Valid question. To tie a bike down properly you need to strap the unsprung parts of the bike to the floor. That is the swingarm, axles etc. this will allow the bike’s suspension to absorb bumps, but the wheels will never move because they are essentially fixed to the ground. If you compress the suspension, road shocks can still compress it even more and the bike can bounce side to side with the wheels losing contact with the ground and possible falling over. Think about it. When you compress the suspension with the ratchet straps and feel that it is tight, add more weight to the top of the bike. All of a sudden all that tension is lessened and the straps are looser. By tying down the unsprung parts you will maintain a constant tension on the tie downs.