Clutches slip due to total load on an engine. If an engine has less power than clutch it stalls. If it has more power than clutch it slips. How much tire, what size arm, bike weight and realistic hp figures all come into play. I would guess a 421 with a plus 10 and 12-14 paddle could use stock springs with around 15 grams per arm. also would depend on sand fluffy or wet.