Many ovens have a self-cleaning feature that takes six hours or so, and you have to remove the oven racks while that happens. Instead of trying to scrub those oversize racks at your sink, use the time to give them a good soak in the tub.
Whether you let your oven self-clean or clean it by hand, you still have to deal with cleaning the racks. The folks over at theKitchn recommend soaking them in your bathtub to loosen up all the baked-on grime.
Lay a towel or two in the bottom of the tub to prevent scratches and place the oven racks on the towels. Fill the tub with very hot water (enough to cover the racks, anyway) and mix in 1/2 cup of dishwashing liquid. Let it soak for at least 4 hours and then brush off whatever remains on the racks.
How To Clean Oven Racks In the Bathtub [theKitchn]