[jacquesm]

Good co-founders you take on where and when you can until you have all the skills you need to make your team complete.

Compensation should be relative to value added rather than relative to time-to-get-on-board. That's because typically both sides know the value contributed and if one side tries to slant the division in their favour this will inevitably lead to trouble down the road.

Start-ups are fragile, if you build in tensions right at the beginning you'll get that back when you need it least.

For instance, during negotiations about an acquisition your key co-founder that you successfully screwed over during the founding days now has you over a barrel. After all he/she has very little to lose and you will stand to lose a lot. You could of course try to take care of that in your articles of incorporation and your shareholder agreements but you can't actually force someone to perform unless they are willing.

Keeping your goals aligned is good for everybody and fairly sharing a much larger pie is the best way to have the story end happily.

"After you gain traction" is sufficiently vague that I don't think that is a good criterion. Better would be that you would take into account the amount of capital already sunk and the risk that you perceive at the moment of joining. Lots of traction can actually mean more risk rather than less.