Old bicycle box: cheap; great if you're cycle touring and leaving from a different airport to that which you're arriving from although will need sourcing of a box at the other end. Reasonable level of protection although pipe lagging is recommended and will not provide guaranteed protection.
GroundEffect/Cinelli Bike Bag: great for Eurostar trips where their compact size means that you can get your bike in luggage racks; provides the their ability to fold up allowing youmeans you the opportunity to ride away from the station/airport. Needs additional measures (pipe lagging minimum) if you're going to entrust your bike to baggage handlers.
Padded bike bag: affordable alternative to a hard case and likely to weigh less than the former (issue with baggage allowances). Reasonable protection for bikes although will not provide guaranteed protection. Takes up a lot of storage space at destination and in home. May also have problems with space on the Eurostar.
Hard case: gold standard as far as protection but expensive and space intensive. If you've got a valuable bike, could be worth the investment although an alternative is hiring. May also have problems with space on the Eurostar
Different options each have their merits
Old bicycle box: cheap; great if you're cycle touring and leaving from a different airport to that which you're arriving from although will need sourcing of a box at the other end. Reasonable level of protection although pipe lagging is recommended and will not provide guaranteed protection.
GroundEffect/Cinelli Bike Bag: great for Eurostar trips where their compact size means that you can get your bike in luggage racks; provides the their ability to fold up allowing youmeans you the opportunity to ride away from the station/airport. Needs additional measures (pipe lagging minimum) if you're going to entrust your bike to baggage handlers.
Padded bike bag: affordable alternative to a hard case and likely to weigh less than the former (issue with baggage allowances). Reasonable protection for bikes although will not provide guaranteed protection. Takes up a lot of storage space at destination and in home. May also have problems with space on the Eurostar.
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Hard case: gold standard as far as protection but expensive and space intensive. If you've got a valuable bike, could be worth the investment although an alternative is hiring. May also have problems with space on the Eurostar