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Wheel Size and Performance
Name: Katie
Status: Student
Grade: 9-12
Location: CA
Country: United States
Date: January 2009
Question:
What is the effect of different wheel sizes on the
performance of a vehicle?
Replies:
Hi Katie,
Larger diameter tires on a vehicle will result in the engine turning
slower, as a result of the tire's larger circumference. However, in
practice, when a larger tire diameter is used on a vehicle, the
manufacturer will change the transmission's gear ratios to correct for
the changes caused by tire diameter change.
Larger diameter tires have slightly lower rolling resistance, so a
larger tire will usually result in slightly better gas mileage. But
the difference is very small.
A wider tire, on the other hand, will result in more rubber contact on
the road, and that will reduce gas mileage slightly. The benefit of a
wider tire is mainly better adhesion to the road; this means that the
tires grip the road better. One can accelerate faster before the tires
"break loose", stop faster, and corner faster.
Larger diameter and wider tires, both result in increased tire and
wheel weight, which both increase "unsprung weight". Unsprung weight
is a car suspension designer's term for the weight of the components
(mainly the tire and its wheel) that will bounce up and down when
going over a bump. Greater unsprung weight requires a stronger shock
absorber to control the "bouncing" of the heavier tire and wheel as
you hit a bump.
As you can see, there are a lot of "tradeoffs" here. One cannot change
the wheel size without compensating with several other changes as
well.
Regards,
Bob Wilson
By "wheel," I assume that you mean the entire tire/wheel assembly. Big
wheels and tires generally support more weight, but they cost more and give
poorer fuel economy than small wheels. When tires roll they generate heat,
and when overloaded, the high temperature causes them to fail.
When engineers design a vehicle, they typically choose the most economical
wheel and tire that is necessary to do the job of supporting the vehicle for
the intended task. For example, a mining truck that is as large as a house
requires tires that are ten feet tall to support the heavy weight. Larger
tires would cost too much, but smaller tires would be quickly ruined. Farm
tractor tires tend to be large to give more traction and to reduce the
amount of soil compaction when the tire passes. Automobile and truck tires
are sized to give the load capacity and/or "performance" that is required.
For vehicles in which appearance is a selling point, wheels may be made
larger because a large wheel may attract more buyers. Wheels and tires of
high-performance cars tend to be larger and/or wider to enable the car to
accelerate and turn corners more quickly, but not so large that they get too
heavy. The size of motorcycle wheels and tires tends to follow the size of
the tires used for racing, and the diameter, profile and width of motorcycle
racing tires are carefully chosen for the best track performance, which may
not be the best for use on streets.
A performance requirement for airplane wheels is that they must be small and
light, and so they are made small, light, strong, (and expensive.)
Robert Erck
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Update: June 2012
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