I have a blog that ranks quite well in its niche and have close to a thousand posts. The problem is that it attracts a lot of comments, many of which are very long. I had wanted to move all these comments to a forum on a subdomain. Then I could place links on the blog posts to different forum threads with related questions and content. I am wondering if this will hurt my blog on the main domain in terms of its ranking on Google? Will Google view these links to the subdomain forum as linking to an external site? Plus a new subdomain will be a PR0 so will it cause me to lose PR on my blog? Any advice would be appreciated.