Set of eight extra shock absorbing kit can be added by the factory only on the 3RS/3RR series to increase shock attenuation from 150-300 lbs. The 3SKB-SA fits all of the 3SKB 20” with wheels and on the 1SKB 20” standard shock racks, they can be added by the factory or customer only on the 1SKB-R910U20 and larger sizes.