Tractor means (traction + motor)so in case of tractor the
requirement of torque is much higher then the other
vehical, as the application where the vehical uses farm,
off-road application dozer and loder, so it needs high
torque in driver wheel to prevent the machine to get slipe.
in tractor there are few speed reduction stages like
transmission, differentail, and final reduction (with bull
gear or planetory) thru bigger tyre can increse the contact
area with soil, which help to increse torque as the torque
(T= r(is the displacement)* f(is the force vector)
its not only a case of tractor, but for max. vehicles.
Basically driver wheel is always kept larger than the
driven where all-wheel-drive is not used. Driver wheel is
larger to utilize and conserve torque, as larger the wheel,
more the M.I. ,hence it can attain rotation for longer
duration. But as front wheel is not powered instead it is
just pushed, its M.I. should be less so that it can be
easily rotated,and thus less power is wasted in moving the
wheel. hence to keep MI low, it is of lower size. thats
Tractors are mainly used for farming purposes where the soil
When the driven wheel (rear wheels in tractor) exerts
pressure on the ground the force applied on the ground
depends on the area of inpression the wheel makes on the ground.
Larger the wheel size is, then larger the area of
impression, hence smaller the force on loose soil whch helps
the wheels not to get into the ground.
Reasons for Having so is
1)Tractor needs Tractive force with low speeds which is
meant for soft terrains.
2)Since the Coefficient of friction between tyre and
surface contact is low in order to overcome this,tractor
produces high tractive force with low speed.
3)For steering stability and power consumption the front
wheel is designed small size.
4)Torque=Force * Distance(T=F*R) by keeping Force constant
and increasing Distance between Wheel axis and force
applied will increase torque which is desired
rear wheel is needed for producing maximum traction i.e.
grip on the ground therefore its area should be more.In
addition to this fact,it is also imp to give max torque and
min speed which is possible only when the size of the wheel
is large.On the other hand,front wheel is only used for
steering purpose so it's size should be kept small.