We rarely get inquiries about hitch extensions or swing away adapters, until a customer has a problem.  We design and test the racks to fit directly into the hitch, but have no control what happens after the rack leaves the building.  When you use a hitch extension, the forces on the rack are multiplied as you are increasing the lever arm forces on the rack.  All hitch racks pivot around the rear axle.  The average vehicle has the hitch 32" from the rear axle and with two bicycles at 30lbs each and a 40lb rack, there is a force of 373 foot pounds of torque on the rack.  Adding an 8" hitch extension increases this force to 466 foot pounds, a 25% increase in force (an additional 93 foot pounds).  

We only make hitch extensions at 8" as anything longer will push the rack beyond its safety factor.  What complicates this further is cheap hitch extensions using standard hitch pins instead of anti-wobble bolts or other means to keep the hitch extension from rattling.  We reluctantly started making our own hitch extensions as most of the ones on the market are detrimental to the hitch rack.  The worst thing you can do is buy a 12" hitch extension like this from Harbor Freight or other seller  https://www.harborfreight.com/12-in-hitch-extender...

Sure, it is only $23 and has hundreds of positive reviews, but this will undoubtably ruin your hitch rack and you will eventually damage your bike.  Most hitch extensions are sold for towing, bike racks are a different animal.  Between the additional forces of the lever arm and the hitch extension rattling around like a loose headset, you are asking for trouble.  We post this in our instruction manual, but people rarely heed the warnings.  Everything bolts togwether and appears to fit, but there are problems lurking.

This same applies to swing out extensions that are available from other brands. These add at least 12" of lever arm and double the amount of play in the rack as there are now two couplers. We had a customer with a "we do not want to trash this brand"  swing out adapter secured with a cheap hitch pin, and within 18 months, the rack looked like it had ten years of hard use and we would not trust it with any bike. We explained the situation to him, and he told us "they said it was fine to use". Sadly, these businesses just want to sell and do not realize the downstream problems their products cause.  

There are many accessories available for sale that we cannot recommend using in conjunction with our hitch racks.  We looked at testing and certifying certain after market accesories with our racks, but it a daunting proposal.  We would have to purchase hundreds of products and test every combination of rack, which is impossible.  If you have any questions, please contact use at ride@rockymounts.com  Your rack will last a very long time if used properly, if not, all bets are off.