Engines are generally too wide to fit between your knees, since an engine is usually a vaguely cubic shape. In early mopeds the engine was below your feet[1],

Engines are generally too wide to fit between your knees, since an engine is usually a vaguely cubic shape. In early mopeds the engine was below your feet[1], so there was no point in having a crossmember where your knees would be. [1]: https://en.wikipedia.org/wiki/File:ZweiRadMuseumNSU_Hildebrand_Wolfmueller.JPG https://en.wikipedia.org/wiki/File:ZweiRadMuseumNSU_Hildebra...