In the past, the saddle position in BikeCAD was largely controlled by the seat angle and the saddle height. Once these values were established, we could further adjust the saddle position by changing the seatpost setback and the position of the saddle clamp on the rails.
In BikeCAD version 10.5, we now have two additional options. We can directly enter values for saddle position relative to the bottom bracket in X and Y coordinates. When we choose this option, the saddle height is automatically established because it is simply the hypotenuse side of the triangle formed by the X and Y coordinates. Because we also allow for the input of saddle dimensions and seatpost setback, the seat angle, just like the saddle height, is determined automatically.
Entering the saddle position in X and Y coordinates is useful when replicating measurements taken off a fit bike or through bicycle measuring devices such as those used in the Specialized BG Fit system. With the X and Y measurements shown on the screen, you can see where on the saddle these measurements are being taken. If you would like to reference a different point on the saddle, you can shift the reference location using the Saddle reference point X and Y input fields.
Even if you are still used to thinking more in terms of saddle height and seat angle, it can still be useful to use the Saddle X and Y method. For example, if we've designed a bike such as this with a 74 degree seat angle and a saddle height of 700mm, we may like this geometry. But what if we want to apply a curved seat tube? As we add the curve to our seat tube, our saddle will move back as BikeCAD maintains our specified seat angle. (Just as an aside, not everyone might agree on how to measure seat angle when you're dealing with a curved seat tube. But in BikeCAD, we take the seat angle as the angle of our seat post. In cases where we have a curved seat tube it is often more meaningful to speak in terms of effective seat tube angle and we can show this dimension as well in BikeCAD Pro.) Going back to our example, if we back up and switch to the Saddle X and Y option before adding the curve to our seat tube, BikeCAD will leave the saddle in the same position and calculate the required seat angle to suit the curved seat tube. If we make further revisions to our model through changing the position of the saddle clamp on the rails, or by changing the seatpost setback, we'll notice that BikeCAD will automatically adjust the seat angle to maintain the specified saddle position.
In addition to the saddle X and Y method, we can also specify the position of the saddle clamp in X and Y coordinates. This option is available because while some fit bikes are largely concerned with the X and Y position of the top of the saddle, other fit bikes focus on the X and Y coordinates of the saddle clamp.
The Saddle clamp X and Y method is similar to the Saddle X and Y method in that it will automatically adjust the seat angle as the saddle clamp is shifted or as the seatpost setback is changed. However, shifting the saddle fore and aft on the rails will not impact the geometry of the frame.
One more note about the saddle height input field: When the saddle height method is chosen in the Primary dimensions dialog box, that same input field is also available in the Saddle dialog box. If you change the saddle height in one dialog box, it will also change in the other. If you choose the saddle X and Y option, or the Saddle clamp X and Y option, the Saddle height input field in the Saddle dialog box will be replaced by a button. When you click this button, the Primary dimensions dialog box will appear allowing you to control the saddle in any of the three methods already discussed.