Parts take a hit each season when the rules change for them. You just don't see the full details of what has been restricted on a chassis or what those areas of the chassis are like.
Interesting if true. Do we know which parts saw a big change between S3 to S4? There doesn't seem to be anyway to view the previous rules? (they're not visible in the Rulebook under 'Championship'
@Tom: All the restrictions are on the Development page. You can see next season's from when they are announced which is before the end of the current season.
What I meant about not seeing the full details is you don't see where you can have winglets on a front wing for example, likewise, you don't know where you have winglets. You can apply things like this to all the other parts.
Ok the research system has changed again , its not following any logic. I've been investing in parts, which went up after dropping, but then dipped a level after going up. All whilst keeping the same research staff. Things just don't make sense, I officially give in, there's no point working it out when the goalposts change and there isn't an answer!