Increasing caster will also increase turning effort, as the wheel tries to lift the vehicle as it's turned away from straight ahead- - - -more caster, more lift. Try 3 degrees as a good starting point with radials, and only go to 4 or more if the vehicle tries to wander when driving straight ahead. Be sure the "center point" adjustment on the worm and sector gear is correct before tinkering with the caster.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!