i understand in multiplayer only some packets are sent within some time intervals, but on user's machine actual simulation is happening, with opponents car as obstacles. I mean, its always server synchronizing to user, making cars jump and jitter. On a client side client car moves smoothly as if it was hotlap just with abruptly moving obstacles of other player's.
Client can alter parameters whatever he wants, but if client is obliged to store his input during the race, which is then sent to the server, server would have: initial car setup, client's input, moving obstacles (other cars) positions which server itself was sending to the client - there is no unknown parameters, we just re-run the sim on the server - and if there is a difference more then 1-2 meter at the end of the race (or we could shrink it to per-lap check) player either sent wrong setup, changed it during the race, altered positions his client was sending or any other trick. Basically the only way to overcome this system (to generate a valid user input which is in sync with all the hacking) would be to write a driving bot faster then native lfs AI, which seems like pretty non-trivial task to put it mildly.