http://www.mountainbikestoday.com/mountain-bike-tips/how-to-be-sure-to-get-a-mountain-bike-that-fits-you (for above image)
Overview
The sport of mountain biking is physically demanding. It requires prior training towards competition, including: healthy diet with all essential micro and macro nutrients as well as high intensity exercises. The whole body needs to be in shape from the legs through the core to the upper body. Fatigue is one of the main causes that inhibit an athlete’s ability to perform at its peak potential. Mountains bikes are specially designed to reduce the level of fatigue and enable them to perform. There are many aspects to the bike that significantly influence a rider’s chance of victory. For example, gears, tires and brakes are some of the main attributes of a mountain bike that definitely affect the performance of each rider. This sport covers a variety of areas in physics which if are understood thoroughly, can substantially increase the chances to victory.
The Frame
The first generation mountain bikes consisted of steel alloys. This material is used in entry level bicycles as it is a cheaper material. Despite being one of the heaviest materials used, it supplies inbuilt shock absorption and a high strength to weight ratio (3). Aluminum alloys on the other hand are extremely light weight but still offer its durability. This inexpensive material is non-corrosive, easy to maintain, and offers tough rigidity which is essential when making a mountain with full suspension. Some other materials used include carbon-fiber and Kevlar which are two composites used to make the frame. These materials are made from a mold which makes it easy to design and customize to a bikers preference. The downside is it is irreparable and does not react well with solvents and acids therefore high maintenance (3). Lastly, titanium, the highest quality material and used by professional mountain biking teams is the most durable and light weight materials. Since they are used primarily by professionals, they are also the most expensive bike frame you can get (3).
Tires
Mountain bike tires depends on the three factors; size, pressure, and tread pattern. All of which are chosen based on the rider and the type of terrain. Firstly, the standard diameter of a mountain bike tire is 26 inches (66cm) and has a width of 1.5 to 2.6 inches (3.8-6.6cm) (9). A greater diameter means it’s easier (more efficient) to maintain an average speed top speed but does make it more difficult to accelerate. A 26 inch tire seems to be the optimal size for a bike and most ergonomic. The width of a tire has significant impact as a narrower tire produces less resistance which makes it easier to maintain a top speed but more difficult to maintain control. Conversely, a wider tire produces more resistance, decreasing the average top speed but increasing traction/control of bike. A wider tire is generally used in mounting to account for the rough terrains. Secondly, pressure is something that be easily changed but is something that also plays a significant role in mountain biking. Some mountain bikes have tubes in them and have a range in pressure (PSI). Less control of the bike because of poor contact with the ground could be related in the pressure being too high (6). Too low of a pressure and the tire is more vulnerable to pinch flats (6). Generally tires should be firm, however, they should also be someone deformable to give control for the rider. Determining this is usually based on the mass of the rider. Lastly, the tread pattern which is the only thing making contact with the ground is another important aspect of the tire. There are 3 types of tread; slick, knobbed, and a combination (7). Slick tires (tread-less) are smooth and offer a high rolling efficiency since there is less resistance. This however decreases the control of the bike due to loss of traction (7). Knobbed tread is tires that contain a variety of heights, depths and widths which increase the resistance or traction. Knobbed increases the traction but lowers the efficiency. Combination is a mixture of both. Mountain bikes generally need a combination but probably more knobbed to account for the rougher terrains.
Gear ratio
Gear ratios vary depending on the bicycle and are limited to the number of gears on the bike. Ones with more gears contain higher gear ratios making it more versatile for several different terrain including the material that is being rode on and the slope in the each hill. Low gears make the pedals easier to spin and vise versa. Gear ratio refers to the number of teeth on the front and rear sprocket. The front sprocket has 1, 2, or 3 sprockets; the higher the gear the larger the sprocket is which contains more teeth. The rear sprocket is the opposite and the higher the gear means the sprocket is smaller. For example, if there are 36 teeth on the front and 12 teeth on the rear, then your ratio is 3:1 (3 teeth in the front: 1 tooth in the rear). So if the pedal goes through one revolution, there are 36 teeth being spun which means the chain moves 36 chain links. Since there are 12 teeth in the back, it has to spin 3 times to go through 36 teeth. This means that the rear wheel spins 3 times for every one revolution in the pedals. This gear would be used on level ground or going down a declines since the rear wheel is spinning rapidly relative to the pedals. To put this in perspective, the circumference of an ever bike tire is 2.1 meters and if it spins 3 times, then the bike if going to travel 6.3 meters a per pedal rotation. Pedaling at 75 revolutions per minute means the bike is going to go 472.5 meters in one minute. This produces a speed of 28.4km/hr. Each gear ratio will produce a different speed at a set number of revolutions per minute.
brakes
Brakes on a bicycle generally work in the same way by stretching a cable which contracts a brake pad producing a higher coefficient of friction, therefore, more frictional force. Mountain bikes will have front and rear brakes to ensure braking is done safely and effectively. By applying too much rear brake could result in loss of control due to loss of traction (skidding). In addition, applying too much brake in the front would result in the rear of the bike coming off of the ground, throwing you off the bike. This takes practice in terms of being able anticipate how much to brake for each wheel which can be challenging when biking on different changes in elevation and type of terrain. Adjusting your weight on the bike can help prevent the rear of the bike from popping up and stop the bike from skidding. Shifting your weight to the back means more force is exerted on the rear tire which ultimately increase the force of friction between the tire and the ground thus making it harder for the wheels to lock. Transfer the weight to the back makes it more difficult for the rear to pop up as it requires a great amount of force to lift a heavier load. These scenarios only occur on level ground and on declines. There are two types of the breaks: cantilever and disc brakes. Cantilever bakes are older technology and usually appear on lower end bikes. Disc brakes however are highly effective and are used on the higher end bikes. Both work very similarly except in disc bakes, the actual brake pad pinches the disk which is connected to the wheel whereas cantilever directly is squeezed the tire. Disc brakes apply a more even and consistent brake force in comparison to the cantilever which create an inconsistent, jerky brake force.
Suspension
Many mountain bikes contain suspension kits which allows the wheels to move up and down while the wheels stay in contact with the ground, giving more control for the rider. Some mountain bikes contain only front suspension (hard tail) and others contain front and rear suspension (full suspension). Hard tail mountain bikes contain a fork in the front whereas the full suspension models contain a fork in the front as well as a frame separated into two pieces connected by a pivot. This enables the two pieces to move more freely and are controlled by the rear shock absorber. Now the question is... which one is better? This is a challenging question to answer as there are many factors that influence which one would be a better choice. Each one has its advantages as well as its disadvantages. Hard tail models are more efficient in converting pedal power to the rear wheel (8). This of coarse makes it easier to main an average top velocity. On rugged terrain, it is better to have a full suspension mountain bike because more wheel power is able to reach the ground increase level of traction. It also keeps the rider seated when going over bumps. Full suspension bikes are generally significantly heavier as the rear shock absorber is made of a coil of steel to increase rigidity. This bike puts less stress on the body giving more of a comfortable ride *8). The choice is based on what the bicycle will be rode on and personal preference.