I would like to know where are the parameters that are controlling the FF of the bumps, hits, etc, because the FF strenght in game options necessary for me to feel them realistic, increases the steering FF making driving almost impossible.
Thank you.

I don't think it is possible to change the separate parameters of the ff in lfs, unless you have a lot of experience in programming and you'd have to dig pretty deep in the program to do it. However there is a value in the cfg.txt file that relates to FF called FF step num. I'm not entirely sure what it does but I have read about people changing it from 128 to 256 or 512 and getting different results. I tried it but didn't notice any change, but it may work for you. But I think it would be nice to be able to change separate properties of the FF because that way we could get the the most realistic feel from the wheel. I also find the wheel difficult to turn sometimes, and when i turn down the FF I lose all feel of the bumps. It only seems to happen in lfs for some reason.

I had the same issue with my momo black, i can not feel some rumbles strips, i will try the karl advice...

The FF ingame is purely based on what forces act on the steering rack, so there is no way to separately increase the FF the bumps on the track generate as opposed to the turn resistance, because they are both the same force.

What you can do is use the Logitech Profiler (if you have a Logitech wheel) and set the FF strength to something like 105 - 110%, which will cause the wheel software to exaggerate small forces. This can help you feel bumps when driving straight over them, however the wheel will still be dead when driving over bumps while cornering, because the turn resistance drowns out all the other forces.

Maybe Scawen will at some point redo the way bumps are handled which could maybe increase their presence in the FF, but that's not guaranteed.

Setting more than 100% of ff strenght in logitech profiler causes forces to oscilate around centre.(people used to set 101-110 to exaggerate low forces around centre, in my opinion it is a mess setup)

Instead im using LFS ff step num 64, logitech profiler 100%, and lfs 10-30%(depends on car im driving) it provides far more realistic fellings. Setting more than 30% in LFS and more than 100 in logitech gives too much countersteer to feel real!!

Furthermore i noticed, that its sometimes better when logitech and lfs have the same ff level(once again it depends on car), then i tune both, logitech and lfs somwhere around 20-60%.

Another discovery?
logitech set to value somwhere around 70-80% improves bumps for dfp,
or its just my personal feeling( xfg so2 ffstep num 64 logitech 70% lfs 50%