After you get the suggestions in post #15 implemented, I suggest you pause and revisit your relations. I did not look at your DB but I sense a need for an additional table.
I am going to guess a single rider, that enters a single event, may want to enter multiple classes. There are several approaches to manage these relations. Bear in mind, you can use VBA to update tables. It is not mandatory to bind all controls on a form (like June mentioned, use your bound form to update one table). For instance, you may deem it beneficial to bind the Rider Combo and not bind the Class combo, using VBA to update a record with Class data in a table not part of the form's recordset.