Well, we might be looking at a misunderstanding here.
When you hear folks talking about a 20 inch bike, a 24 inch bike, a 26 inch bike, etc., they're referring to the size of the wheels. I've never heard of an 18 inch bike in that sense. And such a thing would be a child's bike.
So I'm wondering if your 18 inch Peugeot is actually 18 inches from the bottom bracket to the top tube. Or to the seatpost.
If that's the case, then you should be able to fit an engine in there.
Photos of your bike would help.[/QUOT
yes , i mean the distance between the center of the driving sprocket to the top tube , not the wheel size its( 26 )
thanx a looooooooooooooooot