The manual gearboxes in these cars are behaving like sequential. You can't shift from 4th to 2nd for instance. You can't skip any gear.
That is a known item, but not so much a bug as how the new shift protection works with a manual style transmission. Hopefully in the future they can improve the shift protection routine so that rpm matching allows skipping a gear during downshifts.
Oh, I think it's a bug. Not car specific, but a bug. And probably hasn't been bug-reported before (just discussed in a number of threads), so fair enough.
The clutch requirement comes from a parameter in the [GEARPROTECTION] section of the .hdv. So in that way it's related to 'shift protection', but it's not shift protection, no. None of those parameters suggest any actual gear restrictions, either.
Old cars don't need clutch at all. Ever. Just lift the throttle and throw it into gear. Many cars don't even need the lift. They attempted to add a clutch requirement, and they did, but it has this sequential-manual bug (except changing into 1st, which you can do from any gear). As Dave mentioned this simplistic approach also ignores subtleties like rev-matching, but seemed like most people were ok with that when at least you actually needed a clutch. Closer to realistic than we were.
Ok so it's worse than I thought. I was blissfully unaware of the clutch issue and shifting gears happily in the older cars. I really hope the devs are planning a rework of the clutch and manual gearbox code?
I hope so too, and I hope they're aware of this bug and going to fix it, but if they say something here it'll be the first thing I've seen. I make use of the Realistic Gearshift plugin/program to make me use clutch when driving old cars, and to help identify bad shifts in the caterham. Unfortunately that's what we're limited to until they do actually make some changes in this area. I think we can assume that when they do a drivetrain overhaul it'll be very good, but I think that was identified as on the list in 2013, so it's certainly been a game of patience.
I can't wait for the drivetrain overhaul. I was hoping that it would've came as part of the inclusion of hybrid but that is seemingly not what's going to happen. AMS2 has things like half shaft flex, a variety of differential types, prop shaft flex, clutch wear/heat.. ect.. It's a leap ahead in terms of realism, and these things really add to the driving and realism.
I think many people are blissfully unaware of this clutch requirement because so many of them use the auto clutch aid. But I'd also like to see this fixed and the gearbox changes for realistic shifting along with an output that would allow hardware makers to implement a grinding miss shift.
AMS2's drivetrain physics have improved but, there's still some missing pieces to that puzzle. It remains to be seen if Reiza are able to leverage the game-engine to full effect (such as iRacing does). Gear-box oscillation is non-existent in motion effects but, can be heard in some car audio samples, so it's clearly not physically linked at this point. Some cars do produce a shudder (Flex) effect upon up-shifting but, is missing in other cases. It certainly may be a matter of WIP. rF2 seems to have some form of drive-train effect (surface bump oscillation via audio, at least) although, it's hard to know if that is done physically or via some audio hack alone (Like AC does). The real tell comes when the evidence can be felt, as well as seen and heard. Currently, there's not much to suggest that the physical drive-train model inrF2 has progressed much, if at all. There is motion related to hitting the rev-limiter though, which is nice. The manual shift bug certainly is a problem with these cars, though. Dropping a couple gears when braking very hard is second nature and I can't tell you how many times this bug has caused me to lose my focus, as well as immersion.
Quick correction to something I said: Paul was quoted as saying the shifting bug is a known issue to be looked at, in the Caterham thread. I assume it was probably on discord, I might search later just to confirm.