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Try to make realistic rain drop behavior on Windshield depending on car speed

Hi

I have an instance material of rain drop with some parameters.

The parameter I want to change depending on the car speed is the Speed_rain.
When Speed_rain is greater than 0, the rain drops are supposed to go downward then when Rain_speed is less than 0, the rain drops must go upward.

What I want to do is to make a smooth transition if the car speed is greater than or equal to 20km/h and vice versa if the car speed is less than 20 km/h.

I try to make this rain drop behavior with a timeline but there is a bug in the animation.
I also try with curve float and I have the same bug.

The bug is when the car speed reaches 20km/h, the rain drops instantly go upward before Rain_speed is less than 0 but Rain_speed value decrease gradually without transition.
And vice versa when the car speed is less than 20 km/h, the rain drops instantly go downward but Rain_speed value increase gradually without transition.

Thank you in advance for your help.