Skateboard Wheels

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Skateboard Wheels

• Case Study
• CHE 620

2
Introduction

• From the first push forward on a home driveway to
  intricate freestyle routines in a purpose-built
  park, and at all levels in between, skateboarding
  is a very exhilarating and fun-filled activity.
• In synthesizes the best features of many sports,
  including surfing, skiing, dance, gymnastics, and
  martial arts, combining strength, grace, and
  concentration.

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Main Parts of a Skateboard

• Three main parts
• the deck or flexboard
• the wheels
• the truck

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Main Parts of a Skateboard

• Common skateboard decks
• (a) basic flat board
• (b) kicktail board for rear foot leverage
• (c) rocker board for lower center of gravity
• (d) cambered board for slalom and downhill racing

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Main Parts of a Skateboard

• Common deck materials
• wood (ash, teak, oak)
• polymers (polypropylene, polycarbonate)
• composites (glass fiber reinforced epoxy resin)
• wood and fiberglass combinations
• aluminum and its alloys

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The Wheels

• Modern skateboarding probably would not be
  possible without major advances in both the
  design of the wheels and the materials used in
  their manufacture.
• The wheels generally range in size from 29 mm
  width by 38 mm diameter to 58 mm width by 67 mm
  diameter.

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The Wheels

• The wheels contain either loose ball bearings or
  double-sealed precision bearings.
• The former have the disadvantage of requiring
  regular lubrication and the possibility of the
  wheels being slowed by sand, road grit, and other
  particles.

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The Wheels

• Connecting the deck to the wheels is a steering
  and suspension system called a truck.
• While trucks come in different designs, they all
  feature shock absorbers of elastomeric or
  polymeric bushings or cushions or metallic coiled
  springs.
• Some trucks are adjustable in length and can be
  tightened or loosened to accommodate the weight
  of the rider.

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The Wheels

• Various parts of the truck are made of cast iron,
  forged iron, heat-treated aluminum, or steel.
• (a) standard wheel with open loose bearings
• (b) flex wheel with double-sealed precision
  bearings

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Materials Selection Considerations

• To perform satisfactorily, the materials for the
  wheel (and bearing assembly) must have the
  following characteristics
• lowest possible rolling friction resistance, in
  order to maximize the speed of the skateboard
  (the primary requirement)
• high frictional resistance to lateral movement
  (important for slalom and freestyle motions)
• good shock absorption qualities to make riding on
  rough surfaces more comfortable
• high resistance to abrasive wear
• high resistance to corrosives such as lubricating
  oil

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Materials Selection Considerations

• At right is shown the contact stress on a
  perfectly elastic wheel in motion on a rigid
  surface and carrying a static load.
• Upon rolling, one part of the flat suffers an
  increase in strain while the strain on other
  parts is reduced.

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Materials Selection Considerations

• For real materials, however, the stress in the
  section of the flat where the strain is
  constantly decreasing lags behind that in the
  other section of the flat, resulting in a stress
  distribution as shown at right.

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Materials Selection Considerations

• The final outcome is that the center of pressure,
  O, moves forward of the center of the wheel O by
  a small amount s, giving rise to a bending
  moment, Ps, that resists rolling.

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Materials Selection Considerations.

• The rolling resistance force R is the equivalent
  force acting at the center of the wheel in the
  direction opposite to the bending moment

where Dw is the diameter of the wheel
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Materials Selection Considerations

• The coefficient of friction, mrs is thus

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Materials Selection Considerations

• Some knowledge of s is needed in order to
  estimate mrs from the above equation.
• For a perfectly elastic wheel material, s 0.
• For a worst case material, with the stress
  distribution shown at right, s ac, where c is
  the half-width of the flat and a is a numerical
  constant.

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Materials Selection Considerations

• For many real materials, therefore, we can
  assume that, as a first approximation,
• s a(1-A)c
• where A is the percentage rebound and is equal to
  the square of the coefficient of restitution e of
  the wheel material.

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Materials Selection Considerations

• Now

where b is a numerical constant, usually about
0.8, h is the width of the wheel, and Eef is the
effective modulus of the wheel.
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Materials Selection Considerations

• Eef is given by

where Ew and nw are the modulus of elasticity and
Poissons ratio of the wheel material,
respectively, and Ers and nrs are
the corresponding values for the road
surface material.
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Materials Selection Considerations

• From the above, we can obtain an expression for
  mrs

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Materials Selection Considerations
an examination of the above shows that, for a
given wheel with specified values of wheel area
(h x Dw) and wheel load P, the primary design
requirement of low friction may be obtained by
using a material with high values of modulus of
elasticity and coefficient of restitution
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Materials Selection Considerations

• On rougher surfaces, such as tarmac or concrete,
  an important additional factor affects friction,
  which leads to a reconsideration of the desirable
  properties for a wheel material.
• Upon rolling over a rough surface, the wheel load
  will be supported on road asperities where the
  contact pressure is high.
• The resulting penetration must be countered by
  the elastic deflection of the wheel if energy
  loss and friction are to be minimized.
• This requires a wheel material with a large
  elastic strain and high elastic modulus, with the
  former becoming more important as road roughness
  increases.
• For a rough surface, mrs is increased by about
  50 (mrs 1.5mrs)

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• From the above, the best material from the
  viewpoint of both mrs and mrs is glass fiber
  reinforced polyimide.
• However, because of its relatively high cost, low
  shock absorption qualities, and relatively low
  wear resistance, the material currently favored
  for the construction of skateboard wheels are
  hot-cured polyether-based polyurethanes.

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The End

• The End