RLKS spacer blocks are used to level a 3 bar roof rack system on a vehicle such as a pickup that may have rear posts or a cap/topper rack system. Attached between the Rhino-Rack 2500 leg and either a Rhino-Rack Heavy Duty or Vortex cross bar. These leg height spacers have been designed for use on a support bar, set up for long loads - they are not to be used as a 2 bar system under a load bearing cross bar. Made from glass re-enforced nylon, just like our legs, they are strong and flexible.