You mean the very annying "sound" that only breaks your ears and headphones? Definately the most annoying thing in LFS. It's not actually a crashing sound, there isn't one but because all the sounds in LFS are generated, when you crash it uses (as far as I know) the same scraping sound which would be heard when you damage the suspension.
Anyway, I don't even know from what sound files that sounds comes (maybe from all of the tyre scrub sounds?), actually would be nice to hear from Scawen because looks like this issue isn't very high on his to do list. But at least lowering volume in ScrapeHard.raw and ScrapeSoft.raw files helps a bit. I've attached files that has only 2% volume compared to the original files and the annoying sound isn't completely gone but at least much lower sound when you go over chicane curbs etc. And crashing a wall doesn't break your ears anymore. Drawback is that you can't hear the scraping when your suspension is broken...
put your Live For Speed\data\snd folder and make first backups of the original files.