The design of tire treads is very complex, otherwise there wouldn't
so many differently designed patterns of tire treads in the tire store.
The surface area of contact with the pavement is only one factor.
Others that come to mind are dissipation of heat by air flow through
the tread, depth to give some "grip" in sand or snow, and flexing of
the tire under acceleration or braking.
A major factor is a design that lets water on the pavement flow
through the tread. If it does not, the car will hydroplane, rise up on
top the water layer, even at rather slow speeds. The result is total
loss of traction and control.
What is a good test procedure to determine the
best tire pressure?
Tire pressure is a compromise. Lower pressure gives a smoother
and quieter ride, but with more tire noise and less tire tread life.
Higher tire pressures give more precise steering and better fuel
economy. Higher rear pressure than front pressure will bias a
vehicle to understeer. This is where the front of the vehicle slides
out in a sharp turn before the rear. Higher front tire pressures than
rear biases the vehicle to oversteer. This is where the rear axle
slides out first, leading to a spin.
I would suggest that you experiment with your tire pressure to get
the performance compromise that is acceptable to you. You might
want to use the door sticker recommendations for a lower bound and
the maximum recommended tire pressure number (from the side
wall) as the high limit. Get a good tire pressure gage, and use it
regularly. Try different pressures and see what combination of ride
and control makes you most comfortable.
You should find that your best pressure settings change from
summer to winter, and even for different road surface conditions.
What factors are involved in automotive
engineers/designers choices of wheel sizes. What are the effects
of increasing or decreasing wheel size in relationship to engine
power and vehicle performance?
As far as power transmission is concerned, you can regard the wheel
as just another gear. Among other considerations, enlarging the
wheel would necessarily raise the axle and, thus, the car's center of
mass. Also, changing the wheel size would change the angle of the
force delivered by a bump in the road. If wheel sizes change a lot,
curbs And speed bumps must also change correspondingly, or they
would not have the same effect on a car. The main effect of changing
the wheel diameter on a car is the need to change the gears which
change the ratio of engine speed to wheel rotation speed; larger
wheels clearly rotate more slowly for a given car speed. However,
the acceleration and top speed of a car does not depend on the wheel
diameter if the gear ratio is optimized for that diameter.
Larger diameter wheels have the advantage that they average the
road surface over a larger area so small bumps are not so noticeable.
More important is the weight of the wheel. The lighter the wheel,
the less is the kinetic energy of the wheel at a given speed and so the
more of the work of the engine is used to drive the car forward. It is
also easier to design suspensions, which are more comfortable if the
wheel, is lighter. A lighter wheel can go up and down more rapidly
when subjected to the forces of the bumps in the road and the spring
connecting it to the body of the car.
A larger, heavier tire and wheel assembly will gain more kinetic
energy from each short bump it encounters,which the suspension
then tries to absorb without passing on to the passenger cabin. So
smaller tires give a smoother ride. When tires bounce, a given
amount of suspension hardware can put them back in contact with
the road quicker if they have low Mass. smaller tires should give
better handling. The ground-contact footprint of the tire will grow or
shrink until air pressure bears almost exactly the weight of the car.
[Inflation pressure x Footprint area x Number of tires
= Vehicle weight ] For a given weight you could use small tires
inflated to high pressure,or small tires at low pressure with a long,
soggy footprint,or larger tires with either pressure.
Flexing and smacking heavy rubber on the pavement every revolution
uses some energy, makes some rolling friction, so I think that small
tires at high pressure are known to be the most fuel-efficient
choice.Large flexing of rubber at high speeds also makes heat, which
makes tires wear out or blow out sooner. It is not always engines
pushing tires. Often tires push engines back. Smaller wheels have
less angular inertia, and less torque when skidding,so they present
less threat of damaging the power-train metal parts. This is most
evident in off-roading, where huge tires sometimes break drive shafts
or axles when a tire suddenly lurches into a rock. The axle is caught
between the sudden torque on the tire, and the angular inertia of the
geared-down engine.(The fluid clutch in an automatic transmission is
a partial safety-relief for this stress.)But I am becoming very aware
lately that when you apply gears to an angular inertia (say of an
engine),the apparent rotary inertia goes as the _square_ of the gear
ratio. That figures in somewhere.
On the other hand, small high-pressure tires have more load per
square inch and should wear down faster,and large low-pressure
tires often have better traction in various circumstances.
Large diameter wheels and tires effectively put the car in a slightly
higher gear,which in principle could be compensated for by having a
different set of gears in the power train,but in the real world that
change isn't usually cost-free.If your engine is overpowered or
"torquey" with its given gear set, a larger tire diameter helps you use
that for speed.Conversely many small engines will need small tires
to do their (very modest) best acceleration.The rotary inertia of huge
tires can reduce a vehicle's acceleration,but that's only a factor when
the wheels and tires are a substantial fraction of the vehicle's mass.
Other than monster trucks or beginner-designed RC or robot
vehicles, itusually does not matter much.
Choosing an aspect ratio is a more current issue for engineers.
A tire can protrude for beyond the rim to a distance larger than it is
width,or be "thin and wide" with less sidewall.
Thin and wide can have less lateral flexure during extreme
cornering,and I think there is some technical reason why it is a more
viable option than it used to be. Certainly wide tires must have
radial-ply rather than bias-ply reinforcement,so the wide outer face
can remain "flat".
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