First, the rear tire will always be on the inside of any substantial curve. Your tangents not crossing the other line is a way to demonstrate it. I just think it’s simpler to point out that the rear will be on the inside (just a personal preference probably).

I used the same method as you and decided the lines aren’t well drawn, as the tangent distance from the rear to the front is not constant in either direction. It appears to me that the distance is closer to constant going left than going right.

Your conclusion comes from the fact that you only use 3 points. Using almost any point on the near vertical part on the left would show that going right cannot work. Also, if going right, the inflection point of the rear wheel curve is after it crosses the front wheel’s track. Taking your tangents between the crossing and the inflection point, you would have to conclude that the front wheel was going left as the rear wheel went right.

Your analysis would be good if you knew that the tracks were actually made by a bike, but these tracks were more likely made by 2 unicycles. They are actually more likely an artist’s rendition, and are not a depiction of real tracks at all.

Non the less, you did find the correct method to deduce direction where Sherlock Holmes failed.

Congratulations

As I was moving at low speed, I had a very pronounced front wheel wobble, but the rear wheel showed no noticeable deflection: that is to say the tangent from the rear wheel did not always intersect the track of the front wheel.

I don’t think the distance between the front and rear wheels was changing noticeably: though the speed of rotation of the two wheels relative to each other may have been changing. The wobble may have been timed to match my down-strokes on the pedals. My winter tires do have chevrons, but it is trivial to install the tire backwards! In snow or mud slippage is probably the best indication of direction: The “knobs” (whether directional or not) on the tires will make a track the “points” (or drifts) in the direction of movement (during side to side slipping). Wheel spin will deposit debris behind the point where the tire spins (which shows up as a smooth spot).

A better way to draw the lines would be to run the wheels of a model bicycle through ink, then run that across a sheet of paper. The problem is that such a model would not balance like a real rider. Then there is the more tricky and bandwidth hungry method of photographing actual tracks.

The fact is unless we have atleast 2 sharp curves it will be difficult to assess the direction. Lets say, a very skilled rider going very fast on a smooth terrain will produce no curves and hence no way to judge the direction.