The roulette wheel is fixed before you've even placed your bet, not in a way that will have an effect on your odds at the table. After you press the 'spin' button, the casino software uses a Random Number Generator (RNG) to choose which number the ball is going to land on. So whilst it may appear as though the ball in spinning around and could land anywhere, the result of the spin has already been determined since pressing the 'spin' button.

In spite of this, although the ball already has a set position for it to land in, the result is never determined based on where you have bet your chips. For example, just because you have bet on red, the software will never take this into consideration and think “I'm going to land on black and take their money”.
 

The final result of the spin is always completely random, and is just as random as any land casino. The only difference is that the ball knows exactly where it is going to land after you press spin, and there is no physics involved like there is on a real table. The whole animation of the roulette wheel is for entertainment purposes only.
 

Some people will always complain that something is rigged if the results do not go in their favour, so always take what sceptics have to say with a pinch of salt. It's always going to be much easier to claim an online casino is rigged purely because of the fact that electronics and RNGs are involved, rather than the raw physics of a real table.

How online roulette works