The term "resistance trainer" can refer to several types of fitness equipment, but it most commonly refers to a type of cycling apparatus that affixes to the rear axle of the bike. This unit rests on the ground and holds the rear wheel of the bicycle off the ground so a person sitting on the bicycle can pedal without moving the bicycle forward. The resistance trainer provides resistance by applying a roller to the rear wheel, and a resistance unit provides the force necessary to allow the cyclist to get a workout while cycling indoors.
Several resistance trainer models exist, and they vary both in style and function. The most basic models feature metal stands that stabilize the bicycle while it is attached to the trainer, and a roller unit that adjusts so it can press against the rear wheel of the bicycle with varying amounts of contact. This allows a cyclist to use bicycles with different size tires on the same trainer unit. The resistance unit itself on the trainer can use several methods to apply resistance: magnetic resistance is common, as are wind resistance and fluid resistance. The best option for a realistic ride quality is fluid, but these trainers come with their own disadvantages.
Fluid trainers use a viscous fluid — usually not water, since it can heat up and expand during use — to provide resistance as a spinning unit within the resistance unit pushes against the fluid. This is a quiet way to ride indoors, and it provides a smooth feel akin to actually riding on the road. Some models of fluid trainer do tend to leak, however, so it is important to research the different models before purchasing. Avoid fluid trainers that use water for resistance; find one that uses silicone instead.
Magnetic trainers are the next best option, and they are more affordable than fluid resistance trainer units. These units do tend to be a bit louder, and the ride quality is not quite as good as a fluid trainer, but a magnetic trainer will still provide strong resistance that is often easily adjustable using an on-unit adjuster or using a handlebar mounted resistance lever. The adjustability is not as accurate as a fluid trainer, unfortunately, but it is more adjustable than a wind trainer.
Wind trainers are the least likely to break down, but they are also the noisiest and least impressive when it comes to ride quality. This type is best for the beginner cyclist or a cyclist who is on a tight budget when it comes to purchasing such a unit.