I would box the bike not use a bag, you need to pull off the derailleur, bars, seat+post, wheels etc. Its a good idea to remove the brake discs and put all them as well as all the small bits in a separate box inside your bike box. Also smart to remove the derailleur hanger and put spacers in the brakes and cutting a piece of wood to space out the fork dropouts to protect them.
Take some time to construct a double layer box out of cardboard with a triple layer bottom so it stands upright, I usually ziptie the front wheel to the frame and deflate the tires slightly to 15psi, I usually don't touch the fork pressure...I have never had a problem with them. I turn the forks around so they face backwards and ziptie the handlebars to the top tube so you can leave cables attached.
In general all your doing is duplicating how a bike is shipped in a box from the factory, if you can get an actual bike box it will make your life easier, heavy blankets work well for protection but cardboard and foam is good too. Wrap exposed frame components with foam or cardboard and add bumpers where the hard stuff will hit the outside of the box, I usually use triple layers of cardboard in those locations.
Here's how the factory does it;
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Take some time to construct a double layer box out of cardboard with a triple layer bottom so it stands upright, I usually ziptie the front wheel to the frame and deflate the tires slightly to 15psi, I usually don't touch the fork pressure...I have never had a problem with them. I turn the forks around so they face backwards and ziptie the handlebars to the top tube so you can leave cables attached.
In general all your doing is duplicating how a bike is shipped in a box from the factory, if you can get an actual bike box it will make your life easier, heavy blankets work well for protection but cardboard and foam is good too. Wrap exposed frame components with foam or cardboard and add bumpers where the hard stuff will hit the outside of the box, I usually use triple layers of cardboard in those locations.
Here's how the factory does it;
