The DIY Open Crank Engine Moped

Anyone can strap a two-stroke engine on a bicycle to create a moped. But [robinhooodvsyou] has created something infinitely more awesome. He’s built an inverted open crank engine on a 10 speed bicycle. (YouTube link)  As the name implies, the engine has no crankcase. The crankshaft, camshaft, and just about everything not in the combustion chamber hangs out in the open where it can be seen and appreciated.

[robinhooodvsyou] started with an air-cooled Volkswagen cylinder. He filled the jug with a piston from a diesel car. Camshaft, flywheel, valves, and magneto are courtesy of an old Briggs and Stratton engine. The cylinder head, crankshaft, pushrods, and the engine frame itself are all homemade.

Being an open crank engine, lubrication is an issue. The crankshaft’s ball bearing is lubricated by some thick oil in a gravity fed cup. Even though the engine is a four-stroke,[robinhooodvsyou] adds some oil to the gas to keep the rings happy. The camshaft and connecting rod use Babbit bearings. While they don’t have an automatic oiling system, they do look pretty well lubricated in the video.

Starting the engine is a breeze. [robinhooodvsyou] created a lever which holds the exhaust valve open. This acts as a compression release. He also has a lever which lifts the entire engine and friction drive off the rear wheel. All one has to do is pedal up to cruising speed, engage the friction drive, then disengage the compression release.

We seriously love this hack. Sure, it’s not a practical vehicle, but it works – and from the looks of the video, it works rather well. The unmuffled pops of that low 4:1 compression engine reminds us of old stationary engines. The only thing we can think to add to [robinhooodvsyou’s] creation is a good set of brakes!

50 thoughts on “The DIY Open Crank Engine Moped

    1. Who cares? It’s a DIY engine for a bike that only one person is ever going to ride, and as long as they don’t wear a trench coat it’s fine. If he was kickstarting it, yeah, it would be an issue. But for a garage project?

      1. I think we differ on acceptability of known hazards that have rapid, catastrophic consequences in close proximity to our bodies and are meant to be operated during periods of time in which our attention may be directed at other things. That chain should have a shroud on it, it is far too close to the rider. Mass production or single purpose, basic safety should not be relegated to the backburrner just because the inventor is theoretically the only person who would ever operate it.

        1. Agreed. If one happened to have an accident that involved the groin area coming onto contact with the rapidly moving crank and lubrication reservoir it could rip through the femoral artery very easily, and the rider could then bleed to death quite quickly. A simple shield would suffice.

          1. You guys are either taking the piss or you’re nutters.

            Safety rules are mostly for people who don’t know how things work but who can memorize safety rules. People who pay attention and … Never mind. There is no way in hell I can explain it to you if you don’t already know it.

          2. No and no. Simple safety measures can also help save injuries and lives when the unexpected happens. I suppose nothing unexpected ever happened to you while riding a bike…

      2. He is kind of right though. I mean yes he can just ride it with short shirts but that doesn’t account for a possible accident. Suppose he hits the breaks or something and slips off his seat? That could get incredibly dangerous incredibly quickly. It’s a fantastic invention but even just a simple cover or shroud would probably be a good idea before riding it again.

    2. Safe doesn’t have to look tacky. You could easily toss some welded wire chain guards to keep fingers and clothes out. It already has an interesting look going on, a cage would fit. But, safety isn’t the only concern with the open cylinder head. Seems like the perfect ingress for crud to chew up the cylinder.

      Still, interesting to see it working so well.

    3. Call your congressman. There needs to be a law, or in this case, an international treaty to protect the unwary from this menace. Innovation and the “what just works” spirit will be crushed beneath our imperial heel!

    1. Not “steampunky” enough IMO. This is brilliant! He should clean up his welds and the frame, repaint (or powder-coat), install new tires/brakes/the works, add a nice tooled leather saddle, put on a few stylish (brass, natch) chain-guards and other safety stuff (for crybabies like Waterjet ;) – but nothing that would obscure the beautiful engineering – and make it a show-piece! (It’s already a masterpiece!)

      Even if he didn’t want to ride it anymore, he could sell it for hundreds or even thousands to the right person (assuming some kind of liability waver was involved ;)

  1. The sound is so turn of the century before the last. Hundreds of RPM, one less gear reduction than e or gas modern. However I think it’s bigger than 49cc, by a big margin…legal mumbojumbo.

    1. Interestingly, a few places don’t care about cc’s, just max speed. For example, in Wisconsin, there’s technically no cc limit to a bicycle with a bolt on engine, as long as it doesn’t go over 30mph.

    1. Yep, this definitely comes under my list of things I would never do naked, right below riding any kind of bike naked in public (unless of course it’s a Tuesday). That said depending on the age and quality I might make an exception for the village bike.

  2. I absolutely love the sheer Hack and minimalist nature of this. Oil pump, don’t need it. Starter? I’ve got your starter right here! I have to ask, where does the design concept start when doing something like this. “Oh, look! A piston! Wonder what I can do with that?”

  3. I think he just got anxious to make the video to show off his engine, bike isn’t finished, hence, NO BRAKES. I would bet they are in the plans. Love the engine hacked together from unrelated parts, though timing chain without a guard will oil your back, would be funky cool if he used an old schwinn bike chain guard there.

    1. That said, many of these old engines actually had a pretty ingenious way to reduce compression for starting. Rather than hold open a valve, they’d have a simple petcock installed about halfway down the cylinder bore. For starting the operator would open the petcock (and let it spew atomized fuel) to reduce the effective compression ratio so the engine could be started easier. I can’t imagine they had much larger than a 1/8-3/16 inch hole in the bore, small enough that the piston rings wouldn’t catch on it.

  4. No filter on that carb? Looks like the carb is in a perfect place to suck up road grime and grit, either clogging the carb or scratching the shit out of the cylinder wall and possibly ruining the rings.

      1. This, like many of these types of projects, will probably end up as one of many ‘Hey, remember that thing I made x many years ago, that was awesome… Hmm, I might still have it in the shed somewhere.’ The practicality/safety may be limited, but the hands on fabrication experience and personal rewards of a working project are priceless.

  5. I’d also make sure that fuel hose is properly isolated from the cylinder. Maybe it unlikely, but if that hose gets worn down from rubbing against the cycliner, could be pretty messy if not flamable mess.

    It’s a beautiful design, but I come down on the side of common sense. Protect yourself.

  6. Awesome work. Wouldn’t ride it, but with some paint and polish it would look great.

    Apparently this video was filmed in Finland (Pohjolan Liikenne in the background), and 49cc is the upper limit for registering an vehicle as moped in Finland. That may be the reasoning behind stating that it’s 49cc engine.

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