micky amok-crossposted to sci.math, sci.physics, and rec.autos.tech:
^^^^^
Please post here using your real name.
Post by mickyWhen I was in high school I was taught, or I read, that it's bad to turn
the steering wheel when the car is not moving. It's hard on the front
tires, wears out the tread, and one should be moving the car at least a
little when turning the wheel. Did they say that? Do they still?
Yes, of course. However, this is just a rule of thumb; the amount of wear
depends on the surface and the type of tread. For example, the wear from
turning a still tire on ice or snow is negligibly small compared to the
turning on asphalt.
Post by mickyI've been thinking about this and now I have doubts.
You have not thought this through.
Post by mickySeems to me any extra wear on the tread because of turning the direction
in which the tires point will be the same whether the car is moving or
still.
It is not. When the car is moving relative to the ground surface (road),
and the wheel and tire are rotating the tire’s tread is experiencing mostly
rolling resistance/friction/drag with the road. When the car is at rest
relative to the road, if the wheel is turned, the tread is experiencing
mostly kinetic friction with the road.
The magnitude of the friction (a force) between two surfaces is calculated
as the friction coefficient (commonly: µ, mu) for the contact of the two
surfaces for the respective situation times the magnitude of the normal
force F_n on the body with significantly less mass (lighter body):
F_f = µ F_n,
whereas
F_n = F_g cos α = m g cos α
is the force with which a body is pressed against the ground surface by
gravity (actually the force that the ground surface must exert on the
lighter body to prevent it from continuing to fall freely towards the center
of energy–momentum of the heavier body, e.g. the center-of-mass of Earth).
α is then the angle of the ground surface to the tangent surface of the
heavier body:
.
:`.
: `.
: `.
: `. m
: `* cos(α) = F_n/F_g
: F_n .^:`. F_n = F_g cos(α)
: .' α: `.
: .' : `.
: `. : F_g `.
: `. : `.
: `.: `. ^
:__ v `. : n __
: | α`. : |PE
'--------------------------`----'--
(α = 0 ⇒ F_n = F_g cos(0) = F_g × 1 = F_g as expected, so this works.)
The coefficient of rolling resistance is generally much smaller than that of
kinetic friction – which is why the wheel was invented in the first place.
For example, the coefficient of kinetic friction for car tire rubber on
concrete is 0.6 to 0.85, while the coefficient of rolling resistance is only
0.01 to 0.015.
<https://en.wikipedia.org/wiki/Friction#Kinetic_friction>
<https://en.wikipedia.org/wiki/Rolling_resistance>
For a car that has an average mass of 1 metric ton, on a horizontal road
that makes a difference of friction of at least
F_fs = µ_s m g = 0.6 × m g = 0.6 × 1'000 kg × 9.82 m/s² ≈ 5'892 N
to
F_fr = µ_r m g = 0.01 × 1'000 kg × 9.82 m/s² ≈ 98.2 N,
i.e. at least 60:1. The greater the friction, the greater the wear. So,
roughly speaking, turning a still tire wears it off 60 times more than
turning it while driving, which means that its lifetime is reduced to 1/60
of its normal lifetime if this would be done continuously.
Post by mickyIt's harder to relate to the sliding motion of the tire on the
road surface when the car is moving,
A tire is usually *rolling*, NOT sliding, on the road surface.
[If it would be sliding, then the respective vehicle would be out of
control. One possibility for this condition is aquaplaning: the tire
is sliding on the water on the road instead of rolling in proper
contact with the road. Tires with a pronounced profile and suitable
tread pattern reduce or avoid aquaplaning as the water can be displaced
into the tread pattern so that the tire keeps in contact with the
road.
<https://en.wikipedia.org/wiki/Aquaplaning#Prevention_by_the_driver>]
Post by mickybut it's clear when the car is still. That seems to me to be the
difference, but the vectors that indicate rubbing seem the same either way.
Most certainly they are not. In the rolling case there is an additional
non-zero force vector in the direction of the wheel’s axial rotation:
___ ___
: : : :
: .-:----> F_fk : -------> F_fk = F_res
^ : : :
F_fr : : : :
: : :
=:=*=:= =:=*=:=
: : : :
: : : :
: : : :
: : : :
:___: :___:
rolling, at rest,
turning left turning left
(friction is always opposite to the direction of motion)
Since the tread profile is optimized for the wheel rolling in the direction
of axial rotation, NOT sliding, sidewards sliding of the tire at rest is
detrimental to the lifetime of the tire and quality of the tread,
particularly when the vehicle has a great mass and it is done on a
horizontal road (as then the friction is greater; see above). Also, one can
imagine that the greater torque required to turn a still wheel (to work
against the greater friction) produces additional stress and wear for the
steering.
Post by mickyI included the math group first because it seems like they would have
opinions.
Please do not do that again.
“No article in the world is relevant for more than
a few newsgroups. If World War Ⅲ is announced,
it will be announced in news.announce.important.”
–attributed to Peter da Silva
F’up2 sci.physics
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