Wheel Sizes in Vehicles
Name: Oum Prakash S.
Date: July 2004
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.
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
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.
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
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.
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Update: June 2012