Ah, I see! That might work as intended... While wouldn't work while going up ramps, that would just be something rather minor, I believe... (Or you could also try linking that to specific instances of the bike's motion, seeing as when going up a ramp, it needs to switch over to different sprites... Implementing this would be the difficult part though. 
Here are a few images in two instances, where they go up a hill and change a sprite while also moving along y. The second one shows the shadow sprite, and how it moves sideways along the sprite. Will that script help solve this? And if so, how will I implement it...? (I wish I had more experience with scripting, but stuff tends to blow up in my face when I attempt it...)