If you've been on a biking trail in the last few years,
you've probably seen all kinds of crazy-looking bikes. If
you've read "How Bicycles
Work," you already know the basics, so in this article
we'll take a look at some of the new stuff on today's mountain
bikes, including:
More gears (Some bikes have as many as 27!)
Advances in sprocket technology that make changing gears
easier
Shift levers that automatically go up or down one gear
at a time
Front and rear suspension
New frame designs and materials
New brakes, including hydraulic disc brakes
Mountain bikes are very popular and boast
some pretty cool
technology.
In this edition of HowStuffWorks,
we'll take a look at all of this new high-tech gear. Let's
start with the gears.
Gears Mountain bikes just keep accumulating
more gears. Today,
some bikes have as many as 27 gear
ratios. Mountain bikes use a combination of three
different-sized sprockets in front and nine in back to produce
these gear ratios.
The idea behind having all of these gears is to allow the
rider to crank the pedals at a constant pace (cadence)
no matter what kind of slope the bike is on. You can
understand this idea by imagining a bike with just one gear.
On this one-gear bike, each time you rotate the pedals one
turn, the rear wheel would rotate one turn as well (a 1:1 gear
ratio).
If the rear wheel is 26 inches in diameter (66 cm), then
with 1:1 gearing, one revolution of the pedals would cause the
wheel to cover 26 * 3.14 = 81.6 inches (207 cm) of ground. If
you are pedaling at a cadence of 50 RPM, that means that the
bike can cover 81.6 * 50 = 4,080 inches (340 feet) (103 m) of
ground per minute. That is only 3.8 MPH (6.2 KPH), which is
about walking speed. That's great for climbing a steep hill,
but bad for level ground or downhill stretches.
To go faster, you need a different ratio. For example, to
ride downhill at 25 MPH (40 KPH) with a 50 RPM cadence at the
pedals, you need about a 6.5:1 gear ratio. A bike with lots of
gears gives you a large number of increments between a 1:1
gear ratio and a 6.5:1 gear ratio so that you can always pedal
at about 50 RPM (or whatever cadence feels comfortable to you)
no matter how fast the bike is going.
The Derailleurs The derailleur is the
device that changes gears by moving the chain from one
sprocket to another. There are two derailleurs: one on the
rear and one on the front. The highest
ratio (when the bike can go fastest) is produced when the
chain is on the biggest sprocket in the front and the smallest
in back. The lowest ratio (the bike is easiest to pedal
up hills, but very slow) is produced when the chain is on the
smallest sprocket in front and the biggest in back.
Rear Derailleur The
rear derailleur has two main tasks: keep the chain
tense and switch gears.
The rear
derailleur
The rear derailleur adjusts its position to maintain
tension in the chain no matter which gear you are in. If the
chain is on the biggest sprocket in front and in back, more of
the chain is wrapped around sprockets and the derailleur has
less slack to deal with. If the chain is on the smallest
sprockets in the front and back, the derailleur has more slack
to deal with.
How the derailleur takes up slack in the
chain
The rear derailleur switches gears by moving the bottom of
the chain from side to side. When you pedal the bike, the top
of the chain is in tension -- the force of your legs pedaling
pulls it tight. It is this part of the chain that transmits
the force from the front sprockets to the rear sprockets. The
bottom of the chain is kept in light tension by the rear
derailleur. Since the bottom of the chain is not under much
load, the derailleur can move the chain to another sprocket
even if you're pedaling hard.
One of the rear sprockets, note the "ramps"
in the sprocket that help with gear
shifts.
Sprocket technology has improved the ability of bikes to
shift under load. The sprocket in the picture above is one of
the nine sprockets in the rear. Some of the teeth are shorter
and wider than others -- these teeth grab the chain first
during a shift and pull it up onto the sprocket. The sprocket
also has "ramps," special grooves in the side that help pull
the chain onto the sprocket.
Front Derailleur The
front derailleur moves the chain between the three
front sprockets. Unlike the rear derailleur, the front
derailleur moves the top part of the chain, which is under
tension when you are pedaling. This means that in order to
switch sprockets on the front, you have to ease off on the
pedals.
