If you've got enough tread on your tyre, it'd probably push it foward and snap it, or the mounts on your frame. It looks an, alright idea, just needs more thought into it. How are you going to support it all the way along? I know you said it will be stiff plastic, but it will still flex if you sit near the back of it. Also make the plastic to stiff and it will probably be brittler and a little easier to snap. Good idea thou, keep working at it Try drawing using proper equiptment too, I know it's abit kiddish, but once you get used to drawing a circle with a potractor or template sheet, your freehand drawing gets better, same with rulers ect.