In BikeCAD, you can modify the rear dropouts using the Rear dropouts dialog box accessible from this icon: 
. You can model dropouts with the slot oriented in any direction you like. Some examples are shown below.
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Note that the slot angle should be entered in the input field labeled by the Greek letter Xi (ξ). There is another angular input field labeled Theta (θ). However, Theta simply rotates the entire dropout. This field is necessary because when the frame is locked and longer or shorter forks are mounted on the frame, the entire frame will rotate about the rear axle as shown below.
Also note that in the first two examples, the chain stay is attached at a point lower than the rear axle, while in the third example, the chain stay is attached at a point that is higher than the rear axle. In the first two examples, we have used a negative value for Cy while in the third example we have used a positive value for Cy.