Rear dropout spacing for thru axle dropouts

When inputting rear dropout spacing for a thru axle hub, one might assume that 142mm should be specified, as this is the over-lock-nut dimension for a typical thru axle hub. However, when using the Export to FreeCAD option, it is important to enter 135mm instead. This might be controversial, but this decision was made with the intent of making the design process simpler. Traditionally, mountain bike rear dropout spacing (the distance between the inner faces of the dropouts) has been 135mm. Thru axle dropouts still have 135mm spacing between the inner faces of the dropouts, but they also have a 3.5mm recess on either side to accommodate the 142mm hubs. In a future update of BikeCAD, the rear dropout spacing dimension may be renamed to eliminate this confusion.

Since BikeCAD uses dimensions Cz and Sz to locate the chain stays and seat stays some distance outward from the inner surface of the dropout defined by rear dropout spacing, it is just simpler if that inner surface is the same for both quick release and thru axle dropouts.

To illustrate how much smoother it is when we use rear dropout spacing to define the distance between the innermost faces of quick release as well as thru axle dropouts, consider the Paragon DS0001 sliding dropout which we can use with various quick release compatible inserts but then also select from one of the various thru axle compatible inserts without having to adjust the rear dropout spacing dimension and also change dimensions Cz and Sz to account for what would otherwise be a shifting reference surface.