There are 2 different types of roulette wheel, European and American. Both wheels are made up of alternating black and red pockets which are numbered 1-36. The European roulette wheel has a single green 0 whereas the American roulette wheel has a green single 0 and a green double 00.
The numbers on the wheel look like they are randomly spread out with no logical intent. However they are arranged in a fashion to split up the key bets as best possible so that the wheel layout is indeed as random as it could possibly be designed.
Upon creating the European roulette wheel and losing the green double 00 they could not stick with the same layout as the American roulette wheel because two red pockets (numbers 1 and 27) would have been next to each other. The creators of the new version therefore had to totally recreate the wheel layout and and again make it as random as best possible. So the difference between the 2 roulette wheels is not just the addition of the double zero but the numbers run in a different order on both wheels.
As soon as we can we will roulette wheel diagrams of both types added to the site.
Biased Roulette Wheels
A biased wheel is one that has either specific numbers or group of numbers from a certain sector of the wheel come up more often than you would normally expect. Working out if a wheel has a bias in anyway takes thousands of spins.
Some players in the past have dedicated alot of time and effort to finding a biased roulette wheel and whole books have been written on the subject. A biased wheel in a casino is not something you are easily going to find, as it's certainly in the interests of the casino not to have one and keep all roulette wheels as random as possible.
In the past finding a biased wheel may have been more likely. Natural wear and tear may have resulted in slight imperfections on particular numbered pockets or the mechanics of the wheel may have been effected in some way over time. Just the slightest change could result in wheel bias and the ball being slightly more prone to land in certain pockets than others.
As lessons from pass bias wheels have been learnt modern wheels are manufactured in such a way as to being much less likely to be susceptible to any kind of bias.