I see where you're coming from and other forums do it successfully, but it isn't necessary. There is plenty of money out there in other forms.
The last car forum I used has over 38,000 registered users but it has never asked the forum members for donations. The servers are pretty big too, so the forum is very fast. All of this is paid for with rolling ads and sponsorship from tuning companies, and Google ads at the bottom of every page. Being clever with analytics helps you get up to the top of Google, so even forum browsers can earn you cash. The extra cash means you can buy bigger banners and flags for the shows, raising the forums profile even more.