Knobby.ws or KeepTheRubberSideDown.com?
Both and capitalization does not matter. No “www.” either, but should work if added.
KeepTheRubberSideDown.com is fun, but too long. RubberSideDown.com was already taken and still is being used by a motorcycle parts supplier.
Knobby because it is a short term associated with mountain biking. Knobs are the raised bumps on mountain bike tires that give traction in the dirt. From that knobby and knobbies are frequently used in the mountain bike world.
ws? Where did that come from? Western Samoa if you must know. Since all the good dot coms and dot orgs are taken up, some web site hosts started pushing ws for “world site” or “web site.” And Western Samoa is small enough that they aren’t likely to use all their names.