Is there somewhere I can store luggage (one or two normal suitcases) in San Francisco for a few days?

My girlfriend and I are taking a trip to California in a few weeks, and will spend a few days in San Francisco.

My next plan is then to rent a motorcycle and spend several days riding down the Pacific coast and then back up again to SF. The problem is that we can't put a suitcase on the back of the bike! Where can I store a suitcase for 3 or 4 days in San Francisco?

I know there are several questions with similar topics but I wasn't able to find any others that covered storage for several days. I heard the Airport Travel Agency at SFO mentioned but it's shockingly expensive: as much as $35 per day per bag. If we've got two bags I reckon it'd be cheaper to rent a hotel room just to store the luggage in!