Getting the diff to lock on acceleration, even with a 21.5t, is the real issue...The slipping on black carpet is terrible. I heard steel balls will have more bite than ceramics or carbide, but they will not last as long...I am still experimenting to find a good solution, but the diff bolt keeps snapping with a 4.5t on black carpet...The key is to find enough friction to lock the diff gear ring and the outdrives when power is applied, without much diff spring tension....I will be moving up to 3.5t on crc black carpet to experiment some more....