Frame geometry question

[toaster]

Does descending down a hill increase the effective head angle and thus decrease trail?

[jerk]

ahh....you should really go to this awesome place in santa cruz california called the mystery spot which was once on ripley's believe it or not. gravity does not apply there in the regular manner in which it applies at every other spot on earth....the jerk doesn't know if frame geometry changes there as well...but the important thing is that eddy merckx made sure his bikes had very strange and odd fork rakes for milan-san remo because the race ended with a descent....so there you go.
jerk

[slowgoing]

It does if you crash the front end.

[Climb01742]

all i know is my MXL descends better than any bike i'll ever ridden...and this comes from a HUGE descending wuss.

[Ti Designs]

A change ininclination changes the handling but not the geometry. Trail is defined as the distance between the steering axis and the point of contact. If you picture the front end of a bike, continue the line of the steerer tube (the steering axis) to the ground. The distance from that point to the wheel's contact patch is the trail. Now if you take this picture and rotate it say 45 degrees, what changes? As long as both wheels stay on the ground the geometry doesn't change.


That's not to say that how the bike handles doesn't change. Let's add one more point of interest to this picture, the rider's center of gravity. That would be the balance point along X, Y and Z, probably just in front of their torso (the center of gravity doesn't have to be a point on the rider, just some point in space). While the geometry of the bike is based in points of intersection with the ground based on angles and offsets, the direction of force of the center of gravity is based on gravity (was that redundent or am I repeating myself?). So, on this picture that has been rotated 45 degrees forward you'll notice that the force vector is now pointed at the front wheel. This changes the weight balance of the bike and how the bike handles.

[toaster]

OK, as long as both wheels stay on the ground. I guess what I thought was happening when descending is not the same as lifting the rear wheel off the ground which definitely changes the head angle as it rotates around the wheel axis and changes trail. Right?

[Louis]

Right

[slowgoing]

Trail depends on the head tube angle and the fork rake. Those don't change. Not when you ride, not when you turn the bike upside down, and not even if you take the bike up in space where there's less gravity. You may initially measure these parameters when bike is on level ground but that's just to establish a reference point for the measurement.