Title: Skateboard Wheels
1Skateboard Wheels
2Introduction
- 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.
3Main Parts of a Skateboard
- Three main parts
- the deck or flexboard
- the wheels
- the truck
4Main 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
5Main 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
6The 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.
7The 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.
8The 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.
9The 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
10Materials 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
11Materials 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.
12Materials 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.
13Materials 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.
14Materials 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
15Materials Selection Considerations
- The coefficient of friction, mrs is thus
16Materials 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.
17Materials 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.
18Materials Selection Considerations
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.
19Materials Selection Considerations
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.
20Materials Selection Considerations
- From the above, we can obtain an expression for
mrs
21Materials 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
22Materials 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)
23- 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.
24The End