I still doubt the validity of the 'real' bump claims. The laser scanning won't be accurate enough to get them from there, and the datalogging from real cars will have too much noise to make plotting bumps accurately (except possibly the largest bumps). So I think iRacing is mostly made up of random bumps added to the contact mesh, rather than 'real' bumps.
So LFS/nKP could add lots of random bump noise to achieve the same effect.
Then, when you drive on a track in real life, even a 'bumpy' one, you'll see that you don't feel the bumps through steering weight changes...
There are of course different ways to add random bumps. Rfactor's and ISI's "random road waves" is probably the worst example and imho iracing's method is so far the best. Just the fact alone that there are bumps gives a ton of character to a track.
I've been doing some tracks with bob's track builder and it has quite neat feature which basically adds bumps simply by changing the 3d mesh of the track (you just edit the maximum of the randomness). Plus you can easily add bumps by manually editing the vertices of the road surface. It takes literally few minutes, maybe 30 minutes to do the whole track and it totally escapes me why so many btb and rfactor tracks are so smooth. I can't believe it is much harder to do with purpose-built 3d-editing software if it is so easy with a 50$ app.
Apart from some other reasons (or failures of not being able to have bumps on tracks) imho it is just plain lazyness or done on purpose that many tracks are just so smooth.
I have no idea why nkpro or lfs devs don't do that. Nkpro's Aosta for example is just plain smooth with few accidental "sharp edges" in the 3d-model. Just like lfs' south city has only few clearly accidental bumps. Imho the tools are there but developers in most cases have simply chosen to have smooth tracks instead of having any bumps.
last time i tried iracing it made me feel more and bigger bumps through the wheel than ive ever felt even on the crappiest of public roads (granted german roads by and large are excellent but they certainly are a lot worse than your average race track in terms of bumpyness)
Laser scanning is actually that precise to be able to collect information with enough accuracy to collect data about the bumps. It is not millimeter-precise (iR says that the scanner they use has the precision of one inch for the distances they collect scans from) but gives enough information to create a physical surface that is far more real-feeling than any scratch made surface. Many real drivers (not only the guys from their adverts) said that the bumps they feel during driving IRL are in the same places on the virtual iRacing tracks.
The inpact of bumps on force feedback is other thing though. It varies from car to car. Solstice feels really smooth. Legends car, on the other hand, feels like a kart. Maybe the bumps influence on force feedback is too much in some cases, but FF is having other role here as well - it gives more feedback though the wheel to the driver - the feedback you would get when you sit in a car with your own bottom.
But it's the tire modelling / physics that's the problem, right? When it comes to iRacing FFB generally, sometimes I wish it were a little more like LFS FFB, reacting to everything, but other times the iRacing's FFB feels nice too. The wheel resistance thing for example, is one of the things of the iRacing FFB that I like.