Normally, I'd use a box covered in parcel tape and padded out with bits of foam. However, and this will make you laugh, two boxed bikes don't fit in our little car (actually, one doesn't fit in our car). We can get two bikes in there for a ride though so I assume padded bags would be the way forward. Any advice on which ones will give us the best chance of getting a bike that works at the other end?
The wife so far has found these: -
http://www.chainreactioncycles.com/Models.aspx?ModelID=15325
http://www.evanscycles.com/products/tifosi/xl-padded-bike-bag-ec011781
http://www.chainreactioncycles.com/Models.aspx?PartnerID=79&ModelID=23169
but I thought it best to consult the oracle of STW first!

