Okay, so the procedure of adding pit boxes and start positions is not very complicated, but can be time consuming. It can also get a little bit repetitive, but I guess repetition is a vital part of racing, so nobody should complain about that
Basically what you have to do is import the track to 3DSimEd and locate the pitlane. Check how the pit boxes are placed – they will usually look like cubes or flat arrows. Then find a place in which you would like to place your next pit box.
First, right click on the surface (ground) and select "Memorize XYZ". Then, right click on the last possible pit box, for example in Cadwell Park there were originally 18 pit boxes, which means the last possible will be named AC_PIT_17 (as the first one is always named AC_PIT_0). Right click on it, select "Object...", then "AC_PIT_17". A window called "Object instance edit" should pop up on your right. Select from it the "Clone object" option (the purple cross, like on Swiss flag), this should create a new object, in this example it will be called "AC_PIT_18". Then, select "Recall memorized vertex" option from the same window (it's shortened to blue MR letters). This option will move the pit box you just created to the coordinates you chose before.
Voila, you have a new pit box, where you want it. Then rinse and repeat, until you reach a number that satisfies you. Sometimes pit boxes created this way will be placed a little bit too high in the air or a little bit too low into the ground. Not to worry, what you need to do then is choose an original pit box, and copy it's Z Coord into all your new pit boxes. To copy, just highlight the Z Coord value, for example with double mouse click, ctrl+c, then select your new pit box, double click its Z Coord value, ctrl+v, and it should be right. It’s especially important not to place the pit box below the track level, as sometimes it might mean that the car will spawn under the ground and fall down into oblivion, probably not the best scenario for the race weekend.
Before you start adding pit boxes in 3DSimEd, I recommend that you first visit in Assetto Corsa the pitlane of the track you want to edit, just to see how much space in the pitlane you really have and where does the "ghosting" end (cars going through each other in the pitlane). For example the first time I edited a track this way, I added a pit box outside of the space where the "ghosting" happens, which resulted in collisions inside the pitlane, something you probably want to avoid
Also, sometimes there is something strange going on with the distances and dimensions with some tracks. For example, when I was editing Cadwell Park, I wanted to add one more pit box close to the pit exit, because it looked like there was space for it. When I loaded the game, it turned out that the car spawned on a pit wall, which was weird, because in 3DSimEd the wall looked like it was a few meters back than in reality. This is why it’s very important to check the pitlane before adding more pit boxes, because it can save you some time you would have to otherwise spend on fixing your own mistakes (happened to me more than I like to admit).
The procedure of adding pit boxes applies to start positons as well, which are called AC_START_number. Once again the first one is named as AC_START_0. One very important thing with start positions is not to place them too close to each other. If you do, AI cars on race start will slow down heavily just after the start of the race and possibly ruin your start in the process. If it happens to you, you will have to spread the start positions apart, which is not very difficult, but can be extremely annoying, because you will need to select all your start positions, place them on the side of the track, and then place them back onto the track, one by one, into their new, revised positions.
If you want to learn which distance between start positions is best, look for a track in Assetto Corsa that has a very smooth and rapid race start, load that track in 3DSimEd and check how the start positions are placed. Roughly measure the distance between them and try to implement that into your desired track.
One more thing about "ghosting effect" in the pitlane, that I mentioned before. On some tracks this effect doesn’t happen, which is not a good thing, because then AI cars will crash with each other in the pits and it will ruin the whole race weekend. You can identify it quickly by yourself in practice mode, because once your car loads into the game and you spawn in the pits, you should see the pit box info on the right of your screen (the table with tyres pressures, wings and repairs). If it’s not there, then it means that the pitlane is broken and "ghosting" doesn’t happen as well. This is the main problem with Anglesey Circuit for example, because if the pitlane worked as it should, then the player car would be “ghosted” when it’s in the pits and the AI cars would never crash into it.
Fortunately, there is an easy fix to this problem. Once again 3DSimEd comes to your help. Right click on the ground in the pitlane and select the object that covers the ground in the pitlane. It should be called nPIT (where n is any number except zero) for Assetto Corsa to recognize that it’s in fact the pitlane. It probably isn’t, so you need to rename it correctly. To make sure you are doing this right, once again look for a track in Assetto Corsa that has a working pitlane. Open it in 3DSimEd, check how the objects that form the pitlane are called. With this knowledge, come back to the broken track and rename the objects that form the pitlane in a similar manner to the objects in the track you just checked. It should work, if it doesn’t then try again, this time choose a different track and see how the pitlane is named in that instance.
Please let me know if you understood my instructions, english is not my first language, so I might have said something in a not precise way, for which I apologize.
This is the general rule of adding pit boxes and if you confirm that you understood everything, I will try to explain the more advanced way of adding pit boxes without editing the original track (it's not very complicated, but first you need to understand the basics).
Tagging
@unpierrot , because he might find this helpful as well.
Also, sorry everyone else for creating this horrible wall of text, I probably should have done it in a private message.
