Alright to explain this... We had a character with lower item level gear, therefore his health in the bike was extremely low.
What you do is have a person with higher item level gear get into the bike. They then jump into the passenger seat with the bike icon to the right.
After that, have the lower item level character jump in the driver seat and shoot the passenger out by pressing 4.
This leaves the bike with still the same amount of health the higher level item gear passenger gave it. Rinse and repeat if you have very low level item geared people!
This is not just a display bug, the health on the bikes drops in percentage along with the fight. We used this on 4 tower flame leviathan.