I have State Farm and I've tried to take out a personnal article on my bikes. my agent told me I didn't need one, that they were covered under home owners. I even made sure to mention the cost to replace them and he said I was still fine.
I have a Sportworks rack, mounts on the bakc of the car. I did ask about that and I believe it is not covered by my auto coverage, because it's outside the bumpers of the vehicle. I don't really think I'll back into someone fast enough to damage a bike, but I guess anything can happen. one thing I did hear if you were rear end that they might only pay a part of the replacement because it is used, only pay a percentage of the cost of a new one.
Call your agent & ask about a "hypothetical" incident. You don't want anything entered into the computer until you know you want to file a claim. AND read your contract. The agent may give an off the cuff incorrect answer & regardless of agent's statements, carrier is bound by the contract.