I'm animating a bicycle riding downhill (from the top of a hill to the even ground). What expression should i use to gradually accelerate bicycle's wheels? And how do i gradually slow down the wheels after the bicycle has reached the even terrain?
Well, you could just use a handful of keyframes. Unless this is a super-convoluted animation with multiple ups and downs and exact rotation relation to the travelled distance there would be no point in crafting elaborate expressions. If it were do be done "correctly" it would have to be based on rotational impulse, anyway, and that can get convoluted if you want to figure in acceleration and deceleration. Really, it's one of those "why bother brooding over code for hours when you can do it in 3 minutes with keyframes" things.