The sub forums are normally decided on by Muhammed the Adminisrator of the site. It is mostly due to how many members are accessing information on Bob-Cat brand mowers. The more, the more likely he will have a sub forum for them, For now you would have to post in the general forum. An idea would be to post a message for all Bob-Cat users to reply to your message to determine if the need is high enough to create it's own sub forum.
Oh and welcome to the forum as you just increased the need for Bob-Cat sub forum.