Cap space issues. Simple as that. Unlike other major sports, trades are uncommon in the NFL. The reason for that is simple: the league’s salary cap system makes is very difficult to trade players until they’re near the end of their contracts.

NFL contracts generally aren’t guaranteed. Instead players sign for a number of years and get a signing bonus. For salary cap purposes, the signing bonus is pro-rated over the life of the contract. DeAmgelo Williams signed a five-year, $43 million deal last season, including a $16 million signing bonus. Williams carries a $8.45 million cap figure this season — $5.25 in salary plus a $3.2 million portion (one-fifth) of the signing bonus. Trading a player is treated the same as cutting a player for salary cap purposes: All the remaining signing bonus money ($9.6 million in Williams’ case) comes forward as an immediate or nearly-immediate cap hit. That’s $7 million currently. Williams’ cap figure for next season is $7.95 million or a $9.6 million cap hit to release him or trade him. Those are huge numbers for a team with limited salary cap space.

Bonus observation
: The same problem exists with all those big contracts Marty Hurney handed out recently. Cutting Jon Beason, for example, during the off season would net out to a $4.75 million cap hit ($8.6 million cap value if he’s on the roster, $13.3 million cap figure if he’s cut).