Essay/Term paper: Mountain bikes

Essay, term paper, research paper:  Term Papers

Mountain Bikes

It has been a decade in the making, but the mountain bike has become a passion for many people.
Along with this passion it has also become the most environmental way to get from point A to B. It has
gone through a very intense evolution process over the past decade.
It all started with some guys from California who took their bikes out for a ride on their day off,
they modified their bikes and turned a hobby of theirs into a worldwide phenomenon. The mountain bike's
rapid increase in popularity was influenced by social and economic situations, and by technological
improvements that had the needs of bike riders in mind. The introduction of the mountain bike at a bike
convention in Long Beach, California, early in the 1980's coincided with the need for a bike that combined
technical superiority, ease of care, and multipurpose use.
Technological advances came extremely fast after it's introduction into the world. The advances
have made riding mountain bikes easier, which makes it possible for the rider to explore new terrain. I hope
that this report will be able to provide some information on the subject of mountain bikes and the advances
in technology that the bike has gone through, and what might be in the future of the mountain bike.
The road bike has taken more than a hundred years to evolve into the frame that it is being used on
todays bicycle. Because of the increasingly popularity of the mountain bike the demand for advances to be
made have come very rapidly.
The evolution of the mountain bike has been a stormy one over the past decade. Within one
decade the design has changed radically; this is due to three reasons. First , because geometry and design
were copied from the first "Stone-Age bikes"; second, because off-road riding created different problems ;
and third, because innovative frame design mirrored the "spirit of the times": young, new, dynamic, and
strong. The off-road bike required extra stability.
Frame
It is important to know the basic frame geometry and how to measure it. The combination of tube
length and angle determines not only the maneuverability of the bike, but also determines the seating
position and the transfer of power. Variances of 1° of the headset angle, or a 1' (2cm) difference in the
distance between the rear-wheel axle and the center of the bottom bracket, can have very serious
consequences.
Frame Geometry
The basic elements of frame geometry are: A- Height of the seat tube; B- Length of the top tube;
C- Seat-tube angle; D- Headset-tube angle; E- Trail, F- Distance between the rear-wheel axle and the bottom
bracket; G- Distance between the front-wheel axle and the bottom bracket; H- Wheelbase; I- Height of the
bottom bracket; J- Stem angle; K- Length of the headset tube.
A. Height of the Seat Tube
This is determined by the length of the biker's inseam. This measurement is only of little
importance, because of the different frame designs and the different methods of construction used by
different manufacturers.
B. Length of the Top Tube
This length should correspond to the rider's trunk (length from the seat to the shoulders). With
mountain bikes this measurement should be increased by a few inches. This increases the distance between
the two axles, which increases the riding comfort and makes for a straight and stable ride. Some times the
top tube is slightly slanted, this is because some bike frames are designed so high off the ground, the slanted
top tube makes for an easier dismount.
C. Seat-Tube Angle
This angle basically determines how the bike will handle. Today the standard for a seat-tube is to
be set at a 72° to 73° angle. At 69° to 71°, it is a more comfortable ride, but a sharper angle increases the
bike's agility and ability to climb.
D. Headset-Tube Angle
Along with the fork and trail, the headset-tube angle determines the steering characteristics of the
bike. A steep angle together with a curved fork reacts more sensitively when steering; a flatter angle reacts
less sensitively. In the past the angle was set at 68°, but today the standard angle of the headset-tube is 71°.


E. Trail
The trail is the distance between two points marked from the center of the headset to the floor and
by the extension of a line from the center of the front axle to the floor. This distance depends on the
curvature of the fork and the angle of the headset tube. A longer trail makes for easy steering; a short trail
causes the bike to react quickly to every movement of the handlebars.
F. Distance Between the Rear-Wheel Axle and the Bottom Bracket
The longer this distance is, the more comfortable the ride. A shorter distance creates a "lively
action" and a good climbing ability. The average span from the bottom bracket to the rear wheel for a
mountain bike is 17" (43 cm).
