To many variables to give a very good answer. When setting up the backdrop based on light falloff from your light source you need to know how much ambient light will be coming into the canopy. This is going to change throughout the day as the sun moves around also. What type of lens will you be using? If you move the backdrop further back you will need to be shooting with a longer lens to keep from showing the ends of the canvas. How much space in front of the canopy will you have to work with?
Given a choice I would want as much room as I could get behind the bikes (minimum of 6') and shoot with a longer lens if you have room in front... but that's just me!
Great idea, good luck pulling it off!
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