I'm afraid I don't understand your question either. Assists have little or nothing to do with how your wheel functions. The only "assist" in LFS is auto-clutch, if I'm not mistaken?
Do you mean that you want your steering wheel to stay in place after you've turned it, instead of centering itself again? If so, that's the FFB. Setting the FFB force in the game to 0% should fix this, I think. Also make sure you have the centering spring disabled in the Logitech Profiler.