Mechanics have no tie in to this rental system. They are always guessing at which bikes went out. While the hours may go up, they don't indicate the last time a bike was worked on. At a basic level, you could have one additional field on tracked products "hours since last service"; that goes up in tandem with the "hours rented out" field. But, that we can reset to 0 when the bike has been worked on. That way the mechanics know what items to work on, and which they can ignore.