An inexpensive way to stop the majority of the rocks is by adding mud flap behind the dual tires. Cost is less than $120. Stop the rocks where they start. The need to be at least as wide as the tires, and they should ride within 6" of the ground.
The hardest part of this is sometimes finding a metal base to attach them to.
Then when the money comes in add the mud flap on the back. Just make sure it is 4-5" off the ground. If it is too close, it can end up creating a wind tunnel effect and throwing rocks instead of stopping them.
The rear mud flap stops more than just rocks. We hear stories all the time from folks that find nails embedded in the back of the mud flap and those that have t-boned the mud flap with a tire or piece of lumber in the middle of their lane.
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