.
All of the spokes in a wheel undergo stress cycles for everything that
affects the wheel while it is being ridden. A 700c wheel rotates about 770
times per mile traveled, 480 times per kilometer (the exact number of
rotations depends on the tire size). The weight of the bike and rider
reduces the static build tension of the spokes at the bottom of the wheel,
and increases the tension on the spokes at the top of the wheel. For each
rotation of the wheel, each spoke has it's tension cycled twice, once when
the tension is increased as the spoke circles from the bottom to the top,
and once when the tension gets decreased as the spoke circles from the top
to the bottom.
Road shock, braking forces, and drive torque in rear wheels all add stresses
to the wheel. All of these stresses affect all of the spokes at the same
time, whether the stress is a shock stress (from road roughness) or a
cyclical stress (from the rotation of, or drive power to, the wheel). Drive
torque stresses essentially affect all of the spokes the same amount, while
shock stresses affect the spokes the closest to, and the furthest away, from
the shock the most. Spokes can experience hundreds of thousands of stress
cycles in their lifetime. These stress cycles cause the spokes to age.
Spoked wheels have triangular force vector patterns, both physical and
non-physical, with the main components being the spokes, the rim, and the
hub. Crossed lacing patterns have more of these force vector triangles than
radial lacing. Also in crossed lacing patterns, the outermost cross, where
two spokes come into physical contact, cause the crossing spokes to bend
away from the direct force vector from the hub to the rim along the spoke.
Even though a triangle is the strongest shape possible for an object
comprised of individual pieces, when you combine the stretchability of the
spokes with deformation of the rim and the bend in the spokes path from
contact with the spoke it touches, you end up with a formation that has some
give. This give allows a cross laced wheel to absorb the shock and stresses
seen when the bicycle is ridden. The greater the number of spoke crossings,
the larger the physical and force vector triangles become, and the more give
the wheel has. Smaller spokes are stretched more easily, this applies to the
thinner section of butted spokes as well, which will give a wheel made with
thinner, or butted, spokes a plusher ride due to the increased give of the
system. However, a spoke that is too small can be overstressed easily, this
is discussed more at the bottom of this chapter.
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The figure to the right represents a spoke in a cross laced pattern and it's
major force vectors. The spoke is the
BLACK line, the spokes main
force vector to the center of the hub flange is the
GREEN line, and the effective
torque transfer force is the
RED line. This triangle creates
a torque transfer angle, represented by the
BLUE arc labeled
"a", which is a major factor in the
wheels ability to transfer drive and/or braking torque between the hub and
rim. The closer this torque transfer angle is to 90 degrees, the more
efficiently the spoke transfers torque. Another way to think of it is that
this torque transfer angle in association with the torque force vector (the
RED line) creates a leverage
factor. The greater the leverage factor the stronger the wheel for
transferring torque. The leverage factor is at it's highest when the torque
transfer angle is 90 degrees. The greater the number of crosses the stronger
the wheel is for transferring torque, up to a point. If you laced a wheel
with so many crosses the torque transfer angle passed the 90 degree point,
the leverage factor decreases. The torque transfer capability is then the
same as the spoke it directly crosses. This angle can appear to be
backwards, a radial wheel has this angle at 180 degrees. Each incremental
cross pattern (1-cross, 2-cross, etc.) then reduces the value of this angle.
A pattern with too many crosses results in an angle less than 90 degrees.
The lacing that gives the best torque transfer has an angle that is close
too, but not less, than 90 degrees.
Dish is a term that is usually misunderstood, even though everyone knows
what it refers to. Many think dish only applies to multi-speed rear wheels
and front wheels with disc brakes. In reality all spoked wheels have dish,
and dish really has nothing to do with the drive or braking system on the
wheel. A spoked wheels actually has two occurrences of dish. What dish
really means is the distance from the rim to the hub flange at the points
where the spokes anchor, as shown below.
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The figure to the right shows how dish is determined. Imagine that the rim
and hub are of a built wheel, the spokes are not shown. The dish is the
distance from the center of the hub flange, the
RED line, to the center of the
spokes anchor on the rim, the
BLUE line. There is also dish
for the side of the wheel that is not referenced. The larger the value of
the dish the greater the lateral strength of that side of the wheel.
When people normally talk about dish they are discussing the difference of
the dish values of the two sides of the wheel. This difference can also be
called offset. In a wheel with a very small offset, such as a front disc
brake wheel, it is better to lace both sides with the same pattern and
balance the offset (center the rim in the wheel) with slightly different
spoke tensions and/or different sized spokes. When the offset becomes large,
such as with modern 9 and 10 speed rear wheels, it can do the wheel benefit
to have a mixed lacing.
Wheels with a large offset have great differences in the lateral strengths
of each side of the wheel. The different force vectors caused by a mixed
lacing pattern can compensate for the main force vectors and lateral
strength differences caused by the dish offset. Different lacing patterns
have different amounts of lateral strength, and they have differences in all
of the different force vectors created by, and controlled by, the spokes.
Mixed lacing patterns utilize these differences to increase the strength of
the wheel when compared to standard rear wheel lacing's where each side has
the same pattern.
Typically a front wheel has two equal amounts of dish, with the exception
of front disc brake wheels that usually have a small dish offset. Since the
difference in the dish values of a disc brake front wheel are very small, a
mixed lacing pattern is not something that will do the wheel any good.
