I dunno if you can call them "large", but SovietSteeds.com is a decently active forum for fans of Russian motorcycles, especially as the name suggests Ural and Dnepr bikes made during the Soviet Union. It's a spinoff of the older website Russian Iron, apparently there was some drama with that site's owner but I've never dug into it. It's an absolutely mandatory repository of information if your an English-speaking owner of one of those bikes, since the Owner's Manuals are notoriously thin on details for a lot of essential maintenance tasks.

I get the feeling that most of the active posters are :grillman: -types from Washington state.