Bicycle trainer
A bicycle trainer is a piece of equipment that makes it possible to ride a bicycle while it remains stationary. They are commonly used to warm up before races, or when riding conditions outside are not favorable. Operation. A trainer consists of a frame, a clamp to hold the bicycle securely, a roller that presses up against the rear wheel, and a mechanism that provides resistance when the pedals are turned. In a wind trainer, the roller drives fan blades that create air resistance. These are typically the least expensive and noisiest trainers. Magnetic trainers have magnets and  a conducting flywheel operating as an eddy current brake. They are moderately expensive and moderately noisy. Some magnetic trainers have handlebar-mounted control boxes to change the level of resistance during a training session. Fluid trainers use liquid-filled chambers to create resistance. They are the most expensive and quietest trainers. A small number of trainers use a centrifugal pressure mechanism to create resistance, involving pressure plates, ball bearings and specially shaped grooves. These are similar to fluid trainers in price and performance. Function. Trainers make it possible to build bicycle skills and power very efficiently in a highly controlled  environment, without the unavoidable interruptions of outdoor riding. For instance, in hill training, instead of being limited to whatever hills are around one's home, one can simulate any size and steepness. Trainers provide better preparation for racing than stationary bicycles. Trainers require better technique than stationary bicycles, and they provide a more realistic-feeling ride. The geometry and resulting body position of a stationary bicycle may be significantly different from a racing bike; of course, if one uses the racing bike itself in an indoor trainer, the body position is nearly identical. Some trainers are equipped with sensors that  monitor the rider's performance. Power output, cadence, virtual speed and heart rate are among the metrics that can be transmitted electronically. Analyzing these figures can help to fine-tune the athlete's training. Types. Bicycle trainers are categorized by how the unit provides resistance. There are two broad categories: "wheel on" trainers use the bicycle's own rear wheel, whereas "wheel off" or direct-drive trainers replace the rear wheel with the trainer's own machinery. Wheel off. Usually all trainers can be adjusted for most sizes of road and mountain bikes. However, knobby tires can cause vibration and  noise, defeating the purpose of noiseless units. Virtual reality. This is a very comprehensive simulator. Examples include Zwift. The rear wheel sits on a motorized roller, the front forks fit in a frame equipped with steering sensors, and the whole system is linked to a computer with 'virtual world' software. Riders steer their way through this virtual world and pedalling gets harder (the motorized roller 'loads' the rear wheel) when going uphill. The sophistication of the computer system allows it to be linked to the internet to provide additional information.