The front
derailleur
Some front sprockets also employ a clever design to allow
shifting under load. The sprocket in the photo below is the
center of the three front sprockets. Notice the small pegs
sticking out from the side of the sprocket -- these pegs catch
the chain and pull it up onto the sprocket. Like the rear
sprockets, this front sprocket has ramps that help pull the
chain up.
One of the front sprockets, this one also has
"ramps" and small pegs to help change
gears.
Now let's see how all those gears are used.
Using the Gears On a typical 27-speed
mountain bike, about six of the gear
ratios are so close to one another than you would never
notice a difference between them. So why all the hoopla over
more speeds?
In actual use, riders tend to choose a front sprocket
suitable to the slope they're riding on and stick with it. The
front sprocket is difficult to shift under load. It is much
easier to shift between the gears on the rear. If riders are
cranking up a hill, they'll probably chose the smallest
sprocket on the front and shift between the nine gears that
are available on the rear (since they can do this without
easing off on the pedals). Having more speeds on the back
sprocket can be advantageous.
Shifters The
shifter design used on mountain bikes today recognizes
that the point of shifting is to maintain a constant cadence.
This means that most of the time you'll just be making small
adjustments to the gear in order to increase or decrease your
cadence. The front and rear shifters are equipped with
switches that shift gears one at a time, either higher or
lower.
The switch on top shifts to the next larger
sprocket; the switch on bottom shifts to the next
smaller
sprocket.
Each shift lever adjusts a cable that determines the
position of the derailleur. Both the front and rear
derailleurs contain strong springs that force them to one side
or the other. The shift lever either pulls against those
springs to move the derailleur one way, or lets the springs
pull the cable to move it the other way.
Now let's take a look at some mountain-bike suspension
systems.
Suspension Many bikes today have both
front and rear suspension systems. The
suspension lets the wheels move up and down to absorb
small bumps while keeping the tires in contact with the ground
for better control. It also helps the rider and bike absorb
large shocks when landing jumps.
A downhill racing
bike
Both the front and rear suspension systems contain two
essential elements: a spring and a damper.
Sometimes these components are collectively referred to as a
shock absorber.
Spring The spring
allows the suspension to move up when the wheel encounters a
bump, and to quickly move back down after the wheel passes the
bump.
The spring can be a coil of steel, like most springs we're
familiar with, or it could be a cylinder containing
pressurized air. In either case, the further the spring it is
compressed, the more force it takes to compress it, which is
exactly what we need for a mountain-bike suspension. You don't
want the spring bottoming out when you land a big jump.
Damper If the suspension
were equipped with just a spring, it would bounce up and down
several times after each bump. When compressed by a bump, a
suspension system needs a way to dissipate the energy that is
stored in the spring. The damper is the device that
dissipates the energy and keeps the suspension from bouncing
out of control.
The damper in a mountain bike shock absorber. It
pumps oil through small holes as the piston moves up and
down.
The most common type of damper is oil-filled. This
type of damper is used in car suspensions as well as bike
suspensions. When the shock absorber is compressed, a piston
inside it forces oil to pass through a small hole, called an
orifice. It takes energy to force the fluid to pass
through the orifice, and this energy is converted to heat in
the oil. The cool thing about oil-filled dampers is that they
dissipate more energy and give more resistance to motion the
faster you try to compress the shock absorber.
When the shock absorber compresses faster, a greater volume
of fluid has to flow through the orifice, so more pressure is
required to force the fluid through. This increased flow does
two things: It increases the stiffness of the
suspension (because the pressure resists the motion of the
shock absorber), and it dissipates more energy.
Designing a good shock absorber is partially about finding
the best balance between the spring rate (the stiffness of the
spring) and the damping. For this reason, many shock absorbers
have adjustable spring rates and damping. Some compressed-air
springs can be adjusted by increasing or decreasing the air
pressure.
Now let's see how the damper is incorporated into a
mountain-bike suspension.
Front Suspension The
predominant type of front suspension is the suspension
fork. It works like the front suspension on a motorcycle.
