I made a 39 tooth rear cog out of a chainring my camera is busted but here is a photo of a 48 tooth rear cog I made http://share.ovi.com/media/currentre...resident.10073 the other cogs are 14 16 18 21 24 and 28 the shifting from the 28 to the 39 is instant strangely enough the 39 tooth cog clips the derailer cage it looks like a 10 tooth difference is the maximum jump for shimano derailers. Ritchey made an offset 33 tooth cog that was used by santana to allow 9 cogs with 8 cog spacing. The extra cog was shifted to with an Erickson 9th cog gizmo I think this is a photo of it http://share.ovi.com/media/currentre...resident.10025 perhaps we can pursuade action tec to build offset cogs for us. The reason for this is a standard derailer will have problems climbing these large cogs unless they are far enough away from the dropouts offsetting them is one way 160mm wide dropouts is another. The jump from the 2 largest cogs is not unreasonable if you can live with a 30 and 39 on the front then you should be able to live with a 30 and 39 on the rear. The reason for all this is a 25 tooth on the front and a 39 tooth on the back will double the life of your chain and cogs compared to a 20 tooth on the front and a 32 tooth on the back. This is standard enginerring where cog life and chain life is the cube of the difference. My feeling is these large cogs should be made out of aluminum. A dished cog like Ritchey made can rest against the second largest cog this will reduce the deflection of it.