[JesseK]

Belts rely on alot of friction to transfer power from pulley to pulley. By design, they're less efficient (albeit only slightly) than chain/sprocket setups.

The reason chains break is due to lack of tension. Chains in general are most vulnerable to a break during a sudden change in direction (e.g. full power forward to full power backwards, or from dead stop to full forward). The slack in the chain creates a huge change in force that is spread (use integral physics to exactly calculate) almost equally among the links when the slack becomes taut.

If the chain were already in tension, the change in force is almost negligible, but due to the slack before the change in direction, it is MUCH greater. The weakest link will then break -- usually it's the master link, which is why you only ever want 1 master link per chain run.

If you never change directions usually you're ok if there's not too much slack, but that is never the case in a FIRST robot -- you WILL be bumped, you WILL change directions & speeds. Bicycles on the other hand, do not receive such change in force, therefore by design bicycles can use #25 chain and undergo the same torques as a FIRST bot but the chain will never break.

Tension is your friend. You also want to pre-stretch the chain during practice so that it doesn't stretch during a match. This can be done by tensioning the chain very well, then running a battery down by running the drive train and making it change directions alot. This will prevent the chain from stretching during a match and inadvertently giving you some undesired slack.