my suggestion when doing your alignment is to line up the background instead of the rider. Add each layer up and make it 50% transparent and line it up, then return the transparency back to 100% and use the eraser to erase out the layer (using like a 30% strength) to eat away at the top layer to blend into the layer behind it.
Other tips: if you know you are going to do this type of sequence, stick your camera on full manual. White balance, f-stop, shutter speed. That way each exposure levels are the same.
For this particular shot, it looks like you shot at high iso and low shutter speed.