The thick part of the caster shim needs to go toward the front, tilting the axle and the king pins toward the rear at the top. Caster allows the wheels to return to center after a turn by lifting the front of the vehicle slightly as the wheels are turned either direction away from straight ahead. The weight of the vehicle tends to force the wheels back toward center. Think about the fork on a bicycle- - - -the forward slant of the fork pivot does the same thing- - -the weight of the rider makes the bike want to track straight ahead. Zero caster would cause the driver to constantly "hunt" for straight ahead with the steering wheel.

Too much caster will make the vehicle difficult to turn since the driver is lifting the front end with the steering wheel. It will also cause a too-fast return to center after a turn.
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!
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