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.
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)