With micro web frameworks you're doomed to implement a subset of Rails, or have to dig around to find and configure one of the gems usually associated to it.
For those that are ok with this, or already have an understanding of the limited scope of your app, I strongly recommend Roda [0], it's a much more straightforward routing system with better memory/cpu usage.
I can't find a single instance for which Sinatra can be used where Roda is not a better choice.