Snap ups are usually held in by pressure of a bolt in the back. If you're are sliding, either the back bolt is not tight enough, or, you don't have enough links between your bars and the snap ups.
TV - 2015 Ram Truck EcoDiesel
TV - 2006 Jeep Liberty Turbo Diesel (Retired to Daily Driver)
TT - 2015 Rockwood Roo 183
Nights Camping --- 2015 - 19 Camped | Winterized -- 2014 -18
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