G. Distance Between the Front-Wheel Axle and the Bottom Bracket
This distance determines the amount of toe clearance. Toe clearance means that the front tire and
the tips of the rider's shoes never come into contact as the rider pedals and turns at the same time. To do
this, measure the distance between the center of the axle at the front-wheel hub and the center of the axle of
the bottom bracket.
H. Wheelbase
This is the distance between the centers of both the front-wheel and rear-wheel axles. A long
wheelbase makes for ease of handling and good straight-ahead riding. A short wheelbase makes for
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 aluminum's weight advantage.
The advantages that aluminum tubing for bike frames are, that aluminum alloys are rust-free, they
absorb shocks five times better than steel, and they're light. Because the aluminum absorbs shock better
than steel, the result is a more comfortable ride. Because of these reasons, the aluminum tubed bike frame
is being used more and more frequently in the industry.
Titanium
This material is used most often in fighter planes, but it is now being used for mountain bike
frames. Because of its superior strength vs. its weight, the finished frame is very light and very strong. In
the past titanium turned brittle after time, resulting in small cracks when under heavy loads. The adjustment
in the combinations of the metals that were used it the alloys, titanium is now stronger than steel. The
problem with this frame material was its price and its complicated manufacturing process, but the alloy and
production problems were solved, and, together with a new welding technique, the production of titanium
frames has become much easier.
The high price of titanium is titanium's greatest disadvantage. Titanium is three times as expensive
as CrMo steel. The welding method that weakens steel and aluminum has almost no effect on titanium. In
the past it was necessary to do the welding in a vacuum chamber to protect the material against oxygen. A
good titanium alloy has approximately the same strength as steel, but it achieves only 60% of steel's rigidity.
This problem is also solved by increasing the diameter of the tube. A high torsion strength is titanium's
greatest advantage, a problem that was solved by making the walls of the tube thicker. Also, titanium is
rust-free and is 40% lighter than steel.
Carbon-Fibre
The future of mountain bike frames is in fibre and resin. The superiority of carbon tubes over steel,
aluminum, and titanium is no longer a secret to mountain bike riders. This material provides great riding
comfort, increased rigidity, and amazing shock absorption. Two types of tube stand out: round tubes that
are glued together with aluminum sleeves and one-piece Monocoque frames. Besides carbon fibre,
manufacturers are also using glass fibre, graphite fibre, Kevlar, and Spectra. It is important in the
manufacturing process that a correct, multi-directional arrangement of the fibres is made to increase the
torsion stress. Poorly made carbon-fibre tubes will fracture when exposed to heavy loads.
There are only two small disadvantages a carbon-fibre frame has. One is that it costs about four or
five times more than a steel frame, and the other is that Monocoque frames have a very limited number of
sizes. Other than those a lot better. Carbon-fibre frames are three times stronger than steel frames, and
have 35% more rigidity. They are also well protected from corrosion. Carbon tubes are 20% lighter than
steel tubes; the sleeves used for joining the tubes add some weight, however, carbon frames are still 60%
lighter than steel frames. Their excellent ability to absorb shocks (the energy flow of the shocks is diffused
by traveling from fibre to fibre) doesn't diminish the frame's rigidity. Carbon-fibre is considered to be the
ultimate material for frame tubes.
Suspension
In recent years the trend for mountain bikes is to be equipped with a suspension system.
However, it will take some time before all the problems with suspension are worked out. The high demand
for some kind of suspension is because of the heavy load that the mountain bike's material must bear. Until
now, the solution was to increase the rigidity of the frame, but strengthening the material compromised
comfort. The solution on today's mountain bikes is by adding suspension to the wheels. Suspension was
first used on racing bikes, but because of the harsh conditions a mountain bike goes through, it was only a
matter of time before a suspension system was added to the mountain bike.

Front Shock Absorbers
The front shock was the creation of Paul Turner, who engineered the "Rock Shox". This type of
suspension is similar to the suspension used for motocross forks. This suspension consists of an aluminum
fork crown with two telescoping blades that slide into each other when under pressure. The blades are
either made from aluminum or steel. The distance of the spring action is about 21/8" (5.5 cm). The degree of
tension can be adjusted. There are two ways to absorb shocks: oil-pressure or air-pressure suspension, or
with springs and oil. Plastic parts can also give good results.
Bikes that are equipped with front-wheel shock absorbers don't lose contact with the ground ,
which allows for more control, and thereby making driving at higher speeds possible. However, this
advantage only comes into play when riding at high speed, and when the shocks occur in quick succession.
Suspension prevents shocks from reaching the tire, and thereby prevents damage to the rim; rims aren't as
easily deformed. The greatest disadvantage is the change in the geometry of the bike. The steering-tube
angle gets smaller, anywhere from 2° to 2.5°; the trail gets larger, which changes the handling of the bike
from characteristically quick to a "sluggish" steering reaction. Add to this the additional weight of the shock
absorber. A fork with a shock is around 171/4 oz. to 21/4 lbs (500 to 1000 g) more than a Unicrown or
switchblade fork.
Suspension forks are particularly useful for a biker who doesn't or can't avoid obstacles and when
riding at high speeds is the goal of the rider, like in racing competition, and especially in downhill races. For
the average biker the suspension system won't become useful until the system has been improved to : 1.
Minimize geometrical changes; 2. Design the suspension in such a way that it can be turned on or off as
circumstances require; 3. Reduce weight.
Rear-Wheel Suspension
After the front-wheel suspension systems gained acceptance, it was only a matter of time before
engineers designed a suspension system for the rear-wheel. This was considered to be an ambitious
undertaking, because it meant jeopardizing the stability of the rear frame, a vital part of the frame structure.
At the end of 1990, Cannondale, Offroad, and Gary Fisher introduced the first rear-wheel suspension.
Cannondale and Offroad used similar systems. They both have elevated chain stays providing lateral sway,
with the pivot point located in the front of the seat tube. Cannondale uses an oil-pressure suspension, the
Offroad rear frame is protected against shocks by plastic devices. These suspension systems are well made
and designed, but they also contribute to some problems: Stiff wishbone construction at the rear frame
lessens lateral stability; interference with the important geometry of the rear frame by adding shock
absorbers will also cause considerable loss to the bike's lateral stability, changing the ride of the bike. Gary
Fisher installed plastic devices to absorb shocks. They're located behind the bottom bracket. Chain tension,
however, makes the rear frame more rigid (due to the lowered seat-stay position); traction is not affected.
Rear-wheel suspension is great for riding downhill, because potholes are smoothed out, and tires
are protected from severe punishment. However, uphill riding can be an ordeal when the rear of the frame
bounces with every pedal stroke. This can drain the energy from the rider quite rapidly. A bike equipped
with rear-suspension is also heavier. At this time no satisfactory solution has been found; the many
different versions are all still in the experimental stages. This technology is still recent and still has room for
improvement. A rear-wheel suspension that is standard to most bikes has not yet been found.
Gears
Shifting and drivetrain have undergone enormous evolutions. Today four different methods of
shifting gears are available: single shift, double shift, rotation-grip, and grip shift. All four of the systems are
different. The one thing that they all have in common is that they are all indexed. The functions of the front
and rear derailleurs have reached high standards, technologically and functionally. In combination with
numerous gear positions this is (at this time ) the most perfect gear shifting system. The only disadvantage
is that it needs frequent attention and adjustment.
To shift gears smoothly and silently before the invention of the indexed system was truly difficult.
It was a process of slow learning, and only professionals knew how to do it properly. The indexed system,
however, made it possible for even a novice rider to master the art of shifting gears properly and with ease.
The indexed system has a built-in mechanism that enables the derailleur to move in such a way that the
chain rests securely on the chain ring as well as on the sprockets.


