Dropouts (Choosing from static drawings)

When designing a bike with a particular set of rear dropouts, one of the most critical elements to define is where the rear ends of the seat stays and chain stays attach to those dropouts. This can be done in BikeCAD using dimensional input fields Sx, Sy, Sz, Cx, Cy and Cz. Different styles of dropouts can be represented by selecting from the menu containing options like plate, socket and hooded dropouts. The shape of each of these styles of dropouts can be further refined using a few other dimensional input fields. However, if the goal is to fully capture all the nuances of a particular dropout including cutouts and complex contours, it is impractical to control these elements parametrically within BikeCAD.

Therefore, an alternative approach was introduced in BikeCAD version 12.0 and has been improved upon in BikeCAD version 12.5. While the option to define dropouts parametrically still exists, we now also have the option to select from a collection of commercially available dropouts stored within BikeCAD as static drawings.

To access these dropouts switch the menu from “Define by parameters” to “Define by static drawing”. A new menu will appear below with all the available dropouts. As you switch from one dropout to the next, dimensions Sx, Sy, Sz, Cx, Cy and Cz will update to position the ends of the stays in a way that roughly suits each dropout. However, in many cases, a builder can use their own discretion to decide exactly where the stays will attach. Therefore, the input fields for seat stay and chain stay endpoints remain enabled for the builder to make slight adjustments.

The angle of the dropout can also be changed to better suit the angles of the chain stays and seat stays.

Some dropouts are designed to work with inserts. These inserts will be listed in a menu below the selected dropout.

If the dropout is a sliding dropout, the insert can be moved by modifying dimension Z. If the dropout is a rocker style, the insert can be moved using the available slide controller. Note that by default, moving the insert does not change the distance between the rear axle and the bottom bracket. Instead the frame itself adjusts to maintain a constant distance between the rear axle and bottom bracket which is defined in the chain stay length input field. If you wish to simulate the behaviour of an actual bike in which the chain stay length would change as the insert is slid back and forth, you can turn on the lock frame icon. Note that in this example, the chain stay length value changes as we rock the insert back and forth. Also, because the movement of the rocker has the effect of slightly raising and lowering the rear end of the bike as the insert slides through its range of motion, the head angle and seat angle will change by a few hundredths of a degree and the bottom bracket drop will change by a few tenths of a millimeter. These changes occur because we've chosen to lock the frame yet the entire frame is still free to rotate about the front axle.

New in version 12.5 is the option to save any of these dropouts to the list of standard dropouts. Doing this could save the time of finding your favourite dropout and insert combination from the long list of static drawings. It will also save your preferred values for Sx, Sy, Sz, Cx, Cy and Cz which may be different than the default values.