Like you say, there is enough material there that I'm decently comfortable with the amount of strength, and it feels stable at speed (knock on wood). I even took it to a local bmx track -- but then I chickened out and rode it super slow. Still, it did great!

My dad used to say he made ten foot boats because they looked good if you stood ten feet away. This is a ten foot bike.

That said, your observations sent me back to the shop tonight, where I spent a couple hours sanding and recoating the epoxy in an attempt to fill the weave. Did I mention it's my birthday? Obsessive behavior.

After several hours of sanding and reapplying epoxy and varnish, here is the wuddbike in its (mostly) final form. On this day, I put the removable front seat on and took the boy on a ride through the bike's natural habitat.

Since the photo was taken, I did swap the seat cushion material again for something even gellier. The flesh is weak, but the ride is oh so comfortable.

The wuddbike in its final form. Pretty great little ride and nimble as all getout.