Belts vs Magnets

• There are two main ways exercise bikes provide resistance--the pressure against which you push in order to "feel the burn." The cheaper of the two ways is by using a tension belt inside the bike. This is a physical solution and is usually relatively static--you can't add more resistance to make the bike harder to pedal, or reduce it for cool-down periods. If you want a proper, gym-style exercise bike, you really want one with magnetic resistance.
  
  

Electrifying

• Magnetic resistance makes use of magnetism and electricity to provide you with variable resistance. Magnets attract or repel each other, depending on the charge of their poles--like poles repel each other, and opposite poles attract.
  But magnetism can be affected by electricity. Passing an electric current through magnets allows you to effectively make the magnets more or less attractive, or allows you to make seemingly "dormant" magnets work--like the big electromagnets used to move cars about at a junkyard.
  
  

Lord of the Flywheels

• To make this a practical asset in your exercise bike, you need a flywheel. Flywheels are wheels that resist changes in their rotational velocity--the speed at which they turn. When you peddle a variable resistance bike at its basic level, you supply energy to overcome the resistance, and turn the wheel--the energy comes from the movement of your muscles, and that's what helps you burn calories.
  
  

Push it

• The flywheel in a magnetic resistance bike is linked to the magnets, and the level of resistance is controlled by the level of electric current going through those magnets. So when you turn the dial or press the button for increased resistance on the bike, you increase the current going through the magnets. More current equals a stronger magnetic force, which increases the resistance in the flywheel, and means you have to push harde. If you use more energy you will then burn more calories with every turn of the wheel.
  
  

Virtual Riding

• It's this mechanism, too, which allows for pre-programmed routes--the computer in the bike adjusts the level of current, and so the levels of resistance, so you can simulate riding along flat ground, up steep hills, down the other side, or any other likely environment.
  
  So you can get the workout you want at the turn of a dial or the push of a button, moving up and down the resistance scale as you require.