Re: built a low rider, but bike is unstable. anyone know why ?
Bent springer forks introduce more than a mere trail issue, they actually cause the the front wheel to "sweep" instead of "twist."
Imagine standing astride a bicycle with normal forks looking down at the front wheel.
Twist the handle bars, and the wheel twists along directly underneath.
The same experiment with the bent forks will show the wheel to be some distance out in front of the bike so twisting the handlebars and head set results in the wheel "sweeping" in a concentric circle (arc) around the headset. The longer the distance, the greater the effect.
(straight chopper forks introduce another effect as the wheel stops turning, and simply lays or "flops" over instead of turning the bike )
The upshot is; bicycles have evolved to look the way they do for a reason, but it sure is fun to change 'em - you can really feel the geometry!