A typical mountain-bike suspension
fork
The bottom part of the fork, which holds the wheel, fits
over the tubes that connect the fork to the frame.
Inside each tube on the fork is a shock similar to the one in
the diagram we saw earlier. When the fork moves up (when the
bike hits a bump), the spring gets compressed and the piston
forces fluid through the orifice.
Rear Suspension There are
as many different rear-suspension designs as there are bicycle
makers. Most of them use a shock absorber similar to the one
in our diagram, sometimes with a larger coil spring. It is
mostly the design of the frame and the linkage that makes one
bike different from another.
Some mountain bikes have a rear-suspension
setup with geometry similar to a conventional bicycle
frame.
The rear suspensions usually need bigger springs because
the linkage gives the wheel a mechanical advantage over the
spring. The rear wheel might have to move 3 inches (7.62 cm)
for the spring and shock to be compressed 1 inch (2.54 cm).
This means that the force on the shock is three times the
force on the tire. In the front, there are two shock
absorbers, and the force on each shock is half the force
pushing on the tire.
Some bikes have a rear suspension like a
motorcycle.
Going hand-in-hand with developments in suspension are
developments in frame design.
Frames Front and rear suspensions have
enabled riders to subject their bikes (and themselves) to
incredible abuse. There are high-speed races down mountains
strewn with rocks, fallen trees and gullies.
The increased abuse and rear-suspension systems and have
placed additional stress on the frames of mountain
bikes. Frames constructed simply by welding together steel or
aluminum tubing are quickly being replaced by more complicated
structures.
A welded tube bike
frame
The round tube is not the most efficient shape of tubing
for use in bike frames. A round tube is equally strong in
side-to-side and up-and-down bending, but the stresses in
frames tend to be more in the up-and-down direction and less
in the side-to-side. The rectangular beams and joists used in
building your house can support lots of up-and-down weight
without bending much, but if you turn them on their side they
can barely support any weight at all.
If the frame is made using shapes that are more
rectangular, taller than they are wide, you can gain strength
in the up-and-down direction, sacrificing a little
side-to-side strength that you didn't need anyway.
Making these more complicated shapes required some advances
in material-forming technology. Some of these pieces are made
by hydroforming the metal. This is a process whereby sheets or
tubes of metal are placed inside strong dies (kind of like
molds) and then pressurized with water. The pressure of the
water forces the metal to conform to the shape of the die.
This technique allows for the formation of complex metal
shapes.
Some bike frames are constructed from carbon fiber.
This is a material that is built up in sheets over foam forms.
Sheets of carbon-fiber cloth are placed over the foam forms
and epoxied in place. The result is a very light, strong
structure that can have almost any shape.
These non-circular-section frame structures make certain
rear-suspension designs possible. One example is the
motorcycle-style suspensions that have no vertical tubes tying
them into the frame.
The main section of the frame on this
downhill racing bike is made by welding two stamped
aluminum pieces back to
back.
A suspension like the one in the picture above makes it
difficult to find a place to mount conventional brakes. That's
one of the reasons why some bikes have been using disc
brakes, as we'll see in the next section.
Brakes The brakes on mountain bikes have
evolved over the years. The predominant design is a variation
of the cantilever brakes found on most bikes.
A typical mountain-bike
brake
A cable running from a lever on the handlebars pulls the
two levers on the brakes together. This squeezes the brake
pads against the outside of the wheel.
Some bikes, especially those with suspension systems, use
disc brakes. These work just like the disc
brakes on a car.
A disc brake on the rear wheel of a mountain
bike
The brake lever uses hydraulic fluid to transmit the
force from your hand to the brake shoes. The handle presses a
small piston that applies pressure to the fluid in the line.
At the wheels, a larger piston squeezes the pads onto the
disc. Since this piston is larger, the force is multiplied at
the wheels. (Click here
for details on hydraulic force multiplication.)
The handle contains a small device that works something
like the master
cylinder in your car brakes. It makes sure that there is
enough fluid in the reservoir that if the pads wear, or the
fluid expands or contracts (as it does when heated or cooled),
there will still be enough fluid in the system to actuate the
brakes.
