When we started with gearboxes, these were chain driven. Later we added Gates carbon drive. Why? In short: With belts we really only see advantages over chains. Belts last three times longer than a chain, are lighter and they are super quiet. There is no flex in it as people sometimes think. The CDX belts are therefore equipped with strong carbon fibers. Oh yes, ok, if one does break, you can't repair it, so we recommend that you bring a spare belt on multi-day tours or on holiday. Or just always leave it in your bag... it's only 80 grams.
Lab tests show that chains are more efficient at low power outputs (under 212 watts), while belts are more efficient at high power outputs (over 212 watts). In general a brand-new chain is 0.3% to 1% more efficient than a belt, in a straight line. But the minute the chain is muddy, dry or not in a straight line, it’s efficiency falls off rapidly. Belts shed mud and do not have rotating parts like chains so remain efficient in the worst of conditions. Chains stretch and a worn chain drops 4% efficiency. Belts never stretch and a belt at 100% wear is less than 0.5% different to a new belt in efficiency. Furthermore chain lubricants can cause a few % difference in efficiency. Belts don't suffer from this, since they may not be lubricated at all.