US Flag Only
If only the US Flag is on your bike, it should either be at the center, or to its "marching right" - on the right side of the motorcycle to the rider's perspective when facing forward.
US Flag And One Other Flag Of Any Type
If the US Flag is on your bike with another, it should be to its "marching right" - on the right side of the motorcycle to the rider's perspective. If the other flag is that of another nation, it should be the same size and at the same height of the US Flag - NO flag should ever be displayed higher than the US Flag.
US Flag And More Than One Other Non-National Flag
If the US Flag is on your bike with several other non-national flags (POW/MIA, ALR, Eagles, Service Banners, etc.) , it should be at center and higher than any of the other flags.
US Flag And More Than One Flag Including Those Of Other Nations
If the US Flag is displayed on your bike with those of any other nation, the flags should be same size and at the same height, with the US Flag to marching right (right side of the vehicle), and others arranged in alphabetical order to the left. Other flags should be arranged in order of decreasing importance - Nations first, states (in order of admittance) and territories second, military third (in order of establishment), and then any others. Again, no flag should fly higher than the US Flag, but the US Flag should be no higher than that of any other nations displayed.
I and others have argued that, since the small bike flags we use are all but invisible from the front (when mounted on the rear), the concept of "Flag's own right" should be used with the vantage from the rear of the bike. (This would place the Flag on the on left-hand, rear of your bike). This concept, unfortunately, overlooks a more applicable concept.
If you equate the motion of your bike with marching, and you equate traffic with a procession, another portion of the Flag Code becomes the obvious choice for display of the Flag alone, or with another:
Rule 9: "The Flag, when carried in a procession with another flag or flags, should be either on the marching right; that is, the Flag's own right, or, if there is a line of other flags, in front of the center of that line."
The second portion of this rule does not work well with most motorcycles, since there usually is no means to mount the Flag in front (in the direction of travel) of the others if all flags are to be mounted at the rear of the bike. In this case, we rely on:
Rule 10: "The Flag of the United States of America should be at the center and at the highest point of the group when a number of flags of States or localities or pennants of societies are grouped and displayed from staffs." Included in this would be POW/MIA flags and American Legion Riders flags.
Finally, if you are displaying another country's flag along with the US Flag, they both should be at the same height and the same size, and the US Flag should be displayed on the right side (again, marching right). This is basically Rule 11: "When flags of two or more nations are displayed, they are to be flown from separate staffs of the same height. The flags should be of approximately equal size. International usage forbids the display of the flag of one nation above that of another nation in time of peace. The order of precedence for flags generally is National flags (US first, then others in alphabetical order in English), State (host state first, then others in the order of admission) and territories (Washington DC, Puerto Rico, etc.), Military (in order of establishment: Army, Marine Corps, Navy, Air Force, Coast Guard), then other."