Why do some bikes stay on line when bar pressure is released, and some don't?

[Cobie Fair]

OK, anyone think they know exactly why (and can explain it clearly) one can release the countersteering pressure to initiate the turn, and in other bikes (once the bike is on line) one has to hold a little pressure on the inside bar to keep it on line?

OK geeks, come out of the woodwork! 

[tzrider]

One of the stronger factors I've encountered is the amount of trail a bike has. I first noticed this on a make and model I knew well, where my bike held its line and a friend's bike wanted to stand up in the corner. Same tires, in similar condition. The difference was the rear ride height; his was lower. When we raised it to spec, his bike also held its line.  It made me recall riding other bikes with a lot of rake, which had the same feeling; I had to keep some bar pressure to hold the line. What the two scenarios have in common is that there is more trail on the front wheel.

For anyone not familiar, trail is the distance between the pivot axis of the front wheel's steering and the contact patch of its tire. The pivot axis can be visualized by projecting a line along the axis of the steering stem in the frame down to the ground. You'll find that this axis intersects with the ground at a point ahead of the front wheel. The distance between this point and the contact patch is referred to as trail.

When the bike is vertical and underway, trail influences the bike's stability. Longer trail tends to be more stable and harder to steer. The front wheel behaves like a shopping cart wheel, where it wants to center its contact patch behind he steering axis, aligning the wheel with the bike's direction of travel. Longer trail gives the contact patch a longer lever to apply this centering force to the steering axis, so the self aligning tendency (and resulting stability) is stronger.

When the bike is leaned over, this centering force is still in play, but an additional force is introduced. Because the steering axis is leaned off vertical, the force of gravity on the contact patch and steering pivot point applies a sideways torque that twists the front wheel into the turn. This tends to countersteer the bike upright. Bikes with shorter trail don't apply as much of this twisting force because the "lever" by which the contact patch can apply sideways force to the steering pivot point is shorter. While there is a twisting force, it's too small to notice. With longer trail, the lever is longer, the force is greater and it does affect the bike's line.

The bike also tends to stand up more the steeper the lean angle is. The rider may notice that at high lean angles, it takes more bar pressure to hold the line than at mild lean angles. This is consistent with the geometry of these forces, since at high lean angles, the force of gravity gets closer to perpendicular to the steering axis and a higher percentage of the weight on the contact patch applies the twisting force around the steering axis.