Aluminum vs Brass, I have seen some debate over the years touting brass as a superior frame material, but I have to disagree!
My chosen material for machining GP9 frames is 6061-T6 Aluminum alloy, and for good reason. Some argue that the brass frame being sold by others is a better solution because it adds additional weight, but that is a short-sited argument.
In reality, the brass frames I have seen offered, raise the profile of the locomotive, and to me, that negates any benefit of additional weight gain. Placement of the bolsters is critical when assembling the frame.
Relief Pockets and Bolsters
Unlike the flat brass frame counterpart, the FineScale 360 aluminum frame features machined relief pockets on the underside of the frame to facilitate proper placement of the bolster (truck mount).
The bolsters included with the original GP9 kit, seen in the image below, were designed to mount on the top side of the plastic frame. The relief pockets I have machined into the aluminum frame allow space for the locomotive trucks to rotate without rubbing the frame. As a result, the trucks sit lower on the frame and give the locomotive a more prototypical stance.
In addition to the relief pockets for the trucks, I have added relief pockets along the sides of the frame. These side frame reliefs let the frame tuck under the locomotive shell and create the side profile of the steel beam that can be seen in the section drawing of a prototype GP9 underframe.
If you need the extra weight, stick-on lead weight can be added to my frame in any configuration needed to fit the many types of DCC decoder installations, battery onboard installations, and sound installations.
Once you add in the absorbent cost of quality brass, along with the details listed above, I see a clear winner in the aluminum vs brass debate.