Not if your fare includes a checked bag on some routes: US to Europe: Free. Within Europe: Free.
How much does Brussels Airlines charge for a bike between the US and Europe?
Free (counts as a checked bag). Included in your free checked bag allowance. Economy Light fares don't include a free checked bag. On Economy Light the bike costs $235 each way.
How much does Brussels Airlines charge for a bike within Europe?
Free (counts as a checked bag). Included in your free checked bag allowance. Economy Basic and Light fares don't include a free checked bag. On those fares the bike costs $95 each way.