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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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