I mean, it's not
required to use them at all. But if you want to get enough positive caster for a decent alignment with a set of radial tires you'll need them in both locations.
Even using 2 eccentric bushings per side it's unlikely you'll be able to get more than +3.5° to +4° caster. Which is the most the SKOSH chart shows, although honestly I think the SKOSH chart is really conservative on caster. Personally I think you can add +1.5° to its caster recommendations across the board even for manual steering. I'd never run less than +3° even for a cruiser with radials.
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