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Name:  Oum Prakash S.
Status: Educator
Age: 40s
Location: N/A
Country: N/A
Date: July 2004


Question:
All the wheels of most auto-vehicles are equal in size, per vehicle. But are the wheels of a tractor used by farmers not the same size? The front wheels are generally smaller than the back wheels.



Replies:
It is not required that all four wheels are the same diameter (or the three wheels on a tricycle or the two wheels on a motorcycle or bicycle). In fact, early bicycles had very large diameter rear wheels. It is a convenience if all wheels are the same diameter as then only one spare tire is necessary.

The early bicycles had a large diameter rear wheel, which supported most of the weight of the rider, in order to smooth the ride. The larger the diameter, the less the rider is jostled by bumps in the road.

This is also part of the reason for large diameter rear wheels on tractors. I suspect an even more important reason is to support the tractor on soft and muddy fields. It is also easier for the tractor to avoid getting stuck in the mud if the wheel is larger.

Best, Dick Plano, Professor of Physics emeritus, Rutgers University


Historically, tractors used the rear wheels to propel themselves. Farmers need large tires to avoid compressing the earth, and to avoid digging in. Thus only the rear tires really need to be large and the front tires can be small and smooth. Large four-wheel drive farm tractors have equal size front and rear tires. Some small tractors are four-wheel drive, and these have medium size lugged tires in front for extra pulling power.

Bob Erck


Oum -

An educated guess about your question... The larger wheels, which are used as drive wheels, spread the weight of the tractor over a larger area. This makes is less likely to sink in soft dirt and get stuck. On the other hand, it is more difficult for the steering mechanism to turn the large wheels making it more desirable to keep them smaller.

Now days, you sometimes see tractors with large wheels (as many at six drive wheels) to get into fields early in the season when the fields are still wet and muddy. And with the use of power steering you even sometimes see rather large, multiple wheels on the front when the tractors are 4-wheel drive. It is likely this would not be possible without a power assist for steering.

I would guess that if the fields were always firm the choice would be to go for moderate size wheels simply to save money, but getting stuck will lose money for the farmer.

If you are discussing this with your students, you might want to try calculating the weight of a vehicle by tracing the foot print of the tires while parked. Measure the air pressure in the tires. Then multiply the PSI of pressure times the number of square inches that the tires touches the ground.

Back to your original question... the greater the force and the smaller the area the more likely a wheel is to sink into the surface.

Larry Krengel



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