This is the third and final installment of the series on the All-City Space Horse dropout. We have so far covered initial concept exploration and design development both in the sketchbook as well as in virtual mediums.
After we received our rapid prototype pieces and verified their functionality, we began the process of collaborating with our dropout supplier. Because no issues were apparent, they were comfortable with going straight to tooling.
These dropouts are investment cast. It’s a fabulous process to use if you need to make low-volume intricate parts that are not easily made in a conventional casting mold (popular for jewelry, sewing machine and gun parts, and lugs).
If you are not familiar with the process, here is an incredibly simplistic account of how it works in the case of this dropout:
A wax version of the dropout is molded and is attached to what is essentially a wax funnel (a sprue). The sprue and dropout assembly are then repeatedly dipped in a fine ceramic slurry (small particles of ceramic in water) and allowed to dry. The ceramic particles are deposited on all surfaces of the wax dropout and sprue, creating a hardened coating. Gradually, the ceramic coating is thickened into a thick, hard shell around the wax at which time the wax removed through melting or some other process.
What remains is a ceramic shell with the relatively intricate void of a dropout. Steel is poured in. The shell is broken away. Sprue is cut or broken off.
(Here is a more thorough explanation: http://en.wikipedia.org/wiki/Investment_casting)
You will notice in the image below of the non-drive side some funny seams in the surface of the part. These tooling lines are caused when parts of the mold are uneven- they are resolved in the final product (our personal prototypes have them but your frames will not). The lines appear around the axle pad and the AC of each dropout side and they illustrate the fact that this dropout is designed so that the branding can be removed so the dropout can be made without branding.
Why did we take this approach? Well, our frame-part manufacturer also recognized the lack of attractive and functional dropouts of this type available openly to builders so they requested that we allow them to produce blank versions of this dropout to support that market. As we are huge fans of steel hand-made bicycles (Jeff and I could easily be considered junkies) we thought this was a fabulous way to share the love.
Next, we had to produce frame prototypes to verify that our dropouts could be built into frames the way we wanted them to be. For me, this was the most terrifying part.
In this picture you can most easily see that the cylindrical seat stay interface is rotated in a few degrees so that the seat stay would interface with the dropout at more of a right angle. You can also see why we made the interface a little larger than the seat stay diameter… so that we would have just enough room for a weld bead.
Bonus Notebook Sketch: