Why do tractors have big rear wheels and small front wheels?

You need maximum traction to plow and pull (hence the name 'tractor'). For that purpose, the rear wheels need to be large and wide enough. Why only the rear wheels? Because usually only they are powered by the engines.

That being said,four wheel drive tractors have all their wheels tall and wide. 

While transmitting power (in mechanical engineering) you deal in two type of coins: speed and torque. You can sacrifice speed for more torque, and vice versa. How can you do that? Say a small diameter gear drives a large diameter gear ..for visualization purposes imagine a ceiling fan motor geared to a ferris wheel. The motor has more speed but less torque, the ferris wheel has more torque but less speed.

So a diesel engine powers a shaft with small diameter, high speed and low torque. It is then geared to a wheel with a large diameter. The result? The wheel acquires more torque. What does that mean physically? It means that although the wheel has a slow rpm, it has enough "force" to do heavy duty stuff like pulling out of mud and plowing the field and pulling heavy things

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