This is how rfactor works much different mods and tracks many leauge use mods that they have tweaked self tweaked=chance psysics engine and so on so wanna drive in liga on this servers usually have pw on and is just for leauge members to drive on.
Servers that is public=not have password on usually is mods that can be download from rfactorcentral and tracks from same site.
Then it is servers that is public and only use default mods & tracks this servers you can join
rfactor full install comes with default mods and 14 tracks .
What i suggest you do is when you see a server that you wanna drive on a special mod maybe see info = motd if server has write the link to the site they have, or look what server name they use many servers has write site name.
Google the site name and find site, mod and tracks you must install to enter sites servers and often they not use mods and tracks you can find on rfactor central.
On many sites you maybe must sign up=be member maybe donate money for cost of sites rfactor servers.
One thing to enter a server you must have same Mod/Car & Same track that servers use. when not have it you enter server then kicks out from it aoutomatic for not have files you need.
Tripp Team host servers and have a site offcourse so drivers that wanna race on our servers get links to the mods and track they need !
You ask " If u install a particular series you are automatically installing all type of cars involved in that series?" off course they give you or have link to mods and tracks on the leauge site
The bad part is that uninstall mods and tracks is usually need to delete them manually i hope to see more exe install with uninstall exe in the future .
