

Effects of Wheel Radius
Name: Jessica
Status: Student
Grade: 68
Location: CA
Country: United States
Date: August 2007
Question:
Does the radius of wheel effects the
speed of car? How?
Replies:
Hi Jessica,
As you probably know from Math class, the circumference of a
circular object, like a wheel, is found by multiplying its
diameter by "pi" ("Pi" = 3.1416). This is the same as 2 times
the radius multiplied by "pi". As a car rolls down the road,
every time its wheels rotate one complete revolution, it
travels forward a distance exactly equal to the circumference
of its wheels. To give an example, I just measured the tires
on my car, and they are 22.5 inches in diameter (radius is
11.25 inches). Their circumference is "pi" x diameter = 70.68
inches. This means that every time the wheels rotate once,
the car moves ahead 70.68 inches or about 5.9 feet.
If I were to be able to replace my tires with huge truck
tires whose radius was two times larger than the radius they
are now, then for every rotation of these giant wheels, the
car would move forward 11.8 feet (twice as far as before).
So to summarize, if the speed of rotation of the car's wheels
does not change, the speed of the car is directly
proportional to the wheel's radius. Larger radius means
higher speed, and smaller radius means slower speed. Of
course, you cannot make a car with a top speed of (say) 100
MPH, do 200 MPH, just by putting wheels that are twice as big
on it! The motor has to have a lot more power to keep the
larger wheels turning the same speed as before, because higher
speed (no matter how you achieve it) requires more power.
Regards,
Bob Wilson.
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Update: June 2012

