I have a 2003 Gary Fisher Sugar 4+ with Genesis geometry (still font know how to find the right measurements, it feels small) I have post mounts on my fork but not my rear brake how do I make it compatible? Can you make it compatible?
Sadly there isn't any safe way to make this happen. Even if you could fashion something to clamp around the seat stay the stay just isn't thick enoungh to handle the forces of brake.
I don't think anyone makes brake mount adaptors for non disk frames anymore , even if they did it's risky because frames with disc tabs are strenghtened at that point with thicker walled tubes or a brace to take the load of braking.
I have a 2003 Gary Fisher Sugar 4+ with Genesis geometry (still font know how to find the right measurements, it feels small) I have post mounts on my fork but not my rear brake how do I make it compatible? Can you make it compatible?
You should just need the factory disc mount, like this: http://www.ebay.com/itm/New-Trek-Fisher-Klein-Rear-International-Disc-Brake-Mount-Black-160-mm-/170801945312?pt=Cycling_Parts_Accessories&hash=item27c496dae0#ht_500wt_853
you can physically convert most non-disc brake frames to run a disc brake using Brake Therapy or A2Z adapter systems but as others have commented, its not worth the expense because typically using a disc brake will cause your frame to fail on the seatstay / dropout weld
even the early frames with disc brake mounts suffered this issue, until the frame designers learned to use a thicker walled or larger diameter tube on the rear stays, or reinforce the dropout area with a bridge
it can be luck really whether your frame will crack?
I have seen a few old frames ridden for several years with disc adapters and no issues
but have seen more old frames crack within a few weeks / months of installing the adapter