I just discovered SolveSpace (it is Open Source - search Git or just the web).  I used to use DesignView from the 90s and have at last found its replacement! *
1. It is a 2D and 3D parametric modeller.
2. It is a single EXE that you just download and run; PC, Linux and Mac.
3. The people who make it know what they are doing - bigtime.
Basically you use SolveSpace as a construction tool for geometry - unlike Inkspace it is very accurate. You draw lines/curves like in Inkspace and add nodes to join them, acting as pivots or sliding joints. You add dimensions that you know or want and the geometry adjusts itself to fit to your dimensions. You can tweak the dimensions or drag nodes to adjust shapes and it updates live. Finally, you can draw onto any other plane you want in 3D, and extrude your shapes into 3D.
You can export your bicycle or door-hinge or widget to SVG, PDF and other formats (but import is limited as yet; that is a surprisingly complex development challenge). 
The idea is not to use SolveSpace for full CAD, but as a fast, accurate sketching tool to get something prototyped and tested; you can animate machines and tarck the movement of pivots etc and export the result as CSV for display as graphs. One example would be tracking the velocity of an engine piston for various bore/stroke ratios.    

Note: I have no connection with the program providers. I just like it!
* I can finally retire my last Win98 ACER laptop, preserved only to run DesignView!