In the primary dimensions dialog box, we can control the rear end of the bike by chainstay length or by rear wheel gap. A third option has been modified in BikeCAD version 17.5. If you choose "Rear axle to seat tube" an extra input field will appear allowing you to also enter a dimension for chainstay length. By necessity, the seat tube section of the tubing dialog box will change to reveal a set of inputs for controlling the bend in the seat tube. In order to allow independent control of both chainstay length and "rear axle to seat tube", some amount of seat tube bend is required. If a bend angle of 0° is entered, the angle will automatically switch to 20°. If at any point a change is made to the chainstay length or "Rear axle to seat tube" dimension that would require the bend to go in the opposite direction, BikeCAD will flip the sign on the bend angle. Consequently, it is impossible to directly modify the bend angle from a positive to a negative value. Instead, this is done by changing the relative lengths of the chainstay and rear axle to seat tube dimensions.