first i would like to say that this proyect is amazing, suddenly in some short time it improved amazingly, being honest i used to install it to have it there just in case i would get bored, but now it has my attention entirely. I'm used to make mods for some games or at least i used to, mainly for GTA, so somehow modeling and other minor task became my second nature, not meaning necesarily i'm good at it, but you know, i'm always giving my best, and since i changed to linux i never really had the chance of doing such thing till now with STK, so you may be listening from me more than once.
now, i would like to make some suggestions, from my humble point of view about the information for the kart tutorials, nothing too critical but certainly little helpfull details that would help in the making of the Karts at least (haven't tried making a track yet)
-Having an empty template in .blend format for starters/as reference, but not mentioned before the instalation of the addons, otherwise the template itself would be useless, escentially an empty project with a visual help regarding the average kart dimensions, i made my own Blender file template with something like this:
- boundaries
so i wont be making more tests in game than neccessary, also i'm not quite sure if Tux is the average midle size racer, i mean, not the tiniest nor the biggest, nothing would be more useful than having a "phantom" copy of the current Tux in the blend file or available as a reference even if it's in .obj format, while i have yet to finish the one i'm making, i'm expecting a big diference of size between my kart and the other karts, specially Tux.
-Maybe this is one of the less important parts for the average Kart contribuitor but i think some details could be mentioned like how the game handles certain parts of the karts,for example with the wheels, will it be using it's "origin" for the steering or simply calculating a new one with it's current bounds, if the number of wheels is limited by car, how the game knows which wheels are the steering and which are statics (now that i think about it, i haven't checked if the karts actually use some kind of steering for the front wheels, have to check out that at home later) how the collision is calculated for the karts or if any other kind of data can be used to specify the exhausts emiter or things like that, maybe it's because i'm used to check out all those sort of details, but just saying, no harm is done talking about these details.
that would be pretty much about what i have found, out of that i should say i never went too far from the main pages for succesfully starting to build my own kart, and what kind of member would i be if i don't show you my WIP:
- WIP from FoxyProxy
i'm making or at least trying to make FoxyProxy,i kinda have Coco Bandicoot (Crash TagTeam Racing or something) like look/animations planed for her, but i'm trying not to go too ahead of myslef, i aready could make the vehicle low-poly yet somehow decent looking, i'm not aiming for it to be included at the official racers, like i said in my first lines, i'm really making it for the joy of doing stuff and puting it inside a game, i find it very rewarding and amusing, and is something i enjoy.
well, i'm pretty out of words for now, great job STK team, i finally can back to modding, something i really missed to do. greetings.
but instead of Foxyproxy why not make the Fox guy himself? 
these details are ours (programmers) and as much as possible should be transparent to artists.
