This sounds harder than it is– as soon as you start drawing bikes on top of the tracks it becomes very easy to see the answer.
The front wheel’s track will always be the longer one (though you don’t need to know that as you can just try both).
If you put your pencil at any point on the back wheel track, and draw a straight line tangent to the track, then if you’re going in the right direction, you will always hit the front wheel track and your lines will always be the same length. (Because the bike is a fixed length, and always parallel to the back wheel’s heading).
