As a way to practice some Solidworks skills I quickly designed a childrens bike where the bike is able to grow with the child. The saddle height can be adapted by assembling the bike with the frame flipped 180 degrees. 

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The seat tube is designed so that the post can be inserted in both sides allowing the bike to be made taller for when the child grows. When the child is ready to move onto pedalling the bike a crankset can be mounted in the underside of the seat tube, with use of a simple belt driven single speed system. 

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