The u24 bonus = 0,02 point on every skill automatically. Training or resting does not matter, every update on every skill + 0,02.
That is exactly 91 x 0,02 = 1,82 per season on every skill.
Experienced managers train the mainskill to the 90 as soon as possible, and then switch training and train that skill to an interval.
The reason = in this way the u24 bonus is the most effective.
Because of the intervalnumber of training speed ; 0,02 above 90 is more worth/effective than for example 0,02 above 40.
It has everything to do with the total number of trainings, and training to 90 as soon as possible will "save" trainings due to the u24 bonus.
Training above 90 is much slower ; 0,095 point per training.
Than training from 40 to 60 ; 0,320 point per training.
Therefore the 1,82 extra u24 bonus per season is much more effective above 90 when you "translate" that into "save" trainings ;
1,82 point above 90 takes approximately 19 trainings.
1,82 point above 40 takes approximately 5½ trainings.
This shows that the u24 bonus is more effective above 90, and the cyclist "saved" trainings that he can use for the other skills.
