In Formula 1, in the event of a safety car appearing, you often see cars racing to the pits in order to get a pit stop which is effectively free.
How does this circumstance result in the pitstop being free?
For instance, in the most recent race in Australia, Vettel managed to pass Hamilton during this phase, as Hamilton pitted just before the safety car came out, whilst Vettel pitted during the safety car.
The "free" pit-stop doesn't exist any more, especially since the rules were changed last year that meant drivers had to stick to a certain speed when the safety car was on track, I will explain that in more detail later on.
The basic premise of the "free" pit-stop is that under normal circumstances you lose more time relative to the cars not stopping than you would do under the safety car. For example, let's assume a track where the average speed is 100 (doesn't matter if it's kmh or mph) for the lap and the average lap time 100 seconds. If you stopped in the pits let's assume it adds another 25 seconds to your lap time, this means you've taken 25% more time compared to the other cars.
However, if the safety car is out, the laptimes double to 200 seconds, your pit-stop now only adds 12.5% to the average lap as it is the same duration, so the penalty for stopping is much less, but there is still a penalty.
Another example is once everyone is behind the SC and you pit, let's say from 1st place, assuming the SC stays out you would rejoin at the back of the pack. If the pack isn't too large then that's only a lost of around 10 seconds on the track.
With regards to the recent rule changes - in the past the drivers were told they had to acknowledge the fact that the safety car was on track and drive accordingly. In reality they were still driving at pretty close to 100% until they actually caught the safety car up, so laptimes could be close to normal racing speeds at least for the lap or so after the SC was deployed, they would still be catching the SC and it wasn't until they actually caught it that speeds would be seriously affected.
The new rules mean driver MUST obey a speed limit while the SC is on track, this works by forcing them to drive each sector within a certain amount of time, this is checked by telemetry and any driver not obeying this will be penalised. The drivers can see the time on their steering wheel display - so the advantage of pitting under the SC isn't as great as it used to be and vice versa the advantage isn't so big either.
What happened in the last race was simply that Vettel made up around 25 seconds over Hamilton due to his pit-stop and Hamilton couldn't make that time back up when Vettel pitted as he was limited by the new SC rules as to how fast he could go when trying to catch the SC. He did catch up Vettel once they both caught the SC but the damage was done as you can't overtake under SC conditions.
Consider this scenario: You are leading the race and there is an incident that calls for a safety car. You rush into the pits and get out, let's say having lost six places while in the pits, just as the current leader reaches the safety car and the queue starts to form.
You may have dropped at seventh place, but with an extra pit stop and with the queue eliminating distances between cars. When the race restarts you will be at a slight disadvantage because of the heavier tank but if all goes well you will regain the top spot quite easily when everyone in front of you goes into the pits.
Obviously, if you don't manage to get out of the pits before the queue forms, you may have lost quite a few places and even dropped at the end of the race. The strategy benefits the leading cars, is a judgement call for the cars in the middle of the race and it's worthless to the tail cars.
Of course the scenario above is oversimplified, if all the leaders rush into the pits, pit stop times play a major role in the strategy's success and it may prove disastrous for the team. It's a probability game, teams will have to guess correctly that the safety car will appear before the marshals wave the yellow flag.