Single Shifter
Today, as in the past, the single shifter is the one that most bikers prefer. It is close to the
handgrip, and top mounted, this one is the lightest (51/4 oz or 150 g) and reaches every sprocket within a turn
of 90°. This system also makes it possible to disengage the indexed system, so that in case of difficulties,
the gears and derailleur can be used manually, using the friction system. The only disadvantage is that the
position of the lever isn't ergonomically perfect. The thumb has to move up above the handlebars each time
the gears have to be shifted. However, the single shifter system is preferred for all racing bikes.
Double Shifter
For ergonomic reasons, a few of the professional mountain bike racers, moved the shifter below
the handlebars. The lever worked well of the biker pushed the lever away from himself. It was pulling it
back that was the problem. To solve this problem the double shifter was introduced in 1989. The shifter
was split into two separate levers. The lower lever moved the chain to a larger sprocket and the upper lever
moved the chain to a smaller sprocket. The whole procedure became more complicated; instead of one
movement in two directions, using one lever; now two movements, using two levers in two directions,, was
necessary. To shift gears it was necessary, even for trained bikers, to learn the whole new procedure.
despite the improved position of the shifter the double shifter system has a disadvantage; although by using
the lower lever the largest sprocket or chain ring can be reached, to shift to a smaller sprocket (to the right),
it's necessary to push the lever six or seven times, causing a slight slowdown. Although it is a minor
inconvenience for the recreational biker, it is a concern for mountain bike racers.
Rotation-Grip Shifter
Handle bars with a diameter of 7/8" (22.2 and 22.7 mm) are equipped with a 61/4" (16 cm) long
rotation grip with two or three mechanisms inside. The springs, activated by pressure, cause a mechanism
either to tighten or to loosen the gear cable. In order to shift to another gear, the grip must be rotated. A
dial lets the rider know on which sprocket the chain is riding on. Every sprocket can be reached within a
90° turn of the shifter. Later a lever inside the rotation grip was made that prevents the gears from jumping
when riding in rough terrain. Despite the perfect ergonomical placement of the shifter, it does have two
disadvantages; the increasing number of handlebar accessories leaves little room for mounting new ones,
and accidental shifting can't be totally eliminated.
Grip Shift
The "Grip Shift" is a system that can be mounted at several different places on the handlebars. A
21/8" (5.5 cm) wide by 13/4" thick rotation ring can be mounted on either the inside or outside of the grip and
used on any handlebars that have a 7/8" diameter (22,2 and 22.6). This system has an intricate system
consisting of three ring-cups that turn within each other that tightens and loosens the gear cable by pulling it
across a wedge. The only disadvantages is that a 270° turning radius is needed to reach all the sprocket.
The greatest advantage is its light weight. At only 2 oz (66 g) the "Grip Shift" is even lighter than the single
shifter. Other handlebar accessories may also be added if desired.
Front Derailleur
The front derailleur transports the chain rings. This is done by a chain guide, which can be moved
from side to side by a cable, and is moved back with a retracting spring. Indexed systems also function
with the derailleur, but still need further refinement. All too often the chain rubs against the cage and must
be adjusted at the shifter. While it is quite easy at the shifter, it's much more complicated with the rotation-
grip shifter. Adjustments don't last, and frequent attention is necessary. This is a main complaint about the
rotation-grip shifter.
Rear Derailleur
In order to accommodate the wide arrangement of the gears, the mountain bike's chain housing has
to be much longer than that of a road bike. The chain housing has to accommodate the largest sprocket.
The most popular type of mechanism is the "slant" mechanism, almost all rear derailleurs are built according
to this model. With the slant mechanism, a much better functioning shifting system has evolved because
the guide pulley "wanders" back and fort at the same distance over every sprocket.
Brakes
Brakes are the only components that haven't significantly changed in the evolution of the mountain
bike. Today, the simple cantilever brake system has proven the most reliable for off-road riding. The
future, however, belongs to disc brakes, which at this time, are still going through a trial and error period.
