hmmm... alright, I've gotta give my two cents on this. My profession is in SEO which completely revolves around getting people to come to a specific website and getting the sites higher ranked on Google. Of course those sites are consumer sites, whereas these are knowledge forums. However the more people you have coming to forums, the more knowledge that is shared.
I wouldn't have setup a completely other site. Except for the people who are already on this forum who own twins it'll be difficult to recruit newbies for awhile as your site won't have any value on the engines. Ie, nobody is going to find it.
What would have been better is to completely hitch onto this site IMO. I'm a newb to this site, however it was easy to find.
It would have been MUCH better to have branched off the Other Bikes forum in here, the sohc4 site, to have one or more child forums... one being Honda Twins. Then you could have pointed the hondatwins.com domain directly to the index page of that child forum for now. That way it would have benefited everyone. People looking for twins find a twins forum.... which happens to be part of a much larger forum site which could also hold valuable information. People looking for the fours, newbies and existing alike, are most likely like myself and check out other parts of the site as well since there are always good discussions.
If it grew to the point of needing its own forum then you would have had a dedicated following already. There would be some issues with breaking away later and pointing the domain to a "free standing" forum... such as bringing old threads and user data over ( with permission of course ), but your userbase would have already been there.
As previously stated though, that's just my two cents. I see you're using phpbb. If you're going ahead with it and need any help to customize it then let me know. I've used it a number of times.
Cheers,
XsTatiC