When large numbers of participants have all registered to race at the same time, the field is split. In short, you don't have to do anything. Which race you'll be assigned to is automatically determined by a number of factors, including your iRating, Internet Latency, and Friends. The greatest weight is given to iRatings, so as to create close racing between similarly skilled drivers.When the field is split, you won't know who else you'll be racing with until the countdown timer gets to zero and you press the JOIN button at the top right of the Racing panel. That will automatically take you to the correct race, where you can see who is in your session by clicking the ENTRIES tab once the simulation has loaded.
In the 2015 Season 4 release, we added a new method by which sessions get split into run-groups. The prior method typically chooses the "best" race server farm (US, Sydney, Amsterdam) based on the average ping time of all the registrants to the farms, and then splits the teams into run-groups based on iRating. This method prevented high iRating drivers from the APAC region from competing against high iRating drivers in other regions when there were enough registrants to have multiple splits.
The new method splits the teams into run-groups based on iRating, and then chooses the best farm for each run-group. Additionally, the new method tries to avoid assigning the group onto a farm if any of the drivers in the group would generally prefer to avoid that farm. In practice this will mean that if anyone in the run group prefers the Sydney farm, and anyone else in the run-group prefers the Amsterdam farm, the run-group will be assigned to the US server farm.
Ex. If a split has at least one member whose best ping is to our Sydney farm, and also at least one member whose ping is best to our Frankfurt farm, then both of those farms are excluded as targets for the split. Leaving just the Boston and US-West race farms (as of July 2021). As long as there is not that specific mix in a split, it is put onto the race farm that gives the best average ping (essentially).
Splitting Multi-car-class Fields
Prior to 2015 Sesson 4, the algorithm for forming splits for multi-class sessions attempted to ensure that as many splits as possible would be multi-class, down to some minimum number of drivers in each class within a split. This made for some not-so-great racing for drivers in classes where their split had this minimum number of drivers.
With the 2015 Sesson 4 build, we have implemented a new split-forming algorithm for multi-class sessions that instead attempts to ensure that all car classes are packed relatively evenly into the higher-rated splits, potentially at the expense of the lower-rated splits being devoid of one or more car classes. The algorithm for choosing the overall field size for each of the splits is unchanged - each of the higher-rated splits formed for a session will have the same overall number of teams, with the lower-rated splits potentially having one less team. Since the higher-rated splits might "consume" all the teams in one or more of the less-used car classes, and since the lower-rated splits will still have the same overall number of teams, the lower-rated splits could thus have more teams in each of their car classes than the higher-rated splits have in those same car classes.