The concept of the disc brake is of interest for mountain bikers, because mountain biking makes such great
demands on the brakes. These demands are best served by disc brakes for three reasons: First, the amount
of space that disc brakes allow for the fat tires, so that mud accumulation won't create problems; second,
the brakes should weigh as little as possible; and third, they must function under both wet and dry
conditions. But first we have to the learn the basic, and still the most common type of brake system.
Cantilever Brakes
The best system is also a simple one, and one that works. The cantilever brake is a perfect
example. Two moveable brake arms with brake shoes are mounted on bosses that are soldered to the seat
stays, or to the chain stays. On many models both brake arms are connected by cables. At the end of the
cable, which originates at the brake lever on the handlebars, are cable carriers to which a linking wire is
attached. The link cable can be disconnected either at the left or right brake carriers. This release the
tension and allows the rear of front wheel to be removed. On newer models the brake cable, which comes
from the brake lever, is attached directly to one of the brake arms, and guided by a round cable carrier,
connected to the other brake arm.
On traditional cantilever brakes, brake arms extend rather far to the outside for the best possible
leverage. Sometimes this causes the rider's feet to come in contact with the brake arms. This problem was
solved by "Low Profile" brakes. Brake arms became longer, but the angles became much tighter. The
Pedersen cantilever brake makes use of the direction of the rim rotation to give more power to the brake
shoes. The brake shoes are pulled in the direction of the wheel's forward movement, creating a
correspondingly higher brake action. When releasing the brake shoes, a spring action pulls them back into
the neutral position, which results in an energy saving of 20%.
Brake Shoes
Most brake shoes are made from a hard, friction-resistant, special material consisting of vulcanized
rubberlike plastic, which has been constantly been improved over the years. New combinations made from
synthetic rubber and pheol ressin have increased deceleration, but overall they lose an enormous amount of
effectiveness when the rims are wet. Effectiveness when the rims are wet is the big disadvantage of all rim
brakes.
Since the rim becomes part of the brakes in cable-carrying systems, the effectiveness of the brakes
very much depends on the surface condition of the rim. The most recent rims have a layer ceramic on the
outside which have improved the effectiveness of the brakes under all weather conditions.
Hydraulic Brakes
Hydraulic brakes operate by an enclosed oil tube made from polyamide. Pressure applied to the
brake lever is transferred to a cylinder and the brake shoes. In spite of many advantages, these brakes are
being used less and less, even though the last disadvantage has been eliminated. The disadvantage was that
in order to remove the wheels, you would have to let the air out of the tires. This was solved by designing a
brake so that the brake arms could be opened up so that the wheel could be taken off with out letting the air
out.
Disc Brakes
Despite the good track record of the cantilever brake, the search for an effective disc-brake system
has started. A new bike company in California, Mountain Cycles, introduced a hydraulic "Pro Stop" disc-
brake system in 1990. Aluminum discs (located at the hub of the wheel) have brake shoes made from a
low-temperature fibre material. These brake shoes grip the disc in a "pinching" fashion. The brake shoes,
together with the aluminum disc, don't lose power under wet conditions. Power from hand pressure is
perfectly transferred to the brake shoes. These disc brakes were developed in conjunction with a front-
wheel suspension system. Their weight including fork is 53/4 lbs (2.6 kg). This system can also be mounted
on conventional Unicrown forks.
Brake Levers
The brake lever has been used ever since the mountain bike was invented. It has gone through
improvement over the years in ergonomics, size, weight, and the way it performs. The lever pulls a brake
cable, which transfers the pulling action of the brake arm of the cantilever to the brake shoe. The lever was
shortened after it was discovered that it can be operated with only two fingers. There is also a brake lever
with a roller mechanism, called the "Servo Wave". When this lever is used, the pivot point changes the
relation to the cable carrier, which causes the brake shoes to come closer to the rim. The closer the brake
shoes get to the rim, the more effective the transfer of power from the lever to the brake shoes. All
accomplished with a minimum amount of pressure applied to the brake lever at the handle bars.