Grease caps are the small, round, metal covers you see at the very center of a wheel. Grease caps protect your wheel bearings from the dirt, dust, sand, water, salt and road grime that can (and will) destroy bearings. Without grease caps, your wheel bearings simply wouldn't survive for very long out on the road.
Trojan have available a range of Grease caps to suit most common Trailer hubs.