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Name: Jessica
Status: Student
Grade: 6-8
Location: CA
Country: United States
Date: August 2007

Does the radius of wheel effects the speed of car? How?

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.


Bob Wilson.

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