I have my own take on "gravel grinder". When I have it complete I am definitely going to share, but because I want to try painting it myself, is going to be a month at least before done. As far as I'm aware, there really isn't any rule for frame geometry, but just would be something that doesn't sacrifice much performance on the road, yet is capable of handling pretty horrid road surfaces. In my mind at least, a critical component is the right tires.
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