why do tractors have big rear wheels

1- Driving wheels are bigger in dia, it may be in front or rear. Commonly tractors have rear wheels bigger as majority of implements are connected at rear and the implement weight (in part) gets added to tractor rear weight and u get added traction through rear wheels due to increased weight on rear wheels. 2- Whereas in combine harvesters, front wheel is larger than rear as most of m/c weight in concentrated in front. To enhance driving wheel traction, rear wheels are filled with water, add cast weights etc, as and when how much is required. 3- One aspect is common that smaller dia wheel will be the steering wheel.

This is so that it is easy to steer smaller wheel, has lesser weight on top of it. More over, smaller wheels while in steering mode, do not come in contact with body parts and we get max turn angle for a smaller turn radius requirements. 4- Now for driving wheels, we need to turn it slow in terms of angular rotation to get higher traction, but to achieve a particular speed it s radius need to be increased without increasing rotation speed.

So larger wheel gives us Traction due to slow revolutions we also get required speed due to bigger radius. High ground clearance which is one of the main off road operational needs comes as an added advantage. 5- With 2WD tractors, only part of engine power can be utilized as wheels start slipping beyond a certain load being pulled (traction has limits) which is approx 35-40% of engine power.

This is one reason that now 4WD tractors have gained popularity, which can trap approx 55% engine power. 6- To utilize engine power beyond 4WD drive tractors, PTO drive implements have gained popularity which can utilize approx 80% of engine power.

When PTO implements are milling the soil, traction power required through wheels is 10% and bulk of power is utilized by PTO (power take off shaft)
There are more or less specialist tractors for different purposes and these deviate from the 'norm', but a general purpose farm tractor needs good ground clearance if might be used for cultivating row crops. The large rear tyres can also be ballasted with water for traction, though I don't know if this is still common - I've seen a lot od weights attached which serve the same purpose and are presumably easier to remove/refit.

Ground clearance can be maintained with smaller wheels at the front as stub axles can be used. I haven't given it much thought but I think he hitch height also needs to be below the axle height to prevent the tractor rotating when pulling a heavy trailer or ploughing.