Trail — marked green — is the “on ground” measurement between the plumb line from the hub and the projection of the steering axle.
(More precise parameter is the mechanical trail, which is measured from the ground underneath the hub perpendicular to the steering axle — but we will put it aside for now).
Apparently, Trail is quite a modest measurement — normally within 10 centimeters. So, why it is that important?
You won’t believe it, but it is still a scientific mystery why a bicycle gravitates to staying upright. In short, there’re two theories:
• due to the gyroscope effect,
• because of the caster effect — or Trail.
A further explanation is here:https://www.fastcompany.com/3062239/the-bicycle-is-still-a-scientific-mystery-heres-why
Anyhow, Trail affects how the bike handles. Same as a caster is straightened when a shopping cart or an office chair is pushed forward, the bicycle front wheel is stabilised straight when it is pushed from the rear.
And the larger the Trail, the more stable the wheel — at a given speed: a bicycle is normally moving fast enough to be stable at more or less any positive Trail. But, as I will explain, there are nuances.
Okay, less words & more pictures already. And right away — the wild card :)