There have been many attempts to manufacture and market plastic bicycles. all have been too flexible and soft. which design-limiting property is insufficiently large?

The design-limiting property that insufficiently large is the elastic modulus (Young modulus)

Explanation:

Plastic usually have a relatively low elastic modulus, this couses the material to deform too much under stress. In the case of a bicycle, a little weight you put on it or little bumps will cause the bicycle to deform too much.

oil flow is a highly convective flow compared to water flow at normal temperature and pressure.

unlike the threaded fasteners which are used to clamp the machine members, power screws are used to transmit power. common applications of power screws are: screw jacks: to lift weight. lead screw of lathe: for axial movement of tool and its precise positioning.