Originally Posted by
Road Fan
I mainly mess around with road bikes, so for most of my tires there isn't a lot of center-tread build up, at least not so my clearances are challenged. When I calculate trail I have both measured tire diamter and estimated it with a simple equation. Like all models the equation is not perfect, but it is useful:
diameter = bead seat diameter + 2 * measured tire width.
Sometimes it matches reality and sometimes it doesn't.
For the head of a cycle engineering department, it makes sense to need to understand the loss of trail with tire lean. It also makes sense to study a Vehicle Dynamics text book, most of which analyze tire contact patch issues for single-track vehicles, and how the tire interacts with the road under vehicle and dynamic loading.
No disagreement there. My point is that I agree with the speaker that obsessing over small differences is pointless (if that's what he said) since there's decent latitude before it becomes material. Logically, if small differences were material, we'd expect to have more analysis of how things like tire section affect handling. The fact that there's not so much of that is evidence that there's some degree of forgiveness.
Fork rake tolerance also has to be considered in the context of head tube angle, and wheel diameter tolerance, since trail depends on all three. If you look at a sine chart, it'll be obvious that tiny changes in head tube angle can be more significant than a 2mm change in fork rake. (very roughly equal to 1mm of trail / 0.1° of head angle difference)