How do I increase my B spec drivers strengths during the long races

[Tiny65]

Help! Every time I choose 90% strength level when he pits, nothing happens! Can anybody tell me why?

 

[Blitz24]

Remove it and put it at 10% or off.

 

[1 tYp3 L1k3 D15]

I would stay away from that pit strength feature myself.

It will make your driver pit when their strength reaches the selected level, however in a long enough race your drivers strength wont recover fast enough, and they will pit every lap which is not condusive to winning.