After the British GP, when Hamilton benefitted from being able to repair his car under the red flag after a collision he was found guilty of by the stewards, there has been a lot of discussion about what should and what should not be allowed under a red flag, and by extension also under SC. One of the points being brought under discussion was if it should be possible to change tyres during a red flag or SC. What I have realised is that some people are under the impression that disallowing to change tyres would make things fairer. After all it’s easy to focus on those instances where somebody has gained positions due to being able to have a “free” tyre change. What’s more difficult to realise is what would happen if the rules were changed, because that has not happened in F1, yet.
I have prepared a few mini-scenarios to let you visualise how forbidding tyre changes under SC or red flag will throw some races completely upside down.
Let’s assume this is the situation somewhere mid race before the pit stops start.
Drivers in green managed to pit before the SC/red flag and drivers in red after SC/red flag. It doesn’t really matter if it’s one or the other because by the time of restart, they bunch up the field in a similar way ultimately. I assumed a pit stop takes 25 secs. The situation looks like this now.
Now the SC/red flag has appeared. Drivers are not allowed to change tyres. Once the restart happens, the cars are bunched up so the situation looks something like this.
The drivers who have not pitted are in a very tough situation. Of course they can’t drive away from drivers who are on much fresher tyres. Most likely they are about to get overtaken left right and centre if they stay out. The only sensible thing to do is to pit for new tyres right away. They have to change tyres at some point anyway. Better to do it now than later when they lose even more time and positions on worn tyres.The situation after they pit look like this.
As you see it makes basically all the difference at what point you were scheduled to pit should a red flag or SC appear in the middle of a pit window and tyre changes are not allowed.
Second scenario: perhaps an example when a race leader decided to go long and is the only driver yet to pit can visualize even better how unfair forbidding changing tyres can be:
And here you can see what happens if you can change tyres under red flag instead (compared to the first scenario). The drivers in red have just a free pit stop. The average position change from an initial pecking order is 2.64 positions, while in the “no tyre change” scenario, it was 4 positions. During a SC the average position change would've been even smaller if tyre changes are allowed (because a pit stop under SC "costs" you more than 0 time), thus a difference in "unfairness" even bigger.
Of course I made some simplifications along the way but I don't think any of it should matter much to what I'm saying. I hope everybody understands the concept now. You can still be in favour of forbidding tyre changes but I hope you're aware of what it brings to the races.