a bicycle frame is just that loading DOT approved wheels and tires along with engines that weigh above the little 66 cc cast aluminum China Doll is more a gamble than not it would be better to have a wheel fold under the bike's frame than the bike's frame fold under the seat
I think the vast increase of popularity of motorized bicycles in recent years probably has had a lot to do with he easy availably of those inexpensive little Chinese 2 strokes engines.
But the hobby really can go way beyond that nowadays if you want.
I’ve noticed lately you’ve have made several posts about engine weight vs. bicycle frame strength, etc.... While I honestly do appreciate your concerns about safety I do know it is quite possible to build heavy, fast, or both heavy and fast.
Several builders right here on the forum have done so and shared their experience & successes.
In fact I’d put sportscarpat in the top tier of those builders (research some of his achievements). If he recommends something I pay attention because I know he has done thorough real world testing.
While I’m not saying I’d throw a GX690 on a stock Walmart bike with a rag-joint attached sprocket, with some additional gusseting, quality drivetrain, and quality wheels I’d certainly would load a 212cc engine onto a bicycle frame.
Sorry if it seems like I’m busting your chops, really not my intention. But to quote one of your recent postings- “I’m only being polite” in hopes of persuading you and other builders to please not think in such a limited fashion.
The possibilities are endless.
-Kirk