I am about creating a spin-off from University. I will quit my job later even. I talked with all my colleagues and they like the idea and support it. My experience with them in academic projects is that they always say "oh, we like your idea is nice and you can count on us", but at the end I always end doing all the job and their names appear as co-authors in all publications and the like. Now with the spin-off I feel same thing is going to happen. I will have a meeting with all them in a few weeks and somehow I want to tell them I am really serious about the spin-off and leaving the University. At that point I would like to tell them something like "if you want to be in the spin off, you need to contribute either with a minumum % of your time or money", since I am tired of their vague words and parasitic attitude. What do you think is the best approach here? Perhaps should I force them to sign some written document?
You are going about this all wrong.
Forcing them to sign a written document under these circumstances is only going to create ill will. So you'll be starting off on bad terms - not a good thing. Second, just because they sign the document saying they'll do work, doesn't mean they are actually going to do work. The document in itself can't force them to do any more work than they would have without it. In fact, you already know from experience that they will promise to work, but not actually follow through with it. What makes you think this will be any different?
Trust me, these are not the people you want to work with. You want to be in business with people whose word you can count on. If you go through with it, you're likely to fail and lose the friendships in the process.
To answer your question:
What do you think is the best approach here?Don't work with them. Either work on this by yourself or find other co-founders. But if you find others, make sure you can count on them.
Forcing them into a written commitment is likely to be less than optimum.
A better approach would be to assume you'll be on your own, but welcome help as and when it becomes available. So, no equity/salary for your colleagues, unless they step up.
Consider the following:
You can lead a horse to water, but you can't make them drink.Why would you ever want to force someone to do something? Wrong attitude. You need to uncover their true aspirations, determine if there is a match to yours and then gain their commitment (be it with salary, options, shared future, etc.).
If can't do this, then move on. Backing them into a corner isn't going to prove anything.