I see that the wheel "spokes" are cast, and hollow.
How do they make such a casting hollow, what is used to form the hollow part? How do they get whatever-it-is out of the spoke later?
Thanks
Lots of different methods. Most common form of hollow casting involves pouring molten metal into a two-piece cast to let cool briefly. The liquid closest to the cast cools fastest and forms basically a shell. A hole is then punched through the shell and the still liquid material inside is poured out. This method was first used by bakers to make candies look bigger but use less "stuff" and thus the first mass produced commercial rip-off was born.
The recognised and common way to mass produce accurate intricate, hollow non ferrous parts such as your wheels is by using the "Investment casting" method....sometimes known as the "Lost Wax process" and I would guess the wheels are made this way. Its too complicated to explain the process here and different manufactures have slightly differing techniques but in simple terms the voids in the casting are formed by a medium that slowly melts and is absorbed into the surrounding metal as it cools. Or the medium can be melted and drained out leaving a void in the mould prior to the molten metal entering. There is plenty of info on the net that explains how it is done in more detail. Do a search on the subject and you will find an explaination.
I suspect the wheels on the GS as well as most other cast motorcycle wheels I have seen are sand cast. The surface finish of the inside and outside of the spokes suggests this, as well as a few other clues such as the core clearance holes located on the outside of the spokes. The cores are likely a special core compound or wax, as it is cheap and quick to reproduce. The wheels are free from the stress marks I would expect to see from an investment mold and lack the surface definition an investment tool would provide.
Casting a part such as a motorcycle wheel is fairly straightforward once the process is tweaked. First, a cope and drag are rammed up with a master pattern to produce the outside of the wheel. Then, a pre-made core is installed and the mold clamped shut. The core forms hollow spaces within the casting.
After the pour and a suitable cooling period the boxes are released and the part removed from the sand. The core is then removed from the inside of the spokes with a bent wire and compressed air. The holes on the outside of the spokes help with this process, as does the hollow hub.
In the old days, casting sand was usually a mixture of 60 mesh silica sand and Molasses combined with very small amounts of water. It suffered from relatively poor bond strength and low resolution. Later, casting mixtures including oil came along. This reduced the risk of explosiion due to water vapor. A common mix today consists of 60 mesh silica sand, isopropyl alcohol, Bentone 34, and 2-cycle oil. By using higher mesh sand and a larger proportion of Bentone you can increase the resolution of the sand to almost invenstment quality.
Sand casting and patternmaking have become a lost art in this country. If it interests you, check with your library for books on patternmaking and foundry work. A wealth of information is available though the statewide catalogs. Most of the books you will find were written before 1940. Although the materials have changed, the basics of sand mold and pattern design have not changed significantly for over a century.