Suppose you have 2 tangent arcs where the end points of the 'combined arc' are 2 defined points on a 2D sketch, the arcs are also tangent to straight lines:

straight line (defined end points) - tangent - arc (undefined) - tangent - arc (undefined) - tangent - straight line (defined end points)

Now the arcs are only partially constrained. With one more constraint, be it a relation or a dimension, the sketch will be fully defined. However, my aim is to set the length of the first arc to be double the length of the second arc:

straight line (defined end points) - tangent - arc (length X) - tangent - arc (length X/2=Y) - tangent - parallel straight line (defined end points)

I tried my way by using dimensions and equations, but it doesn't seem to work. If I assign a value to dimension1, then dimension2 is automatically defined (provided the endpoints are fixed), which would break the Y=X/2 link (unless the correct calculated value is known). And I can't leave a dimension out, because then I wouldn't be able to select one of the lengths to create the equation. If I delete a dimension, the equation is broken because the dimension doesn't exist

Can a dimension be created without value? Or is there a way to link both dimensions to each other without assigning a predefined or entered value?

You can easily achieve any ratio you wish using this methodology. The small triangle with dimensions defines the ratio. The larger triangle has a parallel relation between its hypotenuse and the hypotenuse of the smaller one. Therefore, the side length ratio of the larger will always be the same as the smaller. Now the short arc has an equal length relation to the short leg of the large triangle, and the long arc is equal to the long leg. By changing the dimensions of the small triangle, the ratio of arc lengths of the two tangent arcs is defined.