Though it galls me so, i would have to agree with something such as BN has suggested - an across-the-frame inline 4 cylinder engine mounted in a tubular steel duplex cradle frame, with wire wheels and a front disc and rear drum brake. Why? Well, if you were attempting to familiarise someone with motorcycles from a total outsider's point of view, you have a simple machine with all of the BASIC components of every other motorcycle. You could get the little green man comfortable with it and be able to introduce all other features as variations on this theme. For instance - why's that one buzzing? he asks, pointing at a KH125 commuter special. He can still see the basic shape of the motorcycle, but you can now explain it is a smaller version possessed of a slightly different engine. Why does that one look so odd he asks? Well you say, it is an R1, it is basically the same, but the frame goes round the sides and it is wrapped in plastic to make it look faster. Why is that one so strange? It is a motard, it is made to be thin and scare small children. I could go on, but i think BN did nominate an good archetype.
FWIW i know there's a lot of grounds to nominate a single cylinder machine in favour of a 4 cylinder machine. I would take the position that from a 2004 based alien visitation, the 4 cylinder is more relevant to be an introduction. If we were talking 30 years prior, i would say a single cylinder machine almost certainly.
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