Having a front disc wheel with mixed lacing, such as half radial, would give
the wheel more imbalance than the wheel would have if it was laced with a
standard lacing. The wheel could end up being weaker wheel.
A common mixed lacing for mass produced rear wheels is the half radial
lacing. Most of these wheels have the non drive side radially laced. This is
really the wrong side to lace radially in a half radial rear wheel. Radial
lacing has the highest lateral strength of any lacing pattern. Not only are
the spokes shorter, the attachment point at the hub gives the spokes an
anchor point the greatest distance away from the exact center of the wheel.
If you wish to make a half radial rear wheel you should make it radially
laced on the drive side. This will give the drive side the highest lateral
strength it can have. (The spoke heads should be on the inside of the
flange.) Claims that you need cross lacing on the drive side for increased
power transfer are wrong. A rear hub is so strong and stiff that there is no
way it would absorb any drive power if the power had to travel the short
distance from the drive side flange to the non drive side flange.
One thing every wheel builder needs to know is that drive and hub braked
wheels cannot be fully radially laced. It would work, but it puts the spokes
and the hubs under a high stress situation that shortens spoke life and
could cause the spokes to 'pull out' of (tear through) the hub. Drive wheels
will turn out to be 'spongy' under power, that is the wheel would have wind
up and will not transmit drive torque efficiently. The pedals would feel as
though they were a big rubber sponge. The wind up will be the worst under
high power situations like sprints, starts and hill climbs. A wheel with the
braking at the hub will also suffer from wind up during braking, and the
harder you applied the brake the worse the effect will become. During
braking the wind up could cause brake modulation, and under really hard
braking the modulation could cause the wheel to unexpectedly lock up.
In a cross laced pattern on a rear wheel there are 'push' spokes and 'pull'
spokes. The 'pull' spokes are the spokes that transfer all the drive power.
The power that enters the hub increases the tension at the hub end of the
spoke. The spoke then transfers this tension to the rim. Since the rim isn't
attached to the ground the rim starts turning. This all happens
instantaneously. The 'push' spokes don't really push, but their build
tension is reduced from the reaction to the increase of the tension on the
'pull' spokes (every action has an equal an opposite reaction law of
physics). A hub braked wheel has the same principle, only the direction of
the force on the spoke is reversed, it's traveling from the rim to the hub.
If you fully radially laced a rear or hub braked wheel, there would be no
'pull' spokes. The torque forces at the hub and rim would cause the hub to
turn in relation to the rim. The spokes will turn at the hub and pivot on
the rim until the force of the increased spoke tension equals the force of
the torque being transferred. The harder you pedal or brake the more the
spokes will stretch. A cross lacing doesn't have this stretch problem
because of the triangular shape of the force vectors acting on the spoke
The drive torque transfer ages rear wheel spokes. If you have ridden a wheel
for many years and it will no longer hold true, this is because the spokes
have aged in the wheel from transferring drive torque and they easily
stretch. The 'pull' spokes undergo hundreds of stress cycles each mile, or
kilometer, the bike is ridden. The harder and faster you ride, the higher
the stresses on, and the shorter the life of, the spokes. Also the fewer
spokes a drive wheel has the faster it will suffer from this fatigue.
More information about the dynamics of the forces that affect the torque
transfer and ride quality of a cross-laced wheel in comparison to a radially
laced wheel, and information about the lateral strength differences, are
discussed here:
TRIG PAGE; CROSS - LACED WHEEL STRESS FACTORS.
Spokes are a very important part of the wheel. You need to choose the right
spoke for the application. I always lace the driven side of a rear wheel
with 14 gauge spokes. I would never lace a non-drive side with less than a
14-15-14 spoke. 15-16-15's are really only useable with front wheels, but
only if the front wheel has 28 spokes or more, and then only if you are a
light person. Low spoke count wheels need heavy spokes. If you replace a
spoke in a low spoke count wheel, if you have a choice, you should always
use the strongest spoke. Spokes all work together, and the forces they
control while they hold the wheel together are unfathomable. One should
never underestimate the importance of the spokes.
In a cross laced wheel, since the spokes come off the rim at an angle
relative to the major force vectors holding the wheel together (reference
the figure at the top), the tension on the spoke and nipple threads is
uneven side-to-side. This is one aspect that allows cross laced spoke
threads to be greased so they don't seize as the wheel ages. Another aspect
is that since the spokes cross others they are not free to vibrate. Any
vibration a spoke sees from shock gets dampened by the spoke it crosses,
just like lightly placing a finger on a guitar string will stops it's
resonance. When you grease the threads in a cross laced pattern it makes the
wheel easier to build initially, and easier to true when the wheel needs
service. It prevents the spoke and nipple threads from cold welding together
which can make truing difficult or impossible without replacing the spoke.
In radially laced wheels, the spokes are in line with the main force vectors
holding the wheel together, so the spoke threads have even tension all the
way around. Any vibration from road shock will also resonate freely. When
you combine the evenly tensioned spoke threads with the shock and vibration
the spokes see, you end up with the potential that the spokes will loosen in
the nipples. Greasing radial spokes will hasten this action. Either a spoke
prep or nothing at all should be used for a radially laced spoke, either in
a full radial wheel or in a complex pattern like a Crow's Foot. It is also
not a good idea to grease spoke threads in a 1-cross lacing, or any spoke
that does not physically touch another spoke in a crossing away from the
hub.
