Most elliptical trainers share the same basic build: A bulbous shell hides the drive mechanism; a pair of pedals protrude in front of, behind or in very rare cases directly on top of that shell; and moving handlebars flank an upright console.
But once you've stepped onto a few elliptical trainers, you begin to notice the differences between various models. A loud whirr from the resistance mechanism. An unexpected blast of air. A silent ride, except for that brief mechanical noise when you adjust resistance. You've stumbled upon what really sets one elliptical trainer apart from another: differences in the drive mechanism.
Air resistance elliptical trainers are fairly straightforward. Your pedaling turns a fan and the air itself provides resistance against your pedaling as it resists the fan blades. The faster you pedal your air resistance elliptical trainer, the more resistance you get.
The very highest-end magnetic-resistance elliptical trainers use an electromagnet, positioned near a flywheel, to provide resistance. Pushing a button directs more current to the magnet for more resistance, or reduces current for less resistance. This is called eddy current resistance or eddy current braking.
Less expensive forms of magnetic resistance, which may be marketed under any number of names, mechanically adjust a magnet closer to the flywheel for more resistance, or further from it for less resistance.
The faster and harder you pedal your air resistance elliptical trainer, the louder it will get. Eddy current braking is completely silent, both as you pedal and as you adjust resistance. Mechanically-adjusted magnetic resistance is usually silent as you pedal, but may be noisier when the magnet is moving.
A magnetic-resistance elliptical trainer's silent ride means that you can watch television or listen to music as you ride. An air-resistance elliptical's constant, low roar makes this more difficult; even with headphones on, you'll have to turn the volume up.
A benefit of silent eddy-current resistance is that, with no moving parts to speak of, it requires less maintenance and has less potential for breakdowns than other types of resistance.
Air resistance trainers have the benefit of a much lower cost, the effect of a built-in cooling fan, and unlike some magnetic-resistance trainers, they never need to be plugged in. The air resistance mechanism is also much simpler so, if you're a do-it-yourself whose air-resistance machine is out of warranty, you might be able to repair it yourself.
Air resistance elliptical trainers occupy the lowest end of the elliptical trainer price range, usually selling for well under $500. However, they also tend to suffer from common ailments of less expensive exercise equipment: Flimsy construction, low user weight limits and a tendency to wobble---so maintenance and repair costs might be higher.
Magnetic resistance mechanisms vary widely in price range and type. In general, elliptical trainers in the $500 to $1,000 price range will have mechanically-adjusted magnetic resistance. You will find eddy current braking on a few ellipticals in the $1,000 to $2,000 price range. Almost all elliptical trainers in the $2,000 and up price range will have eddy current braking.
Magnetic-resistance elliptical trainers have a fully enclosed drive mechanism. There is only minimal risk of children's curious fingers getting pinched by exterior moving parts. Air-resistance ellipticals, of necessity, have vents that allow air in and out of the fan blades, so there's an increased risk of injury to children or pets.