I've looked at the Tier 4 Towtector and think it's probably the best hitch mounted system out due to the fact that it combines brushes and a rubber backing. I think this helps alleviate complaints from those that have owned rubber flaps and brush flaps. I have had two sets of Rock Tamers and I still drive slow on the gravel with all of my trailers. The rocks indeed do bounce off and the faster you go, the further they bounce and with more trajectory. of course being solid rubber, the rocks don't get through but inadvertently do when they're bouncing all over. With the brush, lots complain that rocks have a tendency to get through however, the rocks that do get knocked down don't bounce nearly as bad as the rubber systems. the brushes take out the energy better than the rubber flaps.
With the Tier 4 system, you have brushes to slow and dissipate the energy and a rubber flap behind that to knock down any rocks that make it through the brush. I think it would help with the bounce effect also as the rocks would have to go through the brush, hit the flap, and bounce back through the brush a second time. I would think that would eat up most of the energy and keep rocks from bouncing all over. With that said, I haven't owned a set but I will be very tempted the next time I buy. for me, I have 5 trailers so buying a system that only protects the boat like the flack jacket is great for the boat but my other 4 trailers then suffer, I need a hitch mounted system that offers a little more versatility.
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That or I need to hit the loto so I can pave the road out to my house!