I'm under the 26,001 weight limit, but over 10,001 with my racing operation. I'm required to have a DOT number and abide by HOS guidelines. Also required to pass a DOT physical every two years, carry a FedMed card, have current inspections on truck and trailer, and keep log books. Up to date fire extinguisher inspections too.
Why? Because I go out of state for car shows/races. Example: I go to Michigan for a show. I win first place. According to law my vehicle may get a bump in value from that activity/trophy. Even though no money changed hands it is enough reason to say I'm engaged in interstate commerce.
Same applies if I win money at a race.
What it comes down to: When you cross state lines in a potentially money making activity you are bound by the rules of Interstate Commerce. Period, end of discussion.
How do I know this? Myself and a friend spent hours with our states lead Motor Carrier enforcement officer. Really nice guy and patient answering our questions.
If you carry a DOT number on your vehicle DO NOT under any circumstances do your own brake work. EVER!!! It is one of the first things they check in the event of a crash. If something is out of whack and you worked on them your suffering will be legendary.
How picky are they? My truck had two failures its first inspection. One was a tiny nick in the left wiper blade. The other was a small chip on one of the fan blades. Got both fixed then showed receipts, got inspected again and got my sticker.
Take away from all this? Do not think you're smarter than the people who will be writing you tickets and taking you to court.
Old joke:
Know how to become a millionaire racer?
Start with two million then go racing.