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sensitive handling.

I. Height of the Bottom Bracket

This is the distance between the floor and the center of the axle of the bottom bracket. A lower

bottom bracket makes the bike more maneuverable; an elevated bottom bracket means more stability and

better straight-ahead riding. A very high bottom bracket makes it easier to clear obstacles.

J. Stem Angle

This angle is determined by the inclination of the headset tube. A wider angle gives an easy, more

comfortable ride. A narrower angle gives a “sportier” feel. For the comfortable ride the angle should be set

from 15. to 25., for a racer the angle should be between 0. and 10..

K. Length of the Stem

A longer headset (stem) will distribute the weight of the rider more evenly between the front and

rear wheels. Longer headset tubes are more frequently found on racing bikes. A long headset tube is about

51/8″ to 6″ (13 to 15 cm); a short tube measures between 4″ to 43/4″ (10 to 12 cm).

All measurements taken together, and their relationship to each other, define a bike’s

characteristics. The ability to interpret a frame’s dimensions allows someone to predict a bike’s

maneuverability, and allows the biker to determine if a bike will perform to his expectations.

Tube Materials

More than 90% of all the mountain bikes used today are made from steel tubes. The steel tubes are

all made from high quality steel alloys. Other substances have been added make sure the frame is

problem-free as well as having a high degree of stability and flexibility. Although the steel tubes are of

exceptional quality, they have one major disadvantage, their weight. Since weight is one of the basic

problems of a mountain bike, there has been a search for a material that was light in weight as well as

strong.

Aluminum has rapidly become the tube material of choice in the past few years. A decade ago

aluminum was still an “exotic” metal, a term used to describe titanium today. The use of carbon fibre and

kevlar are also being used more for the construction of bike frames. In recent years these materials have

been used in more industries other than the aircraft industry, making them more affordable. Today builders

use these materials because of their qualities: light weight, and good elasticity, both combined with good

strength. Because the tube materials play such an important role in the way a bike reacts and feels it is

important for a buyer to know what the bike frame is made from. It is also important to know the

advantages and disadvantages of each of the materials.

Steel

Since the mountain bike was invented, the frame manufacturers have used chrome-molybdenum-

steel in various thickness’ to build high quality bikes. The two most used steel alloys are 25-CrMo4 and

34-CrMo4. For 25-CrMo4, the 25 means that it contains 25% carbon (carbon makes steel tension-resistant,

and serves as a protection against deformation); CrMo4 indicates how much of the substances that improve

the quality of the steel (chrome and molydenum) have been added. Manganese-molydenum is another

alloy that may also be added. All of these alloys reach very good anti-breakage strength. High-quality steel

tubes have seamless joints, and their ends have been reinforced or “butted”. The strength of the walls of

high-quality CrMo tubes have been tripled. Butted tubes are strongest at the point where two tubes are

joined, and are weakest in the middle of the tube.

The advantages of steel tubes are that it is a relatively inexpensive metal. The soldering the tubes

produces strong, stable connections. Steel also tolerates a great deal of stress before it starts to break down.

There are two disadvantages of using a steel frame: it’s weight and it’s susceptibility to corrosion.

The fight against rust is endless; also, a search for a material that is light is still an ongoing process.

Despite these shortcomings steel remains the most reliable material for the frame industry.

Aluminum

The use of aluminum to make bike frames has increased rapidly in recent years. In order to make

aluminum useful for mountain bike frames, an alloy had to be produced. Copper, magnesium, zinc,

manganese, silicon, and titanium were each added; all of them increased the strength of aluminum.

Aluminum has a very high resistance to breakage. However, the maximum load capacity (the amount of

pressure tolerated by a material before it becomes permanently distorted) of aluminum is not as high as that

of CrMo steel. Because of this the strength of aluminum can be increased by widening the diameter of the

wall of the aluminum tube.

The disadvantages of aluminum is that the price for high-quality aluminum is as high as the price

for steel, but depending on the method used to connect the tubes, aluminum frames require more time to

make, which in turn means that it costs more to the buyer. The tubes are either glued or screwed together

with expensive sleeves, or they’re welded together. Both methods are expensive and time consuming.

Although aluminum tubing has only one-third of the rigidity of steel, when the diameter of aluminum tubes

is doubled, the amount of rigidity is not simply twice but eight times higher. Another disadvantage of

aluminum is its torsion strength. To improve the torsion strength of the aluminum the thickness of the wall

was increased, this however, defeats

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